Aspects of Strike-Slip Tectonics in the Inner Moray Firth Basin, Offshore Scotland

Aspects of strike-slip tectonics in the Inner Moray Firth Basin, offshore Scotland

TIM J. BIRD, ANDREW BELL, ALAN D. GIBBS & JOHN NICHOLSON

Bird, T. J., Bell, A., Gibbs, A. D. & Nicholson, J.: Aspects of strike-slip tectonics in the Inner Moray Firth Basin, offshore Scotland. Norsk Geologisk Tidsskrift, Vol. 67, pp. 353-369. Oslo 1987. ISSN 0029- 196X

Structural interpretation of the Inner Moray Firth basins based on a large database highlights the style, extent and variety of strike-slip related structural geometries in this long-lived basin. Flower geometries, en echelon faults and folds, pull-apart systems, inversion structures, lateral offsetsin stratigraphy, ordered combinations of extensional and compressional faults (including major thrusts and listric faults) all suggest a significantcomponent of strike-slip in the basin. These features are interpreted to develop on an arra y of linked, shaped faults, with several levels of detachment, which relate to the Great Glen Fault system at depth. Lateral movements along this major crustal lineament have led to the formation and deformation of the Inner Moray Firth Basin. Movement of different components of the basin up, down and/or across elements of the linked-fault system has led to compressional, extensional and strike-slip features respectively, according to their relative orientation with respect to movements of the Great Glen Fault.

T. J. Bird, Andrew Bel/, A/anD. Gibbs & John Nicholson, Midland Va/ley Exploration Ltd, 14 Park Circus, Glasgow, G3 6AX, Scotland.

The Inner Moray Firth, located off the northeast anism of formation to that of other North Sea coast of Scotland, is a fault-controlled basin basiris with a more truly extensional origin bounded on its northwest side by the Great GJen (Christie & Sclater 1980; Sclater & Christie Fault and an en echelon branch - the Helmsdale 1980; Barton & Wood 1984; Beach et al. 1987). Fault. Its southern margin is controlled by the The Great GJen Fault is a major crustal lin Banff fault set while the Wick Fault determines eament which bisects Scotland. To the southwest the northern margin of the basin (Fig. 1). Drilled of the Moray Firth it can be traced overland as a sedimentary section indicates that the offshore linear feature with 'singular straightness' for over Moray Firth has been a basin since at !east 100 km. It is then generally interpreted to con Devonian 'times. However, the apparent Jack of tinue offshore through the Malin Sea (Evans et significant crustal thinning beneath the Inner al. 1980) passing off the northwest coast of Ire land Moray Firth (Smith & Bott 1975; Donato & Tully and forming the northern margin of the Porcupine 1981) suggests that this basin has a different mech- Bank at the Atlantic continental margin. North of the Moray Firth it is believed to connect with the Walls Boundary Fault immediately west of Shetland (Flinn 1961, 1969, 1975) continuing northwards again to intersect the continental mar gin (Fig. 2). The Great GJen Fault has long been recognised as a strike-slip fault. Major strike-slip movements in Devonian and pre-Devonian times 'undoubtedly' occurred (McQuillin et al. 1982) and a number of authors have cited evidence for post-Devonian strike-slip movements. Based on onshore geology, Kennedy (1946) suggested a Hercynian sinistral displacement of 105 km, Hol gate (1969) suggested an additional Tertiary dex tral movement of about 30 km, while Garson & Plant (1972) proposed a dextral sense for the Fig. l. Generalised map of principal structural elements of the Hercynian movement. Although the evidence for Inner Moray Firth Basin. lateral movement is in some cases conflicting, the 354 T. J. Bird et al. NORSK GEOLOGISK TIDSSKRIFf 67 (1987)

Modification of the model for basin development Although most features suggest an overall relative dextral couple affecting the basin, a number of others seem to demand a sinistral interpretation. It is here suggested that these correspond to rela tive sinistral phases of strike-slip movement along the Great GJen Fault, leading to periods of partial inversion (the 'uplifts' and unconformities observed in the basin by McQuillin et al. (1982)), in an overall continued strike-slip setting. The two main post-Devonian phases of uplift are Late Jurassic ('Kimmerian' unconformity) and Ter tiary and indeed these are the interpreted ages of the sinistral features observed. Thus, although the McQuillin et al. (1982) model for Mesozoic basin development is for the most part supported, Fig.�. Generalised map showing Great GJen Fault in the con text there are some remaining problems and it is of northwest Euro}>ean crustal-block movements. stressed that the dextral displacement is only over all and relative. Nevertheless, the strike-slip gen general consensus is that overail relative dextral eration of the basin is not in dispute nor is the movement of a few tens of kilometres has notion of lateral movements of the Great GJen occurred since the Devonian. In particular, the Fault as the driving mechanism. work of Speight & Mitchell (1979) seems to have In detail, however, the basin is considered to received wide acceptance. From investigation of develop on a linked system of steeper ramp faults a dyke swarm in northern Argyll they concluded (planar and listric) and flat detachment faults a post-Permo Carboniferous oblique-slip dis which are connected as complex array to the placement incorporating a dextral strike-slip com Great GJen Fault at depth. Movements along the ponent of 7--skm. Great GJen Fault cause different portions of the McQuillin et al. (1982) used this information, basin to move up, down and/or across elements along with considerations of the regional tectonic of this linked-fault system, and according to their setting of the basin in the context of NW Euro relative orientation, generate the compressional, pean crustal-block movements and stresses, to extensional and strike-slip features. derive a model for Ioner Moray Firth Basin devel A number of examples of strike-slip-related opment in terms of a dextral pull-apart along features are briefly described and illustrated in the Great GJen Fault. They proposed that from this paper in order to emphasise the prevalence Triassic times onwards the basin developed prin of strike-slip phenomena in the basin while at cipally as a result of about 8 km dextral shift along the same time attempting briefly to illustrate the the Great GJen Fault and that this resulted in control of deformation by a linked-fault system. 5--6 km approximately north-south extension Several examples show how resultant syntectonic between the sub-parallel Wick and Banff faults. sedimentation can be used to monitor the struc (Barr (1985) derived similar post-Triassic exten tural development of the basin. sion from 3D palinspastic restoration of the basin.) From detailed local seismic mapping of the area, McQuillin et al. (1982) pointed out many features indicative of strike-slip movement; 'Flower' geometries flower structures, 'scissor' faults, en echelon Flower geometries are well documented in some faults, pull-apart features, fault geometries and areas (e.g. Lowe 111972; Bol & Van Hoorn 1978; tip-line effects. However, they concluded that Harding & Lowe111979; Harding 1983, 1985) and the magnitude of this movement was 'clearly not increasingly recognised in others. Indeed, recent large' and that 'there is no evidence for large publications by McQuillin et al. (1982) and Barr scale wrench faulting in Mesozoic rocks'. (1985) refer to flower structures in the Ioner NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 355

Fig. 3. Seismic panel from GECO line GMF 25B (1977, migrated) showing ftower structure.

Moray Firth Basin. (McQuillin et al. include a seismic example along the Great GJen Fault.) There are good examples of flower geometries along many of the major faults in the Inner Mora y Firth. Fig. 3 illustrates a well-developed example from the Central Ridge. Flower structures tend to localise at large orien tation changes (bends) opposing lateral move ment of the major faults. Flower structures thus develop as a result of sidewall collapse on 'restraining bends' (Crowell 1974; Woodcock & Fischer 1986) working effectively to smooth out these bends. Using the terminology for fault bounded arrays firstderived in thrust fault terrains (e.g. Dahlstrom 1970; Butler 1982), flower struc 50 KM tures are the plan-equivalent of 'horses' being accreted on the bends. Fig. Fig. shows the location of a seismic section 4. Skctch rnap of Moray Firth showing approximate location 4 of sections (Fig. 5(a), S(b), 9) with respect to principal faults.

crossing a 'restraining' bend on the Great GJen GGF = Great Glen Fault; HF = Helmsdale Fault; WF = Wick Fault. The seismic section, Fig. 5(a), shows clear Fault. Inset shows approximate location of map Fig. 10. U> VI 0\

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Fig. 5 (b). Comparison of structural style with onshore cross section between Strathpeffer and Inverness (see location map Fig. 4) constructed from geological map and field data.

flower geometries. As the Great Glen Fault runs offshore here, its throw decreases rapidly while an en echelon fault to the northwest, the Helmsdale Fault, takes up the displacement (see map Fig. 4). Alow-angle 'leaf fault (Gibbs 1987) branching fromthe main flower can be interpreted as trans Fig. 6. En echelon fold sel formed in hanging wall at 'restraining bend' of major fault. Low-angle thrust faults have developed ferring the displacement from one strike-slip with continued local compression. North-south extension boundary fault to its en echelon neighbour. This (major Upper Jurassic fault) and contemporaneous east-west makes the elongate area between the Great Glen shortening indicate dextral sense of shear. and Helmsdale faults a local sinistral pull-apart basin. The continuity of this style onshore to the southwest is illustrated in Fig. 5(b), a geological cross-section independently-derived from on shore mapping and field data.

En echelon structures t En echelon faults and folds are a classic strike slip phenomenon (e.g. Harding 1973, 1974, 1976; Wilcox et al. 1973). En echelon patterns of both faults and folds can be found in the Inner Moray Firth. En echelon fault sets in the basin generally have a dextral sense and generate small pull apart features indicative of local dextral strike-slip displacement. Fold sets (also generally having a dextral sense) tend to develop at sharp orientation changes of a fault ('restraining' bends) the folds representing, this time, hanging-wall deformation response to sidewall or footwall 'restraining' bends. An example is shown in Fig. 6. In the case illustrated continued compression has resulted in folds being breached by low-angle thrust faults. An important implication of the strain-ellipse (Fig. 7) is that not only should both extensional and compressional elements be found in strike slip basins, but that they should co-exist spatially and in time in a predictable way. Fig. 6 illustrates this point. The fold orientation indicates a dextral Fig. 7. Strain ellipse illustrating synchronous co-existence of couple parallel to the northeast-trending fault extensional and compressional elements within a strike-slip segment which is effectively acting as a sidewall basin. 358 T. l. Bird et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

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w Fig. 8. Part of GECO line GMF 23B (1977, migrated) across the Banff Fault showing reverse nature of this fault and inversion on this fault set continuing to present day, deforming Vl present seabed. (a) Blank, (b) interpreted. \Q 360 T. !. Bird et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

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>< "' � o 'It � C) � b � Fig. 9. Part of GECO line GMF 28 (1977, migrated) across thrust portion of Great Glen Fault (see Fig. lO for approximate line location). (a) Un-interpreted. (b) Interpret ed Section. � Fault ftattens to bedding-parallel detachment (D) ahead of the thrust ramp portion. Upper fault (F) affecting Base Cretaceous (BK) decouples on this decollement horizon. This � faulting is completely unrelated (detached) to the lower levels of faulting below the detachment; for example the fault which rotates Middle Jurassic beds (MJ) at right of section, i:" ;:æ which detaches on top Triassic (T )...... � w 0\ ...... 362 T. J. Bird et al. NORSK GEOLOGISK TIDSSKRIFT' 67 (1987) fault. However, the fault as a whole is a listric growth fault with major north-south extension jjjjjjjjjjjjjjjjijjjjjjijjjjjjijjjjj BK across the east-west trending arm of the fault. MJ The growing Upper Jurassic sedimentary wedge can thus be seen extending north-south while at the same time shortening east-west.

Compressional features Compressional features are well represented in the lnner Moray Firth Basin. In addition to the folding associated with 'restraining' bends and en echelon transfers, thrusts and reverse faults are recognised. The Banff Fault controlling the southem margin of the Moray Firth can be seen to be a major reverse fault, as evidenced on seismic data (Fig. 8) and onshore control, which substantially contradicts a simple dextral pull apart model for the basin. 1 O KM McQuillin et al. (1982) state that 'the Great GJen Fault is now considered to be not a simple transcurrent fault but a complex of thrusts and Fig. JO. Sketch map of localised area offshoreLybster (see Fig. faults both normal and transcurrent'. Fig. 9 shows 4 for location). Solid lines represent fault pattern at Middle a seismic section across such a thrust portion of Jurassic (MJ) leve!. Hatched line represents trace of principal Base Cretaceous (BK) fault (F on seismic line Fig. 9). Fault the Great Glen Fault as it swings inshore near patterns are virtually orthogonal, indicating the existence of an Lybster, changing from a northeast strike to a intervening detachment, as interpreted on the seismic line. northwest strike (see location map Fig. 4). The occurrence of a thrust here is compatible with this being a 'restraining' bend during the dextral closing of the local (sinistral) pull-apart basin within the Upper Jurassic and is one of several between the Helmsdale and Great GJen faults, regional decoupling horizons utilising zones of which re-join here. The thrust is seen, then, as lithological weakness. The validity of this par the linking transfer-structure which accomplishes ticular detachment can be tested by comparing this convergence and is thus the partner of the a map of the Middle Jurassic fault pattem (a low-angle 'leaf' fault described at the opposite prominent faulted horizon below the interpreted end of the local pull-apart basin between the detachment) with the trace of the Base Cre Great GJen and Helmsdale faults (see above, taceous fault above the interpreted detachment. Fig. 5). Fig. 10 shows that the two fault patterns are virtually orthogonal and therefore unrelated. The existence of an intervening detachment is thus demonstrated. Detachments To the northeast of the thrust described in the preceding section the fault Hattens into a bedding paraBel detachment. Faults affecting the over Linked fault systems lying Upper Jurassic rocks can be seen decoupling Shaped fault segments are often imaged in the on this flatdetachment, and develop entirely inde Inn er Moray Firth as on the seisrnicline (Fig. 11). pendently of the underlying Permian to Middle A portion of imaged listric fault is interpreted at Jurassic fault pattem, as depicted on the right the right-hand end of the .line. In addition, the hand (northeast) side of line GMF 28 (Fig. 9). very strong reflector dipping obliquely beneath The detachment separating these two levels of the crest of the structure is interpreted as an faulting occurs at a prominent lithological marker imaged footwall deformation fault joining the NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 363

Fig. 11. Panel from GECO line GMF 23B (1977 migrated). Imaged listric fault interpreted at right-hand end of line (rotation of beds and growth can be seen in hanging-wall sediments). Strong reflector dipping obliquely beneath crest of structure is also interpreted as imaged fault (rotation seen across it) deforming footwall. Such 'break-back' faults can be important inversion structures and may be the mechanism of footwaU uplift observed in the basin. listric fault at depth. Such 'break-back' structures result balancing techniques may also be used with are preferentially developed at 'restraining' bends some confidence to reconstruct fault geometries of major faults in a strike-slip setting, and may where they are not seismically imaged. lead to local inversion or 'footwall uplift' of crestal structures, without necessitating the isostatic mechanisms discussed by Barr et al. (1985) and Barr (1987). Lateral offsets The seismic panel in Fig. 12 contains an imaged When sediment supply occurs across an active portion of a sinuous ramp-flat fault (Gibbs 1987) oblique-slip fault fan accumulations build up on demonstrating the relationship of shaped beds to shaped faults. Ramp-flat combinations forming staircase-like fault arrays are frequentlyimaged in the Inner Moray Firth. In places these 'staircase' fault arrays are imaged frequently enough on neighbouring seismic lines to build up a 3-dimen sional picture of the linked fault system locally. Where the stratigraphy on either side of such faults is known and the section is approximately in the direction of tectonic transport (e.g. Figs. 13 and 14), the validity of such reflectors as fault traces can be verifiedusing balanced section tech niques (Gibbs 1983) to calculate fault shape from hanging-wall geometry. Fig. 12. Seismic panel from GECO line GMF 9 (1977, migrated) Such calculations place fault planes very dose showing imaged example of sinuous ramp-flat fault, and hang to observed events on the seismic data, and as a ing-wall response to 'staircase' geometry of shaped footwall. w �

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z o c; :>:: o 2 rn o 5 o Vi :>:: Fig. 13. Part of GECO line GMF 11 (1977, migrated) showing ramp-flat-ramp 'staircase' fault imaged. Event can be confirmed as fault plane retlector by section balance. (Section ::l approximately in direction of transport.) VlCl Vl :>:: :;.l � � � :0 � - - z o NW ::<:1 ' 1 km .SE (l) :o<: o o tTl o ["" o o til :o<: ::l o (l) (l) :o<: ::<:1

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·-E 2- � � (Ø C) V) � � o � � � - i::' 3 ...... & Fig. 14. Part of IGS line 4 (1972, GECO re-processed, migrated} across extensional listric fault. Fault (F) has well-imaged, strongly listric ramp portion at left-hand side (covcr !.;.) basement juxtaposition), diffusely-imaged central'ftat' portion at 1.45 seconds (basement-basement interface) and variably-imaged gentle 'ramp' segment at right-hand side (dipping 0\ to 1.9 seconds}. Depth to detachment calculation and section balance techniques confirm geometry of shaped 'staircase' fault as interpreted. Vl 366 T. J. Bird et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

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Fig. 16 (a). Schematic cross-section parallel to oblique-slip fault with laterally-offset stacked build-ups developed across it. Notional location shown in Fig. 15.

Fig. 15. Sketch showing sequential development of laterally stepping sinistrally across the section. The base offset stacked fan build-ups across oblique-slip fault: Fan builds of these, a distinct lithological boundary, appears out across oblique-slip fault. During active strike-slip episodes to have been utilised subsequently as a low-angle fans are decapitated and moved off laterally. A new fan is then thrust fault. built out along strike from the first, and so on. Dotted line shows position of schematic cross-section parallel to controlling fault (Fig. 16). Inversion The lnner Moray Firth has experienced several the downthrown side of the fault. During active periods of partial inversion. Uplift occurred in strike-slip episodes these fans are decapitated and the Late Jurassic and Tertiary (McQuillin et al. moved off laterally as illustrated in Fig. 15. This 1982) as well as earlier Devonian episodes. Inver phenomenon is well-known in many strike-slip sion can be seen strongly affecting the fault-con basins both past (e.g. Greenland: Surlyk & Hurst trolled margins of the Inner Moray Firth. Steep 1983) and present (e.g. Turkey: Mann et al. 1983, reverse structures such as the Banff Fault (Fig. and California, e.g. Crowell 1974, 1976 and this 8), low-angle thrust faults and shallow-dipping volume). As the process continues, a new fan is reversed normal faults can be seen accomplishing built out along strike from the first, and so on. this inversion. Several of the inversion phenom A cross-section parallel to the controlling fault enon which can be constrained to the 'uplift' towards the margin of such a basin will show a periods of Late Jurassic or Tertiary are also series of stacked build-ups offset laterally, as in indicative of a sinistral sense of movement. It is Fig. 16(a). Fig. 16(b) illustrates an equivalent suggested that these uplift periods correspond to seismic section fromthe Moray Firth, parallel to phases of sinistral relative displacement along the and just offshore from the Great Glen Fault. A Moray Firth portion of the Great Glen Fault series of laterally-offset anomalies can be seen leading to partial inversion to the basin.

Fig. 16 (b). Part of GECO seismic line GMF 28 (1977, migrated) parallel to and just offshorefrom the Great Gl en Fault showing series of laterally offset, stacked anomalies, interpreted as sedimentary build-ups developed across the Great Glen Fault during a phase of sinistral oblique-slip move ment in the Late Jurassic. The base of these sedimentary build-ups, a distinct lithological boundary, appears to have been utilised sub sequently to activate a low-angle thrust fault. NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 367

.. 8 N o � ..... o • ..... o 6 . . C') E .... - c o >- • Q. :s lØ � ... o CD � Cll .... > o c ... Fig. 17. Panel from GECO line GMF B (1977, migrated) show ing fault-controlled channel build-up at base of the Cretaceous. Fig. 19. Sketch showing restoration of channel-fan complex Location of line shown in Fig. 18. from present position to original deposition geometry. Suggests some 3,000 feet of post-Lower Cretaceous inversion ( probably Tertiary) across the structure.

Overall amounts of inversion are generally dif ficult to estimate within the basin, but there is one place where at !east a minimum estimate of Conclusions Tertiary inversion can be obtained. Fig. 17 is a seismic panel showing a distinctive fault-con A structural approach to seismic data has been trolled channel build-up at the base of the Cre used to evaluate the Inner Moray Firth Basin. taceous. This runs southwestwards along the axis The wide range of classic strike-slip phenomena of the basin before broadening into a fan-delta . described identifythis as a basin with a significant system (see Fig. 18). However, the delta 'head' component of strike-slip movement. Indeed, the is now considerably elevated above the channel formation and subsequent deformation of the 'tail' of the system, so the channel is now effec Inner Moray Firth Basin are considered to be tively fiowing uphill (and the re is no thermal principally controlled by strike-slip movements of sag basin showing differentialsubsidence above). the Great Glen Fault acting as a basin-bounding Restoring this to its original deposition geometry, sidewall. The basin is considered to open (grow) as illustrated in Fig. 19, gives an estimated 3,000 ft during periods of relative dextral movement along of differential uplift across the structure. This is the Moray Firth portion of the Great Glen Fault in very close agreement with uplift estimates and to partially dose (inversion/uplift) during which can be calculated from overcompaction in periods of sinistral strike-slip movement along the sediments drilled by wells on the Beatrice Field, Moray Firth portion of the Great GJen Fault. which is immediately adjacent to the fan delta. However, these may be simply relative senses of movement and may not represent the overall sense of strike-slip movement across the Great GJen Fault. The basin is interpreted to develop and deform as a series of linked, shaped faults at different Ievels. Several such faults appear imaged on seis mic data in the basin and locally their three dimensional geometry can be constrained. Bal anced section techniques are a powerful tool to confirm the validity of the ramp-flat fault ge ometries identified. Such a linked network of shaped faults is seen as a mechanical and ge ometrical necessity in the evolution of strike-slip basins, as in extensional basins (Gibbs 1984). Fig. 18. Sketch map of channel-fan build-up at base of They represent an essential component in any Cretaceous, showing present elevation of ends, position of line GMF B (Fig. 17) and proximity to Beatrice Field. Delta 'head' strike-slip system, an element not conventionally now considerably elevated above channel 'tail'. recognised. 368 T. J. Bird et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

The approach taken here has resulted in the Donato, J. A. & Tully, M. C. 1981: A regional interpretation recognition of a much larger family of structural of North Sea gravity data. In Uling, L. V. & Hobson, G. D. styles than previously and hence has had a pro (eds.): Petroleum Geology of the Continental Shelf of NW Europe. 67-75. Institute of Petroleum London. found inftuence on views of the prospectivity in Evans, D. , Kenolty, N., Dobson, M. R. & Whittingham, R. J. the basin. It is suspected that the integrated struc 1980: The geology of the Malin Sea. Report of the Institute of tural approach used in evaluating the Mora y Firth Geological Sciences No. 79/15. would be equally successful in identifying new Flinn, D. 1961: Continuation of the Great Glen Fault beyond the Moray Firth. Nature 191, 589-591. components in other basins where the extent, Flinn, D. 1969: A geological interpretation of the aeromagnetic variety and style of strike-slip deformation has maps of the Continental Shelf around Orkney and Shetland. not previously been recognised. Geological Journal 6, 279-292. Flinn,D. 1975. Evidence for post-Hercynian transcurrent move Acknowledgements. -This paper is based on ideas evolved over ment of the Great Glen Fault in the Moray Firth. Scottish a considerable period of time by staff of MYE. All Seismic Journal of Geology 11, 266-267. data are from a GECO 1977 survey (plus one IGS 1972 line Garson, M. S. & Plant, J. 1972: Possible dextral movements on reprocessed by GECO). I am grateful to GECO (UK) and the Great Glen and Minch Faults in Scotland. Nature of particularly Henry Rivers for supplying lines and for permission Physical Science 240, 31-35. to publish. Gibbs, A. D. 1983: Balanced cross-section construction from seismic sections in areas of extensional tectonics. Journal of Structural Geology 5, 153-160. Gibbs, A. D. 1984: Structural evolution of extensional basin References margins. Journal of the Geological Society 141, 609-620. Gibbs, A. D. 1987: Development of extensional and mixed Barr, D. 1985: 3-D Palinspastic restoration of normal faults in mode sedimentary basins. In Coward, M. P., Dewey, J. F. the Ioner Moray Firth: implications for extensional basin & Hancock, P. L. (eds.): Continental Extensional Tectonics. development. Earth and Planetary Science Letters 75, 191- 640 pp. Geological Society of London Special Publication. 203. Harding, T. P. 1973: Newport-Inglewood trend, California-an Barr, D., McQuillin, R. & Donato, J. A. 1985: Footwall uplift example of wrenching style of deformation. Bulletin of the in the loner Moray Firth basin, offshore Scotland. Journal of American Association of Petroleum Geologists 57, 97-116. Structural Geology7, 267-268. Harding, T. P. 1974: Petroleum traps associated with wrench Barr, D. 1987: Lithospheric stretching, detached normal faults. Bulletin of the American Association of Petroleum faulting and footwall uplift. In, Coward, Dewey & Hancock Geologists 58, 129(}-1304. (eds.): Continental Extensional Tectonics, 640 pp. Geological Harding, T. P. 1976: Tectonic significance and hydrocarbon Society of London Special Publication. trapping consequences of sequential folding synchronous with Barton� P. & Wood, R. 1984: Tectonic evolution of the North San Andreas faulting, San Joaquin Valley, California. Bull Sea Basin: crustal stretching and subsidence. Geophysical etin of the American Association of Petroleum Geologists 60, Journal of the Royal Astronomical Society 79, 987-1022. 356-378. Beach, A., Bird, T. & Gibbs, A.D. 1987: Extensional tectonics Harding, T. P. 1983: Divergent wrench fault and negative tlower and crustal structure: deep seismic retlection data from the structure, Andaman Sea. In Bally, A. W. (ed.): Seismic northem North Sea Viking Graben. In Coward, Dewey & Expression of Structural Styles. American Association of Pet Hancock (eds.): Continental Extensional Tectonics, 640 pp. roleum Geologists Special Publication. Geological Society of London Special Publication. Harding, T. P. 1985: Seismic characteristics and identification Bol, A. J. & Van Hoorn, B. 1978: Structural Styles in Western of negative tlower structures, positive tlower structures and Sabah Offshore. Geological Society Malaysia 2nd Petroleum positive structural inversion. Bulletin of the American Associ Seminar, Kuala Lumpur. ation of Petroleum Geologists 69, 582-600. Butler, R. W. H. 1982: The terminology of structures in thrust Harding, T. P. & Lowell, J. D. 1979: Structural styles, their beits. journal of Structural Geology 4, 239-245. plate tectonic habitats, and hydrocarbon traps in petroleum Christie, P. A. F. & Sclater, J. G. 1980: An extensional origin provinces. Bulletin of the American Association of Petroleum for the Buchan and Witchground Graben in the North Sea. Geologists 63, 1016-1058. Nature 283, 729-732. Holgate, N. 1969: Paleozoic and Tertiary transcurrent move Crowell, J. C. 1974: Sedimentation along the San Andreas ments on the Great Glen Fault. ScottishJournal of Geology Fault, California, and origin of late Cenozoic basins in 5, 97-139. Southern California. In Dott, R. H. & Shaver, R. H. (eds.): Kennedy, W. Q. 1946: The Great Glen Fault. QuarterlyJournal Modem and Ancient Geosynclinal Sedimentation. 292-303. of the Geological Society of London 102, 41-72. Special Publication of the Society of Economic Palaentology Lowell, J. D. 1972: Spitsbergen Tertiary orogenic belt and the and Mineralogy 19. Spitsbergen fracture zone. Bulletin of the Geological Society Crowell, J. C. 1976: lmplications of crustal stretching and short of America 82, 3091-3102. ening of coastal Ventura Basin, California. Pacific Section of Mann, P., Hempton, M. R., Bradley, D. C. & Burke, K. 1983: the American Association of Petroleum Geologists, Mis Development of pull-apart basins. Journal of Geology 91, cellaneous Publication 24, 365-382. 529-554. Dahlstrom, C. D. A. 1970: Structural geology in the eastern McQuillin, R. , Barclay, W. & Bacon, M. 1979: An Introduction margin of the Canadian Rocky Mountains. Bulletin of to Seismic Interpretation. 199 pp. Graham & Trotman. Canadian Petroleum Geology 18, 332-406. McQuillin R., Donato, J. A. & Tulstrup, J. 1982: Development NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 369

of basins in the lnner Moray Firth and North Sea by crustal dyke-swarm of northern Argyll and its bearing on dextral extension and dextral displacement of the Great GJen Fault. displacement of the Great GJen Fault. Journal of the Geo Earth and Planetary Science Letters 60, 127-139. logical Society of London 136, 3-11. Sclater, J. G. & Christie, P. A. F. 1980: Continental stretching; Surlyk, F. & Hurst, J. M. 1983: Evolution of the earry Palaeozoic an explanation of the post mid-Cretaceous subsidence of the deep-water basin of north Greenland - aulacogen or narrow Central North Sea Basin. Journal of Geophysical Research ocean? Geology 11, 77-81. 85, 3711-3739. Wilcox, R. E., Harding, T. P. & Seely, D. R. 1973: Basic Smith, P. J. & Bott, M. H. P. 1975: Structure of the crust wrench tectonics. Bulletin of the American Association of beneath Foreland and Caledonian Belt of the North Scottish Petroleum Geologists 57(1), 74-96. Shelf region. Geophysical Journal of the Royal Astronomical Woodcock, N. H. & Fischer, M. 1986: Strike-slip duplexes. Society 40, 187-205. Journal of Structural Geology 8, 725--735. Speight, J. M. & Mitchell, J. G. 1979: the Permo-Carboniferous