tectonic evolution of devonian basins in northern scotland ... · devonian conglomerates with sharp...

Tectonic evolution of Devonian basins in northern Scotland and southern Norway

MIKE G. NORTON, KEN R. McCLAY & NICK A. WAY

Norton, M. G., McClay, K. R. & Way, N. A.: Tectonic evolution of Devonian basins in northern Scotland and southern Norway. Norsk Geologisk Tidsskrift, Vol. 67, pp. 323-338. Oslo 1987. ISSN 0029-196X.

The geometries of the Devonian basins of northern Scotland and southern Norway are analysed to determine the regional tectonic control on their formation. The Orcadian Basin in northern Scotland consists of distinct half-graben sub-basins controlled by separate, closely spaced, sub-parallel arcuate extensional fault elements which are parti y con tro lied by the Caledonian crustal fabric. The unconformity at the base of the middle Devonian in this area is interpreted as being formed by a sudden increase in the rate of extension. In southern Norway, in con trast, extensional structures are more widely spaced and larger in scale. Very deep (>8 km) half-graben basins were developed of which only the bottom parts are now preserved. In the Devonian of Norway the Jack of a major stratigraphic break suggests a constant extension rate. The observed differences in style are thought to be due to different initial crustal thicknesses.

M. G. Norton, K. R. McCiay and N. A. Way, Department of Geology, Royal Hol/oway and Bedford

New College, Egham, Egham Hill, Surrey, TW20 OEX, U.K.

The broad outlines of the Devonian 'Old Red Sandstone' basins of northern Scotland, western Norway and the northern North Sea have been known for some time (e.g. Ziegler 1982) (Fig. la). The tectonic setting of the Devonian sediments has been variously interpreted as related to megashears (Ziegler 1982; Bukovics et al. 1984) or regional extension (Dewey 1982; McClay et al. 1986) (Fig. 1). Large basins like the Orcadian Basin of northern Scotland consist of many similar sub-basins each exhibiting separate tectonic contro! and distinct sedimentation and subsidence patterns. This paper provides a detailed analysis of some of the individual Devonian sub-basins and uses the results to constrain a general model for Devonian tectonics in the north west European continental margin.

The analysis has been carried out by: the reinterpretation of published onshore data, fieldwork at various critical localities, the interpretation of offshore geophysical and well data together with available structural and sedimentological data.

Northern Scotland North of the Highland Boundary Fault, Devonian sediments define the approximate extent of the 'Orcadian Basin' (Fig. 2). Within this region two

main aspects have been studied; the pattern of basin-forming faults and the significance of the unconformity between lower and middle Devonian. In defining the basin-forming fault network, use has been made of the work of Enfield & Coward (1987) in the West Orkney Basin. Fieldwork has been carried out in the region bordering the Moray Firth and in Caithness, Orkney and Shetland. lnterpretation in the Mora y Firth itself has been carried out using data provided by Fina (UK). In the offshore area around Shetland the interpretation is based on the BIRPS SHET survey (McGeary 1987).

The identification of faults active during Devonian sedimentation is based on observed or inferred faults which correlate well with major thickness and facies variations, and published sediment dispersal patterns (Mykura 1983). On some of the published geological maps faulted contacts between Devonian sediments and basement are shown to pass laterally into unconformable contacts; this is particularly so along the north western boundary of the Devonian between Inverness and Helmsdale and in east Shetland. In most cases unconformity has been inferred along unexposed contacts in which the interface is seen to dip at an angle of less than 45°. Now that the existence of low-angle extensional faults has become recognized such contacts can be con-

� () .

OIST

RIBU

TIO

SE

DIME

NT$.

N OF

OEV

ONIA

N

�lA-._� .... _._.·· ·· .·J ···· · .·· .. ·• ... •· ... · ·,·

��

!3

�j·/

,/ V

O

200K

M.

'-----'

a

SINI

STRA

L ME

GASH

EAR

S.

-Lo

cal

ised

øun

-aøa

rt b

asin

s

-V

aryi

tlg e

xte

naio

n d

irec

tions

au

b-pa

ralle

t to

tau

lts

-�·

,:

_)'J

.-/

�

\

0 20

0KM.

'-

--'

HB

F -

Hlg

hla

nd

B

ou

nd

ary

Fa

un

GG

F

-G

rea

t G

lan

Fa

un

MF

-

Mln

ch

Fa

ull

WB

F -

Wa

11a B

ou

nd

ary

F

au

ll

MT

FZ

-M

ore

-T

røn

de

lag

Fa

ult

Z

on

e

BF

-

Bll

lel)

ord

afl

Fa

ult

b

�

-W

lde

ap

rea

d

h.alf

-g

rabe

na

-C

on

ait

tan

t e

Kte

ns

lon

dir

ec

llo

nl

-O

rea

tea

t e

Jtte

na

ion

in

th

e

zo

ne

of

gre

ate

st

cru

ata

l th

ick

en

ing

,&.:

O 20

0KM.

'-

----'

� i � " .. X

� � �

. "

z

o o

..

;;; !:3

�8

"� �

�0

��

.,o

æ

er "

�

��

.. ;::

zz

o

o

.{"

Fig

. l.

Nor

thea

st A

tlant

ic r

egio

n in

pre

-rif

t co

nfigu

ratio

n sh

owin

g (a

) D

istr

ibut

ion

of D

evon

ian

sedi

men

ts o

f no

rthe

rn S

cotla

nd,

sout

hern

Nor

way

, E

ast

Gre

enla

nd a

nd S

valb

ard,

(b

) Si

nist

ral

Meg

ashe

ar H

ypot

hesi

s, a

nd (

c) E

xten

sion

al C

olla

pse

Hyp

othe

sis.

(b)

and

(c)

als

o in

dica

te e

xpec

ted

basi

n ch

arac

teri

stic

s fr

om t

he t

wo

mod

els.

c l

Ul

� � � � � �

�

z

o

�

V>

::-:

o

m

o

t""'

o 8 V>

::-:

::l

o

V>

V>

::-:

� � O>

......

....._ �

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

fidently reinterpreted as tectoni'c. In some cases fracture sets are developed sub-parallel to the contact with individual fractures showing extensional displacement providing supporting evidence for the interpretation of these boundaries as extensional faults.

The fault pattem established by this study is shown in Fig. 3. This has been constructed by assuming the following overall strike-slip movements since the lower Devonian: Great GJen Fault - 20 km dextral (Rogers 1987), Walls Boundary Fault - 75 km dextral (Astin 1982), Nestings Fault - 15 km dextral (Astin 1982); no allowance has been made for movement on either the Helmsdale or Melby Faults whose movements are insufficiently constrained. The overall pattem would not be greatly affected by movements on these faults of 50 km or less. Three major faults incorporated into the network were also important structures during the Mesozoic development of the Inner Moray Firth Basin; the Smith Bank Fault, the Lossiemouth Fault and the Buckie Fault. The evidence for an earlier phase of movement on these faults comes from seismic interpretation of Devonian sediments in half-grabens on their hanging walls and from well data showing age and facies variations across the faults. Fig. 4 shows a generalized geoseismic section across two of these faults taken from commercial seismic data (the position of the section is shown in Fig. 2).

One of the most interesting aspects of the fault network deduced in the Orcadian Basin is the presence of several arcuate fault elements which define the separate sub-basins. These have a characteristic aspect ratio and size with a strike extent of ca. 120 km. These arcuate fault elements are very similar in scale to those observed in the Lake Tanganyika region of the East African Rift (Rosendahl et al. 1986). It is not clear, however, how all of these faults link together either along strike or at depth.

The observed arcuate fault geometry implies that along much of the fault lengths there is a significant strike-slip component to the displacement. At the ends of some of these faults the displacement must be almost pure strike-slip in character. The orientation of these high! y oblique segments can be used to constrain the overall extension direction as approximately northwest-southeast trending (see also Enfield & Coward 1987).

The overall structural framework shows no sign

TSGS Symposium 1986 325

of a significant strike-slip component in the regional tectonic setting; the Great GJen Fault itself has no clear expression in either the observed fault network or in the pattern of sedimentation (Rogers 1987). However, the interpretation of widespread unconformities at the base of the middle and upper Devonian successions in the Orcadian Basin has been used as evidence for rna jor periods of uplift which have in turn been used to imply periods of transpression.

The unconformity at the base of the middle Devonian is best exposed on the southern shore of the Moray Firth in the excellent coastal exposures at Pennan (Trewin 1987) within the Turriff half-graben sub-basin (Fig. 2). At Pennan lower Devonian conglomeratic sandstones dip moderate! y steeply (ca. 40°) towards the west and are overlain by shallowly dipping middle Devonian conglomerates with sharp angular unconformity, this is the on! y clear example of angular unconformity in the Orcadian Basin, elsewhere it is marked only by a facies change (Rogers 1987). The base of the middle Devonian is irregular and strongly erosive. The lower Devonian is affected by a series of small-scale faults with displacements of a few cms up to a few metres which do not cut the unconformity and clearly predate this erosional event. All of the fault zones are extensional in geometry, most of them dipping towards the east. This is the only deformation, for which there is evidence, that can explain the rotation of the lower Devonian strata. The fault zones themselves are rather diffuse zones in which no fault rock is developed and appear to have been formed in only part-consolidated sediments. We interpret this unconformity not as a period of general uplift but as a period of rejuvenation along pre-existing extensional faults probably with the initiation of some new extensional structures. In the Turriff region this rejuvenation led to the progradation of alluvial fans from the new fault scarps eroding down into the previously deposited and on! y partly consolidated sediments. This fits well with the interpretation by Rogers (1987) of a major change in the drainage pattern at this time. It is possible that both the small-scale faulting and the fan progradation date from this same extensional pulse. Fig. 5 shows a schematic cross-section over the Turriff half-graben at end middle Devonian times to illustrate the proposed interpretation. The recognition of the middle Devonian basal unconformity throughout the

326 M. G. Norton et al.

KEY

� Mesozoic

D U. Oevonian

[] M. Devonian

� L. Devonian

"'v Volcanics

Irr] Granite

D Basement

;-"'""

Unconformity

/ Extensional fault

/ Contractional fault

/P Strike-slip fault

/ Han_ging�wall anticline

1

.... 'Q

MORA Y FIRTH ·,.


UNST W

50 km


\ \

/ 25 KM.

'====='

Fig. 3. Devonian fault pattern map of the Orcadian Basin as

deduced in this study using data from Enfield & Coward (1987)

in .the West Orkney Basin (extensional faults, downthrow on ornament ed side).

Orcadian Basin suggests that this extensional rejuvenation was of regional extent.

I,n the region lying northwest of the Great GJen Fault a series of contractional structures have been known for some time, e.g. at Struie Hill (Armstrong 1964; Mykura 1983), which are probably related to transpression along the fault. These are best exposed in Iower Devonian strata and a genetic link has been suggested between them and the unconformity at the base of the middle Devonian. However, it is clear that some of these structures (in common with those in Caithness) involve upper Devonian strata as well and must be at !east as young as lower Carboniferous in age. There seems no reason to suggest a separate period of transpression at the

. end of the lower Devonian to explain these structures.

In the Elgin area the unconformity at the base of the upper Devonian involves overstep across


8KM. SW

BANFF F.

l l • l sees

� l TWT

l

Fig. 4. Schematic geoseismic section ( taken from various commercial surveys) across the Smith Bank and Lossiemouth Faults

showing Devonian half-graben basins and reactivation of their

marginal faults during the Mesozoic. For approximate location

see Fig. 2.

TURRIFF SUB-BASIN AT END MIODLE DEVONIAN.

PENN"N NEW ABERDO\JR

PRESENT EROS ION LEYEL

Fig. 5. Schematic cross-section across the Turriff sub-basin at the end of the middle Devonian as deri ved in this study.

the middle Devonian sub-basins onto the basement. The marginal fault system to this subbasin was still clearly active as a growth fault at this time as shown by thickness and facies variations across it (Mykura 1983). However, fault activity appears to be relative! y subdued and much of the upper Devonian sediments represent reworking of the previously deposited Devonian (Watson 1984). The unconformity is not well exposed except on Hoy where it is associated with the Hoy Javas but no strong discordance between the middle and upper Devonian has been described.

To the south of the Highland Boundary Fault (i.e. south of the area described above) the sedimentary and tectonic history of the Devonian are quite different to that described above. It seems that subduction was still active at the start of the Devonian period with eventual closure and smallscale collision (in comparison to the rest of the Caledonides) marking the start of the middle Devonian. Various lines of evidence have been put forward to show that this convergence was associated with significant sinistral strike-slip

Fig. 2. Geological map of the Orcadian Basin modified after various BGS maps showing SHET lines, the Western Geophysical

line (WG) as figured by Andrews (1985) and the location of Fig. 4. Key to faults: GGF Great Glen Fault, HF Helmsdale Fault, NSF North Scapa Fault, WBF Walls Boundary Fault, MF Melby Fault & NF Nesting Fault.


motion of the Highland Boundary and other faults (including the Great GJen Fault and faults in the Grampian Highlands) (Bluck 1983; Soper & Hutton 1984). We do not deny the existence of important sinistral strike-slip movements south of the Highland Boundary Fault during late Silurian and into lower Devonian times but we suggest that north of this structure such movements were of reduced scale, predated most of the sedimentation (which began in the Siegennian at the earliest) (Mykura 1983), and have very little contro! on basin formation. We suggest that at the beginning of the middle Devonian the ending of convergence to the south may have acted as a trigger for the pulse of renewed extension to the north.

Shetland

The sub-basin geometry both onshore Shetland and in the surrounding shelf area is not as clear as that to the south. The results from the SHET survey (Fig. 2) (McGeary 1987) have been disappointing in delineating Devonian sediments in the shelf area due mainly to unfavourable acquisition conditions, particularly the presence of relatively high velocity rocks (Devonian sediments or Caledonian basement) at the surface in shallow water. Onshore detailed fieldwork has been carried out studying basement-cover contacts and fault patterns within the Devonian particularly in East Shetland and on the island of Foula (Fig. 2). As mentioned in the previous section the western boundary of the East Shetland Devonian is here interpreted as an extensional fault, probably active during sedimentation. Observations of movement indicators from the marginal fault zone and minor faults in the hanging wall indicate dominant E-W extensional displacement which supports this interpretation. In the region around Lerwick this extensional fault activity is expressed in alluvial fanglomerates but the effect on sedimentation dies out towards the south and the throw on the fault is presumed to do the same. The clear unconformity at the base of the Devonian at Quarff (Fig. 6) is explained as representing part of the unconformity on the hanging wall of the marginal fault.

This basin marginal fault follows the line of the 'Quarff Thrust' and 'Quarff Melange' from Lerwick to south of Quarff. The possibility that some of the high-strain rocks in the zone could be related to the observed fault as an extensional


Ej o .

KEY

Devonlan

Conglomerate

Sandatone

Dalradlan

Quarff Nappe

� Major lntruaion

/ ExtenalonaJ Fault

_/ Strlke-SIIp Fault

Fig. 6. Geologi ca( map of the east Shetland Devonian, modified after BGS 250,000 sheet, Shetland.

detachment (i.e. as in the model of Davis 1983) wm investigated. Although the high strain zone is poorly exposed, lineations were found to be mainly sub-parallel to strike (NNW-SSE ), similar to that developed in much of the Caledonides of Shetland. Unequivocal sense of shear indicators were lacking but as yet we can find no evidence for a genetic link between the high strain rocks and the marginal fault except that the fault has locally reactivated the older structure.

On Foula middle Devonian sediments are in contact with Caledonian basement, the contact being interpreted as either a slightly modified unconformity (Blackbourn & Marshall in prep.) or as an extensional detachment (Beach 1985). As in East Shetland the boundary between Devonian and basement is parallel to a strong, in this case mylonitic, foliation in the basement (Fig. 7). Here, however, the mylonites contain unequivo-


\

DNOOPSANOSTONfFH EZI ""''"

DSHfl..(iSANJSTONEfJ'1 DBLOfli!SBURNftl DsæERLJ(SAI()SfONEFH EEl il'NESSFM (Z::]BASfl"lNT l:::) lt(AiliR.tNITf --- 1'\\ØIEXTEHSØW..

'"'-''

GEOUlGUL MAP CF FOOLA

95

!KM L.....---'

Fig. 7. Geological map of Foula, modified after Blackbourn & Marshall (in prep).

ca! sense of shear indicators showing thrust sense displacement towards the northeast. The pattern of minor faults in the Devonian of the hanging wall and in the upper part of the basement of the footwall shows an unequivocal extensional displacement approximately E-W in orientation. It is clear again that the orientation of the extensional fault has been controlled by the pre-existing basement anisotropy.

This preliminary work on Shetland has identified a number of extensional faults probably active during the Devonian and these have been included in the final compilation (see last section). The structure of the west Shetland Devonian (Mykura 1976; Astin 1982) with two phases of folding and small-scale thrusting is complex and this, together with the emplacement of the Sandsting Granite Complex, obscures the original basin geometry. The origin and timing of these structures and their relationship to the granite (Mykura 1976, 1983: deformation during and after intrusion; Astin 1982: deformation during intrusion; Enfield & Coward 1987: deformation


before intrusion) must be clarified before this geometry can be firmly established.

The middle Devonian volcanics of Shetland are an apparent anomaly within the basins, whether extensional or transtensional in origin, due to their tholeiitic to calc-alkaline chemistry (Thirlwall 1981). Any interpretation in terms of subduction active at this time is very difficult to fit in with the known geology of the area. One method of generating such magmas during extension would involve partial melting of previously metasomatized upper mantle (Simpson et al. 1979; Gill 1982) resulting from earlier subduction.

Southern Norway Devonian sediments in southern Norway (i.e. south of 64°N) are found in three areas (Fig. 8)

l

( '

\

Oevonlan aedlmenta

Upper and uppermoat '"ochthon Lower and mlddle allochthon Par/autochthon

tOOKM "\ Extenelonallaull

l _,

' '

Fig. 8. Simplified geological map of southern Norway modified

from various sources including NGU and SGU maps. Key: M Meråker, T Trondheim, B Bergen, R Røragen, NSD

Nordfjord-Sogn Detachment, RD Røragen Detachment, SD

Sunnhordaland Detachment, LGF Lærdal-Gjende Fault, MTFZ Møre-Trøndelag Fault Zone.


(exduding the uppermost Silurian ORS Facies Ringerike sandstone which formed in a foreland basin generated by the advancing Caledonian thrust sheets) (Hossack et al. 1985). These are; on the west coast between Sognefjord and Nordfjord, in the outer Trondheimsfjord, and at Røragen. The ages of these sedimentary sequences are not well constrained although the ·

dating available suggests that these are mostly lower to middle Devonian with possibly uppermost Silurian on Hitra, outer Trondheimsfjord.

The four west coast basins and Røragen share the following features:

(l) Unconformable boundary to the west onto rocks of the middle or upper allochthon.

(2) Tectonic boundary to the east consisting of an intense mylonite zone, in most cases involving deformed Devonian at the top of the zone, against a thick zone of mylonites.

(3) Clear tectonic control on sedimentation with alluvial fanglomerates derived from positions dose to the present tectonic margins.

( 4) Deformation of the Devonian sediments with local formation of deavage, open to dose folding with local small-scale thrusting accompanied by Prehnite-Pumpellyite possibly into Greenschist facies metamorphism.

The main models for basin formation are (see also Steel et al. 1985):

• Extensional half-graben basins with most of the deformation and metamorphism caused by hanging-wall accommodation structures (Hossack 1984; Norton 1986, 1987; Seranne & Seguret 1985, 1987; Michelsen & Tillung pers. comm. 1987).

• Transtensional basins formed on releasing bends on strike-slip faults with deformation caused by local transpression (Steel 1976; Steel et al. 1977; Steel & Gloppen 1980).

• Allochthonous basins emplaced into their present position by a major thrusting event with related folding and metamorphism, the 'Solundian', of late Devonian age (Sturt 1983; Torsvik et al. 1986); this model does not predict an y link between basin formation and structure affecting the sediments.

The arguments for and against thrustirrg have been recently critically assessed by Norton (1987) who found no unequivocal evidence for thrusting and the third model is not considered further


here. The strike-slip model fails to explain the presence of mylonites along the eastern tectonic contact and only in some of the basins have possible strike-slip faults been identified. Man y of the features of the strike-slip mod el are, however, induded in the extensional model in which the extensional detachments have several higher angle strongly oblique segments (Hossack 1984; Norton 1987; Michelsen & Tillung in prep.).

Large-scale extension is thought to have been generated due to the collapse of overthickened crust at the end of the Caledonian continentcontinent collision (Norton 1986). The !argest structure formed during the extensional phase was the Nordfjord-Sogn Detachment described in detail by Norton (1987). A large half-graben basin developed in the hanging wall of this structure filled with the erosional products of the uplifting footwall. The effect of irregularities in the footwall geometry was to give locally distinct depositional systems now recognized as the four west coast sub-basins Hornelen, Håsteinen, Kvamshesten and Solund; for the larger basin unit the name Vestland Basin is proposed. The fault geometry at the surface during sedimentation may have bad similarities to that seen in the Orcadian Basin (Fig. 3) with the individual arcuate fault segments linking together laterally and at depth to form the observed detachment.

Norton (1987) proposed the existence of another major extensional structure in southern Norway, the Røragen Detachment (Fig. 8). In this paper we describe in more detail the evidence for this interpretation. The Røragen Devonian (Fig. 9) consists of ca. 1.5 km preserved stratigraphic thickness of sediments interpreted as alluvial fan deposits (Roberts 1974; Steel et al. 1985). The overall structure of the sediments is simple with a fairly constant 40°-70° dip towards the southeast. As indicated earlier an unconformable boundary is present to the northwest against rocks of the Levanger-Øyfell Nappe (Koli equivalent (Gee et al. 1985), upper part of upper allochthon) while the southern and southeastern boundaries are apparently tectonic. In the southwestern part of the area an almost monomict serpentinite conglomerate, the Svartberg Conglomerate, is in high angle faulted contact with serpentinite of the Skjøtingen-Essandjø Nappe (Seve equivalent, lower part of the upper allochthon, (Gee et al. 1985)). In the northeast of the area the Devonian is in presumed tectonic contact with a mylonite zone previously interpreted as


AURSUNDEN


UPPER PLATE

� Sandstone

Polymict conglomerate

Serpentinite conglomerate

Upper Allochthon

Røros schist

Gabbro

Serpentin i te

Semi-pel i te

PETACHMENT

l Augen mylonite

Q.uartz phyllonite

Calcareous phyllonite

Psammite mylonite

LOWER PLATE

D Eocambrian psammite

Fig. 9. Geological rna p of Røragen di stri et showing outcrop with )akes and glacial drift left unornamented, modified after Roberts (1974).

the thrust at the base of the Trondheim Nappe. This high strain zone places rocks of the Koli equivalent against sparagmite arkoses of the Osen-Røa Nappe of the lower allochthon. Along this structure the whole of the Gula Group is missing, as is the Seve equivalent north of Røragen, and the middle allochthon is only represented by the ca. 100 m thick augen mylonite presumed to be equivalent to the Tiinniis Augen Gneiss in Sweden (Gee 1978). Due to rather patchy exposure, a continuous transect across the mylonite zone is only possible in the northeast of the area shown in Fig. 9 along the section line A. The mylonites dip shallowly (15°-30°) towards WNW and have a tectonostratigraphy as shown in Figs. 9 and 10.

The lowest exposed mylonite is derived from an arkosic psammite presumed to be the underlying sparagmite although a transition between them is not exposed. A strong mineral elongation lineation is developed within the mylonite foliation plunging 28° to 300°. In thin section quartz is seen to be almost completely recrystallized to an

aggregate of slightly inequant grains. These define an oblique grain shape fabric consistent with an extensional sense (i.e. hanging wall displaced down to NW ) to the zone (Lister & Snoke 1984). A quartz c-axis fabric measured from a sample taken from near the top of the psammite mylonite shows a strongly asymmetric cross-girdle fabric implying the same sense of shear (Bouchez et al. 1983).

Above the psammite mylonite there is a ca. 30 m thick band of calcareous phyllonite. The more micaceous parts of this band have a welldeveloped secondary shear band cleavage whose asymmetry again gives a down to the NW sense of shear; more carbonate rich layers and quartz veins are affected by northwest verging asymmetric folds. The origin of this band is not clear but it could possibly represent a high! y attenuated equivalent of the Cambro-Ordovician shelf sequence of the lower allochthon. Continuing up the section there is ca. 50 m of quartz rich phyllonite whose dominant small-scale structures are northwest verging microfolds. Following the pre-


NW

MyaamOrbullen


SE

Fig. JO. Schematic cross-section along line A, Fig. 9, showing small-scale structures developed. The quartz c-axis diagram is contoured at l, 2, 4, 8%, 151 observations.

vious logic this layer may represent the Silurian part of the shelf sequence.

The topmost part of the exposed sequence consists of ca. 100 m of augen mylonite. K-feldspar porphyroclasts (up to 10 cm in size) form the augen in this rock, the only kinematic indicators observed are north west verging asymmetric folds. As previously suggested this is thought to represent the lower allochthon. The metamorphic grade at which mylonitization occurred is not clear but the recrystallization of the quartz throughout the section suggests conditions of at !east greenschist facies. There is an unexposed zone, at its narrowest only a few metres wide, between the augen mylonite and the Devonian sediments which is here interpreted as a brittle fault zone. It is possible to trace this mylonite stratigraphy into the poorly exposed ground for 10 km to the south although no other continuous sections were found.

The presence of southeasterly dipping Devonian sediments in the hanging wall, the omission or thinning of tectonostratigraphy and the small-scale structures developed in the mylonites are all consistent with the interpretation of this structure as an extensional detachment. Supporting evidence for this interpretation comes from the observation of clast populations from the Brekkefjell Conglomerate as reported by Roberts (1974) . The alluvial fan system which deposited this conglomerate is thought to be derived from

the northeast (Steel et al. 1985), but the observed clast lithologies of 'Røros Group phyllites and schists as well as quartzites, chert, sandstone, metagreywacke, vein quartz and other rock-types' (Roberts 1974) all indicate derivation from an area with exposed geology similar to that seen to the west beneath the basal unconformity. This suggests that when sedimentation was initiated there was no pre-existing major structure separating the present basin floor from the area immediately to the southeast.

Roberts (op. cit.) reported a series of smallscale structures developed within the Devonian sediments. The most important of these observations were of locally intense deformation in the sandstone/silt layers which are interbedded with the fanglomerates. Close, angular northwest verging asymmetric folds are developed. In the extensional model such deformation is explained in terms of hanging wall accommodation. With the geometry as depicted in Fig. 11 hanging wall strains would necessarily be developed at the point at which the high angle fault along the south of the Røragen Devonian joined the top of the . detachment. This could be accommodated by bedding parallel shear which, in the conglomeratic units, would be concentrated in the interbedded sandstone/siltstone layers.

The synclinal closure mentioned by Steel et al. (1985) observed in the Svartberg Conglomerate is probably a result of so-called 'normal fault drag'

NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 333

Fig. 11. Schematic cross-section along line B, Fig. 9, showing the assumed relationship between the high angle fault and the mylonite zone.

against the nearby high angle fault. The other features of the Røragen Devonian previously interpreted as related to a Svalbardian deformation phase (i.e. the presence of a cleavage and very Iow grade to ? low grade metamorphism) have already been considered by Norton (1987). The main importance of these observations is that it suggests that the original Devonian basin in this region was very much bigger than that presently exposed. It was pro ba bly 8 km deep and extended along strike for as much as 300 km, the present strike extent of the mylonite zone with its associated omission/thinning of tectonostratigraphy, from the Grong Culmination in the north to the culmination near Otta in the south.

The Lærdal-Gjende Fault (Fig. 8) and the associated 'Faltungsgraben' (Milnes & Koestler 1985) are also thought to be extensional structures of Devonian age. It is very likely that a smaU basin was developed in the hanging wa11 of the Lærdal-Gjende Fault which, although comparatively sha11ow (2-3 km), probably extended along the whole strike length of the fault i.e. ca. 150 km. Milnes & Koestler (1985) also report reactivation of the thrusts in this area with displacement to the northwest, movements of pro bable Devonian age.

Devonian sediments of the outer Trondheimsfjord are associated with the Møre-Trøn-

delag Fault Zone but these faults were clearly active during the Mesozoic and probably did not control Devonian sedimentation. Due to this later faulting the relationship between the sediments and the basement is not clear. Detailed fieldwork onshore combined with seismic interpretation in the immediate offshore area will be necessary to establish a basin model for this area.

Devonian fault patterns and palaeogeography

The network of faults interpreted in northern Scotland and southern Norway have been compiled together in Fig. 12. In east Greenland the approximate position of some of the faults thought to be responsible for Devonian basin formation are marked along the trend of the zone of low-angle extensional faults as reported by Frank] (1953) ca1led by McClay et al. (1986) the East Greenland Extensional Zone. These westdipping extensional systems are not seen to be directly associated with Devonian sediments but are interpreted as Devonian in age (Ha11er 1971, 1985). The present outcrop pattern of the Devonian in east Greenland is contro11ed by eastdipping post-Devonian extensional faults.


DEVONIAN

BASIN-FORMING

FAULTS

O 200KM. L-....1

Fig. 12. Pattem of Devonian basin forming faults in northeast Atlantic region on pre-rift configuration.

The main difference between the basins of northern Scotland and southern Norway is one of scale. The basin forming faults are in both cases arcuate extensional faults but the Norwegian examples have generally longer strike extent, greater spacing between faults, larger inferred fault displacements and thicker hauging wall basin fills. This is presumed to be a refiection of the much larger extension resulting from the much greater crustal thickening produced in Norway at the end of the Silurian.

One other change in style of extension is noteworthy, the sense of asymmetry. In northern Scotland the extensional faults are dominantly east to southeastward dipping except for a poorly defined zone between Orkney and Shetland where they dip to the west. In southern Norway and east Greenland all the extensional structures dip to the west. This con trast in asymmetry is also shown in Fig. 13 linking the interpreted palaeogeography to the underlying structure. In both cases the direction of di p of the extensional structures matches that of the preceding contractional structures. This is not thought to imply direct


reactivation of the thrusts as extensional faults, although locally this has undoubtedly taken place. Rather it is suggested that the contractional tectonics induced an overall anisotropy in the middle to upper crust which had an important con tro! on the direction of di p of the succeeding extensional structures.

Fig. 13 shows the palaeogeography as derived from the observed fault network and published sediment dispersal patterns (Mykura 1983; Astin 1985; Steel et al. 1985). In Scotland drainage systems running axially along half-graben subbasins interconnect and have an outlet to the sea (as represented by the marine Devonian of the central North Sea (Ziegler 1982)). Such a connection has been suggested to explain features of the Achanarras fish bed (Trewin 1986).

Fig. 13 also shows the main areas in which little or no data is available on the nature of Devonian basins, if present. These areas naturally coincide with the areas of greatest Mesozoic and Tertiary extension where any Devonian strata will be too deep to be imaged on standard seismic refiection data. The increasing availability of deep seismic data should allow at !east some of these gaps to be filled in.

Discussion

The megashear hypothesis was originally developed to explain apparent palaeomagnetic evidence of large discrepancies in palaeolatitude for Devonian sediments from either side of structures such as the Great GJen Fault in Scotland (Van der Voo & Scotese 1981) and the Billefjorden Fault in Svalbard (Harland et al. 1974). More recent work has east doubt on the significance of these discrepancies suggesting that no resolvable (i.e. >200 km) movement has occurred on these faults since the lower Devonian (Torsvik et al. 1985; Lamar et al. 1986; Tarling 1985). This does not, however, preclude movements of this smaller scale which are quite sufficient to generate the observed sedimentary basins.

The main arguments against the dominance of strike-slip related sedimentation in the Devonian are:

(l) The very widespread area in which basins developed often far from any known strikeslip structures e.g. the Turriff and Røragen basins.

MID

DL

E-

DE

VO

NIA

N

PA

LE

OG

EO

GR

AP

HY

&

TE

CT

ON

ICS

��

- ,

�·

�OKM

Fig

. 13

. E

nd m

iddl

e D

evon

ian

pala

eoge

ogra

phy

from

nor

ther

n Sc

otla

nd t

o ce

ntra

l N

orw

ay a

nd e

ast

Gre

enla

nd i

ncor

pora

ting

stri

ke-s

lip m

ovem

ents

as

liste

d in

the

tex

t. A

llow

ance

has

als

o be

en m

ade

for

ca.

100 k

m e

xten

sion

pos

t-D

evon

ian

in t

he V

ikin

g G

rabe

n. (

Vol

cano

es in

dica

te a

reas

of

mid

dle

Dev

onia

n m

agm

atis

m.)

z

o � :><:

o

tT1

o 5 o

c;;

:><: :l o

Vl

Vl � � �

�

..... � �

C)

Vl

�

� � ;::·

3 ....... � ....,

....,

Vl


(2) The lack of a clear sedimentary signature within Devonian strata deposited adjacent to the postulated major strike-slip faults, e.g. along the Great Glen Fault in the Inverness area (Rogers 1987). Strong evidence exists for strike-slip motion along some basin margin faults (e.g. the northern margin of Hornelen (Steel & Gloppen 1980) but these are readily explained as oblique segments on arcuate extensional structures.

(3) The consistency of implied extension directions at high angles to the postulated strikeslip faults.

The extensional model is also favoured by a correlation between the area of greatest crustal thickening with the !argest extensional structures and the deepest apparent basin fills.

One of the implications of this model is that Devonian extension and, therefore, basin formation is likely to have occurred throughout the area affected by late Caledonian crustal thickening. Devonian half-grabens should be expected beneath areas of Mesozoic and Tertiary sedimentation.

An aspect of the Devonian extensional structures which has only been touched on is the effect on subsequent basin formation. We have proposed partial reactivation of Devonian basin marginal faults during the development of the Mesozoic lnner Moray Firth Basin. If such relatively minor structures can exhibit such control, a structure of the scale of the Nordfjord-Sogn Detachment would be expected to have a very marked effect. A study of the published structures offshore suggests that the Øygarden/Horda Fault Zone which forms the eastern margin of the Mesozoic basin off western Norway could represent a short cut fault into the detachment at depth.

Conclusions

(l) Devonian sediments were deposited in sets of linked half-grabens generated during northwest-southeast directed regional extension.

(2) Sinistral strike-slip had no infiuence on sedimentation north of the Highland Boundary Fault after earliest lower Devonian times.

(3) The unconformity at the base of the middle Devonian in the Orcadian Basin represents a rejuvenation of extension.


(4) The basement/cover contact on Foula is a reactivated Caledonian structure, not a Basin and Range type detachment.

(5) The Røragen Devonian outcrop is an erosional remnant of a much larger half-graben basin formed by extensional movement on the Røragen Detachment.

(6) Devonian half-grabens are likely to be present on most of the Norwegian Continental Shelf.

(7) Devonian extensional structures, particularly major detachments, may have had a profound infiuence on Mesozoic basin geometries.

Acknowledgements. - The work reported in this paper was carried out as part of the COCO project funded by Fina (UK) and Fina Exploration Norway. N. A. Way was supported by a NERC research studentship; earlier fieldwork by MGN was funded by a grant from the University of Keele. We thank the following for useful discussions: M. Armstrong (BGS Edinburgh), B. T. Kneller, N. H. Trewin (Aberdeen), T. R. Astin (Reading), D. Rogers (Cambridge), M. Enfield (Imperial), S. McGeary (BIRPS), J. E. A. Marshall (Southampton) and J. Scallon, J. Staffurth, J. Mathalone (Fina UK).

References Andrews, l. J. 1985: The deep structure of the Moine Thrust,

southwest of Shetland. Scottish Journal of Geology 21, 213-217.

Armstrong, M. 1964: The geology of the region between the Alness River and the Dornoch Firth. Ph. D. thesis, Newcastle upon Tyne (unpublished).

Astin, T. R. 1982: The Devonian geology of the Walls peninsula, Shetland. Ph. D. thesis, University of Cambridge (unpublished).

Astin, T. R. 1985: The palaeogeography of the middle Devonian Lower Eday Sandstone, Orkney. Scottish Journal of Geology 21, 353-375.

Beach, A. 1985: Some comments on sedimentary basin development in the Northern North Sea. Scottish Journal of Geo

logy 21, 493-512. Blackbourn, G. A. &Marshall,J. E. A. (in prep.): The Geology

of Foula, Shetland. Bluck, B. J. 1983: Rote of the Midland Valley of Scotland

in the Caledonian orogeny. Transactions Royal Society of

Edinburgh 74, 119-136. Bouchez, J-L., Lister, G. S. & Nicholas, A. 1983. Fabric asym

metry and shear sense in movement zones. Geologische Rund·

schau 72, 401-420. Bukovics, C. , Cartier, E. G. , Shaw, N.D. & Ziegler, P. A. 1984:

Structure and development of the mid-Norway continental margin. In: Spencer et al. (eds.), Petroleum Geology of the North European Margin, Graham and Trotman, London, pp. 407-424.

Davis, G. H. 1983: Shear-zone model for the origin of meta-morphic core complexes. Geology 11, 342-347.

Davis, G. H. & Coney, P. J. 1979: Geologic development of the Cordilleran metamorphic core complexes. Geology 7,

120-124. Dewey, J. F. 1982: Plate tectonics and the evolution of the


British Isles. Journal of the Geological Society of London

139, 371-414. Enfield, M. & Coward, M. P. 1987: The structure of the West

Orkney Basin, northern Scotland. Journal of the Geological

Society of London 144, 871-884. Frank!, E. 1953: Geo1ogische Untersuchungen in Ost-Andrees

Land (Nordostgronland). Meddelser om Gronland 103, 1-35. Gee, D. G. 1978: The Swedish Caledonides - a short synthesis.

In: IGCP Project 27, Caledonian-Appalachian O ro gen of the

North Atlantic Region; Geological Survey of Canada, Paper

78-I3, pp. 63--72. Gee, D. G. , Kumpulainen, R. , Roberts, D. , Stephens, M. B. ,

Thon, A. & Zachrisson, E. 1985: Scandinavian Caledonides, Tectonostratigraphic Map. In: Gee, D. G. & Sturt, B. A. (eds.), The Caledonide Orogen: Scandinavia and Related

Areas. Wiley, New York. Gill, J. B. 1982: Mountain building and volcanism. In: Hsu,

K. J. (ed.), Mountain Building Processes, Academic Press, London, pp. 13--17.

Haller, J. 1971: Geology of the East Green/and Caledonides,

Interscience Publishers, New York, 413 pp. Haller, J. 1985: The East Greenland Caledonides - reviewed.

In: Sturt, B. A. &Gee, D. G. (eds.), The Caledonide Orogen:

Scandinavia and Related Areas, Wiley, New York, pp. 1031-1046.

Harland, W. B. , Cutbill, J. S. , Friend, P. F. , Gobbett, D. J. , Holliday, D. W. , Maton, P. 1., Parker, J. R. & Wallis, R. H. 1974: The Billefjorden fault zone, Spitzbergen. Norsk Polarinstitutt Skrifter 161, 72 pp.

Hossack, J. R. 1984: The geometry of listric growth faults in the Devonian basins of Sunnfjord, W. Norway. Journal of

the Geological Society of London 141, 629-637. Hossack, J. R. , Garton, M. R. & Nickelsen, R. P. 1985: The

geological section from the foreland up to the Jotun thrust sheet in the Valdres area, South Norway. In: Sturt, B. A. & Gee, D. G. (eds.), The Caledonide Orogen: Scandinavia and

Related Areas, Wiley, New York, pp. 443-456. Lamar, D. L. , Reed, W. E. & Douglass, D. N. 1986: Bil

lefjorden fault zone, Spitzbergen: Is it part of a major Late Devonian transform? Geological Society of America Bulletin

97, l 083--1088. Lister, G. S. & Snoke, A. W. 1984: S-C Mylonites. Journal of

Structrual Geology 6, 617-638. McClay, K. R., Norton, M. G., Coney, P. & Davis, G. H.

1986: Collapse of the Caledonian orogen and the Old Red Sandstone. Nature 323, 147-149.

McGeary, S. 1987: Non-typical BIRPS on the margin of the North Sea. In: Matthews, D. H. & BIRPS Group ( eds. ), Deep Seismic Reflection Profiling of the Continental Lithosphere, Geophysical Journal Royal Astronomical Society 89, 231-238.

Milnes, A. G. & Koestler, A. G. 1985: Geological structure of Jotunheim, southern Norway (Sognefjell-Valdres crosssection). In:Sturt,B. A. &Gee, D. G. (eds.), TheCaledonide Orogen: Scandinavia and Related Areas, Wiley, New York, pp. 457-476.

Mykura, W. 1976: British Regional Geology: Orkney and

Shetland. HMSO Edinburgh. Mykura, W. 1983: Old Red Sandstone. In: Craig, G. Y. (ed.),

Geology of Scotland, Scottish Academic Press, pp. 205-251. Norton, M. G. 1986: Late Caledonide extension in western

Norway: a response to extreme crustal thickening. Tectonics

5, 195-204. Norton, M. G. 1987: Nordfjord-SognDetachment, W. Norway.

Norsk geologisk Tidsskrift 67, 93--106. Roberts, D. 1974: Sedimentary, tectonic and metamorphic fea-


tures of the Devonian of Røragen, Sør-Trøndelag. Norsk

geologisk Tidsskrift 54, 89-108. Rogers D. A. 1987: Devonian correlations, environments and

tectonics across the Great GJen Fault. Ph. D. thesis, University of Cambridge (unpublished).

Rosendahl, B. R. , Reynolds, D. J. , Lorber, P. M. , Burgess, C. ·

F. , McGill, J. M. , Scott, D. , Lambiase, J. J. & Derksen, S. J. 1986: Structural expressions of rifting: lessons from Lake Tanganyika, Africa. In: Frostick, L. E. et al. (eds.), Sedi

mentation in the African Rifts, Geological Society Special

Publication 23, 29-43. Seranne, M. & Seguret, M. 1985: Etirement ductile et cisail

lement basal dans les bassins Devoniens de l'Ouest Norvege. Comptes Ren dues Academie des Sciences Paris, 300, Serie 11,

373--378. Seranne, M. & Seguret, M. 1987: The Devonian basins of

western Norway, tectonics and kinematics of an extending crust. In: Coward, M. P. , Dewey, J. F. & Hancock, P. (eds.), Continental Extension Tectonics, Special Publication of the

Geological Society of London 28, 537-548. Simpson, P. R. , Brown, G. C. , Plant, J. & Ostle, D. 1979:

Uranium mineralisation and granite magmatism in Britain. Philosophical Transactions of the Royal Society of London

291A, 385-412. Soper, N. J. & Hutton, D. H. W. 1984: Late Caledonian sinistral

displacements in Britain: implications for a three-plate collision model. Tectonics 3, 781-794.

Steet, R. J. 1976: Devonian basins of western Norway; sedimentary response to tectonism and varying tectonic context. Tectonophysics 36, 207-224.

Steel, R. J. , Mæhle, S. , Nilsen,H., Røe, S. L. & Spinnanger, Å. 1977: Coarsening-upward cycles in the alluvium of Hornelen Basin (Devonian), Norway: sedimentary response to tectonic events. Geological Society of America Bulletin 88, 1124--1134.

Steel, R. J. & Gloppen, T. G. 1980: Late Caledonian (Devonian) basin formation, western Norway: signs of strikeslip tectonics during infilling. In: Ballance, P. F. & Reading, H. G. (eds.), Sedimentation in Ob/ique-slip Mobile-Zones,

Special Publication of the International Association of Sedi

mentologists 4, 79-103. Steet, R. J. , Siedlecka, A. & Roberts, D. 1985: The Old Red

Sands tone basins of Norway and their deformation: a review. In: Sturt, B. A. & Gee, D. G. (eds.), The Caledonide Orogen: Scandinavia and Related Areas, Wiley, New York, pp. 293--316.

Sturt, B. A. 1983: Late Caledonian and possible Variscan stages in the orogenic evolution of the Scandinavian Caledonides. The Caledonian Orogen-IGCP Project 27, Symposium de Rabat ( Abstract).

Tarling, D. H. 1985: Palaeomagnetic studies of the Orcadian Basin. Scottish Journal of Geology 21, 261-274.

Thirlwall, M. F. 1981: Implications for Caledonian plate tectonic models of chemical data from volcanic rocks in the British Old Red Sandstone. Journal of the Geological Society of

London 138, 123--128. Torsvik, T.H. , Løvlie, R. & Sturt, B. A. 1985: Palaeomagnetic

argument for a stationary Spitzbergen relative to the British Isles (Western Europe) since late Devonian and its bearing on North Atlantic reconstruction. Earth Planetary Science

Letters 75, 278-288. Torsvik, T. H. , Sturt, B. A. , Ramsay, D. M. , Kisch, H. J.

& Bering, D. 1986: The tectonic implications of Solundian (Upper Devonian) magnetization of the Devonian rocks of Kvamshesten, western Norway. Earth Planetary Science Let

ters 80, 337-347.


Trewin, N. H. 1986: Palaeoecology and sedimentology of the Achanarras fish bed of the Middle Old Red Sandstone, Scotland. Transactions of the Royal Society of Edinburgh 77, 21-46.

Trewin, N. H. 1987: Pennan, unconformity within the Old Red Sandstone. Excursion 9/n: Trewin, N. H. , Kneller, B. T. & Guillen, C. (eds.), Excursion Guide to the Geology of the

Aberdeen Area, Scottish Academic Press (in press).


Van der Voo, R. & Scotese, C. R. 1981: Palaeomagnetic evidence for a large (ca. 2000 km) sinistral offset along the Great Glen Fault during Carboniferous time. Geology 9, 177-180.

Watson, J. 1984: The ending of the Caledonian orogeny in Scotland. Journal of the Geological Society of London 141,

193--214. Ziegler, P. A. 1982: Geo/ogica/ Atlas of Western and Central

Europe. Elsevier, Amsterdam, 130 pp.

tectonic evolution of devonian basins in northern scotland ... · devonian conglomerates with sharp...

Documents