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Eocene Deformation on the Continental Margin SW of the British Isles

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Eocene Deformation on the Continental Margin SW of the British Isles J. geol. Soc. London, Vol. 140, 1983, pp. 913-920, 5 figs. Printed in Northern Ireland. Eocene deformation on the continental margin SW of the British Isles D. G. Masson & L. M.Parson SUMMARY: Two narrow E-W trending zones of late Eocene compressional deformation, one on the northern margin of the Bay of Biscay, the other in the southern Porcupine Seabight, are described. Both appear to follow pre-existing geological lineaments. The deformation is related to the NW movement of Iberia between latest Cretaceous and Late Eocene time. Theearly Tertiary convergence between Iberia and (Fig. 2) and that it appears to die outlaterally both to Europe, whichgave rise tothe Pyrenean orogenic the E and W (Montadert ef al. 1979). belt, is marked offshore by the overthrusting of Iberia Inthe area where the deformation is strongest, onto the oceanic crust of the Bay of Biscay (Sibuet & along the Trevelyan Escarpment, compressional buck- Le Pichon1971; Montadert et al. 1971;Boillot & ling of thepre-late Eocene sequence has created a Capdevila1977; Boillot et al. 1979; Grimaud et al. complex faultedmonocline, with crustal shortening 1982). The major zone of overthrusting and reverse accommodated by two zones of reverse faulting (Fig. faulting follows the base of the northern continental 2). To theW, the trend of thisdeformation zone slope of Iberia, and trends E-W in the E but swings to graduallychanges from E-W to NW-SE. This is a NE-SW strike N of Galicia Bank in the W. Within accompanied by an apparent decrease in the intensity theBay of Biscay, there is abundantevidence for of deformationtowards the NW (Fig. 3), until the Eocene movement on major faults along the flanks of effects of the compression disappear completely near the Biscay, Cantabria, and North and South Charcot 47"40'N, ll"2O'W (Fig. 1). Seamounts. All of thefaulting, including theover- Montadert et al. (1979) havespeculated that the thrusting,can be dated by reference to DSDP drill change in trend of the deformation zone from E-W to holes in the Bay of Biscay (Laughton, Berggren et al. NW-SEis marked by atransition to a strike-slip 1972). Overall,the structures are consistent witha dominatedstructure, although this cannotbe un- NWmovement of Iberia of about 120km between equivocallyestablished using seismic sections. How- latest Cretaceous and Late Eocene time (Grimaud et ever, such a transition is clearly compatible with the al. 1982). observed lateral changes in the deformation style and The purpose of this short paper is to summarize the with a simple predicted compressional regime resulting distribution of Late Eocene tectonic structures on the from the NW movement of Iberia. northern margin of the Bay of Biscay, and to describe a previously undescribed zone of Eocene folding in the The southern Porcupine Seabight southern Porcupine Seabight (Fig. 1). A previously undescribed E-W trending linear zoneof Early Tertiary deformation in the southern Porcupine The northern continental Seabight (Fig. 1) is observed on six seismic reflection margin of the Bay of Biscay profiles, two of which are illustrated here (Figs 4 and 5). Theform of thiszone changes from a complex Eocenedeformation along thenorthern continental faultedanticline in theW (Fig. 4) to simplea margin of the Bay of Biscay was briefly discussed by monoclinein the E (Fig. 5). Itappears to die out Montadert et al. (1979). Theydescribed a complex rapidly both E of 12'30'W and W of lPW, although zone of faultingand foldingconfined to anarrow, the mechanism by whichshortening is takenup at arcuate belt along the Trevelyan Escarpment (Fig. l), these end points is not obvious from the current data moreor less coincidentwith the continent-ocean set. transition(see Montadert et al. 1979, fig. 21). They Fig. 4a,a seismicreflection profile across the alsodescribed re-activation of EarlyCretaceous western end of this deformationzone, illustrates an rift-phasefaults on the continental margin. This anticlinal structure overlying a broad zoneof uncertain reactivationwas particularly prevalent where such basement configuration. Over the crest of the fold, the structures have an E-W trend (e.g. see Montadert et readilyrecognized, well-stratified section which de- al. 1979, fig. 7b) but, in regional terms, such secondary fines the closure extends to approximately l s below movement is apparently of minorimportance. De- the level of the Early-Middle Eocene reflector (depths tailed study of the main deformation zone shows that quoted are in seconds, two-waytravel time). Below it is mostintensely developed near 46"45'N, 9'30'W this,our tentative interpretation showsa series of 0016-7649/83/11004913$02.00 0 1983 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/6/913/4887740/gsjgs.140.6.0913.pdf by guest on 02 October 2021 914 D.G. Masson & L. M. Parson T 5c BISCAY A 4E 150 km L / ILLUSTRATEDPROFILES 400e IPOD DRILL SITES AREASOF LATE EOCENE DEFORMATION 0 BISCAY I 1 I 1 I I 46" 14" 12" 10" 1" FIG.1. Areas of Late Eocene deformation, IPOD drilling sites and illustrated seismic reflection profiles locatedon a bathymetric chart of the study area (depths in metres). For full extent of seismic reflection coverage see Robertset al. (1981), fig. 2. minor, predominantly reversed faults cutting the lower and 8 S towards the SE of the illustrated profile (R in Tertiaryand older sediments and the acoustic base- Fig. 4a); theseare apparently continuous beneath a ment.Note that the interpretation of thedeep faultedacoustic basement surface. Interval velocities structure is complicated by apparentvariations in measured from theseismic reflection profile within this thickness of theLower Cretaceous and ?older rift- group of reflectors are in the range 5.5 to 6.5 km/s, phase sediments (almost certainly an original feature seemingly confirming thatthey do occur within the of deposition). Further complications are introduced acoustic basement. At the present time, we are unable by the recognition of a prominent group of flat-lying to offer an explanation of this sequence of reflectors. reflectors within theacoustic basementbetween 6.5 Fig. 5 illustrates a typical profile across the eastern Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/6/913/4887740/gsjgs.140.6.0913.pdf by guest on 02 October 2021 b 'SW NA 3.0- E MIDDLEEOCENE - EARLYOLIGOCENE REFLECTOR 4.0- 1 LATEEOCENE FAULTS m ACOUSTICBASEMENT 5.0- + + <+ + + + y/+ + + + 5 km 1 CONTINENTAL 9.0 OCEANIC CONTINENT -OCEAN l B A SE M E NT TRANSITION BASEMENT BASEMENT 10.0 FIG. 2. a, Part of a migrated multichannel seismic reflection profile (line CM15, Fig. 1) across the continent-ocean transition on the northern margin of the Bay of Biscay; b, Simplified line drawing interpretation of CM15 showing the localized zone of deformation in the area of the continent-ocean transition. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/6/913/4887740/gsjgs.140.6.0913.pdf by guest on 02 October 2021 a ssw NNE W r_ c 6.0- 5 3 0 E 7.0- (I)n - - z 0 0 8.0- + + + _- J --- + + +? + + 9.0- + + + ? _- CONTINENT CONTINENTAL + + OCEAN BASEMENT + l 5 km I + 10.0- TRANSITION + 11.0 FIG.3. a, Part of a migrated multichannel seismic reflection profile (line CM14, Fig. 1) across the continental margin of the northern Bay of Biscay; b, Simplified line drawing of profile CM14 showing a localized zone of deformation just N of the continent-ocean transition. Note that the apparent degree of deformation is much reduced relative to profile CM15. For key see Fig. 2b. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/6/913/4887740/gsjgs.140.6.0913.pdf by guest on 02 October 2021 Eocene deformation on the continental margin 917 0 SE 2.0- ;60 0 0 w 7.0 E' EARLY/MIDDLE EOCENE REFLECTOR / LATE EOCENE FAULTS l 5 km J ,' EARLYCRETACEOUS FAULTS + ACOUSTIC BASEMENT FIG. 4. a, Part of a migrated multichannel seismic reflection profile (line 10s 1, Fig. 1) in the southern Porcupine Seabight;, b, Tentative interpretation of profile 10s-1 illustrating possible disposition of fault structures beneath a broad anticline. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/6/913/4887740/gsjgs.140.6.0913.pdf by guest on 02 October 2021 a b NW SE 01 E' 5km LI FIG. 5. a, Part of a multichannel seismic reflection profile (line CM19, Fig. 1) in the southern Porcupine Seabight, b, Simplified line drawing of profile CM19, illustrating an open monocline. For key see Fig. 4b. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/6/913/4887740/gsjgs.140.6.0913.pdf by guest on 02 October 2021 Eocene deformationcontinental on marginthe 919 end of the deformation zone. The monocline results in Discussion and Conclusions an 800 m relative vertical displacement of the Early- Middle Eocene reflector; subsequent control of sedi- A zone of late Eocene deformation along the southern mentation resulted in a much thicker section to the N PorcupineSeabight, dated using information from of the deformation zone than to theS. Weak basement IPOD Leg 80, has been found to be contemporaneous reflectors are present beneath the southern limb of the with one on the northern margin of the Bay of Biscay, monocline but are absent to the N. This locationof the previously described by Montadert et al. (1979). Both deformationover a basement structure may suggest these E-W zonesappear to havedeveloped along reactivation of anEarly Cretaceous rift-related fea- pre-existinggeological lineaments associated with ture. changes in crustal structure. In contrast, reactivation of earlyCretaceous rift-phase faults is relatively The age of the compressional unimportant on a regional scale,occurring only locally phase where such faults have an E-W trend. Inthe N Biscay examples,the continent-ocean The seismic stratigraphy of the continental margins of transitionhas become the locus of thelater move- the northern Bay of Biscay and Goban Spur can be ments.In the Porcupine Seabight, the pre-existing calibrated using the drilling results of IPOD legs 48 trend is associatedwith thenorthern edge of the and 80 (Fig.
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