triangle zones – geometry, kinematics, mechanics, and the ...special structural elements in the...

Contents lists available at ScienceDirect

Earth-Science Reviews

journal homepage: www.elsevier.com/locate/earscirev

Triangle zones – Geometry, kinematics, mechanics, and the need forappreciation of uncertainties

Christoph von Hagkea,⁎, Alexander Malzb

a Lehr-und Forschungsgebiet für Geologie – Endogene Dynamik (GED) Rheinisch-Westfälische Technische Hochschule, Aachen University, Germanyb Landesamt für Geologie und Bergwesen Sachsen-Anhalt, Köthener Straße 38, 06118 Halle (Saale), Germany

A B S T R A C T

Triangle zones are important structures found in foreland fold-and-thrust belts all over the world and arecommonly associated with tectonic wedging. However, tectonic wedging and consequently the formation ofpassive hinterland verging roof thrusts requires particular mechanic conditions such as pre-fractured rocks,syntectonic sedimentation, or a specific stratigraphic layering with variable mechanically weak and rigid for-mations. In this contribution models of triangle zones and hypotheses regarding their formation are reviewed.Our results show that the term “triangle zone” is often used in a contradictory sense and interpretations areaffected by large uncertainties. Passive roof thrusts are not necessarily required in all cases and suggested modelsof triangle zones, even if they are geometrically and kinematically viable, are hampered by their mechanicalimplications. With respect to a large number of published triangle zones and associated kinematic as well asmechanic models we present a new definition and classification scheme, which is discussed and applied tonatural examples worldwide. We show that natural examples of triangle zones can be separated into two types:(1) detachment dominated and (2) ramp dominated triangle zones. Both types imply particular mechanicconditions of involved detachments and regional dynamics. Regarding the large uncertainties associated withtriangle zone interpretation we suggest to only use the term “triangle zone” in a very stringent manner and onlyif irrefutable evidences for stratigraphic repetitions within a duplex are given. The results should then be va-lidated with additional kinematic and mechanic considerations. Geometric and kinematic uncertainties as wellas implications of the proposed model explaining observed triangular structures should be reflected in the no-menclature used.

1. Introduction

Foreland fold-and-thrust belts are well-explored and intensivelystudied parts of orogenic systems worldwide as they comprise a com-plex structural inventory and play an important role as hydrocarbonplay (e.g. Nemcok and Henk, 2006). This research resulted in devel-opment and application of essential techniques in geosciences, such assection balancing (Bally et al., 1966; Boyer and Elliott, 1982;Dahlstrom, 1969; Laubscher, 1972). This method allows for quantita-tive prediction of structures in the underground and constructing self-consistently restorable sections. However, they require geometricidealization of structures and assumption of an underlying kinematicmodel (or several models). In addition to geometric and kinematic in-sights, a deeper level of understanding the formation of structures re-quires mechanic considerations. In their seminal paper on the devel-opment of chevron folds and echelon veins, Fletcher and Pollard (1999)showed that geometric and kinematic approaches often fall short of

acknowledging physical principles underlying the deformation process.They argue, even though geometric and kinematic models provideimportant insights, understanding the processes responsible for pro-ducing a particular structure requires appreciation of constitutive re-lations, boundary conditions and initial conditions together with fun-damental physical laws. Mechanics of fold-and-thrust belt may beaddressed using critical wedge analysis (Dahlen, 1990; Davis et al.,1983), physical (see e.g. Koyi, 1997 for a review), or numerical models(e.g. Finch et al., 2003; Hughes et al., 2014; Jaquet et al., 2014). Theseefforts have tremendously increased our understanding of geometries,fault development, propagation and linkage in fold-and-thrust beltsincluding fault and fold mechanics. A general result of these studies isthat it is not only of critical importance to understand individualstructures and thrust belt geometries, but also to untangle the sequenceof thrusting and tectonic events (Smit et al., 2003), and furthermorethat also kinematic solutions should at best be substantiated with me-chanical models.

https://doi.org/10.1016/j.earscirev.2017.11.003Received 31 May 2017; Received in revised form 1 November 2017; Accepted 1 November 2017

⁎ Corresponding author at: Institute of Structural Geology, Tectonics and Geomechanics RWTH Aachen University, Germany.E-mail addresses: [email protected] (C. von Hagke), [email protected] (A. Malz).

Earth-Science Reviews 177 (2018) 24–42

Available online 07 November 20170012-8252/ © 2017 Elsevier B.V. All rights reserved.

T

http://www.sciencedirect.com/science/journal/00128252

https://www.elsevier.com/locate/earscirev

https://doi.org/10.1016/j.earscirev.2017.11.003


mailto:[email protected]

mailto:[email protected]


http://crossmark.crossref.org/dialog/?doi=10.1016/j.earscirev.2017.11.003&domain=pdf

Critical taper theory describes taper of a wedge, i.e. the sum of to-pographic slope and basal dip, as a function of basal friction of thedetachment and strength of the deforming overlying wedge forCoulomb materials (Dahlen, 1990).Geometry and evolution of fold-and-thrust belts to first order depend on the basal (detachment) and internal(overlying section) friction of a critically tapered wedge and thus reflectthe rheology of the involved stratigraphic pile. Low taper angles areassociated with weak detachments (Davis et al., 1983). This has beenextensively explored in analogue experiments (see Buiter, 2012 for areview), as well as using numerical models (e.g. Willett, 1999), and wassuccessfully applied to many field areas around the globe, e.g. Taiwan(Suppe, 1981), the European Alps (Burkhard and Sommaruga, 1998;von Hagke et al., 2014a), Borneo (Morley, 2007), Zagros (Mouthereauet al., 2006), or the Andes (Weiss et al., 2015). In addition to these firstorder controls, geometry and kinematics may be influenced by the pre-thrusting history affecting the existence of structural and rheologicalheterogeneities and the formation of local mechanical weaknesses (e.g.Giambiagi et al., 2003; Madritsch et al., 2008). Furthermore thrust beltgeometries and kinematics are affected by the interplay of syn-tectonicsedimentation and erosion (e.g. Bonnet et al., 2007; Fillon et al., 2013;Mugnier et al., 1997).

Special structural elements in the focus of research on fold-and-thrust belts are triangle zones, which are described worldwide (Fig. 1)and form, among other structural elements, typical locations for hy-drocarbon traps. Often observed at the front of fold-and-thrust belts,triangle zones may provide insights on the processes at play during faultpropagation (MacKay et al., 1996). They were first described asmarking the boundary between plains and foothills of parts of thesouthern Canadian Cordillera (Gordy et al., 1977), and have been ori-ginally defined as a seismic expression at the frontal edge of a fold-and-thrust belt. Their definition was soon modified and expanded to de-scribe tectonic structures (MacKay et al., 1996). Since then, multipleterms for triangle zones have been used interchangeably, for instance“underthrusts”, “delta structure”, or “wedge into split-apart structures”(Laubscher and Bernoulli, 1977; McClay, 1992; Price, 1986; Roeder,1967). Partly reflecting the different interpretations of their internalgeometry or evolution, under the umbrella “triangle zone” multipletypes of structures in fold-and-thrust belts have been described (seecross sections in Fig. 2). Whereas Morley (1986) suggested trianglezones to form only at the frontal toe of fold-and-thrust belts (thrustfronts type 1 sensu Morley), other authors expand the term to triangulargeometries, which have formed behind the orogenic front. Further-more, several terms have been coined for particular triangle zones:Tectonic wedges imply progressive furcation and underthrusting ofautochthonous cover strata (Price, 1986). If internal shortening bystacking of tectonic slices within the tectonic wedge occurs, the re-sulting structure is called a “passive-roof duplex” (Banks andWarburton, 1986; Jones, 1996). “Intracutaneous wedges” may be usedas synonym for “triangle zone” but sometimes implies low taper ex-tension of tectonic wedges (Lawton et al., 1994). If several detachmentsare involved, triangle zones have been called “fishtail structures”(Harrison, 1995; Sans and Vergés, 1995). In contrast to the classicalthin-skinned scenario linked to foreland fold-and-thrust belts, the con-cept of triangle zones was extended to orogen-wide scales, where entirenappes have been classified as tectonic wedges (Fallot, 1949). Severalauthors adopted the process of tectonic wedging to lithospheric scalesin many orogens like the European Alps (Schmid et al., 1996) or theCantabria Mountains, Spain (Gallastegui et al., 2016; Pedreira et al.,2015). Lithosphere-scale triangles have been termed “crocodile struc-tures” mostly in the Alpine community (Laubscher, 1990; Meissner,1989), whereas elsewhere they are referred to as “flakes” (Oxburgh,1972). It was suggested that tectonic wedging may lead to reversals inpolarity of subduction zones (Oxburgh, 1974). In this model, the platesswitch roles as the upper plate migrates through the lower platebounded by two crustal scale faults of opposite vergence. However,subduction polarity reversals seem to be much more complicated, do

not involve tectonic wedges, and are a 4D rather than a 2D problem(Clift et al., 2003; von Hagke et al., 2016).

The enormous variability of terms, concepts and structures sum-marized as triangle zones implies that an overarching definition oftriangle zones encompassing its entire current usage in literature maybe “a triangular structure bound by thrusts of opposite vergence in fold-and-thrust belts or orogens”. Apart from its breadth, a shortcoming ofthis definition is its pure geometric nature. This invites possibly mis-leading or purely hypothetic interpretations of the structure of theunderground, as often geometric models lack appreciation of their ki-nematic consequences, and kinematic models commonly do not discussuncertainties as well as geologic implications. At the orogen-scale it hasbeen shown that mechanical validation of the crustal wedge in theEuropean Alps may dramatically decrease possible geometries, and thatmechanically consistent models must be closer to reality than scenariosmapped from a combination of geophysical or geological data (Burovet al., 1999). The ultimate goal of this contribution is to show thatappreciation of uncertainty in structures, and in particular discussingmechanic implications of kinematic models, is required to prevent suchmisleading interpretations. This is of general importance, but is espe-cially true for triangle zones, which require particular mechanic con-straints to form. We suggest a new classification scheme of trianglezones with the hope to trigger critical assessment of any triangularshape observed in fold-and-thrust belts or orogens.

In this contribution we first describe natural examples of differenttriangle zones, covering many of the basic geometric and kinematicmodels proposed around the world. We then discuss geometric, kine-matic and mechanic models and use these to suggest a new classifica-tion scheme. We then apply the classification to natural examples be-fore ending with a discussion on the respective uncertainties andpossible 3D extension of triangle zones.

2. Exemplary models of suggested triangle zones

Since their first description in the foothills of the southern CanadianCordillera (Gordy et al., 1977) triangle zones have been interpreted onseismic data and in the field in many fold-and-thrust belts and accre-tionary prisms worldwide, including for instance the Sulaiman Moun-tain belts, Pakistan (Banks and Warburton, 1986), Ural, Russia(Sobornov, 1992), the Rocky Mountains, USA (Chapin et al., 2014), theAlps, Europe (Müller et al., 1988; Ortner et al., 2015), the Jura fold-and-thrust belt, Switzerland (Malz et al., 2016; Sommaruga, 1997,1999), the Himalayas (Yin, 2006), the Hikurangi subduction margin,New Zealand (Morley et al., 2017), or the Argentinian Andes (Zamora-Valcarce and Zapata, 2015; Zapata and Allmendinger, 1996), (see Fig. 1for locations and examples of published triangle zones). The type ex-ample of triangle zones forms the foothills of the Canadian Cordillera(Gordy et al., 1977), where an approximately 20 km wide tectonicwedge with internal stacking of thrust sheets is topped by a hinterland-verging back-thrust (Fig. 1A) (Bégin et al., 1996). The structure isdominantly of thin-skinned style and all thrust faults of the internalwedge lead into an upper detachment level, where the back-thrustprogressively evolved. Such back-thrusts in which the sequence aboveremains attached to the foreland (i.e. has not been displaced) com-monly called “passive” (McClay, 1992). This seems confusing, as theyaccommodate shortening at the same time as the “active” forethrusts;however, as this term is very common and deeply ingrained in literaturewe do not attempt any corrective usage recommendation and adopt it inthe following.

Barnes and Nicol (2004) describe an active subduction-related in-version structure in eastern New Zealand. The dominantly thin-skinnedstructure is linked to an approximately 5 km deep weak shale detach-ment (Barnes and Nicol, 2004). In their study they used a purely geo-metrical assumption to describe the structure as a “thrust trianglezone”. However, regarding the structural and kinematic interpretationthis structure is strictly an inverted half-graben (Fig. 1B). A further

C. von Hagke, A. Malz Earth-Science Reviews 177 (2018) 24–42

25

geometric concept for a triangle zone was applied for a polyphasestructure in the Agrio fold-and-thrust belt in the foreland of the Ar-gentinian Andes (Zamora-Valcarce and Zapata, 2015), (Fig. 1C). Incontrast to classical triangle zone interpretations the structure consistsof two thin-skinned thrust faults of opposite vergence framing a trian-gular shaped block in section view (see geometric models, Section 3).During a late-stage phase the structure was affected by thick-skinnedthrusting, which is also assumed to be responsible for triangle zoneformation for other parts of the Andean Precordillera (Zapata andAllmendinger, 1996). Such two thrust faults of opposite vergence havealso been described in the Coalinga region in the central CaliforniaCoast Range (Namson and Davis, 1988).

Based on the tectonic wedging model (Price, 1986) including in-ternal shortening of the wedge, Sobornov (1994) presented a thin-skinned structure in the northeastern Greater Caucasus, Russia. We notethis structure formed towards the north, i.e. at opposite vergence to the

main orogenic thrust direction, which is to the south. Associated thrustfaults ramp up from the basal detachment and lead into a secondarydetachment where the passive back-thrust developed (Fig. 1D). As theauthor could show, a stratigraphic pile with weak horizons favors thedevelopment of single or multiple detachments and influences tectonicwedging. A more or less simple single-detachment scenario is presentedfor the southern Pyrenean foreland (Sans et al., 1996), where one thickevaporitic formation favored thin-skinned thrusting (Fig. 1E). Wherethe evaporitic formations pinch out in the foreland a narrow anticlinewith thrust faults of opposite vergence developed. This variety of tri-angle zones observed in nature is reflected in several geometric models,which we will discuss in the following.

3. Geometric models of triangle zones

It is important to note that all geometric concepts are based on

0°180° 180°

0°0°

BB

AA

DD

CC

EE

Thrust Triangle Zone

10

km

B

10

km

Faulted Triangle Zone

C

5 km

3 s

TW

T

Thrust Triangle Zone

A

10

km

E

Triangle Zone

10

km

Triangle Zone

D

Fig. 1. Compilation of triangle zones worldwide and selectedexamples: A - Canadian Rocky Mountains (Bégin et al., 1996); B- Hawke Bay, New Zealand (Barnes and Nicol, 2004); C - Neu-quén fold-and-thrust belt, Argentina (Zamora-Valcarce andZapata, 2015); D - Northeastern Caucasus (Sobornov, 1996). Wenote thrusting in this section is opposite the main orogenicthrust direction, which is to the S; E - Barbasto anticline,southern Pyrenees (Sans et al., 1996).


26

observations in 2D sections. Extension of these triangles along-strikeresults in wedge-shaped geometries and may include duplexes if nappestacking occurs within the triangle zone. We discuss the three dimen-sional geometry of triangle zones below (Section 6). 2D Geometricconcepts commonly result from interpreted seismic sections orientedperpendicular to the orogenic front. Only limited studies exist whereentire triangle zones are observed in outcrop (e.g. Ortner et al., 2015;Tanner et al., 2010) Often seismic interpretations are supported withfield data, however recognizing a triangle zone at depth may be chal-lenging from field data alone. This is because opposite dips at thesurface do not necessarily require triangle zones at depth, but may re-sult from multiple structural configurations. For instance the fold-and-thrust belt of the European Alps has been studied since the early days ofgeology as a science (Studer, 1825), and detailed structural mappingbased on field data was carried out (Ganss and Schmidt-Thomé, 1953).However it was not until the end of the 20th century the triangle zonehas been recognized due to availability of seismic data (Müller, 1984;Ziegler, 1987).

A simple geometric model of a triangle zone is the tectonic wedge(Fig. 2A) (MacKay et al., 1996). It is the result of two conjugate faults.This model is widely used for triangle zones at all scales and can includeinternal duplexing. Characteristic for this model is the existence of ahinterland-verging thrust. A variation of this model is the “inter-cutaneous wedge” (Fig. 2B) with a flat sole thrust and a passive roof

thrust (McClay, 1992). In the “two-folds” model (Fig. 2C) McClay(1992) suggested use of the term “triangle zone” where two fault-bendfolds of opposite vergence enclose a triangular-shaped structure. Jones(1996) noted that in this model it is unclear what happens when thetwo fault-bend folds interact, as progressive and synchronous thrustingat both faults will lead to a space problem in the crest of the structure asboth faults converge. Likewise, the sequence of fault activity in such atriangle zone is unclear. Assuming a simple in-sequence scenario for theentire structure in Fig. 2C, the foreland verging thrust would becomeinactive when thrusting at the hinterland-verging fault begins, which isexemplified by Zamora-Valcarce and Zapata (2015) for a natural ex-ample. At this point, the shown triangle zone (thick thrusts in Fig. 2C) issimilar to the tectonic wedge model, where a hinterland and a forelandverging (detachment-related) thrust enclose the triangle. A modifica-tion of this is the “pop-up” model of Butler (1982). Note that the termpop-up is also used in strike slip tectonic settings for positive flowerstructures, which is a completely different geometry. However, Butler'spop-up is strictly a “two-folds” model integrated in an in-sequencethrust system (Fig. 2D). Subsequent shortening of this system will leadto the propagation of the entire stack, which will then be folded above aramp-flat geometry in the foreland. While Butler (1982) mentioned thatall thrusts developed asynchronous and only interact geometrically,McClay (1992) defined a pop-up as “a section of hangingwall strata thathas been uplifted by the combination of a foreland verging and a hin-terland verging thrust”. He thus left room for speculation whether thebinding thrusts in a simple pop-up can be active at the same time.However, regarding the similarities of pop-up structures and trianglezones, the most important feature is a foreland and a hinterland vergingthrust enclosing a triangular structure (Fig. 2D). From the CentralApennines an example is known, where transpressional inversion ofeither symmetric fault-bounded grabens or asymmetric half grabenscauses formation of a pop-up structure (Pace and Calamita, 2014).Depending on the amount of thrust displacement, the resulting struc-tures very much resemble positive flower structures as known fromstrike slip faults, and the transition from triangle zone to positive flowerstructure is smooth. Especially if several secondary detachments areinvolved, the similarities of positive flower structures and these trianglezones are striking, as both typically form narrow anticlines and acomplex fault pattern. At depth, these structures may theoretically bedistinguished based on fault dip, as strike slip structures are commonlysub-vertical, whereas thrust dips are more shallow. However, particu-larly steep structures are not well-resolved in seismic data, and inter-pretation of the overall geometry is difficult even when individualstructures are resolved well and described in detail (Madritsch et al.,2013), but additional regional geological information such as paleos-tress analysis is not considered. We discuss the evolution of pop-upstructures during continuous shortening in Section 5.2.

Special types of triangle zones with several detachments involvedare “fishtail” or “zig-zag” structures (Drozdzewski, 1979; Harrison andBally, 1988; Harrison, 1995; Sans and Vergés, 1995). These structuresform when the deeper detachment connects to the uppermost detach-ment by a ladder of thrust ramps of opposite vergence. In a strict sensethese structures consist of several triangle zones on top each other, ormay be seen as triangle zones with a complex triangle tip (Fig. 2E).Predictions of geometric models may be tested with kinematic models,which we discuss in the following.

4. Kinematic models of triangle zones

Starting from geometric considerations various kinematic modelscan be derived, where the apparent geometric simplicity of trianglezones allows for retro-deforming suggested geometries. Here we focuson the kinematic validity of triangle zones consisting of a duplex boundby two faults of opposite vergence, which were active synchronously.For such triangle zones, several kinematic models were suggested.

In general, three different scenarios can be assumed for roof

E

Triangle ZonesTriangle Zones

Fish-tail Structure

Pop-Up

D

Triangle Zone

Triangle Zone

C

intercutaneous wedge

B

Tectonic Wedge

A

Fig. 2. Geometric models of triangle zones taken from literature (dashed lines representinactive thrusts); A: Tectonic wedge (Price, 1986); B: Intercutaneous wedge (McClay,1992); C: Triangle zone (McClay, 1992) The triangle is defined by the triangular shape ofundeformed beds between two thrust faults of opposite vergence.); D: Pop-Up (Butler,1982) as a special case of a triangle zone. Note: All geometric models associated with theterm “triangle zone” can be defined by a foreland and a (passive) hinterland vergingthrust fault (thick black lines).


27

thrusting associated with duplex formation based on a stratigraphiclinkage (Fig. 3): (1) Active roof duplexes require two individual andregionally active detachments, which are decoupled from each other.The upper detachment accommodates more displacement than thelower, and consequently the strata above the roof thrust over-thruststhe entire foreland migrating duplex system. In this model the strati-graphic section in the foreland is allochtonous; a bedding parallel thrustforms in front of the duplex (Fig. 3A). Note that in this model theforeland-dipping part of the upper detachment would be observed as anormal fault in the field. However, active roof duplexes (Couzens andWiltschko, 1996) do not strictly follow the geometric characteristics oftriangle zones, which have a hinterland verging (passive) roof thrust.(2) A passive roof duplex requires two regionally active detachmentsthat are kinematically linked in the foreland (Fig. 3B). Continuousthrusting results in shortening between the lower and the upper de-tachment and consequent formation of a duplex bound by two thrusts ofopposite vergence. (3) In case of a single regional detachment thestratigraphic section in the foreland and hinterland are linked. Upwardmigration of the detachment to a higher stratigraphic level results in thedevelopment of a tectonic wedge, and the hinterland verging roof thrustforms a ramp bounding the front of the duplex (Fig. 3C). These firstorder kinematic models show that for triangle zones with a passive roofthrust, no shortening towards the foreland along the upper detachmentoccurs, i.e. the foreland stratigraphy is undisturbed. In case of a singledetachment, also the hinterland shows an undisturbed stratigraphicsequence.

4.1. Triangle zones with a single-detachment

Charlesworth and Gagnon (1985) provided a variation of the tec-tonic wedge model with two thrust faults of opposite vergence in-cluding a ramp in the hinterland (Fig. 4). In this model, a thrust rampsup through the stratigraphic section and a passive roof thrust forms

close to surface. Triangle formation is associated with a single detach-ment only, and the entire structure can be described as a ramp-flatgeometry with a passive backthrust at the frontal tip of the flat (cf.Fig. 4B). Progressive thrusting along the thrust leads to accretion of newmaterial into the triangle zone (Fig. 4C), and folding at the ramp edgeoccurs due to fault-bend-folding (Suppe, 1983). If further thrust sheetsare integrated into this system, the internal structure of the trianglezone can evolve towards different degrees of complexity and may in-clude duplexes(Fig. 4D). Such structures are sometimes called alloch-tonous roof duplex (Boyer and Elliott, 1982; Jamison, 1993). Couzensand Wiltschko (1996) classified these triangle zones involving only asingle detachment as “Type 1”. This model has been successfully ap-plied in the Rocky Mountain foothills (Soule and Spratt, 1996), or at thethrust front of the eastern Jura fold-and-thrust belt, where the frontalthrust furcates into an upper detachment with associated triangle for-mation near surface (Malz et al., 2015). However, to be kinematically

A

B

C

floor thrust (lower detachment)



passive roof

thrust

passive roof

thrust

active roof

thrust

active roof

thrust

passive roof thrust

(with thrust ramp)

passive roof thrust

(with thrust ramp)

Fig. 3. Different scenarios of duplex structures related to bedding-parallel thrusting andtheir relative fixation of beds; A: active roof duplex evolved due to complete decouplingand thrusting towards the foreland; B: passive roof duplex formed due to complete de-coupling in the hinterland and fixation of beds in the foreland forming an intercutaneouswedge. Note: If further stacking in the duplex occurs, the passive roof thrust will becomeeroded and no further link to the roof thrust is present. The bedding parallel backthrust inthe hinterland will become inactive; C: Triangle zone evolved due to fixation in theforeland and the hinterland. Further thrusting results in growing of the duplex and suc-cessive erosion of the backthrust defining a nearly constant length of the backthrust evenif the triangle zone propagated towards the foreland.

triangle tiptriangle tiptriangle tip


limb rotation


FAFAFA

limb rotation


FAFAFA

A

B

C

D

E

Fig. 4. Kinematic model of a single-detachment triangle zone (inactive and further thrustsare shown as stippled lines); Two conjugate faults associated with a lower and an upperdetachment result in formation of a triangle zone, where backthrusting occurs in front ofa ramp-anticline (Charlesworth and Gagnon, 1985). Due to ongoing shortening the tri-angle tip propagates towards the foreland, further thrust sheets are stacked within aduplex and folding and steepening of the front limb occurs, which also affects thebackthrust (D). After enough shortening is accommodated, the backthrust is too steep toremain active (E). At this point it remains unclear how the structure evolves due to furtherthrusting.


28

viable, some constraints are essential: (1) This model requires a weakdetachment, where progressive underthrusting at the triangle tip canoccur. As a consequence, the triangle tip propagates towards the fore-land, which implies that the length of a well-defined slice of strata lo-cated between the triangle tip and the front limb of the ramp anticlineremains constant (Fig. 4). (2) Deformation causes folding at the rampedge and steepening of the passive roof thrust as well as the back limbdue to limb rotation (Fig. 4D). Continuous deformation will result inprogressive steepening of the roof thrust from its tip migrating down-wards to the triangle tip, eventually leading to a sub-vertical or evenoverturned roof thrust (Fig. 4E). When the acting normal stress σn isoriented perpendicular to the fault surface, there is no shear stress andthe fault becomes stable (see also Discussion in Section 5). Assumingthe stress field remains constant this implies that after a certain angle isattained, the steepened roof thrust becomes mechanically unfavorable.Consequently deformation in such a structure can only be pertained fora limited amount of shortening, and long-lived deformation seems im-plausible.

4.2. Triangle zones with multiple detachments

Similar final geometries may be achieved with involvement of twoor more detachments, possibly related to presence of additional weaklayers within the stratigraphic section (intercutaneous wedge cf.McClay, 1992). These models show in their initial phase two beddingparallel detachments connected by a thrust ramp (Jones, 1982) (Fig. 5).Initially, deformation above the lower detachment is taken up by dis-tributed strain. Ongoing convergence leads to thrusting at a ramp,which connects to the upper detachment, and activates it as a passiveroof thrust (Fig. 5B). Additional thrust slices are accreted resulting in aduplex with a passive roof thrust. Like the single detachment model(Fig. 4), this scenario is only kinematically viable if a weak detachment

exists but propagation of the frontal triangle tip towards the foreland isimpeded (Fig. 5). Nevertheless, it is important to note that the forma-tion of a duplex with several thrust ramps and slices provokes distinctfolding and steepening of the fold limbs at the upper ramp edges. Theseresults in an over-steepened upper detachment, which at some pointmust become mechanically stiff (see discussion below). Furthermore,continuous thrusting will raise the passive-roof thrust above the level oferosion, and the upper detachment in the hinterland becomes inactive(Fig. 5D).

4.3. Triangle zones forming behind the leading edge of fold-and-thrust belts

Starting from the same initial configuration of two bedding paralleldetachments, Tanner et al. (2010) show a combination of several thrustsheets within a triangle zone that does not include an internal duplex asthe result of foreland propagation. Instead, out-of-sequence thrustingassociated with semi-ductile folding is suggested for triangle zone for-mation (Fig. 6). Their cross section is based on area balancing due tointernal strain of the fault blocks. Strain is accommodated along apassive roof thrust, which can also be observed in outcrop (Tanneret al., 2010). Nevertheless, this model strongly varies from the kine-matic models mentioned above. The most important difference is thatTanner et al. (2010) suggest a spatially fixed triangle tip (Fig. 6). In thisscenario, an initial fault-bounded horse is pushed over a ramp. There-after, the entire structure is not able to propagate towards the forelandand the fault-bend fold detaches from the horse block. Ongoing short-ening results in a backbreaking sequence of thrust sheets, i.e. accretionof material occurs behind the fault-bend fold. It was shown elsewherethat formation of triangular structures within fold-and-thrust belts be-hind the orogenic front may be related to out-of-sequence thrusting


limb rotation



A

B

C

D

Fig. 5. Multiple-detachment triangle zone model (Jones, 1982) with a foreland-propa-gating triangle tip. Due to ongoing shortening thrust sheet stacking within a duplex andlimb rotation occurs probably making detachment-related backthrusts mechanically in-effective. Furthermore, the backthrust in the hinterland becomes inactive if the duplex iseroded.


wedge propagationwedge propagationwedge propagation

??


limb rotation

A

B

C

Fig. 6. Kinematic models of an out-of-sequence triangle zone (future and inactive thrustsare shown as stippled lines); Two bedding-parallel detachments result in formation of atriangle zone, where thrust sheet stacking within a duplex occurs in a thrust sequencetowards the hinterland (out-of-sequence thrusting), and the triangle tip is spatially fixed(Tanner et al., 2010). Because the triangle tip is fixed, further thrusts and consequentlythe wedge propagate towards the hinterland. Nevertheless, due to ongoing shorteningfault-related folding and limb rotation occurs. Thrusts evolving in the backlimb of thewedge will be steeper than earlier thrusts, which renders the structure mechanicallydifficult (question mark in C).


29

(e.g. Butler, 1982; Schori et al., 2015). However, a backbreaking se-quence of thrust ramps leads to successive steepening of the anticline'sback-limb, independent of semi-ductile conditions. In this model eachyounger thrust shows a steeper initial ramp angle with respect tobedding dip (see Fig. 6C). Such steep thrusts are mechanically difficult,and in-sequence formation of the duplex seems a viable alternative.Furthermore, even though kinematically possible, out-of-sequencethrusting has mechanic implications not addressed in the out-of-se-quence model, which we will discuss below.

4.4. Multiple triangle zone model

Kinematic models including more than one evolved triangle zoneswere suggested by Banks and Warburton (1986). Starting from a two-detachment configuration similar to the solution presented by Jones(1982) (Fig. 5) the passive roof thrust links to a bedding parallel upperdetachment. Shortening causes activation of a hinterland verging thrustlocated at the front of a narrow ramp anticline at the lower forelandverging thrust (Fig. 7). Further thrusting and stacking at the lowerdetachment leads to propagation of the entire system towards theforeland and additional new triangle zones develop for every thruststack (Fig. 7B). The entire system evolves in-sequence (Fig. 7C). Due tosuccessive thrusting and duplex formation at the lower detachment, theupper detachment will be passively folded and the dip of the upperdetachment changes. As a consequence, ramps of younger passive roofthrusts in the overlying strata become steeper (Fig. 7C), which may

impede their activity (see discussion on mechanics below). Ongoingpropagation leads to a widening of the triangle zone and the roof de-tachment becomes flatter. At this point the roof thrust reaches a fa-vorable position and may be activated along its entire length (Fig. 7D).This causes fault activity behind the thrust front.

In general, models of triangle zones, which evolved during onesingle tectonic event (i.e. do not require changes of kinematic boundaryconditions), can mainly be summarized according to the amount ofdetachments. This systematic only includes geometric and kinematictriangle zone models applicable for fold-and-thrust belts but do notaddress the mechanic conditions necessary for formation of hinterlandverging faults. Therefore, mechanistic approaches must be addressed intriangle zone interpretations, which will be discussed in the following.

5. Mechanic models of triangle zones and passive roof thrusting

Even though geometric and kinematic models can help to improvethe consistency of triangle zones and understand their evolution, theydo not address the mechanical problems posed by the formation andsustainability of hinterland verging thrust faults. What is the mechan-ical reason resulting in passive faults of opposite vergence?Understanding why backthrust form is the essential step towards un-derstanding triangle zone mechanics. Solutions have been providedfocusing on backthrusts related to a single detachment, as well as amechanically layered stratigraphic sequence, and both numerical andanalogue models have been used. These models provide insights on theevolution and distribution of stress and strain in evolving experimentsand, if properly scaled, provide insights on fault mechanics in fold-and-thrust belts. Note however that these models are usually not able tocover all complexity of nature and therefore are simplistic. Even thoughthey are important for our understanding of processes, they as of nowcannot necessarily rule out particular scenarios.

5.1. Mechanic concepts for triangle zone formation

5.1.1. Mechanic concepts and single detachment solutionsIn simple analogue experiments fold-and-thrust belts can be mod-

eled using only one material such as cohesionless sand, or with a singleweak detachment, commonly represented by glass beads or siliconputty, and a mechanically stronger homogenous overburden. Thesemodels are often used to reproduce the general structure of fold-and-thrust belts or to unravel the sequence of thrusting (e.g. Smit et al.,2003). In such models commonly forethrusts (i.e. foreland-directedthrusts) dominate, but occasionally backthrusting is observed, often atthe same time when forethrusts are active (e.g. Lohrmann et al., 2003;Nieuwland et al., 2000). Analogue as well as numerical models showsuch backthrusting occurs where the footwall flat transitions to thefootwall ramp (Ellis et al., 2004). This is because shear stresses localizein fold kinks due to thrusting and rotation of the overburden associatedwith fault-bend-folding (cf. Cristallini and Allmendinger, 2002; Suppe,1983). However, in such models systematic formation of triangle zonesis not observed, particularly not at the leading edge of deformation.This indicates additional mechanic constraints are required for back-thrusting. In general, criteria like fault geometries, fault friction as wellas material properties and anisotropy are invoked to explain the or-ientation of faults and shear planes in thrust systems (Erickson et al.,2001). Using critical taper theory it has been shown that backthrustspreferentially form at low taper angles, where σ1 is roughly parallel tothe basal detachment, and fore- and backthrusts consequently have thesame dip-values and thus the work to create them is similar (Bilotti andShaw, 2005). For brittle-ductile wedges, analogue models show thatvery low basal strength favors orientation of principal stress σ1 towardsthe interior of the wedge (σ1 stress axis dipping towards the hinterland),and thus the formation of backthrusts (Bonini, 2007). In such modelswith a non-Coulomb rheology used for the detachment, hinterlandverging thrust faults only evolve in a small range of low strain rates

limb rotation





roof thrust propagationroof thrust propagationroof thrust propagation

floor thrust propagationfloor thrust propagationfloor thrust propagation

A

B

C

D

Fig. 7. Kinematic multiple-triangle zone model; Several triangle zones develop due toforeland propagation (Banks and Warburton, 1986), where two thrust systems evolverelated to different detachments. Also in this scenario limb rotation occurs in the uppersystem (C), which makes the frontal backthrust mechanically ineffective and possiblyearlier backthrusts are reactivated during ongoing shortening.


30

(Gutscher et al., 2001). This may indicate that low shear stress is ne-cessary for backthrust formation, as supported by numerical modeling(MacKay et al., 1996). Using numerical models of critical tapers it hasbeen show that backthrusts form when differences along dip of thebasal detachment favors variable stress orientation, or if the effectivebasal friction increases towards the foreland (Cubas et al., 2013).Changes in detachment rheology as potential key for triangle zoneformation is corroborated by a case study in the southern Pyrenees,where presence of triangle zones is linked to the distribution of eva-poritic basins (Sans et al., 1996). Similarly, in the Damara Belt, Na-mibia, backthrusts are associated with stratigraphic changes within thebasal detachment (Kukla, 1992).

Alternatively, backthrusts can evolve in the hanging wall of thrustramps. Using analogue and numerical models, respectively, Bonini et al.(2000) and Erickson et al. (2001) show that shallow-dipping back-thrusts evolve due to enhanced ramp friction and bedding-parallelshear bands become reactivated with a hinterland sense of shear whenthe hanging wall reaches the upper ramp edge (Fig. 8). An example ofthis scenario may be the frontal triangle zone in the Venetian Alps,where the backthrust forms exactly at the upper ramp edge (Doglioni,1992). Backthrusts forming due to weak overburden and relatively highramp friction are also known from the Banda Arc (Morley et al., 2017).

5.1.2. Two or more detachment solutionsAll the examples above relate to backthrusts forming over a single

detachment horizon (Fig. 2C and D). The concept of Coulomb failurecriterion can also be applied to mechanically layered sequences. In aninclined foreland basin with a layered stratigraphic sequence and awedge-shaped overburden of sediments the position of fractures is well-defined based on the magnitude of differential stress and associatedshear stresses (Fig. 9). The basal fault dip may be variable, and alsochange down-dip. Typical values from fold-and-thrust belts and accre-tionary wedges range between 1 and 9° (Davis et al., 1983). With theassumption of constant forces in the entire orogenic system (Molnar andLyon-Caen, 1988), differential stresses in a wedge increase towards theforeland and the wedge top, respectively (see Hindle, 2008 for a dis-cussion). Fractures and thrusts in such a detached system can onlyevolve in that part of the wedge where shear stresses τ in the detachedoverburden exceed critical shear stresses τcritical (Fig. 9). Fracture in-itiation may occur in multiple locations within the stratigraphic se-quence, preferentially in the mechanically strong units. As a con-sequence, in front of wedge-shaped foreland basins fractures evolve in a

wide area, and are conjugate and distributed in mechanically stiff layers(Fig. 10). These fractures evolve to proto-thrusts, which may be fore-land or hinterland verging (e.g. MacKay, 1995). Analogue experimentsof thrust initiation show a three-stage evolution, first formation of weakshear bands, second strain localization when peak strength of the ma-terials is reached, and third, stable sliding of the new thrust (Dotareet al., 2016). Furthermore, it has been shown that it requires less worknucleating thrusts at shallow depths than at the basal detachment (DelCastello and Cooke, 2007). Consequently, the proto-thrusts propagatedownwards and connect to the propagating fault in the weak detach-ment (Eisenstadt and De Paor, 1987). These considerations underlinethat presence of mechanically weak layers in the stratigraphic columnmay be a controlling factor for vergence changes in one thrust system(Morley, 1986). We speculate in addition to presence of a weak layer,initial orientation of the weak shear bands during the very early stagesof deformation is important for triangle zone formation. The dominantinfluence of mechanical layering on backthrusting has been supportedby a review of stratigraphy and geometry of selected triangle zonesworldwide, suggesting the existence of weak layers and their mechan-ical characteristics is responsible for the localization of triangle zones(Couzens and Wiltschko, 1996). The smectite-illite transition may besuch a mechanical boundary responsible for backthrust formation(Couzens and Wiltschko, 1996). Similarly, analogue models with aweak basal detachment show that triangle zones and the formation ofbackthrusts are associated with at least one additional weak horizonwithin the cover sequence (Couzens-Schultz et al., 2003; Wang et al.,2013). High fluid pressures within a sedimentary package (i.e. hydro-static pressure / lithostatic pressure ≥ 1) decrease effective stresses.Overpressured sediments are commonly observed in foreland basins(Hubbert and Rubey, 1959). Such weak horizons can localize forelandpropagating deformation, which results in relatively thin, low-tapertriangle zones (Jones, 1996; Lawton et al., 1994). Applying this conceptto the Alberta fold-and-thrust belt Skuce et al. (1992) suggested acombination of mechanical stratigraphy and fluid overpressures is re-sponsible for underthrusting of a relatively thin section of sedimentaryrocks under a sedimentary sequence of an original thickness of>3000 m. Additionally, the shape of the wedge may be favorable forbackthrust formation; in the Alpine foreland it has been shown thatnarrow taper wedges and the associated inclination of σ1 supportsbackthrusting (Sommaruga et al., 2017).

These straight forward mechanical considerations based on theCoulomb failure criterion are necessarily simplifying a thus not able torepresent the mechanical development of all structures. Particularlywhere not purely frictional behavior but ductile processes occur, theapproach may fail to address all scenarios possible for formation oftriangular structures. Ductile or brittle-ductile behavior may alreadyoccur at shallow depths (e.g. carbonates do not require elevated pres-sures or temperatures to develop pressure solution features) (Gratieret al., 2013), and brittle-ductile deformation has been reported for in-stance from the Jura fold-and-thrust belt, which never was burieddeeper than 1-1-5 km (Laubscher, 1977). In 2D mechanical modelsfocusing on dominantly viscous behavior and including mechanicalstratigraphy of a layered sequence of more stiff and weak matrix itcould be shown that the ratio of viscosities of layers and matrix controlsdeformation style (Jaquet et al., 2014). The authors show that if theweak layer is thick enough (in their models as thick as the stiff layer),the layers act independently, and thrusts of both vergence are pro-duced. The resulting structures are very similar to structures observedin analogue models using viscous silicone putty interbedded withquartz sand. However, we still lack comparable numerical and analoguemodels addressing triangle zone formation in the purely brittle regime.

5.1.3. Inherited structures and syn-tectonic sedimentsIn addition to detachment rheology and mechanical layering of the

foreland deposits, the overburden may be pre-structured and includeinherited zones of mechanical weakness (e.g. pre-thrusting normal

shear bands

cross-cutting bedding planes

shear bands

cross-cutting bedding planes

bedding-parallel

shear

bedding-parallel

shearfuture backthrust

A

B

Fig. 8. Mechanical sketch of the evolution of backthrusts and tectonic wedging at a thrustramp (based on numerical mechanical models of Erickson et al., 2001); Hinterland ver-ging shear bands form due to high friction at thrust-fault ramps (A) and act as mechanicalheterogeneities during further thrusting. During continuous shortening the shear bandsmay become connected and are activated as hinterland verging thrust faults (B).


31

faults), which can act as a precursor for future triangle zones (Albaneseand Sulli, 2012). The role of inherited normal faulting on fold-and-thrust belt geometry was tested using analogue experiments with aviscous detachment (Ferrer et al., 2016). Results show that inheritanceis a main controlling factor, and thrust faults localize along oldernormal faults. Small basins may be completely preserved during laterinversion, and triangle zones with pop-up geometry (Fig. 2D) form. Atlarger scale, formation of a lithosphere scale triangle due to inheritedpassive margin structures has been argued for in the European Alps(Mohn et al., 2014). If preferentially oriented and weak layers exist inthe overburden, pre-existing normal faults can even be reactivated aspassive roof thrusts, as shown for the Jura fold-and-thrust belt (Malzet al., 2016). If thrusts face obstacles in the footwall, e.g. basementhighs or graben flanks, so-called “thrust-mills”may develop (Laubscher,1986). According to Laubscher (1986) they form when a mobile seg-ment of a thrust plane is pinned at its sides, and the mobile segmentbows out, eventually looping back on itself, similar to Frank-Readdislocation mills (c.f. Suppe, 1985). Another example is known from theVenetian Alps, where a triangle zone is localized along inherited ten-sional structures that have been reactivated (Doglioni, 1992). This tri-angle zone may also partly be controlled by syn-tectonic sedimentation,which we will discuss in the following, which acts as an additionalcontrolling factor to sustain triangle zones. Once triangle zones evolvedsyn-tectonic sedimentation and erosion (Mugnier et al., 1997) affectsgravitational loading of the wedge and thus the distribution of differ-ential stress in the thrust system (Adam et al., 2004). This is supportedby two-dimensional numerical models (Fillon et al., 2013) and ana-logue models (Driehaus et al., 2014) that showed a significant pre-ference of backthrusts when syntectonic sedimentation rates are higherthan the structural uplift of the thrust zone. In some areas of the fold-and-thrust belt of the European Alps presence of a triangle zone ispossibly related to the distribution of pre- to syntectonic conglomerates

(Ortner et al., 2015). However, in other parts of the basin, the trianglezone is possibly associated with inversion of Permo-Carboniferousbasement grabens (Burkhard and Sommaruga, 1998), lateral changes ofdetachment rheology, or a combination of both (Hinsch, 2013).

5.2. Evolution of triangle zone mechanics

Once a triangle zone developed either in a single- or multi-detach-ment scenario its further evolution and sustainability is highly influ-enced by mechanical characteristics. As shown above, triangle zonesare subject to limb rotation associated with fault-related folding. This isobserved in all kinematic models, but particularly in models where twoor more detachments are involved. This observation may challenge themechanic viability and sustainability of triangle zones. Fractures andfaults forming ramps at an angle that is suitable for thrusting (dip ofapprox. 30°; cf. Anderson, 1905) are rotated with the back limbs of thetriangle zones during continuous deformation (Fig. 5). They thuschange their orientation with respect to the surrounding stress states.Even if these faults can be assumed to be cohesionless and remain activeduring deformation, stress states for frictional activity of them arelimited. Moving along the faults is only possible if surrounding stressescause gliding along the fault rather than frictional failure in the sur-rounding material (cf. Sibson, 1985). If faults become steeper they canonly remain active if their rock friction coefficient is significantly lowerthan the usual value following Byerlee's law (approx. 0.75 to 0.85; cf.Byerlee, 1978; Sibson, 1983), or abnormally high fluid pressure con-ditions exist (Sibson, 1985). Otherwise thrusting at single faults will behampered according to limb rotation, which challenges kinematic sce-narios with single large backthrusts (single- and multiple-detachmenttriangle zones; see above) or accretion of material behind the triangletip (see above). In line with this, in the Venetian Alps it has been ob-served that triangle zones initially formed at the leading edge of de-formation have later been cut by progression of internal thrusts(Doglioni, 1992). In contrast, the multiple triangle zone model (seediscussion on kinematic models in Section 4.4) shows that fracturesonce evolved can be reactivated as large passive roof thrusts due toback-folding and subsequent rotation of fractures in a more ideal po-sition relative to the surrounding stress states. However, generally it hasto be noted that triangle zones are not easily preserved in the geologicrecord. Jamison (1993) used critical wedge equations in numericalmodels to show that backthrusts can only be sustained over geologicaltimescales if the taper of the fold-and-thrust belt is large enough; iftaper is too small the backthrust will be cannibalized by foreland ad-vancing of the wedge.

5.3. General notes on triangle zone mechanics

In summary, different mechanic concepts exist, and various factorsmay be responsible for backthrust formation. There seems no uniquesetting responsible for generation of triangle zones. As reviewed above,triangle zones may be the result of multiple detachment horizons,

τ

σAACC BBB

normal stress

shear s

tress

τ < τcritical

τ < τcritical

τ < τcritical

τ > τcritical

τ > τcritical

τ > τcritical

0 50

0

5

σ3

σ3σ3

σ1

σ1σ1

surface slope

basal dipAA

CC

BB

A B

depth

distance from backstop

Fig. 9. Mechanical concept of fracture development in a wedge-shaped foreland basin (A) and associated ratio of normal and shear stresses (B). Assuming constant forces in the hinterland(push of the orogen) stress distribution is directly linked to the taper of the wedge (thickness of overburden and orogenic push) and fractures can only evolve in area C. If mechanicallyweak layers decouple beds, fractures and thrusts in upper units evolve farther inland.

A

B

Backthrust initiationBackthrust initiation

fracture initiationfracture initiation

fracture connection in detachmentfracture connection in detachment

weak/ductile

stiff/brittle

weak/ductile

stiff/brittle

Fig. 10. Evolution of fractures and thrusts in brittle beds decoupled by mechanicallyweak layers (modified after Eisenstadt and De Paor, 1987). Foreland and hinterlanddipping fractures first evolve randomly distributed in brittle beds (A) and later becomeconnected by fracture propagation through weak layers. The connection of fractures caneither result in ramp-flat-ramp thrusts or triangle zones.


32

mechanical weaknesses in the overburden, inversion of basementstructures, or may be associated with areas of high ramp friction,changes in detachment rheology, or syntectonic sedimentation. None ofthese factors are mutually exclusive. In any case, one or more of thesecontrolling factors are required for formation of backthrusts and con-sequently triangle zones. Consequently, even if geometrically and ki-nematically sound, tectonic wedging may be dynamically not always afavorable solution, and independent evidence for at least one of thecontrolling factors must be sought.

6. Triangle zones in 3D

Triangle zones originally are a 2D concept, but must extend in 3D.There are few locations, where data coverage is extensive enough toimage triangle zones in 3D. Prominent examples are the Wheeler Ridge,CA, USA (Medwedeff, 1992), the Subalpine Molasse, European Alps(Müller et al., 1988; Vollmayr and Wendt, 1987), or the deepwaterNiger delta (Higgins et al., 2009). At the Wheeler Ridge, Medwedeff(1992) constructed five closely spaced (distance of few hundred meters)across the lateral termination of a triangle zone. The general geometryis that of a simple tectonic wedge as shown in Fig. 1A. However, indetail the thrust surface is disrupted by steps and near-vertical tearfaults, resulting in different ramp heights along strike. This results insimilar structural complexity of the associated fold, which is asym-metric and non-cylindrical. Most excitingly, Medwedeff (1992) findsthat at the termination of the structure the backthrust bounding thetriangle is no longer present, and geometry is abruptly changing to asimple forethrust. The author notes the change in thrust vergence co-incides with a lithological change from more shaly parts in the triangleto sandier parts where the simple forethrust is located. He suggestsabsence of shale reduces heterogeneity of the mechanical stratigraphy,and no backthrust forms. In the Subalpine Molasse, as mentionedabove, along-strike absence and presence of a triangle zone has beennoted, and assigned to paleogeography, half graben inversion or lateralchanges in detachment rheology. For the western and central Eastern

Alps recent high-resolution reflection seismic data exists, showingchanges of the geometry of the fold-and-thrust belt along strike theorogen (Hinsch, 2013; Ortner et al., 2015). Whereas in the west (i.e.south of Lake Constance at the German-Swiss boarder) a triangle zonewith potentially multiple thrust imbricates is present, going ~50 kmeastward, the structure slowly pinches out and forms isolated inter-cutaneous wedges, before disappearing completely south of Munich(Ortner et al., 2015). This implies, as opposed to the Wheeler Ridgeexample, in the Subalpine Molasse a transition from a triangle zone toforethrust geometry occurs over a distance of> 100 km. The geometricdifference coincides with a general decrease of the amount of short-ening towards the east. Interestingly, farther east in the Autrian part ofthe basin a new triangle zone develops (Hinsch, 2013). This trianglezone in parts is a result of sedimentary onlap and seals thrusting. Lat-erally, multiple thrust slices form a stack of individual triangle zones,each at a relatively small size of few 100 m. This results in a zigzagpattern between the undeformed and deformed part of the basin. Such azigzag looks similar to the fishtail structure described from theSouthern Pyrenees (Sans et al., 1996), however, formation of fishtailstructures may take place during a single tectonic phase whereas thezigzag pattern evolves from protracted interaction of syn-tectonic se-dimentation and stable position of the leading edge of deformation.

Laterally, the zigzag structure in the Subalpine Molasse evolves intoa buried duplex, similar to what is observed in the western part of thebasin, but also comparable to other triangle zones in the world (e.g.Fig. 1A). Similar to what is observed farther west in the basin, thischange in geometry occurs over a distance of several tens of kilometers.Another case where the 3D structure of triangle zones can be observedat high detail is reported from the Niger delta (Higgins et al., 2009).Here, multiple thrusts form independently and initially at distances offew hundred meters to few kilometers. In the Niger delta, backthrustingtypically occurs where basal detachment, bathymetric slope and max-imum horizontal stress directions are parallel to one another (Corredoret al., 2005). This results in formation of isolated triangle zones, asso-ciated with a detachment fold (Fig. 11). As detachment folds (and

triangle tip line

triangle tip linedetachm

ent

detachment

genera

l direction o

f m

ovem

ent

foreland-vergent

thrust

triangle zone

branch line

transfer zone

along-strike

triangle propagation

B

triangle tip line

triangle tip line

detachment

detachment

genera

l direction o

f m

ovem

ent

triangle zone

triangle tip line

triangle tip line

detachment

detachment

genera

l direction o

f m

ovem

ent

Atriangle zone

along-strike

triangle propagation

Fig. 11. 3D sketch of a triangle zone and triangle propa-gation along strike. The triangle zone is associated with adetachment fold, that grows laterally during continuousshortening. Isolated detachment folds may link, and in casethe triangle zone encounters a fold of opposite vergence acomplex zone forms where the two structures link. Figurepartly based on Higgins et al. (2009).


33

consequently the triangle zones) grow laterally, they may encounter afault-bend-fold of opposite vergence (Higgins et al., 2009), Fig. 11. Thetriangle zone will no longer grow laterally, and a structurally complextransition zone between the two faults of opposite vergence forms. Areview of lateral fold-growth is beyond the scope of this contribution.However, the reader is referred to studies investigating the processusing field data (Bretis et al., 2011; Burbank et al., 1999; Davis et al.,2005), as well as numerical (Grasemann and Schmalholz, 2012) andanalogue models (Liu and Dixon, 1991).

A schematic block diagram of a triangle zone in 3D is provided inFig. 12. A strain gradient at depth results in lateral tapering of thestructure. An interesting geometric implication are the widening limbsand narrowing crests going from undeformed to deformed section. Incase the fold crest is isolated from burial, the age of the sediment on thecrest records passage of the fold tip (Medwedeff, 1992). In summary,the 3D structure of triangle zones is variable and depends on the pro-cesses forming them. They may end abruptly due to abrupt changes ofmechanical stratigraphy along strike. They may slowly fade into fore-thrust-dominated fold-and-thrust belt geometry, which may be causedby smooth changes of mechanical stratigraphy, but also be a result ofpresence or absence of syn-tectonic sedimentation. Eventually they mayresult from lateral fold linkage and only form transition zones betweentwo interacting detachment folds. In all described case studies, trianglezones seem to be highly non-cylindrical. Often data coverage may notbe dense enough to allow for describing the 3D geometry of particulartriangle zones. It is important to keep in mind that lateral projection oftriangular geometries may not be possible over long distances.

7. Definition and classification of triangle zones

As seen from the different models and natural examples described inliterature the term “triangle zone” is currently used for a variety ofstructures, which share a triangular appearance visible in cross-sectionsperpendicular to the orogenic front. It is thus a purely geometric defi-nition, which seems unsatisfactory given the kinematic and mechanicconstraints required for triangle zones. Invoking triangle zones shouldconsequently always include critical assessment of the kinematic and

mechanic consequences of suggested geometries. We try to reflect thiswith suggesting an unambiguous definition of triangle zones that takesinto account kinematic and mechanic implications.

In its original sense the term “triangle zone” should only be used forcontractional structures bound by a foreland- and a hinterland-vergingthrust, while e.g. extensional structures of similar geometry shouldfollow a different nomenclature. Furthermore, a variety of triangularstructures exists that result from individual faults of different age,which affect each other (e.g. crosscutting faults in Fig. 1C). In contrast,triangle zones always feature backthrusts which are active at the sametime as the forethrusts. These two constraints lead us to a general de-finition of triangle zones: “Triangle zones are structures with a triangularshape in section view accommodating shortening by coeval activity of a basalthrust and an associated back-thrust of opposite vergence.” This definitionstresses the importance of the triangle tip, where the fore- and back-thrust join. Based on kinematic and mechanic considerations above,two different types of triangle zones following this definition can bedistinguished based on the number and friction of involved thrusts(Fig. 13): Detachment dominated triangle zones (Type-1) only involve asingle detachment (e.g. the basal detachment of fold-and-thrust belts).During ongoing shortening a hinterland-verging thrust develops, whichroots in this basal detachment. Type-1-triangle zones can only formwhen a weak or ductile detachment is deformed at low strain rates (seeabove).

Ramp dominated triangle zones (Type-2) involve two or more de-tachment horizons. The triangle forms at the transition from a thrustramp to a flat secondary detachment. Moderate to high friction alongthe fault can cause mechanical weakening above the thrust ramp andbedding parallel slip towards the hinterland favors the development ofthe passive backthrust at the ramp-flat transition (Fig. 8). We note thatadditional factors may influence triangle zone formation. In particular,syn-tectonic sediments may play an important role, for both, develop-ment of Type I and Type II triangle zones (see above).

The suggested classification scheme has advantages. First of all, itcleans up nomenclature, as it encompasses many of the previously usedterms, such as tectonic wedge, intercutaneous wedge and others (seeintroduction for a more complete review of suggested terms). Structures

fold axial planes

(projected to surface)

back thrust

narrow

ing c

rest

narrow

ing c

rest

wid

enin

g lim

b

wid

enin

g lim

bnarrow

ing c

rest

narrow

ing c

rest

wid

enin

g lim

b

wid

enin

g lim

b

basal detachment

secondary

detachment

strain gra

dient

strain gra

dient

strain gra

dient


position of triangle tipposition of triangle tipposition of triangle tip

Fig. 12. Schematic 3D diagram of a triangle zone based onMedwedeff (1992). Note the strain gradient at depth causing nar-rowing crest and widening limbs closer to surface.


34

such as fishtails may be seen as stacked triangle zones. Second, andmaybe more importantly, we in a way hope to turn the current ap-proach from its head to its feet. Instead of initial geometric descriptionand only possible later kinematic or mechanic specification using avariety of different terms, our suggestion requires addressing un-certainties in kinematics and discussion of mechanic plausibility. Thismay help to prevent misleading interpretations, which were maybe notinsinuated. Requiring discussion of implications of triangle zones, ourdefinition includes assessment of the possible transient nature of tri-angle zone geometries. We will address this point after applying oursuggested scheme to natural examples.

8. Discussion

8.1. Application of the suggested triangle zone classification

To apply our suggested classification scheme to natural examples,we compiled descriptions of contractional triangle zones from all overthe world (Table 1, Fig. 1). All these triangle zones can be classifiedeither as Type-I or as Type-II triangle zones. Even though extensive, ourcompilation may not be complete, and more triangle zones may exist inother fold-and-thrust belts around the globe. However, based on ourdata set, Type-II triangle zones seem far more abundant, and are de-scribed from various fold-and-thrust belts e.g. the Alberta Foothills(Canada, Fig. 1A), the Sub-Andean thrust belt (Argentina, Bolivia), theTien Shan (China), or the forelands of the European Alps. Their ubi-quity is not surprising, as mechanically heterogeneous layering inforeland basins can be expected. Mechanic considerations above in-dicate if intercalations of weak and rigid layers are present, Type-IItriangle zones may form, making ramp-dominated triangle zones a ty-pical structural feature of thrust systems, rather than an exception.Type-I triangle zones are described in few regions e.g. the CascadiaBasin (offshore N-America), the Salt Range fold-and-thrust belt (Paki-stan), the Central Zagros (Pakistan), or the southern Pyrenean foreland(Spain; Fig. 1E). In all cases the basal detachments are associated withthick evaporites, and in addition to triangle zones foreland vergingthrusts formed, which are linked to salt-cored anticlines. The latter areparticularly dominant in the Central Zagros fold-and-thrust belt. Here,Type-I triangle zones only evolve in the outer and less deformed parts ofthe mountain belt (Sherkati et al., 2006). Based on analogue models byGutscher et al. (2001) that show backthrusting with a single viscouslayer detachment setting is most favorable at relatively low con-vergence rates (see above) we speculate that detachment-dominatedtriangle zones preferentially develop if strain rates are relatively low.This also seems to be the case for the Salt Range fold-and-thrust belt

(Pakistan), where an up to 2 km thick sequence of massive halite formsthe basal detachment, and Type-I triangle zones are observed in areasdeformed at low strain rates (7 mm/yr), as compared to the overalldeformation rates (40 mm/yr) in this region (Pennock et al., 1989). Aninteresting implication of this may be that fold-and-thrust belts coulddevelop Type-I triangle zones at their leading edge of deformationwhen shortening rates decline, e.g. towards the end of their activity orwhen deformation shifts location. They may consequently at leastpartly allow for constraining shortening rates through time and mayoffer an explanation why triangle zones are often reported from theleading edge of deformation of ancient fold-and-thrust belts. However,in many cases this may be speculative and would require substantiationwith independent data, such as age-dating of fault activity or syn-tec-tonic sediments.

8.2. Evolution of triangle zones during ongoing shortening

Triangle zones are often observed at the tip of fold-and-thrust belts.This observation lead to two fundamental interpretations: (1) Once atriangle zone has formed, foreland propagation seems difficult and (2)triangle zones are commonly established during the latest stages of fold-and-thrust belt deformation (e.g. Morley, 1986). However, with moreexamples described around the globe, these interpretations have beenchallenged. Triangle zones have also been described in the interior offold-and-thrust belts, and may form or be reactivated at any time duringdeformation, as for instance shown in the European Alps (Ortner et al.,2015; Schlunegger and Mosar, 2011; von von Hagke et al., 2012; vonHagke et al., 2014b). Consequently, apart from geometric, kinematicand mechanic aspects for triangle zone formation, it remains to bediscussed how a once established triangle zone evolves during furthershortening.

The type example of a large triangle zone in the Alberta Foothills(Canada) shows that several foreland verging thrusts ramp up into astratigraphically higher weak horizon and a passive roof thrust forms(our Type II triangle zone, Fig. 13). Ongoing shortening leads to theaccretion of further stacks in the internal duplex and continuous fore-land propagation of the triangle tip (kinematic model in Fig. 7). Upliftassociated with thrust sheet stacking favors erosion of the duplex in thehinterland, where the passive roof thrust reaches the surface. In asteady state of uplift and erosion the extent of the triangle zone wouldremain constant. In case foreland propagation of the fold-and-thrustbelt after triangle zone formation does not involve backthrusts (e.g.associated with change of the mechanical properties of the upper de-tachment due to facies transition, or reduction of overburden stress dueto absence of syntectonic sediments), the triangle zone may be lost due

Type 1 - Detachment dominated Type 2 - Ramp dominated

A B1

1

2

1 ductile detachment 1 secondary detachment

2bedding parallel slip at

upper ramp edge

low strain rates moderate to high fault friction



Fig. 13. Types of triangle zones; A: Type-1 (Detachment dominated) triangle zones linked to a single ductile detachment typically evolve under low strain rates. B: Type-2 (Rampdominated) triangle zones are linked to a secondary detachment and evolve due to bedding parallel slip at the upper ramp edge. Moderate to high fault friction of the ramp additionallyweakens the overlying section due to backthrusting at the ramp. The resulting structures may act precursor for future backthrusts in the overburden (see also Fig. 8).


35

Table1

Exam

ples

oftriang

lezo

nesan

dap

plicationof

thesugg

estedclassification

.

Con

tine

ntTriang

lezo

ne/system

Referen

ces

Type

Com

men

t

Africa

Dam

arathrust

belt,

Nam

ibia

(Kuk

la,1

992)

INiger

Delta,N

igeria

(Higgins

etal.,20

09)

IKaragwe-Ank

olefold

belt,

Tanzan

ia(K

oege

lenb

ergan

dKisters,2

014)

IN-America

CascadiaBa

sin,

offshoreN-America/

Pacific

(Ada

met

al.,20

04;G

utsche

ret

al.,20

01)

IPa

rryIsland

s,Arctic

Can

ada

(Harrison,

1995

)II

SaltRiver

Ran

ge,W

yoming,

USA

(Che

ster,2

003)

IIAlberta

Foothills,C

anad

a(Jon

es,1

982;

Lawtonet

al.,19

94;L

ebel

etal.,19

96;S

ande

rson

andSp

ratt,1

992;

Soulean

dSp

ratt,1

996)

II

Melville

Island

fold

belt,

Northwestern

Territo

ry,

Can

ada

(Harrisonan

dBa

lly,19

88;H

arrison,

1993

)I

Wasatch

Mou

ntains,C

odilleran

fold-and

-thrustbelt,

Utah,

USA

(Law

ton,

1985

)I

Wheeler

Ridge,C

alifo

rnia,U

SA(M

edwed

eff,1

992)

Ventura

Avenu

eAnticlin

e,Ventura

fold

belt,

Califo

rnia,

USA

(Yeats

etal.,19

88)

II

S-America

MagallanesFo

reland

fold-and

-thrust

belt,

Southern

Chile

(Alvarez-M

arrónet

al.,19

93;Ram

os,1

989)

II

Sub-And

eanthrust

belt,

NorthernArgentin

a(Belottiet

al.,19

95)

IIIncahu

asistructure,S

ub-And

eanthrust

belt,

Bolivia

(Drieh

auset

al.,20

14)

IINeuqu

énfold

andthrust

belt,

Argentin

a(Zam

ora-Valcarcean

dZa

pata,20

15)

ITy

pe-Itriang

lezo

neaff

ectedby

laterfaulting

Asia

SaltRan

gefold

andthrustbelt,

Pakistan

(Pen

nock

etal.,19

89)

I15

%(strainrate

7mm/y

r)of

plateco

nverge

nce

(40mm/y

r)Junggarfold-and

-thrust

belt,

northern

Tian

Shan

foreland

,China

(Gua

net

al.,20

16)

II

Kelasutriangle

zone,sou

thernTian

Shan

foreland

,China

(Xuan

dZh

ou,20

07)

II

Riwat

Thrust,H

imalay

anforeland

,Pak

istan

(Jad

oonan

dFrisch

,199

7)II

Central

Zagros

fold-th

rust

belt,

Iran

(She

rkatiet

al.,20

06)

IOnlyin

surrou

ndingpa

rtsan

din

less

deform

edforeland

Dab

asha

nforeland

triangle

zone,C

entral

China

(Liet

al.,20

07)

IIHaw

asinathrust

wedge,O

man

Mou

ntains,O

man

(Coo

peret

al.,20

14)

IINorth

UralsTh

rust

belt,

Russia

(Sob

orno

v,19

92)

IIAlsoTy

pe-Itriang

lesintegrated

Sulaim

anmou

ntainbelt,

Pakistan

(Ban

ksan

dWarbu

rton

,198

6)II

Band

aArc,Ind

onesia

(Morleyet

al.,20

17)

IIWestern

Foothills,T

aiwan

(Mou

thereauet

al.,20

01)

IIAustralia/N

ewZe

alan

d/Ocean

iaAmad

eusBa

sin,

Australia

(Tey

ssier,

1985

)I

PuriAnticlin

e,Pa

puaNew

Guinea

(Med

d,19

96)

IIHikuran

gisubd

uctio

nmargin,

New

Zealan

d(M

orleyet

al.,20

17)

IEu

rope

Sicilia

n-Maghrebianfold-and

-thrust

belt,

Western

Sicily

(Alban

esean

dSu

lli,2

012)

IIJura

fold-and

-thrust

belt,

Switz

erland

/France

(Malzet

al.,20

15;M

alzet

al.,20

16;S

choriet

al.,20

15;S

ommarug

a,19

99)

IISu

balpineMolasse,A

lpineFo

reland

,German

y(M

üller,

1984

;Ortne

ret

al.,20

15;S

chulleret

al.,20

15)Hinsch,

2013

)II

VenetianAlps,Ita

ly(D

oglio

ni,1

992)

IIInclud

esup

liftedan

dpa

rtly

erod

edtriang

leSo

uthern

Pyrenean

Foreland

,Spa

in(San

set

al.,19

96)

I


36

to erosion. This has been corroborated with analogue models showingloss of imbricates at the orogenic front due to erosion (Bonnet et al.,2007). Consequently it has been argued the observation that trianglezones occur only at front of fold-and-thrust belts is an artifact of in-complete preservation (Medd, 1996). An example where a triangle zonemay have formed and later was uplifted and partly eroded is knownfrom the European Alps, where steep attitude of bedding may not beeasily explained as a footwall syncline (Doglioni, 1992).

Frontal structures of fold-and-thrust belts can be reactivated duringlater stages of deformation, as for instance observed in the triangle zoneof the European Alps (von von Hagke et al., 2012; von Hagke et al.,2014b). Here, however the buried duplex does not show much internalactivity; instead a foreland verging roof-thrust is reactivated. A similarconfiguration is shown in the case of the eastern Jura fold-and-thrustbelt (Switzerland) where the triangle zone formed during an early stageof foreland deformation at a precursory zone of mechanical weakness,which transiently hindered further foreland propagation (Malz et al.,2016). At a later stage, the triangle zone was overridden and thus in-tegrated in the footwall of the fold-and-thrust belt. In summary, trianglezones do not necessarily represent late stage structures of fold-and-thrust belt evolution. Instead, they may form early or continuouslyduring foreland propagation of the system. Depending on the mechanicconfiguration of the foreland sequences, the triangle may localize de-formation (e.g. European Alps, Alberta foothills), may be accreted tothe hangingwall (e.g. Papua New Guinea), or overridden and form partof the footwall (e.g. Jura Mountains; Fig. 14. This stresses that despitethe unifying characteristic of two simultaneously active thrust of op-posite vergence, triangle zones show large kinematic and mechanic

variability that has to be addressed individually for every case.As discussed above, very commonly triangle zones are associated

with pop-up structures due to initiation of a backthrust at the lowerramp edge or above the ramp (Fig. 8). When enough shortening occurs,these backthrusts may be pushed up the entire ramp onto the upperdetachment (Fig. 15). The pop-up structure is preserved at the front ofthe fault-bend fold, where a complex deformation pattern can be ob-served. This is corroborated by analogue modeling studies, showingcomplexity of pop-up structures after they are shortened enough(Couzens-Schultz et al., 2003). However, in natural examples de-pending on the scale of the pop-up, this complexity may not be well-imaged on seismic data, and interpretation of the structure as deformedtriangle zone may not be straight forward.

8.3. Uncertainties of triangle zone interpretation

Although geometric models of triangle zones look like accuratedescriptions of nature, cross-section balancing and numerical modelingsuccessfully tested their kinematics, and dynamic concepts exist, in-terpreting a triangle zone may be associated with large uncertainties.This may be particularly the case when triangle zones are integrated inthe complex structures of entire fold-and-thrust belts including thrustsof opposite vergence (see simplified sketches in Fig. 2C and D).

8.3.1. Data uncertaintyTriangle zones are often interpreted from seismic data. Seismic in-

terpretation commonly allows for different equally viable solutions and,regarding its resolution, it might not be able to resolve for instancedifferences between steeply dipping fold limbs and duplex structures(Brown, 1996; Lawton et al., 1994). Complex seismic velocity varia-tions including shadow zones (Fig. 16B) and seismic reflectors de-formed during migration are typical features of triangle zones, and it isimportant to keep in mind how seismic waves propagate through suchstructures, and how geometrical effects influence seismic processing(Jardin et al., 2007). If acquisition (e.g. design of source and detectorarrays) and processing efforts (e.g. filter, deconvolution and migrationparameters, velocity models) are known uncertainties can be reduced orquantified. Nevertheless, even then it is challenging to create errorbonds once a conceptual model for interpretations is chosen (Alcadeet al., 2017), which we will discuss in the following.

8.3.2. Uncertainty in interpretationAnother problem that may limit our understanding of the geometry

A

B

C

previous TZprevious TZprevious TZ

previous TZprevious TZprevious TZ

Initial Triangle Zone

Fig. 14. Evolution of triangle zones during ongoing shortening. A: The triangle zonepropagates towards the foreland. Further thrust sheets may form a duplex, but the generalstructure remains a triangle zone. B: Ongoing shortening results in foreland thrusting. Thetriangle zone is thrusted along the secondary detachment and may be eroded. C: Thetriangle zone is overthrusted by foreland verging thrust sheets. Parts of the triangle arethrusted to the surface but parts are integrated into the footwall of the thrust belt.

A

B

C

preserved

pop-up structure

preserved

pop-up structure

Fig. 15. Kinematic evolution of a triangle zone that initiated as a pop-up structure (cfFig. 2D). The backthrust is pushed up the ramp and the pop-up is preserved at the tip ofthe fault-bend fold. The complex geometry that forms may however not be reconstructedeasily, particularly as it might not be well-resolved on seismic data.


37

of triangle zones is bias introduced by individual researchers, especiallywhen single 2D reflection seismic data is interpreted (Bond et al.,2007). Even with high resolution seismic images and good field ex-posure, geometric and kinematic analysis of structures is uncertain dueto the inherent subjectivity of interpreting seismic sections (Bond et al.,2007; Oncken et al., 2006). Likewise, even if cross-section balancing isused to validate the kinematics of triangle zones, various geometricconcepts of fault-related folding exist (Brandes and Tanner, 2014), andsuch different models are often rather randomly than systematicallyapplied for individual structures. However, applying different con-ceptual models may result in large differences of estimated shortening,and consequently systematic geometric and kinematic analysis ofstructures visible in seismic sections should be preferred, at best in-cluding regional geologic data such as stratigraphic relationships,thermochronometry, paleo-stress, fault traces, transfer structures orshortening gradients along individual structures (Malz et al., 2015). Forinstance in the Jura fold-and-thrust belt, a triangle zone became in-visible in reflection seismic sections due to larger thrust sheets andmore prominent structural features covering the structure, and onlyadditional data on regional geology allowed for its detection (Malzet al., 2016) Shortening estimates and geometric models may varysubstantially, for instance by interpreting presence or absence of largeburied duplexes, folds, roof thrusts, or doubling of stratigraphicpackages (Oncken et al., 2006). This is particularly the case for triangle

zones, which may or may not contain internal duplexing (Figs. 1 and 2).When structures are interpreted from seismic sections, it is the decisionof the interpreter which conceptual model he/she uses. Especially iftriangle zones with bedding parallel shear in a secondary detachmentare interpreted, passive roof thrusts are often not clearly detectable.

8.3.3. ExamplesFig. 16 shows a single seismic section of a classic triangle zone

(MacKenzie Mountains, Canadian Rocky Mountains). Due to a largeshadow zone at the front limb of an anticline, a fault-propagation-foldwould be an equally viable interpretation of the seismic data (Fig. 16D).We can only be certain this is a triangle zone, because it has been ob-served on many seismic sections and field evidence corroborates itsexistence. Another example of a triangle zone can be found in theforeland fold-and-thrust belt of the European Alps, the Subalpine Mo-lasse. The triangle zone has been imaged on seismic data (Berge andVeal, 2005; Müller, 1984), and geometric models have been testedusing line-length balancing (Ortner et al., 2015; von Hagke et al.,2014b). However, the exact geometry of the triangle zone is unclear,and two different interpretations of the same seismic section have beenproposed (Fig. 17) (Ortner et al., 2015; Schuller et al., 2015). Althoughboth models are viable geometric solutions and the general structuremapped is similar, some differences are striking. The model of Schulleret al. (2015) shows an internal duplex with two thrust sheets (evolvedin an in-sequence scenario) both leading into an upper detachment andforming a long backthrust. In contrast Ortner et al. (2015) suggestedtwo separate thrust imbricates, which probably require backthrustingon their top or distributed deformation in front of the structure. Ifbackthrusting as the fundamental criteria for the triangle zone is as-sumed, the shown interpretation indicates offset of the bedding parallelsecondary detachment (Fig. 17) thus suggesting later internal de-formation of the Subalpine Molasse triangle zone.

8.3.4. Material transfer and strainThe two examples of triangle zones in the Rocky Mountains and the

Subalpine Molasse (Figs. 16 and 17) illustrate that the characteristic

10 km

2.0

1.0

0

2.0

1.0

0

2.0

1.0

0

AnticlineAnticline shallow synclineshallow synclineshadow

zone

shadow

zone

2.0

1.0

0

A

B

C

D

TW

T [s]

TW

T [s]

TW

T [s]

TW

T [s]

Fig. 16. Reflection seismic section of a classic triangle zone (section (A) and interpreta-tion (C) from Taborda and Spratt, 2008) illustrating uncertainties of geophysical data anddifferent viable interpretations; A: seismic section; B: seismic section with clear andcontinuous reflections highlighted (reflection dips are highlighted with dip marks; tri-angles show clearly identifiable reflections; red arrows show positions of faults; Note thehuge shadow zone at the anticlines front limb); C: interpreted triangle zone; D: inter-preted fault-propagation fold. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

~ 2 km

Subalpine Molasse

Triangle Zone

0

2

4

de

pth

[km

] b

. sl.

A

Lower Marine

Molasse

Lower Freshwater

Molasse

0

2

4

de

pth

[km

] b

. sl.

B

Lower Marine

Molasse

Lower Freshwater

Molasse

Fig. 17. Two interpretations of the Subalpine Molasse Triangle Zone (European Alps)with two interpreted thrust sheets. Triangle tips are marked with red dots. A: Triangle tipsare connected by a secondary detachment (Schuller et al., 2015) suggesting in-sequenceformation of the triangle zone. B: Triangle tips are not connected by the same backthrust(Ortner et al., 2015). The wedge in the hinterland offsets the backthrust of the forelandwedge, thus suggesting an out-of-sequence formation of the triangle zone. (For inter-pretation of the references to color in this figure legend, the reader is referred to the webversion of this article.)


38

criterion for triangle zones seen on reflection seismic data, is foreland-dipping strata, which may be cut by hinterland verging thrust faults.However, foreland dipping reflectors do not necessarily require tectonicwedging or passive backthrusting. Foreland-dipping strata and conse-quently triangular-shaped structures in seismic sections are also asso-ciated with fault-bend-folding or fault-propagation-folding (Fig. 18).Especially if the front limb of anticlines is not well imaged in seismicsections or is not exposed at the surface, it is difficult to determinewhether hinterland verging thrusts are present. As illustrated inFig. 18A and B it is important to note how shortening at a thrust ramp isaccumulated. In triangle zones excess material occurs at the anticlinecrest and is accumulated by backthrusting. In contrast, fault-bend-folding requires a distinct amount of material excess in the foreland ofthe anticline, which may be far away in the foreland where the thrustramps up from the secondary detachment or hangingwall strata reachesthe surface. Alternatively, shortening can be accommodated by pene-trative strain in the foreland (e.g. Groshong, 1975); see Burberry (2015)for a recent review and its spatial and temporal variation. Distributeddeformation within fold-and-thrust belts may be significant, particu-larly in areas where pressure solution is prevalent. The importance of

recognizing distributed deformation, as well as the limitations ofmapping strain even when using high-quality seismic data has recentlybeen pointed out (Iacopini and Butler, 2011).

For fault-propagation-folds neither backthrusting nor foreland ver-ging thrusts are necessary to create foreland dipping reflectors, asshortening is accumulated by a decrease of thrust offsets towards theirtip (Fig. 18C). In thick-skinned fold-and-thrust belts triangular-shapedstructures are observed, where steep basement faults pass into a weaksedimentary cover. At the transition between basement and cover re-latively minor extension in the hangingwall anticline and a markedthickening of probably ductile units in foreland synclines can occur. Inless ductile units such thickness variations are not observed and trishearfault-propagation folding (cf. Erslev, 1991) may result in movement ofexcess material out of the foreland syncline and it is dispersed bybedding parallel backthrusting towards the anticline (Fig. 18D).Trishear folding is commonly not associated with a through-goingbackthrust typical for triangle zones. In summary, even though it maybe an appealing model, tectonic wedging is not necessarily the onlyviable interpretation of the structural inventory of different fold-and-thrust belts or orogens. Fault-propagation folds or fault-bend folds thatare later truncated by thrusts, as well as distributed deformation infront of such structures may provide alternative explanations.

9. Conclusions

In this study, we reviewed the different concepts of triangle zones,which are typical elements observed in fold-and-thrust belts all over theworld, and discussed mechanical implications as well as uncertaintiesin their interpretation. Triangle zones are commonly depicted as thefrontal structures of thrust belts, but there are increasing numbers ofnatural examples forming behind the orogenic front. In general trianglezones are interpreted to be associated with tectonic wedging and hin-terland verging (passive) roof thrusts. They can be described geome-trically to form triangular-shaped structures in cross-section view per-pendicular to the orogenic front, where a hinterland verging roof thrustwas active synchronously to the foreland verging basal (or lower)thrust.

This review shows that formation, persistent activity and pre-servation of roof thrusts of opposite vergence has kinematic and me-chanic implications, which should be addressed when triangle zones areinvoked. The multiple different natural examples, as well as the me-chanical observations from analogue and numerical modeling, showthat roof thrusts can evolve in two settings: (1) in brittle-ductile wedgeswith a single weak basal detachment (Type-1 triangle zones) or (2) infold-and-thrust belts were at least two detachments are present (Type-2triangle zones). In brittle-ductile wedges hinterland verging thrustingoccurs under low strain-rate conditions possibly suggesting their de-velopment during early or late stages of thrust belt formation andconfirming the observation that triangle zones are often located at theborders fold-and-thrust belts where the amount of shortening wasconsiderably low. Type-2 triangle zones form at the upper ramp edge offoreland verging thrust ramps due to hinterland directed bedding-par-allel shear. In this scenario moderate to high fault friction at the thrustramp can cause the development of hinterland verging fractures prob-ably acting as a precursor for future backthrusts. Nevertheless, thepreservation of such triangle zones is influenced by subsidence of theentire system or high rates of syn-tectonic sedimentation. The influenceof syn-tectonic sedimentation is two-fold: (1) as a supporting factor forfracture localization in the triangle zone due to lowering of differentialstresses in the foreland and the hinterland and (2) supporting trianglezones preservation due to fast burial of the structure.

Based on observations from natural examples as well as from ki-nematic and mechanical models, we suggest using the term “trianglezone” in a very stringent manner. As the interpretation of triangle zonesis highly affected by uncertainties of geological and geophysical data,irrefutable evidence for passive roof thrusting is essential to favor the

A

B

C

D

Fig. 18. Geometric sketches of foreland dipping and triangular structures typicallyevolving in fold-and-thrust belts. A: (Type-2) Triangle zone; Foreland dipping strata isassociated to fault-related folding at the ramp edge and successive (passive) backthrustingat the roof thrust. B: Fault-bend-fold model resulting in a similar bedding dip, if thenecessary foreland thrust is not observed. C: Fault-propagation-fold with upward de-creasing fault offset results in probably similar bedding geometries. D: Triangular(trishear) geometries in front of a blind basement thrust possibly causing thickening ofductile units in foreland syncline and may results in excess of brittle material dispersed bybedding-parallel backthrusting the steep front limb (Erslev, 1991). Note: All types resultin similar surface expressions and bedding geometries visible in reflection seismic data.


39

interpretation of tectonic wedges and, as a consequence, it should bekept in mind that triangle zones demand particular kinematic andmechanic conditions. To understand better why and how triangle zonesform in particular settings, field studies including seismic interpreta-tions should be substantiated by balanced or kinematically viablemodels including uncertainty analysis, at best supported by a combi-nation of numerical and analogue experiments.

Acknowledgements

Hugo Ortner and Chris Morley are thanked for fruitful discussions.A. Taborda and D. Spratt are thanked for providing the seismic sectionof Fig. 16 in the Virtual Seismic Atlas (see-vsa.leeds.ac.uk). Jon Mosarand David Iacopini are thanked for constructive comments on an earlierversion of the manuscript. We thank Carlo Doglioni for editorialhandling.

References

Adam, J., Klaeschen, D., Kukowski, N., Flueh, E., 2004. Upward delamination of CascadiaBasin sediment infill with landward frontal accretion thrusting caused by rapid gla-cial age material flux. Tectonics 23, TC3009.

Albanese, C., Sulli, A., 2012. Backthrusts and passive roof duplexes in fold-and-thrustbelts: the case of central-western Sicily based on seismic reflection data.Tectonophysics 514–517, 180–198.

Alcade, J., Bond, C.E., Johnson, G., Ellis, J.F., Butler, R.W.H., 2017. Impact of seismicimage quality on fault interpretation uncertainty. GSA Today 27, 4–10.

Alvarez-Marrón, J., McClay, K.R., Harambour, S., Rojas, L., Skarmeta, J., 1993. Geometryand evolution of the frontal part of the Magallanes foreland thrust and Fold Belt(Vicuña area), Tierra del Fuego, southern Chile. AAPG Bull. 77, 1904–1921.

Anderson, E.M., 1905. The dynamics of faulting. Trans. Edinb. Geol. Soc. 8, 387–402.Bally, A.W., Gordy, P., Stewart, G.A., 1966. Structure, seismic data, and orogenic evo-

lution of southern Canadian Rocky Mountains. Bull. Can. Petrol. Geol. 14, 337–381.Banks, C., Warburton, J., 1986. ‘Passive-roof’ duplex geometry in the frontal structures of

the Kirthar and Sulaiman mountain belts, Pakistan. J. Struct. Geol. 8, 229–237.Barnes, P.M., Nicol, A., 2004. Formation of an active thrust triangle zone associated with

structural inversion in a subduction setting, eastern New Zealand. Tectonics 23.Bégin, N.J., Lawton, D.C., Spratt, D.A., 1996. Seismic interpretation of the Rocky

Mountain thrust front near the Crowsnest deflection, southern Alberta. Bull. Can.Petrol. Geol. 44, 1–13.

Belotti, H.J., Saccavino, L.L., Schachner, G.A., 1995. Structural styles and petroleumoccurrence in the Sub-Andean thrust belt of northern Argentina. In: Tankard, A.J.,Suárez, S., Welsink, H.J. (Eds.), Petroleum Basins of South America. Vol. 62. AAPGMemoir, pp. 545–555.

Berge, T.B., Veal, S.L., 2005. Structure of the Alpine foreland. Tectonics 24, TC5011.Bilotti, F., Shaw, J.H., 2005. Deep-water Niger Delta fold and thrust belt modeled as a

critical-taper wedge: the influence of elevated basal fluid pressure on structuralstyles. AAPG Bull. 89, 1475–1491.

Bond, C.E., Gibbs, A.D., Shipton, Z.K., Jones, S., 2007. What do you think this is?“Conceptual uncertainty” in geoscience interpretation. GSA Today 17, 4–10.

Bonini, M., 2007. Deformation patterns and structural vergence in brittle–ductile thrustwedges: an additional analogue modelling perspective. J. Struct. Geol. 29, 141–158.

Bonini, M., Sokoutis, D., Mulugeta, G., Katrivanos, E., 2000. Modelling hanging wallaccommodation above rigid thrust ramps. J. Struct. Geol. 22, 1165–1179.

Bonnet, C., Malavieille, J., Mosar, J., 2007. Interactions between tectonics, erosion, andsedimentation during the recent evolution of the Alpine orogen: analogue modelinginsights. Tectonics 26.

Boyer, S.E., Elliott, D., 1982. Thrust systems. Am. Assoc. Pet. Geol. Bull. 66, 1196–1230.Brandes, C., Tanner, D.C., 2014. Fault-related folding: a review of kinematic models and

their application. Earth-Sci. Rev. 138, 352–370.Bretis, B., Bartl, N., Grasemann, B., 2011. Lateral fold growth and linkage in the Zagros

fold and thrust belt (Kurdistan, NE Iraq). Basin Res. 23, 615–630.Brown, A.R., 1996. Interpretation of Three-dimensional Seismic Data. AAPG Memoir,

Tulsa, Oklahoma, pp. 1–31.Buiter, S.J.H., 2012. A review of brittle compressional wedge models. Tectonophysics

530–531, 1–17.Burbank, D., McLean, J., Bullen, M., Abdrakhmatov, K., Miller, M., 1999. Partitioning of

intermontane basins by thrust-related folding, Tien Shan, Kyrgyzstan. Basin Res. 11,75–92.

Burberry, C.M., 2015. Spatial and temporal variation in penetrative strain during com-pression: insights from analog models. Lithosphere 7, 611–624.

Burkhard, M., Sommaruga, A., 1998. Evolution of the western Swiss Molasse basin:structural relations with the Alps and the Jura belt. In: Mascle, A., Puigdefabregas, C.,Luterbacher, H.P., Fernandez, M. (Eds.), Geological Society, London, SpecialPublications. 134. pp. 279–298.

Burov, E., Podladchkov, Y., Grandjean, G., Burg, J., 1999. Thermo-mechanical approachto validation of deep crustal and lithospheric structures inferred from multi-disciplinary data: application to the western and northern Alps. Terra Nova 11,124–131.

Butler, R.W., 1982. The terminology of structures in thrust belts. J. Struct. Geol. 4,

239–245.Byerlee, J., 1978. Friction of rocks. Pure Appl. Geophys. 116, 615–626.Chapin, C.E., Kelley, S.A., Cather, S.M., 2014. The Rocky Mountain Front, southwestern

USA. Geosphere 10, 1043–1060.Charlesworth, H.A., Gagnon, L., 1985. Intercutaneous wedges, the triangle zone and

structural thickening of the Mynheer coal seam at Coal Valley in the Rocky MountainFoothills of central Alberta. Bull. Can. Petrol. Geol. 33, 22–30.

Chester, J.S., 2003. Mechanical stratigraphy and fault-fold interaction, Absaroka thrustsheet, Salt River Range, Wyoming. J. Struct. Geol. 25, 1171–1192.

Clift, P.D., Schouten, H., Draut, A.E., 2003. A general model of arc-continent collision andsubduction polarity reversal from Taiwan and the Irish Caledonides. Geol. Soc. Lond.,Spec. Publ. 219, 81–98.

Cooper, D.J.W., Ali, M.Y., Searle, M.P., 2014. Structure of northern Oman Mountainsfrom the Semail Ophilite to the Foreland Basin. In: Rollinson, H.R., Searle, M.P.,Abbasi, I.A., Al-Lazki, A., Al Kindi, M.H. (Eds.), Tectonic Evolution of the OmanMountains. 392. Geological Society Special Publications, London, pp. 129–153.

Corredor, F., Shaw, J.H., Bilotti, F., 2005. Structural styles in the deep-water fold andthrust belts of the Niger Delta. AAPG Bull. 89, 753–780.

Couzens, B.A., Wiltschko, D.V., 1996. The control of mechanical stratigraphy on theformation of triangle zones. Bull. Can. Petrol. Geol. 44, 165–179.

Couzens-Schultz, B.A., Vendeville, B.C., Wiltschko, D.V., 2003. Duplex style and trianglezone formation: insights from physical modeling. J. Struct. Geol. 25, 1623–1644.

Cristallini, E.O., Allmendinger, R.W., 2002. Backlimb trishear: a kinematic model forcurved folds developed over angular fault bends. J. Struct. Geol. 24, 289–295.

Cubas, N., Avouac, J.P., Leroy, Y.M., Pons, A., 2013. Low friction along the high slip patchof the 2011 Mw 9.0 Tohoku-Oki earthquake required from the wedge structure andextensional splay faults. Geophys. Res. Lett. 40, 4231–4237.

Dahlen, F.A., 1990. Critical taper model of fold-and-thrust belts and accretionary wedges.Annu. Rev. Earth Planet. Sci. 18, 55–99.

Dahlstrom, C.D.A., 1969. Balanced cross sections. Can. J. Earth Sci. 6, 743–757.Davis, D., Suppe, J., Dahlen, F.A., 1983. Mechanics of fold-and-thrust belts and accre-

tionary wedges. J. Geophys. Res. 88, 1153–1172.Davis, K., Burbank, D.W., Fisher, D., Wallace, S., Nobes, D., 2005. Thrust-fault growth and

segment linkage in the active Ostler fault zone, New Zealand. J. Struct. Geol. 27,1528–1546.

Del Castello, M., Cooke, M.L., 2007. Underthrusting-accretion cycle: work budget as re-vealed by the boundary element method. J. Geophys. Res. Solid Earth 112.

Doglioni, C., 1992. The Venetian Alps Thrust Belt, Thrust Tectonics. Springer, pp.319–324.

Dotare, T., Yamada, Y., Adam, J., Hori, T., Sakaguchi, H., 2016. Initiation of a thrust faultrevealed by analog experiments. Tectonophysics 684, 148–156.

Driehaus, L., Nalpas, T., Ballard, J.-F., 2014. Interaction between deformation and sedi-mentation in a multidecollement thrust zone: analogue modelling and application tothe sub-Andean thrust belt of Bolivia. J. Struct. Geol. 65, 59–68.

Drozdzewski, G., 1979. Grundmuster der Falten-und Bruchstrukturen im Ruhrkarbon. Z.Dtsch. Geol. Ges. 51–67.

Eisenstadt, G., De Paor, D.G., 1987. Alternative model of thrust-fault propagation.Geology 15, 630–633.

Ellis, S., Schreurs, G., Panien, M., 2004. Comparisons between analogue and numericalmodels of thrust wedge development. J. Struct. Geol. 26, 1659–1675.

Erickson, S.G., Strayer, L.M., Suppe, J., 2001. Initiation and reactivation of faults duringmovement over a thrust-fault ramp: numerical mechanical models. J. Struct. Geol.23, 11–23.

Erslev, E.A., 1991. Trishear fault-propagation folding. Geology 19, 617–620.Fallot, P., 1949. Les chevauchements intercutanes de Roya (A.-M.). In: Annales Hérbert et

Hauy. 7. pp. 161–169.Ferrer, O., McClay, K., Sellier, N., 2016. Influence of fault geometries and mechanical

anisotropies on the growth and inversion of hanging-wall synclinal basins: insightsfrom sandbox models and natural examples. Geol. Soc. Lond. Spec. Publ. 439(SP439), 438.

Fillon, C., Huismans, R.S., van der Beek, P., 2013. Syntectonic sedimentation effects onthe growth of fold-and-thrust belts. Geology 41, 83–86.

Finch, E., Hardy, S., Gawthorpe, R., 2003. Discrete element modelling of contractionalfault-propagation folding above rigid basement fault blocks. J. Struct. Geol. 25,515–528.

Fletcher, R.C., Pollard, D.D., 1999. Can we understand structural and tectonic processesand their products without appeal to a complete mechanics? J. Struct. Geol. 21,1071–1088.

Gallastegui, J., Pulgar, J.A., Gallart, J., 2016. Alpine tectonic wedging and crustal dela-mination in the Cantabrian Mountains (NW Spain). Solid Earth Discuss. 2016, 1–26.

Ganss, O., Schmidt-Thomé, P., 1953. Die gefaltete Molasse am Alpenrand zwischenBodensee und Salzach. Z. Dtsch. Geol. Ges. 105, 402–495.

Giambiagi, L.B., Alvarez, P.P., Godoy, E., Ramos, V.A., 2003. The control of pre-existingextensional structures on the evolution of the southern sector of the Aconcagua foldand thrust belt, southern Andes. Tectonophysics 369, 1–19.

Gordy, P., Frey, F., Norris, D.K., 1977. Geological Guide for the CSPG 1977 Waterton-Glacier Park Field Conference. Canadian Society of Petroleum Geologists.

Grasemann, B., Schmalholz, S.M., 2012. Lateral fold growth and fold linkage. Geology 40,1039–1042.

Gratier, J.-P., Dysthe, D.K., Renard, F., 2013. The role of pressure solution creep in theductility of the Earth's upper crust. Adv. Geophys. 54, 47–179.

Groshong, R., 1975. Strain, fractures, and pressure solution in natural single-layer folds.Geol. Soc. Am. Bull. 86, 1363–1376.

Guan, S., Stockmeyer, J.M., Shaw, J.H., Plesch, A., Zhang, J., 2016. Structural inversion,imbricate wedging, and out-of-sequence thrusting in the southern Junggar fold-and-thrust belt, northern Tian Shan, China. AAPG Bull. 100, 1443–1468.


40

http://vsa.leeds.ac.uk

http://refhub.elsevier.com/S0012-8252(17)30286-6/rf0005



















































































































































Gutscher, M.-A., Klaeschen, D., Flueh, E., Malavieille, J., 2001. Non-coulomb wedges,wrong-way thrusting, and natural hazards in Cascadia. Geology 29, 379–382.

von Hagke, C., Cederbom, C.E., Oncken, O., Stöckli, D.F., Rahn, M.K., Schlunegger, F.,2012. Linking the northern Alps with their foreland: the latest exhumation historyresolved by low-temperature thermochronology. Tectonics 31, TC5010.

von Hagke, C., Oncken, O., Evseev, S., 2014a. Critical taper analysis reveals lithologicalcontrol of variations in detachment strength: an analysis of the Alpine basal de-tachment (Swiss Alps). Geochem. Geophys. Geosyst. 15, 176–191.

von Hagke, C., Oncken, O., Ortner, H., Cederbom, C.E., Aichholzer, S., 2014b. LateMiocene to present deformation and erosion of the central Alps— evidence for steadystate mountain building from thermokinematic data. Tectonophysics 632, 250–260.

von Hagke, C., Philippon, M., Avouac, J.-P., Gurnis, M., 2016. Origin and time evolutionof subduction polarity reversal from plate kinematics of Southeast Asia. Geology 44,659–662.

Harrison, J.C., 1993. In: Geologists, A.A.o.P (Ed.), Salt involved tectonics of a forelandfolded belt, Arctic Canada. Hedberg Conference on Salt Tectonics, Bath, England. pp.13–17.

Harrison, J.C., 1995. Tectonics and kinematics of a foreland folded belt influenced by salt,Arctic Canada. AAPG Mem. 65, 379–412.

Harrison, J., Bally, A., 1988. Cross-sections of the Parry Islands fold belt on MelvilleIsland, Canadian Arctic Islands: implications for the timing and kinematic history ofsome thin-skinned decollement systems. Bull. Can. Petrol. Geol. 36, 311–332.

Higgins, S., Clarke, B., Davies, R.J., Cartwright, J., 2009. Internal geometry and growthhistory of a thrust-related anticline in a deep water fold belt. J. Struct. Geol. 31,1597–1611.

Hindle, D., 2008. How hard were the Jura mountains pushed? Swiss J. Geosci. 101,305–310.

Hinsch, R., 2013. Laterally varying structure and kinematics of the Molasse fold andthrust belt of the central eastern Alps: implications for exploration. AAPG Bull. 97,1805–1831.

Hubbert, M.K., Rubey, W.W., 1959. Role of fluid pressure in mechanics of overthrustfaulting I. Mechanics of fluid-filled porous solids and its application to overthrustfaulting. Geol. Soc. Am. Bull. 70, 115–166.

Hughes, A.N., Benesh, N.P., Shaw, J.H., 2014. Factors that control the development offault-bend versus fault-propagation folds: insights from mechanical models based onthe discrete element method (DEM). J. Struct. Geol. 68, 121–141.

Iacopini, D., Butler, R.W., 2011. Imaging deformation in submarine thrust belts usingseismic attributes. Earth Planet. Sci. Lett. 302, 414–422.

Jadoon, I.A.K., Frisch, W., 1997. Hinterland-Vergent tectonic wedge below the Riwatthrust, Himalayan Foreland, Pakistan, implications for hydrocarbon exploration.AAPG Bull. 81, 438–448.

Jamison, W.R., 1993. Mechanical stability of the triangle zone: the backthrust wedge. J.Geophys. Res. Solid Earth 98, 20015–20030.

Jaquet, Y., Bauville, A., Schmalholz, S.M., 2014. Viscous overthrusting versus folding: 2-Dquantitative modeling and its application to the Helvetic and Jura fold and thrustbelts. J. Struct. Geol. 62, 25–37.

Jardin, A., Chaker, R., Krzywiec, P., 2007. Understanding Seismic Propagation ThroughTriangle Zones, Thrust Belts and Foreland Basins. Springer, pp. 63–74.

Jones, P.B., 1982. Oil and gas beneath east-dipping underthrust faults in the Albertafoothills. In: Geologic Studies of the Cordilleran Thrust Belt: Rocky MountainAssociation of Geologists. 1. pp. 61–74.

Jones, P.B., 1996. Triangle zone geometry, terminology and kinematics. Bull. Can. Petrol.Geol. 44, 139–152.

Koegelenberg, C., Kisters, A.F.M., 2014. Tectonic wedging, back-thrusting and basin de-velopment in the frontal parts of the Mesoproterozoic Karagwe-Ankole belt in NWTanzania. J. Afr. Earth Sci. 97, 87–98.

Koyi, H., 1997. Analogue modelling: from a qualitative to a quantitative technique - ahistorical outline. J. Pet. Geol. 20, 223–238.

Kukla, P.A., 1992. Tectonics and sedimentation of late Proterozoic Damaran convergentcontinental margin, Khomas Hochland, central Namibia. In: Memoirs of theGeological Survey, Namibia. 12 (95 pp).

Laubscher, H., 1972. Some overall aspects of Jura dynamics. Am. J. Sci. 272, 293–304.Laubscher, H., 1977. Fold development in the Jura. Tectonophysics 37, 337–362.Laubscher, H., 1986. The eastern Jura: relations between thin-skinned and basement

tectonics, local and regional. Geol. Rundsch. 75, 535–553.Laubscher, H., 1990. The problem of the Moho in the alps. Tectonophysics 182, 9–20.Laubscher, H., Bernoulli, D., 1977. Mediterranean and Tethys, The Ocean Basins and

Margins. Springer, pp. 1–28.Lawton, T.F., 1985. Style and timing of frontal structures, Thrust belt, Central Utah. AAPG

Bull. 69, 1145–1159.Lawton, D.C., Spratt, D.A., Hopkins, J.C., 1994. Tectonic wedging beneath the Rocky

Mountain foreland basin, Alberta, Canada. Geology 22, 519–522.Lebel, D., Langenberg, W., Mountjoy, E.W., 1996. Structure of the central Canadian

cordilleran thrust-and-fold belt, Athabasca-Brazeau area, Alberta: a large, complexintracutaneous wedge. Bull. Can. Petrol. Geol. 44, 282–298.

Li, Z., Liu, S., Luo, Y., Liu, S., Xu, G., 2007. Structural style and deformation mechanism ofthe southern Dabashan foreland fold-and-thrust belt, central China. Front. Earth Sci.China 1, 181–193.

Liu, S., Dixon, J.M., 1991. Centrifuge modelling of thrust faulting: structural variationalong strike in fold-thrust belts. Tectonophysics 188, 39–62.

Lohrmann, J., Kukowski, N., Adam, J., Oncken, O., 2003. The impact of analogue ma-terial properties on the geometry, kinematics, and dynamics of convergent sandwedges. J. Struct. Geol. 25, 1691–1711.

MacKay, M.E., 1995. Structural variation and landward vergence at the toe of the Oregonaccretionary prism. Tectonics 14, 1309–1320.

MacKay, P.A., Varsek, J.L., Kubli, T.E., Dechesne, R.G., Newson, A.C., Reid, J.P., 1996.

Triangle zones and tectonic wedges: an introduction. Bull. Can. Petrol. Geol. 44, I.1–I. 5.

Madritsch, H., Schmid, S.M., Fabbri, O., 2008. Interactions between thin- and thick-skinned tectonics at the northwestern front of the Jura fold-and-thrust belt (easternFrance). Tectonics 27, TC5005.

Madritsch, H., Meier, B., Kuhn, P., Roth, P., Zingg, O., Heuberger, S., Naef, H., Birkhäuser,P., 2013. Regionale Strukturgeologische Zeitinterpretation der Nagra 2D-Seismik2011/12. Nagra Arb. Ber. NAB-13-10. (100 pp).

Malz, A., Madritsch, H., Kley, J., 2015. Improving 2D-seismic interpretation in challen-ging settings by integration of restoration techniques: a case study from the Jura fold-and-thrust belt (Switzerland). Interpret. J. Bible Theol. 3, SAA37–SAA58.

Malz, A., Madritsch, H., Meier, B., Kley, J., 2016. An unusual triangle zone in the externalnorthern Alpine foreland (Switzerland): structural inheritance, kinematics and im-plications for the development of the adjacent Jura fold-and-thrust belt.Tectonophysics 670, 127–143.

McClay, K.R., 1992. Glossary of thrust tectonic terms. In: McClay, K.R. (Ed.), ThrustTectonics. Chapman & Hall, London, pp. 419–433.

Medd, D.M., 1996. Recent triangle zone deformation in Papua New Guinea. Bull. Can.Petrol. Geol. 44, 400–409.

Medwedeff, D.A., 1992. Geometry and kinematics of an active, laterally propagatingwedge-thrust, Wheeler Ridge, California. In: Mitra, S., Fisher, G.W. (Eds.), StructuralGeology of Fold and Thrust Belts. Johns Hopkins University Press.

Meissner, R., 1989. Rupture, creep, lamellae and crocodiles: happenings in the con-tinental crust. Terra Nova 1, 17–28.

Mohn, G., Manatschal, G., Beltrando, M., Haupert, I., 2014. The role of rift-inheritedhyper-extension in Alpine-type orogens. Terra Nova 26, 347–353.

Molnar, P., Lyon-Caen, H., 1988. Some simple physical aspects of the support, structure,and evolution of mountain belts. Geol. Soc. Am. Spec. Pap. 218, 179–207.

Morley, C., 1986. A classification of thrust fronts. AAPG Bull. 70, 12–25.Morley, C., 2007. Interaction between critical wedge geometry and sediment supply in a

deep-water fold belt. Geology 35, 139–142.Morley, C.K., von Hagke, C., Hansberry, R.L., Collins, A.S., Kanitpanyacharoen, W., King,

R., 2017. Review of major shale-dominated detachment and thrust characteristics inthe diagenetic zone: part I, meso- and macro-scopic scale. Earth-Sci. Rev. 173,168–228.

Mouthereau, F., Lacombe, O., Deffontaines, B., Angelier, J., Brusset, S., 2001.Deformation history of the southwestern Taiwan foreland thrust belt: insights fromtectono-sedimentary analyses and balanced cross-sections. Tectonophysics 333,293–318.

Mouthereau, F., Lacombe, O., Meyer, B., 2006. The Zagros folded belt (Fars, Iran): con-straints from topography and critical wedge modelling. Geophys. J. Int. 165,336–356.

Mugnier, J.L., Baby, P., Colletta, B., Vinour, P., Bale, P., Leturmy, P., 1997. Thrust geo-metry controlled by erosion and sedimentation: a view from analogue models.Geology 25, 427–430.

Müller, M., 1984. Bau, Untergrund und Herkunft der Allgäuer Faltenmolasse: Mit 4Abbildungen. In: Jahresberichte und Mitteilungen des Oberrheinischen GeologischenVereines, Neue Folge. 66. pp. 321–328.

Müller, M., Nieberding, F., Wanninger, A., 1988. Tectonic style and pressure distributionat the northern margin of the Alps between Lake Constance and the River Inn. Geol.Rundsch. 77, 787–796.

Namson, J., Davis, T., 1988. Seismically active fold and thrust belt in the San JoaquinValley, central California. Geol. Soc. Am. Bull. 100, 257–273.

Nemcok, M., Henk, A., 2006. Oil reservoirs in foreland basins charged by thrustbeltsource rocks: insights from numerical stress modelling and geometric balancing in theWest Carpathians. In: Buiter, S.J.H., Schreurs, G. (Eds.), Analogue and NumericalModelling of Crustal-Scale Processes. Geol. Soc. Spec. Publ., London, pp. 415–428.

Nieuwland, D.A., Urai, J.L., Knoop, M., 2000. In-situ stress measurements in model ex-periments of tectonic faulting. In: Aspects of Tectonic Faulting. Springer, pp.155–166.

Oncken, O., Hindle, D., Kley, J., Elger, K., Victor, P., Schemmann, K., 2006. Deformationof the Central Andean upper plate system — facts, fiction, and constraints for plateaumodels. In: Oncken, O., Chong, G., Franz, G., Giese, P., Götze, H.-J., Ramos, V.,Strecker, M., Wigger, P. (Eds.), The Andes. Springer, Berlin Heidelberg, pp. 3–27.

Ortner, H., Aichholzer, S., Zerlauth, M., Pilser, R., Fügenschuh, B., 2015. Geometry,amount, and sequence of thrusting in the subalpine Molasse of western Austria andsouthern Germany, European Alps. Tectonics 34, 1–30.

Oxburgh, E., 1972. Flake tectonics and continental collision. Nature 239, 202–204.Oxburgh, E.R., 1974. The plain man's guide to plate tectonics. Proc. Geol. Assoc. 85,

299–357.Pace, P., Calamita, F., 2014. Push-up inversion structures v. fault-bend reactivation an-

ticlines along oblique thrust ramps: examples from the Apennines fold-and-thrust belt(Italy). J. Geol. Soc. 171, 227–238.

Pedreira, D., Afonso, J.C., Pulgar, J.A., Gallastegui, J., Carballo, A., Fernàndez, M.,Garcia-Castellanos, D., Jiménez-Munt, I., Semprich, J., García-Moreno, O., 2015.Geophysical-petrological modeling of the lithosphere beneath the CantabrianMountains and the north-Iberian margin: geodynamic implications. Lithos 230,46–68.

Pennock, E.S., Lillie, R.J., Zaman, A.S.H., Yousaf, M., 1989. Structural interpretation ofseismic reflection data from eastern salt range and Potwar Plateau, Pakistan. AAPGBull. 73, 841–857.

Price, R.A., 1986. The southeastern Canadian Cordillera: thrust faulting, tectonic wed-ging, and delamination of the lithosphere. J. Struct. Geol. 8, 239–254.

Ramos, V.A., 1989. Andean foothills structures in northern Magallanes Basin, Argentina.AAPG Bull. 73, 887–903.

Roeder, D.H., 1967. Rocky Mountains: der geologische Aufbau des kanadischen


41













































































































































































Felsengebirges. Gebr. Borntraeger, Berlin-Nikolassee.Sanderson, D.A., Spratt, D.A., 1992. Triangle zone and displacement transfer structures in

the eastern front ranges, southern Canadian Rocky Mountains. AAPG Bull. 76,828–839.

Sans, M., Vergés, J., 1995. Fold development related to contractional salt tectonics:southeastern Pyrenean thrust front, Spain. AAPG Mem. 65, 369–378.

Sans, M., Muñoz, J., Vergés, J., 1996. Triangle zone and thrust wedge geometries relatedto evaporitic horizons (southern Pyrenees). Bull. Can. Petrol. Geol. 44, 375–384.

Schlunegger, F., Mosar, J., 2011. The last erosional stage of the Molasse Basin and theAlps. Int. J. Earth Sci. 100, 1147–1162.

Schmid, S.M., Pfiffner, O.A., Froitzheim, N., Schönborn, G., Kissling, E., 1996.Geophysical-geological transect and tectonic evolution of the Swiss-Italian Alps.Tectonics 15, 1036–1064.

Schori, M., Mosar, J., Schreurs, G., 2015. Multiple detachments during thin-skinned de-formation of the Swiss Central Jura: a kinematic model across the Chasseral. Swiss J.Geosci. 108, 327–343.

Schuller, V., Frisch, W., Herzog, U., 2015. Critical taper behaviour and out-of-sequencethrusting on orogenic wedges – an example of the eastern Alpine Molasse Basin. TerraNova 27, 231–237.

Sherkati, S., Letouzey, J., Frizon de Lamotte, D., 2006. Central Zagros fold-thrust belt(Iran): new insights from seismic data, field observation, and sandbox modeling.Tectonics 25.

Sibson, R.H., 1983. Continental fault structure and the shallow earthquake source. J.Geol. Soc. Lond. 140, 741–767.

Sibson, R.H., 1985. A note on fault reactivation. J. Struct. Geol. 7, 751–754.Skuce, A.G., Goody, N.P., Maloney, J., 1992. Passive-roof duplexes under the Rocky

Mountain Foreland Basin, Alberta (1). AAPG Bull. 76, 67–80.Smit, J.H.W., Brun, J.P., Sokoutis, D., 2003. Deformation of brittle-ductile thrust wedges

in experiments and nature. J. Geophys. Res. Solid Earth 108.Sobornov, K.O., 1992. Blind duplex structure of the north Urals thrust belt front. J.

Geodyn. 15, 1–11.Sobornov, K.O., 1994. Structure and petroleum potential of the Dagestan thrust belt,

northeastern Caucasus, Russia. Bull. Can. Petrol. Geol. 42, 352–364.Sobornov, K., 1996. Lateral variations in structural styles of tectonic wedging in the

northeastern Caucasus, Russia. Bull. Can. Petrol. Geol. 44, 385–399.Sommaruga, A., 1997. Geology of the central Jura and the Molasse Basin: new insight into

an evaporite-based foreland fold and thrust belt. In: Mémoires de la Société desSciences Naturelles de Neuchatel, pp. 12 (176 pp).

Sommaruga, A., 1999. Décollement tectonics in the Jura forelandfold-and-thrust belt.Mar. Pet. Geol. 16, 111–134.

Sommaruga, A., Mosar, J., Schori, M., Gruber, M., 2017. The role of the Triassic eva-porites underneath the North Alpine Foreland. In: Soto, J., Flinch, J., Tari, G. (Eds.),Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins:

Tectonics and Hydrocarbon Potential. Elsevier, Amsterdam (pp. 447–446).Soule, G.S., Spratt, D.A., 1996. En echelon geometry and two-dimensional model of the

triangle zone, Grease Creek Syncline area, Alberta. Bull. Can. Petrol. Geol. 44,244–257.

Studer, B., 1825. Beiträge zu einer Monographie der Molasse.Suppe, J., 1981. Mechanics of mountain building and metamorphism in Taiwan. Mem.

Geol. Soc. China 4, 67–89.Suppe, J., 1983. Geometry and kinematics of fault-bend folding. Am. J. Sci. 283,

684–721.Suppe, J., 1985. Principles of Structural Geology. Prentice-Hall, Englewood Cliffs, NJ.Taborda, A., Spratt, D., 2008. In: Butler, R., McCaffrey, B., Torvela, T. (Eds.), Mobil057-

79-44: Peel Plateau; Northwest Territories. Virtual Seismic Atlas. http://www.seismicatlas.org/entity?id=5cba926c-3cbf-4de8-8861-18f3335aaede.

Tanner, D.C., Brandes, C., Leiss, B., 2010. Structure and kinematics of an outcrop-scalefold-cored triangle zone. AAPG Bull. 94, 1799–1809.

Teyssier, C., 1985. A crustal thrust system in an intracratonic tectonic environment. J.Struct. Geol. 7, 689–700.

Vollmayr, T., Wendt, A., 1987. Die Erdgasbohrung Entlebuch 1, ein Tiefenaufschluss amAlpennordrand. In: Bulletin der Schweizerischen Vereinigung Petroleum-Geologenund-Ingenieure. 53. pp. 67–79.

Wang, R., Zhang, Y., Xie, G., 2013. Physical modeling of fold-and-Thrust Belt evolutionand triangle zone development: Dabashan Foreland Belt (Northeast Sichuan basin,China) as an example. Acta Geol. Sin. 87, 59–72 (English Edition).

Weiss, J.R., Brooks, B.A., Arrowsmith, J.R., Vergani, G., 2015. Spatial and temporaldistribution of deformation at the front of the Andean orogenic wedge in southernBolivia. J. Geophys. Res. Solid Earth 120, 1909–1931.

Willett, S.D., 1999. Orogeny and orography: the effects of erosion on the structure ofmountain belts. J. Geophys. Res. 104, 28957–28981.

Xu, C., Zhou, X., 2007. Seismic interpretation of the Kelasu triangle zone in the southernTian Shan foothills, northwestern China. AAPG Bull. 91, 161–171.

Yeats, R.S., Huftile, G.J., Grigsby, F.B., 1988. Oak Ridge fault, Ventura fold belt, and theSisar decollement, Ventura basin, California. Geology 16, 1112–1116.

Yin, A., 2006. Cenozoic tectonic evolution of the Himalayan orogen as constrained byalong-strike variation of structural geometry, exhumation history, and foreland se-dimentation. Earth-Sci. Rev. 76, 1–131.

Zamora-Valcarce, G., Zapata, T., 2015. Building a valid structural model in a trianglezone: An example from the Neuquén fold and thrust belt, Argentina. Interpretation 3,SAA117–SAA131.

Zapata, T.R., Allmendinger, R.W., 1996. Growth stratal records of instantaneous andprogressive limb rotation in the Precordillera thrust belt and Bermejo basin,Argentina. Tectonics 15, 1065–1083.

Ziegler, P.A., 1987. Compressional intra-plate deformations in the Alpine foreland–anintroduction. Tectonophysics 137, 1–5.


42






















































http://www.seismicatlas.org/entity?id=5cba926c-3cbf-4de8-8861-18f3335aaede

http://www.seismicatlas.org/entity?id=5cba926c-3cbf-4de8-8861-18f3335aaede































triangle zones – geometry, kinematics, mechanics, and the ...special structural elements in the...

Documents