nature of palaeozoic extension in lofoten, north norwegian continental shelf: insights from 3-d...

Nature of Palaeozoic extension in Lofoten, north NorwegianContinental Shelf: insights from 3-D seismic analysis of aCordilleran-style metamorphic core complex

Gijs Allard Henstra and Atle RotevatnDepartment of Earth Science, University of Bergen, All�egaten 41, Bergen 5007, Norway

ABSTRACT

Analyses of 3-D seismic data reveal that pre-Triassic basins

are present underneath the Mesozoic North Træna Basin (Lo-

foten Margin, Norway). These are linked to a Cordilleran-style

metamorphic core complex that developed in Palaeozoic

times, including rotated fault blocks with hanging wall

‘growth’ wedges, bounded by listric faults that sole into a

sub-horizontal detachment. On the basis of similarity in age,

structural style and transport direction, we propose a kine-

matic link with a Permian mylonitic detachment documented

onshore. This study presents the first offshore evidence for

Palaeozoic detachment faulting, elucidating the mechanisms

behind the long-lived exhumation history of the Lofoten

basement.

Terra Nova, 26, 247–252, 2014

Introduction

Metamorphic core complexes (MCC)are associated with exhumation andjuxtaposition of high-grade metamor-phic basement rocks against lowergrade upper crustal rocks across low-angle detachments, and were firstdescribed in the North AmericanCordilleras (e.g. Lister and Davis,1989). Gneiss-cored exhumationcomplexes of Palaeozoic age, relatedto collapse and unroofing of the Cal-edonian orogen, are well documentedin the Western Gneiss Region ofsouth Norway (e.g. Andersen et al.,1991). However, although theexhumed basement and low-angledetachment are commonly preserved,the classical MCC geometries of theupper plate are rarely seen, owing toeither lack of preservation or lack ofexposure (Osmundsen et al., 2005).In this study, a reprocessed 3-D

seismic dataset (Fig. 1) that offersimaging of the pre-Triassic succes-sion with unprecedented clarity isused to elucidate the nature of Palae-ozoic extension and exhumation inthe Lofoten margin. We describe aprobable-Permian MCC, which fea-tures classical geometries such astransport of severely rotated faultblocks over a low-angle detachment.

The fault blocks are associated withhanging wall basins containing syn-tectonic ‘growth’ strata. Althoughnot directly age-constrained, thesestrata are situated c. 0–1500 msbelow a well-tied near base Triassicreflection. Herein, we present (i) thedocumentation, (ii) a simple recon-struction and (iii) a model for theformation of the MCC in Permiantimes.Previous onshore workers have

suggested that the Lofoten Ridge(LR) forms part of a Permian MCC(Hames and Andresen, 1996; Olesenet al., 2002; Steltenpohl et al., 2004),based on: (i) the presence of lowercrustal rocks at the surface; (ii) anage-constrained Permian, sub-hori-zontal, mylonitic detachment thatcrops out on the islands of Røst andVærøy; and (iii) a shallow Mohounderneath the LR. This studyunderlines that the previously sug-gested MCC is regionally extensive(Steltenpohl et al., 2011), and pro-vides the first offshore evidence forbasement denudation related toprobable-Permian-aged low-angledetachment faulting.

Tectonostratigraphic frameworkof the Lofoten margin

The partly submerged LR representsa c. 200-km-long, NNE-trendingregional basement high, comprisinglower crustal, mangeritic rocks (Ha-mes and Andresen, 1996), which isflanked to the east and west by rift

basins of Cretaceous and older age(Fig. 1). The geology of NorthernNorway is to a large extent domi-nated by the imprint of the Caledo-nian orogeny, which culminatedduring Iapetus closure and the penul-timate collision of Baltica and Laur-entia in Late Silurian to EarlyDevonian times (Gee and Sturt,1985). The contractional phase of theCaledonian orogeny in Scandinaviawas superseded by a stage of syn- topost-orogenic collapse in Devoniantimes (Andersen et al., 1991; Fossen,2000). Extension in the WesternGneiss Region (WGR) of south-westNorway is manifested by the pres-ence of large-scale supradetachmentbasins filled with Devonian clastics,but no deposits of this age have beenreported in the Lofoten. Further-more, while most of the exhumationand cooling of allochthonous rocksthrough ~350 °C took place prior toc. 390 Ma in the southern WGR(Berry et al., 1995), the unroofing ofsimilar structural levels in the Lofo-ten area occurred in several stagesover c. 150 Ma, lasting well into thePermian (Hames and Andresen,1996). Investigating the nature ofPermian extension offshore to thesouth-west of the Lofoten archipel-ago is therefore important to under-stand the regional exhumationhistory.The evolution of the Lofoten mar-

gin since the Palaeozoic is character-ized by an extended rift phase(Hansen et al., 2012) that led to

Correspondence: Mr. Gijs Allard Henstra,

Department of Earth Science, University

of Bergen, All�egaten 41, Bergen 5007,

Norway. Tel.: 00 47 451 85 776; e-mail:

[email protected]

© 2014 John Wiley & Sons Ltd 247

doi: 10.1111/ter.12098

continental break-up in the Eocene(Mosar et al., 2002) and post-break-up uplift in Oligocene–Miocene times

(Dor�e et al., 1999). The oldest sedi-mentary rocks drilled west of theLofoten archipelago are of earliest

Triassic age and resemble alluvial fandeposits directly overlying crystallinebasement, similar to the mangeriticbedrock of the Lofoten islands (Han-sen et al., 1992). The possible pres-ence of a Palaeozoic successionbelow the Mesozoic strata in theNorth Træna basin has been recog-nized previously. However, the sedi-mentary nature and Palaeozoic ageof such a succession is disputed; it istreated as a sedimentary wedge bysome authors (Hansen et al., 1992;Tsikalas et al., 2001; Bergh et al.,2007), while others refer to it asbasement (Færseth, 2012). In thisstudy, we provide evidence for thesedimentary nature of these pre-Tri-assic reflections as well as the rela-tion to Palaeozoic exhumation of theLofoten margin.

Evidence for Palaeozoic extensionin the (proto-) North Træna basin

Our database comprises a repro-cessed three-dimensional, pre-stacktime-migrated seismic reflection sur-vey that was acquired in 1996 overthe northern part of the NorthTræna basin, c. 3 km south of where

Fig. 1 Structural element map of the Lofoten segment of the Norwegian continen-tal margin. The 3-D seismic survey and well 6710/3-U-3 are indicated in blue; thelocations of the sections in Fig. 2 and 4 are shown in red. Modified after Blystadet al. (1995).

?

W ESWNE

10 km

PaleogeneU. CretaceousL. CretaceousJurassicTriassicPaleozoicBasement

Base Paleozoic

A A’

2 km

B B’

East Røst Fault Zone

?growth in minibasin

400

ms

3500 m

(A) (B)

NW SE

?

0.0

0.5

1.0

TWT

(sec

onds

)

0.0

0.5

1.0TWT

(sec

onds

)

Vest

erdj

upet

Faul

t Zon

e

Fig. 2 Geoseismic sections across the North Træna Basin. A–A0 shows a key section across the basin with the Palaeozoic mega-sequence visible below the Mesozoic. B–B’ shows a more detailed image of the Palaeozoic wedge; note the rotated fault blocksseparated by listric faults detaching onto a deeper bundle of reflections, representing a basal detachment. When flattened attop Triassic (see Fig. 3c), the detachment zone would dip to the north-west. A seismic-well tie between B-B0 and 6710/3-U-3 isfound in the data repository.

248 © 2014 John Wiley & Sons Ltd

Palaeozoic extension in Lofoten, Norwegian Continental Shelf • G. A. Henstra and A. Rotevatn Terra Nova, Vol 26, No. 3, 247–252

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the Triassic-basement contact wasdrilled (IKU shallow core 6710/3-U-3; Fig. 1). It has a line spacing of12.5 m and a vertical resolution of c.25 Hz within the stratigraphic inter-val of interest. Stratigraphic controlis provided by shallow core 6710/3-U-3 (Hansen et al., 1992) and tied tothe 3-D survey via 2-D seismic reflec-tion lines (seismic-well tie is availablein the data repository). Within theshallow core, the base Triassic eventis also the sediment–basement inter-face; the seismic-well tie shows howthe two events separate towards thesouth to give way to the pre-Triassicwedge.This Palaeozoic megasequence con-

sists of a westward-expanding wedgeof semi-transparent seismic character;tracing the wedge eastward, itbecomes progressively more con-densed as reflectors coalesce and thewedge thins to almost nothing beforebeing truncated at the erosional baseTriassic event (Fig. 2a). To the west,it thickens to c. 1500 ms two-way tra-vel time (ms); here, the wedge isbounded by the East Røst FaultZone (ERFZ). The dipslope of thisPalaeozoic graben forms the deepestcontinuous reflector here and, basedon a tie to well 6710/3-U-3, is inter-preted to represent the top of acous-tic basement at the base of the wedge(henceforth referred to as ‘top base-ment’). It should, however, be notedthat the presence of older Palaeozoicstrata below this reflector cannot beruled out.The dipslope of the Palaeozoic gra-

ben is itself disrupted by a series ofimbricate, rotated, extensional faultblocks, separated by east-dipping li-stric faults (Fig. 2b). The top base-ment reflector is mappable within arelatively small area of approximately20 km2, revealing the N–S to NNE–SSW strike of the 1- to 2-km-widefault blocks; the faults are about10 km long (Fig. 2; a structure mapis provided in the data repository).Wedges of ‘growth’ strata occupy theaccommodation space (c. 200 ms;Figs 2b and 3) that was generated asthe fault blocks rotated. Theseexpand by a factor of two over a dis-tance of 1 km, clearly too much to beexplained by differential compaction.The listric faults sole out on a dis-

continuous bundle of reflections thatoccurs a few 100 ms below the top

basement reflector. This bundle isinterpreted to represent a detachmenthaving formed within crystalline base-ment, or in mechanically weak layerswithin possibly present older Palaeo-zoic sedimentary rocks. In a strike-normal section, this detachment zoneand reflections within the lower partof the supra-basement sedimentsbroadly parallel one another(Fig. 2b). This means that by the timethe fault blocks moved with respect toone another, the detachment zone wasa sub-horizontal feature a few hun-dred metres beneath the ground sur-face. By flattening on the base Triassicevent, thereby removing the imprintof Mesozoic rifting and Cenozoic

uplift, it becomes evident that the top-to-the-east detachment zone becamewest-dipping (‘warped’) by latest Pal-aeozoic time (Fig. 3).

Discussion and conclusions

With the arrival of improved seismic,it can now be confidently concludedthat a distinct sedimentary megase-quence is present below the base Tri-assic event (see Fig. 2a) as tied toshallow core 6710/3-U-3. The drilledTriassic sediments are assigned aGriesbachian age (Hansen et al.,1992); this is the earliest stage of theTriassic, spanning only 1 Ma follow-ing the peak of the Permian–Triassic

Post- eT ctonic Stage (Latest Triassic)

Stage 1

5 km

0.0

0.5

1.0TWT

(sec

onds

)

Triassic

Basem

entPermian

Stage 2

Snake Range analog

5 km

0

8

Dep

th ( 4

km)

stage 1stage 2

(A)

(B)

(C)

(D)

Fig. 3 Conceptual reconstruction of the Palaeozoic succession of geoseismic sectionA–A0 from Fig. 2. (A) Early stage of fault block rotation. (B) Sedimentationchanges from isolated depocentres to more widespread deposition in response tothe development of listric faults that do not sole out on the detachment. (C) Thesituation at the end of the Triassic: late Permian erosion has removed parts of themetamorphic core; Triassic rift faults preferentially nucleate over the Permianbasin. (D) Structural analogue; initial transport of small fault blocks over a crustaldetachment zone, followed by the development of syn- and antithetic listric faultsand large depocentres ‘upstream’.


Terra Nova, Vol 26, No. 3, 247–252 G. A. Henstra and A. Rotevatn • Palaeozoic extension in Lofoten, Norwegian Continental Shelf

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extinction event. The strata thatmake up the underlying megase-quence must therefore indeed belongto the Palaeozoic.The oldest sediments belonging to

this megasequence were laid down asa syn-tectonic unit in the mini basinsthat formed as a response to rotationof the small fault blocks (Figs 2band 3a). These sediments thus recordthe activity of the listric faults andtherefore the first eastward move-ment over the detachment. This per-iod of localized deposition wasfollowed by more widespread deposi-tion in response to the developmentof a large fault to the west (possiblythe proto-ERFZ), which gave rise tothe wedge-shaped configuration ofthe megasequence observed today(Figs 2a and 3b). More continuousPermian reflections belonging to thelarger wedge (Fig. 2b) indicate thatthe smaller listric faults were lessactive (Fig. 3b); moreover, the devel-opment of the larger halfgrabenresulted in the downthrowing of thedetachment, making it dip to thewest (Fig. 3b and c).The novel interpretation of the

pre-Triassic root to the North Trænabasin presented here closely resem-bles that of the area between thecrustal culmination and the break-away fault of a Cordilleran-stylemetamorphic core complex (MCC),with (i) transport of listric faultblocks over a sub-horizontal basaldetachment and (ii) ‘upstream’ devel-opment of a relatively large depocen-tre, resulting in (iii) down-warping ofthe detachment (Wernicke, 1985; Lis-ter and Davis, 1989). The develop-ment of the up-dip breakaway zoneof the Snake Range MCC as inter-preted by Johnston (2000) bears astriking resemblance to the Permianfeatures seen in the North Trænabasin. Initial small-scale listric fault-ing over the main detachment zone(Fig. 3d) is followed by the forma-tion of larger listric faults that dipmore steeply and do not sole into themain detachment.Given its (inferred) age, structural

style and inferred movement sense, itis likely that the detachment zoneproposed herein connects to the duc-tile detachment zone previouslydescribed onshore Røst and Værøy,which are exposed parts of the LR(Fig. 1). The latter structure led

other workers to strengthen the casefor the existence of an MCC here(Steltenpohl et al., 2004), after it hadfirst been suggested by Coker et al.(1995) and Hames and Andresen(1996). A direct link between theironshore and our offshore workwould make the detachment a signifi-cant basement detachment, stretchingc. 50 km E–W from the VestfjordenBasin to the Røst High. This sup-ports the notion that the MCC is ofpotential regional importance andresponsible for both the exhumationof the crystalline basement in the Lo-foten area (Steltenpohl et al., 2011)and the formation of the probable-Permian proto-North Træna Basin(Fig. 4). Crustal-scale simple shearrelated to MCC development wouldtypically produce an asymmetricstructural template with mantleupwelling closer to the down-dip endof the detachment zone (Wernicke,

1985). This is in good agreementwith the fact that the Moho anomalyculminates underneath the Lofotenridge (Mjelde et al., 1996; Faleideet al., 2008), away from the break-away zone in the North Træna basinand towards the down-dip end of thebroadly east-dipping detachmentzone (Fig. 4).Based on the strike (N–S to NNE–

SSW) of the listric faults that soleout into the Permian detachmentzone in the North Træna basin, thetectonic transport direction is E toESE. This differs by some 45 degreesfrom the transport direction of my-lonitic shear zones at Røst/Værøy(top-to-the-ENE/NE; Steltenpohlet al., 2004). The trend of ductileshear (obtained from strain indica-tors such as elongation lineations,folds and/or porphyroclasts) onMCCs is known to vary by up toseveral tens of degrees within

Latest Triassic

Present Day

Permian

Pre-Tectonic setting

PaleogeneJur. / Cret.TriassicPermianCrustMantle

N. Træna Ribban V.- ordenMarmæle Spur

Lofoten Ridge

ERFZ

VFZ

Mohoiii

i

ii

iv

’AA A’’

Fig. 4 Proposed model for the late Palaeozoic development of a Cordilleran-stylemetamorphic core complex and the younger overprint of the Cretaceous rift. Theleft hand side of the sections is based on observations and interpretations describedin this study (see Fig. 3). The right hand side is more hypothetical, based on acombination of observations made by other workers: (i) Permian brittle faulting(Klein et al., 1999); (ii) Palaeozoic sediments at the base of the Vestfjorden basin(e.g. Færseth, 2012); (iii) a shallow Moho (Mjelde et al., 1996); and (iv) a top-to-the-north-east ductile detachment fault over the Lofoten Ridge (Steltenpohlet al., 2004).



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distances of a few tens of kilometreswith respect to the overall extensionvector (e.g. Bargnesi et al., 2013). Itis likely that neither the transportdirection of fault blocks we observein the North Træna basin nor that ofthe mylonitic shear of Røst/Værøy isfully representative of the overallextension direction, which is believedto have been E–W in Permian times(Dor�e et al., 1999; Mosar et al.,2002).Other observations and previous

work can be explained in conjunctionwith this model and thus offer fur-ther support. Firstly, brittle NE–SWtrending normal faults of Permianage are described further to the NEin the Lofoten archipelago (Kleinet al., 1999), in the hanging wall ofthe detachment zone of Røst/Værøy(Steltenpohl et al., 2004). Such syn-thetic normal faults typically developin the upper plate, down-stream ofthe main core complex where thedetachment zone ramps down tolower crustal levels (Wernicke, 1985).Secondly, the suspected presence ofPalaeozoic sediments in the Vestfjor-den basin (Mjelde et al., 1996; Fær-seth, 2012) suggests that such upperplate, synthetic normal faults areassociated with syn-tectonic sedimen-tation and occur not just onshoreLofoten, but over a large area eastof the LR. Thirdly, the southern partof the Ribban basin (Fig. 1) was notflooded before the Kimmeridgian(Hansen et al., 1992), which is signif-icantly later than neighbouringbasins; the buoyant, denudated crustappears to have produced a long-lived relative high.In summary, on the basis of the

observations and interpretations pre-sented herein and in combinationwith findings of previous workers,we propose that the MCC docu-mented onshore in SW Lofoten(Hames and Andresen, 1996; Stel-tenpohl et al., 2004) forms part of amuch larger, asymmetric structurethat facilitated crustal extension andexhumation in Palaeozoic, probablyPermian, times. In addition to ashallow Moho, lower crustal rocksat the surface and an onshoredetachment fault documentedbefore, we recognize a breakawayzone at the base of the NorthTræna basin. Moreover, we suggestthat the main metamorphic window

developed where the southern Rib-ban basin is located now and thesuspected Palaeozoic lower part ofthe Vestfjorden basin formed inresponse to synthetic normal faultsthat link up with the down-dip endof the detachment (Fig. 4).

Regional Implications

The findings of this study shed lighton the exhumation history of lowercrustal rocks in the Lofoten margin,the timing and mechanisms of whichdiffer from that of the WGR ofsouth Norway. In the latter region,post-Caledonian exhumation oflower crustal rocks was rapid andlargely accomplished by early MiddleDevonian times (Dunlap and Fossen,1998), due to crustal-scale exten-sional collapse and formation oflarge supradetachment basins. In theLofoten region on the other hand,post-Caledonian exhumation lastedwell into the Permian (Hames andAndresen, 1996). This study eluci-dates the nature of this longer livedexhumation, providing the first off-shore evidence for the developmentof a Cordilleran-style metamorphiccore complex in the Lofoten Margin,which was probably the chief drivingmechanism for tectonic unroofing inPermian times. Moreover, there is astriking difference in the size andstructural style of the relatively smallupper plate basins described herein,compared with that of the scoop-shaped orogenic collapse basins ofthe southern WGR.

Acknowledgements

We thank Norske Shell for funding anddata access. Permission to publish the 2-D data was granted by TGS; the 3-Ddata by Norske Shell & PL219 partners.We acknowledge Mark Steltenpohl andan anonymous reviewer for their con-structive reviews that helped improve thefinal version of the manuscript.

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Received 21 August 2013; revised version

accepted 30 January 2014

Supporting Information

Additional Supporting Informationmay be found in the online versionof this article:Fig S1. A composite seismic line pro-viding a tie between shallow core6710/3-U-3, that drilled through thebase Triassic, and section B-B0 ofFig. 2.Fig S2. The acoustic basement asmapped in the area immediatelysouth of section A-A0 of Fig. 2.Fig S3. An additional section, strik-ing SW-NE, displaying the Paleozoiclistric faults from a different angle.



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nature of palaeozoic extension in lofoten, north norwegian continental shelf: insights from 3-d...

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