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Structural Geology Chapter 5: Reverse faults and thrusts

Chapter 5: Reverse faults and thrusts

Reverse faults

Thrusts

Fold-and-thrust belts

Introduction

Reverse faults are faults where the hanging wall moves up relative to the footwall.

They cause a shortening of the crust in the horizontal direction. The maximum

compressive stress is therefore close to horizontal. Since Mohr-Coulomb failure occurs

at an angle


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Salient & re-entrant

Klippe

Fenster

Because thrust are usually shallow dipping, their outcrop pattern may be complicated.

Some terms you need to know (Fig. 5.2a):

A salientis a part of the thrust that is ahead of the main thrust front. A re-entrant isthe opposite

A klippe is a part of the thrust and overlying rocks that is completely isolated, i.e.surrounded by its footwall (Fig. 5.2b).

Afensteris an isolated area of footwall outcrop.

Detachment folds

& pop-ups

Fault-propagation folds

Fault-bend folds

Main types of thrust-related structures

Three types of structures are common in fold-and-thrust belts:

Detachment folds and pop-ups: These structures form to accommodate the spaceproblems that arise at the tip of a blind detachment.

Fault-propagat ion folds : These structures form to accommodate the spaceproblems at the tip of an upwards-propagating thrust or imbricate.

Fault-bend folds: These structures form when a fault is not straight.

Kink bands

Pop-up structure

Box folds

Detachment folds and pop-ups

If there is fault movement along a blind detachment, space problems at the tip

necessitate the formation of accommodation structures (Fig. 5.3). The accommodation

can be by propagation of the detachment fault, or by the formation of folds. An

anticlinal fold can form if material escapes upwards. This is usually possible, since the

surface of the Earth is a free surface.

Since detachments usually form in incompetent units, these units usually accommodate

the space problem by inhomogeneous deformation, including thickening of the units.

Competent layers tend to only rotate in kink bands, while maintaining their original

thickness.

The detachment folds that form when an incompetent layer is missing and all layers

can only fold by rotation of limbs are called pop-up structures. When they have flat

crests they are called box-folds.

Fig. 5.3. (a) Blind detachment. (b) Displacement along the detachment

without any accommodation would lead to an impossible overlap of themoving and non-moving block. (c) Instead, an anticline may form above

the detachment tip. The uplifted excess area must be equal to the overlap

area in (b). This area depends on stratigraphic level relative to the

detachment. (d) Box folds are pop-up structures with a flat crest.

There are two end-member mechanisms for the formation of detachment folds (Fig.

5.4):

(1) Rotation of the limbs, making the fold ever taller and narrower. The kink in the

layers remains in the same point within the folding layers.

(2) Migration of the kink band. The fold limbs maintain a constant angle, but become

longer with progressive fault movement.

(3) In reality, one would often have a combination of these two end-membermechanisms.


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Fig. 5.4. Main mechanisms of the formation of detachment folds. From Shaw, Connors & Suppe (2005) Seismic interpretation of

contractional fault-related faults. An AAPG Seismic Atlas. Studies in Geology 53.

General characteristics of detachment folds

There is usually an incompetent layer at the detachment. This layer is thickened inthe core of the fold.

The frontal axial plane or kink band ends at the tip of the detachment. Detachment folds are usually upright and symmetric to moderately asymmetric,

with the steep limb facing the thrusting direction.

Competent layers usually maintain their original thickness. The fold becomes smaller towards the tip of the detachment (axial planes

converge), because the excess area (A) enclosed by the fold is equal to the

stratigraphic level above the detachment (H) and the offset (F), with A = H!F.

Fig. 5.5. Example of a detachment

fold in a mountain face at Opal

Mountain, Canadian Rocky

Mountains. The fold axial planesconverge to the lower left, where

the tip of the detachment is inferred.

Layers in the core of the fold are

more strongly deformed.


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Fault-propagation folds Faults can propagate. The detachment can propagate in its own plane, but often it

splays upwards towards the surface. As long as the splay does not reach the surface,

folding must accommodate the absence of fault movement in front of the tip.

Characteristic fault-propagation folds form. A typical aspect is that the fault tip

progressively shifts forward and upward, as the fold structure develops (Fig. 5.6).

Fault-propagation folds are characterised by:

A distinct asymmetry with a steep forelimb and a shallow back limb. The frontal synclinal axial plane ends at the fault tip. Folds get tighter downwards. Slip on the fold decreases towards the fault tip. Upward (listric) curvature of the fault produces a syncline at the back of the system

(a fault-bend fold, see next section)

Fig. 5.6. Development of a fault propagation fold. (a)

Beginning situation. (b) After some slip along the fault, a fold

starts to develop. Slip is accommodated in front of the tip byupward escape of material. (c) Situation after another

increment of slip and fault propagation.

Imbricate fans

Leading imbricate fan

Trailing imbricate fan

An upward splay rarely comes alone. Usually splays form sequentially at the tip of a

detachment, with each splay forming a fault-propagation fold. Such a system we call an

imbricate fan.

In a leading imbricate fan the youngest imbricate is in the front, carrying the older

imbricates "on its back", which is sometimes called "piggy-backing". This is the most

common situation. The older imbricates are rotated to a steeper position (Fig. 5.7a)

In a trailing imbricate fan, the youngest imbricate is at the rear, thrusting over older

imbricates (Fig. 5.7b).

Fig. 5.7. (a) Leading imbricate fan. The youngest imbricate is at the front and the older imbricates get progressively steepened. (b)

Trailing imbricate fan, with the youngest imbricate at the back. #1 is the oldest imbricate, #3 the youngest


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Fault-bend folds

Active axial plane

Passive axial plane

Thrusts have ramps and flats, just like normal faults can have. The bends in the faults

produce accommodation folds:fault-bend folds.

Two types of axial planes develop (Fig. 5.8):

Active axial planes: These are fixed relative to the fault ramps and flats. Each bendin the fault is associated with an active axial plane.

Passive axial planes: These are fixed relative to the layers they bend. They movetogether with the material along the fault.

Figure 5.9 shows an example of a ramp-flat system in a small-scale thrust.

Fig. 5.8. Progressive development of a thrust with a ramp and two flats. A = active axial planes, through which materials

moves. P = passive axial planes that move with the material.

Fig. 5.9. Example of a small-scale ramp-flat system in sedimentary rocks from Sestri Levante in Italy. The fault has stepped up

from the bottom (left) to the top (right) of the competent layer. The distance between the two axial planes indicate the offset along

this fault. Compass for scale


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Compressional duplex Duplexes also form under compression. As with extensional duplexes, they consist of a

number of lenses or horses, bounded by a fault on both sides. One of the most famous

textbook examples comes from Crow's Nest Pass in the Canadian Rocky Mountains

(Fig. 5.10). The duplex consists of many small horses, that make a progressively bigger

angle with the roof- and floor-fault from left to right. This means that the youngest

horses are at the leading end in the front (left).

Fig. 5.10. Small-scale duplex structure with 23 horses. The oldest horses on the right have been steepened by riding piggy-back on

the younger horses on the left. Crow's Nest Pass. Canadian Rocky Mountains. Original structure dipping about 40 to the right.

Hinterland-dipping duplex

Foreland-dipping duplex

As with imbricate fans, you can have different types of duplexes, depending on when

the new horses are formed relative to the displacement of the older horses.

a) If new horses are formed at the front (in the slip direction), the older horses aretilted to the back and you get a hinterland-dipping duplex (Figs 5.10 and 5.11a).

This is the most common of the two types.

b) However, if the new horse only forms after the older horses have moved over it,one gets aforeland-dipping duplex. (Fig. 5.11b).

Fig. 5.11. Hinterland-dipping (a) or foreland-dipping (b) duplexes form depending on the timing of the formation of new

horses, relative to the movement of older horses. Active fold is the thick line, de-activated faults are drawn with a medium-

thick line.

Fig. 5.12. Example of a "mini" fold-and-thrust belt on

Santorini Island, Greece. The structure was formed by

slumping of still soft volcanic sediments.


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Fig. 5.13. Fold-and-thrust structure in Mesozoic sediments at Molinos, NE Spain. The trace of the frontal thrust that breachedthe surface is shown on the left. Several fault-propagation folds to the right are the result of the propagation of blind faults

that did not breach the surface. Tectonic transport to the left (north).

Fold and thrust belts Fold and thrust belts (Fig. 5.12-13) are found in many places in the world, usually

associated with subduction. In general there is the situation that a substrate moves

relative to the overlying thrusting rocks. The two are separated by a detachment. The

geometry is similar to a bulldozer pushing a pile of sand.

Fig. 5.14. Section through a

fold-and-thrust belt (eastern

front of Rocky Mountains).

Such belts are normally wedge shaped or tapered (like in an accretionary wedge). They

are thin in the front and thicker at the back (Fig. 5.14). These wedge-shaped or tapered

fold and thrust belt characteristically have:

Shallow dipping thrusts in the front, and steeper ones in the rear. The lattersteepened by the younger imbricates in the front of the belt

Stratigraphically deeper units outcropping in the rear, and youngest strata in thefront.

Metamorphic grade increasing from front to rear, as deeper rocks are exposed inthe rear of the belt.

Angle of taper What determines the wedge shape and the angle of taper (")? Consider a wedge as

shown in figure 5.12 or a bulldozer pushing a layer of sand. The base of the wedge is

the detachment and to get sliding, the critical shear stress must be reached at this

detachment.

The force applied from the rear is balanced by the friction of the material sliding overthe detachment. The differential stress (!#) thus decreases from the rear of the wedge

to the front of the wedge. The size of the Mohr circle for stress is equal to the

differential stress. The size of the circle thus decreases from rear to front.


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The position of the circle along the normal stress axis is determined by the pressure.

The pressure is proportional to depth. The pressure at the detachment is therefore

highest at the rear, where the detachment is deepest. The vertical stress at the

detachment is proportional to depth. The horizontal stress is added by the push from the

rear, so we can take the vertical stress (#yy) as the minimum principal stress.

("= 0)

("= "crit)

Critical angle of taper

(">"crit)

Consider three points A, B, and C on the detachment at the base of a wedge (Fig. 5.15).

Case 1. The angle of taper is zero (" = 0) (Fig. 5.15a). The depth to the detachment

is the same everywhere, and therefore #yy is constant. The minimum stress for

all Mohr circles is the same, so they are all left-aligned. !# is highest for

point C and smallest for point A. If failure occurs, it will first occur at point C

at the rear of the wedge. If the back of the wedge gets thicker by thrusting, but

the front remains undeformed, the taper increases: "gets bigger.

Case 2. The angle of taper is equal to the critical taper (" = "crit) (Fig. 5.15b). The size

of the Mohr circle at C is larger than that at A, but because C is deeper, its

circle is shifted to the right. At the critical angle of taper, all Mohr circlessimultaneously touch the failure envelope. The whole detachment is activated.

Case 3. The angle of taper is larger than the critical taper (" >"crit) (Fig. 5.15c). Now

the pressure increase from point A to C is so large that the Mohr circle for C

does not touch the failure envelope, even though it has the largest differential

stress. In this case the detachment only fails at the front (point A). Movement

at the front and not at the rear means the wedge gets stretched and the angle of

taper decreases.

Erosion effect

We see that the wedge develops towards the critical angle of taper. If the wedge is too

shallow, thrusting at the rear thickens the wedge at the rear and increases the taper. If

the wedge is to steep, thrusting at the front will decrease the taper. The wedge is in

balance at the critical angle of taper. This angle is clearly related to the failure

properties of the material and the friction along the detachment.

Erosion can change the angle of taper, and thus influence thrusting in a wedge.

Fig. 5.15. Explanation of the concept of a critical angle of taper. (a) Taper is zero, so thrusting only occurs at the rear of the

wedge at point C. (b) Wedge at the critical angle of taper, with thrusting along the whole detachment, as the Mohr circles for

all points touch the failure envelope. (c) Wedge at a taper larger than the critical angle of taper. Now point A at the front of

the wedge is the first to reach failure.

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