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 offsets in stratigraphy, orde red combinations of extensional and compressional faults (including major thrusts and listric faults) all suggest a significant component 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 coast of Scotland, is a fault-controlled basin bounded on its northwest side by the Great GJen Fault and an en echelon branch - the Helmsdale Fault. Its southern margin is controlled by the Banff fault set while the Wick Fault determines the northern margin of the basin (Fig. 1). Drilled sedimentary section indicates that the offshore Moray Firth has been a basin since at !east Devonian 'times. However, the apparent Jack of significant crustal thinning beneath the Inner Moray Firth (Smith & Bott 1975; Donato & Tully 1981) suggests that this basin has a different mech-

Fig. l. Generalised map of principal structural elements of the Inner Moray Firth Basin.

anism of formation to that of other North Sea basiris with a more truly extensional origin (Christie & Sclater 1980; Sclater & Christie 1980; Barton & Wood 1984; Beach et al. 1987).

The Great GJen Fault is a major crustal lin­eament which bisects Scotland. To the southwest of the Moray Firth it can be traced overland as a linear feature with 'singular straightness' for over 100 km. It is then generally interpreted to con­tinue offshore through the Malin Sea (Evans et al. 1980) passing off the north west coast of Ire land and forming the northern margin of the Porcupine 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 mar­gin (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, Hol­gate (1969) suggested an additional Tertiary dex­tral movement of about 30 km, while Garson &

Plant (1972) proposed a dextral sense for the Hercynian movement. Although the evidence for lateral movement is in some cases conflicting, the

354 T. J. Bird et al.

Fig.�. Generalised map showing Great GJen Fault in the con text of northwest Euro}>ean crustal-block movements.

general consensus is that overail relative dextral movement of a few tens of kilometres has occurred since the Devonian. In particular, the work of Speight & Mitchell (1979) seems to have received wide acceptance. From investigation of a dyke swarm in northern Argyll they concluded a post-Permo Carboniferous oblique-slip dis­placement incorporating a dextral strike-slip com­ponent of 7--s km.

McQuillin et al. (1982) used this information, along with considerations of the regional tectonic setting of the basin in the context of NW Euro­pean crustal-block movements and stresses, to derive a model for Ioner Mora y Firth Basin devel­opment in terms of a dextral pull-apart along the Great GJen Fault. They proposed that from Triassic times onwards the basin developed prin­cipally as a result of about 8 km dextral shift along the Great GJen Fault and that this resulted in 5--6 km approximately north-south extension between the sub-parallel Wick and Banff faults. (Barr (1985) derived similar post-Triassic exten­sion 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 structures, 'scissor' faults, en echelon faults, pull-apart features, fault geometries and tip-line effects. However, they concluded that the magnitude of this movement was 'clearly not large' and that 'there is no evidence for large­scale wrench faulting in Mesozoic rocks'.

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 rela­tive 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 Ter­tiary 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, there are some remaining problems and it is stressed that the dextral displacement is only over­all and relative. Nevertheless, the strike-slip gen­eration of the basin is not in dispute nor is the notion of lateral movements of the Great GJen Fault as the driving mechanism.

In detail, however, the basin is considered to develop on a linked system of steeper ramp faults (planar and listric) and flat detachment faults which are connected as complex array to the Great GJen Fault at depth. Movements along the Great GJen Fault cause different portions of the basin to move up, down and/or across elements of this linked-fault system, and according to their relative orientation, generate the compressional, extensional and strike-slip features.

A number of examples of strike-slip-related features are briefly described and illustrated in this paper in order to emphasise the prevalence of strike-slip phenomena in the basin while at the same time attempting briefly to illustrate the control of deformation by a linked-fault system. Several examples show how resultant syntectonic sedimentation can be used to monitor the struc­tural development of the basin.

'Flower' geometries

Flower geometries are well documented in some areas (e.g. Lowe 111972; Bol & Van Hoorn 1978; Harding & Lowe111979; Harding 1983, 1985) and increasingly recognised in others. Indeed, recent publications by McQuillin et al. (1982) and Barr (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 orien­tation changes (bends) opposing lateral move­ment 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 fault­bounded arrays first derived in thrust fault terrains (e.g. Dahlstrom 1970; Butler 1982), flower struc­tures are the plan-equivalent of 'horses' being accreted on the bends.

Fig. 4 shows the location of a seismic section crossing a 'restraining' bend on the Great GJen Fault. The seismic section, Fig. 5(a), shows clear

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Fig. 4. Skctch rna p of Mora y Firth showing approximate location of sections (Fig. 5(a), S(b), 9) with respect to principal faults. GGF = Great Glen Fault; HF = Helmsdale Fault; WF = Wick Fault. Inset shows approximate location of map Fig. 10.

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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). A low-angle 'leaf fault (Gibbs 1987) branching from the main flower can be interpreted as trans­ferring the displacement from one strike-slip boundary fault to its en echelon neighbour. This makes the elongate area between the Great Glen 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 on­shore mapping and field data.

En echelon structures

En echelon faults and folds are a classic strike­slip 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 pull­apart 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 strike­slip 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 couple parallel to the northeast-trending fault segment which is effectively acting as a sidewall

TSGS Symposium 1986 357

Fig. 6. En echelon fold sel formed in hanging wall at 'res training bend' of major fault. Low-angle thrust faults have developed with continued local compression. North-south extension (major Upper Jurassic fault) and contemporaneous east-west shortening indicate dextral sense of shear.

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Fig. 7. Strain ellipse illustrating synchronous co-existence of extensional and compressional elements within a strike-slip basin.

358 T. l. Bird et al.

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fault. However, the fault as a whole is a listric growth fault with major north-south extension across the east-west trending arm of the fault. 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 pull­apart model for the basin.

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 faults both normal and transcurrent'. Fig. 9 shows a seismic section across such a thrust portion of the Great Glen Fault as it swings inshore near Lybster, changing from a northeast strike to a 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 between the Helmsdale and Great GJen faults, which re-join here. The thrust is seen, then, as the linking transfer-structure which accomplishes this convergence and is thus the partner of the low-angle 'leaf' fault described at the opposite end of the local pull-apart basin between the Great GJen and Helmsdale faults (see above, Fig. 5).

Detachments

To the northeast of the thrust described in the preceding section the fault Hattens into a bedding­paraBel detachment. Faults affecting the over­lying Upper Jurassic rocks can be seen decoupling on this flat detachment, and develop entirely inde­pendently of the underlying Permian to Middle Jurassic fault pattem, as depicted on the right­hand (northeast) side of line GMF 28 (Fig. 9). The detachment separating these two levels of faulting occurs at a prominent lithological marker

NORSK GEOLOGISK TIDSSKRIFT' 67 (1987)

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Fig. JO. Sketch map of localised area offshore Lybster (see Fig. 4 for location). Solid lines represent fault pattern at Middle Jurassic (MJ) leve!. Hatched line represents trace of principal Base Cretaceous (BK) fault (F on seismic line Fig. 9). Fault patterns are virtually orthogonal, indicating the existence of an intervening detachment, as interpreted on the seismic line.

within the Upper Jurassic and is one of several regional decoupling horizons utilising zones of lithological weakness. The validity of this par­ticular detachment can be tested by comparing a map of the Middle Jurassic fault pattem (a prominent faulted horizon below the interpreted detachment) with the trace of the Base Cre­taceous fault above the interpreted detachment. Fig. 10 shows that the two fault patterns are virtually orthogonal and therefore unrelated. The existence of an intervening detachment is thus demonstrated.

Linked fault systems

Shaped fault segments are often imaged in the Inn er Moray Firth as on the seisrnic line (Fig. 11). A portion of imaged listric fault is interpreted at the right-hand end of the .line. In addition, the very strong reflector dipping obliquely beneath the crest of the structure is interpreted as an 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 are preferentially developed at 'restraining' bends of major faults in a strike-slip setting, and may lead to local inversion or 'footwall uplift' of crestal structures, without necessitating the isostatic mechanisms discussed by Barr et al. (1985) and Barr (1987).

The seismic panel in Fig. 12 contains an imaged portion of a sinuous ramp-flat fault (Gibbs 1987) demonstrating the relationship of shaped beds to shaped faults. Ramp-flat combinations forming staircase-like fault arrays are frequently imaged in the Inner Moray Firth. In places these 'staircase' fault arrays are imaged frequently enough on neighbouring seismic lines to build up a 3-dimen­sional 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 verified using balanced section tech­niques (Gibbs 1983) to calculate fault shape from hanging-wall geometry.

Such calculations place fault planes very dose to observed events on the seismic data, and as a

result balancing techniques may also be used with some confidence to reconstruct fault geometries where they are not seismically imaged.

Lateral offsets

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Fig. 12. Seismic panel from GECO line GMF 9 (1977, migrated) showing imaged example of sinuous ramp-flat fault, and hang­ing-wall response to 'staircase' geometry of shaped footwall.

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Fig. 15. Sketch showing sequential development of laterally offset stacked fan build-ups across oblique-slip fault: Fan builds out across oblique-slip fault. During active strike-slip episodes fans are decapitated and moved off laterally. A new fan is then built out along strike from the first, and so on. Dotted line shows position of schematic cross-section parallel to con trolling fault (Fig. 16).

the downthrown side of the fault. During active strike-slip episodes these fans are decapitated and moved off laterally as illustrated in Fig. 15. This phenomenon is well-known in many strike-slip basins both past (e.g. Greenland: Surlyk & Hurst 1983) and present (e.g. Turkey: Mann et al. 1983, and California, e.g. Crowell 1974, 1976 and this volume). As the process continues, a new fan is built out along strike from the first, and so on. A cross-section parallel to the controlling fault towards the margin of such a basin will show a series of stacked build-ups offset laterally, as in Fig. 16(a). Fig. 16(b) illustrates an equivalent seismic section from the Moray Firth, parallel to and just offshore from the Great Glen Fault. A series of laterally-offset anomalies can be seen

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.

stepping sinistrally across the section. The base of these, a distinct lithological boundary, appears to have been utilised subsequently as a low-angle thrust fault.

Inversion

The lnner Moray Firth has experienced several periods of partial inversion. Uplift occurred in the Late Jurassic and Tertiary (McQuillin et al. 1982) as well as earlier Devonian episodes. Inver­sion can be seen strongly affecting the fault-con­trolled margins of the Inner Moray Firth. Steep reverse structures such as the Banff Fault (Fig. 8), low-angle thrust faults and shallow-dipping reversed normal faults can be seen accomplishing this inversion. Several of the inversion phenom­enon which can be constrained to the 'uplift' periods of Late Jurassic or Tertiary are also indicative of a sinistral sense of movement. It is suggested that these uplift periods correspond to phases of sinistral relative displacement along the Moray Firth portion of the Great Glen Fault leading to partial inversion to the basin.

Fig. 16 (b). Part of GECO seismic line GMF 28 (1977, migrated) parallel to and just offshore from the Great Gl en Fault showing series of laterally­offset, stacked anomalies, interpreted as sedimentary build-ups developed across the Great Glen Fault during a phase of sinistral oblique-slip move­ment in the Late Jurassic. The base of these sedimentary build-ups, a distinct lithological boundary, appears to have been utilised sub­sequently to activate a low-angle thrust fault.

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

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Fig. 17. Panel from GECO line GMF B (1977, migrated) show­ing fault-controlled channel build-up at base of the Cretaceous. Location of line shown in Fig. 18.

Overall amounts of inversion are generally dif­ficult to estimate within the basin, but there is one place where at !east a minimum estimate of Tertiary inversion can be obtained. Fig. 17 is a seismic panel showing a distinctive fault-con­trolled channel build-up at the base of the Cre­taceous. This runs southwestwards along the axis of the basin before broadening into a fan-delta . system (see Fig. 18). However, the delta 'head' is now considerably elevated above the channel 'tail' of the system, so the channel is now effec­tively fiowing up hill (and the re is no thermal sag basin showing differential subsidence above). Restoring this to its original deposition geometry, as illustrated in Fig. 19, gives an estimated 3,000 ft of differential uplift across the structure. This is in very close agreement with uplift estimates which can be calculated from overcompaction in sediments drilled by wells on the Beatrice Field, which is immediately adjacent to the fan delta.

Fig. 18. Sketch map of channel-fan build-up at base of Cretaceous, showing present elevation of ends, position of line GMF B (Fig. 17) and proximity to Beatrice Field. Delta 'head' now considerably elevated above channel 'tail'.

TSGS Symposium 1986 367

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Fig. 19. Sketch showing restoration of channel-fan complex from present position to original deposition geometry. Suggests some 3,000 feet of post-Lower Cretaceous inversion ( probably Tertiary) across the structure.

Conclusions

A structural approach to seismic data has been used to evaluate the Inner Moray Firth Basin. The wide range of classic strike-slip phenomena described identify this as a basin with a significant component of strike-slip movement. Indeed, the formation and subsequent deformation of the Inner Moray Firth Basin are considered to be principally controlled by strike-slip movements of the Great Glen Fault acting as a basin-bounding sidewall. The basin is considered to open (grow) during periods of relative dextral movement along the Moray Firth portion of the Great Glen Fault and to partially dose (inversion/uplift) during periods of sinistral strike-slip movement along the Moray Firth portion of the Great GJen Fault. 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 seis­mic data in the basin and locally their three­dimensional geometry can be constrained. Bal­anced section techniques are a powerful tool to confirm the validity of the ramp-flat fault ge­ometries identified. Such a linked network of shaped faults is seen as a mechanical and ge­ometrical necessity in the evolution of strike-slip basins, as in extensional basins (Gibbs 1984). They represent an essential component in any strike-slip system, an element not conventionally recognised.

368 T. J. Bird et al.

The approach taken here has resulted in the recognition of a much larger family of structural styles than previously and hence has had a pro­found inftuence on views of the prospectivity in the basin. It is suspected that the integrated struc­tural approach used in evaluating the Mora y Firth would be equally successful in identifying new components in other basins where the extent, variety and style of strike-slip deformation has not previously been recognised.

Acknowledgements. - This paper is based on ideas evolved over a considerable period of time by staff of MYE. All Seismic data are from a GECO 1977 survey (plus one IGS 1972 line reprocessed by GECO). I am grateful to GECO (UK) and particularly Henry Ri vers for supplying lines and for permission to publish.

References Barr, D. 1985: 3-D Palinspastic restoration of normal faults in

the Ioner Moray Firth: implications for extensional basin development. Earth and Planetary Science Letters 75, 191-203.

Barr, D., McQuillin, R. & Donato, J. A. 1985: Footwall uplift in the loner Moray Firth basin, offshore Scotland. Journal of Structural Geology 7, 267-268.

Barr, D. 1987: Lithospheric stretching, detached normal faulting and footwall uplift. In, Coward, Dewey & Hancock (eds.): Continental Extensional Tectonics, 640 pp. Geological Society of London Special Publication.

Barton� P. & Wood, R. 1984: Tectonic evolution of the North Sea Basin: crustal stretching and subsidence. Geophysical

Journal of the Royal Astronomical Society 79, 987-1022. Beach, A., Bird, T. & Gibbs, A.D. 1987: Extensional tectonics

and crustal structure: deep seismic retlection data from the northem North Sea Viking Graben. In Coward, Dewey & Hancock (eds.): Continental Extensional Tectonics, 640 pp. Geological Society of London Special Publication.

Bol, A. J. & Van Hoorn, B. 1978: Structural Styles in Western Sabah Offshore. Geological Society Malaysia 2nd Petroleum Seminar, Kuala Lumpur.

Butler, R. W. H. 1982: The terminology of structures in thrust beits. journal of Structural Geology 4, 239-245.

Christie, P. A. F. & Sclater, J. G. 1980: An extensional origin for the Buchan and Witchground Graben in the North Sea. Nature 283, 729-732.

Crowell, J. C. 1974: Sedimentation along the San Andreas Fault, California, and origin of late Cenozoic basins in Southern California. In Dott, R. H. & Shaver, R. H. (eds. ): Modem and Ancient Geosynclinal Sedimentation. 292-303. Special Publication of the Society of Economic Palaentology and Mineralogy 19.

Crowell, J. C. 1976: lmplications of crustal stretching and short­ening of coastal Ventura Basin, California. Pacific Section of the American Association of Petroleum Geologists, Mis­cellaneous Publication 24, 365-382.

Dahlstrom, C. D. A. 1970: Structural geology in the eastern margin of the Canadian Rocky Mountains. Bulletin of Canadian Petroleum Geology 18, 332-406.

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

Donato, J. A. & Tully, M. C. 1981: A regional interpretation of North Sea gravity data. In Uling, L. V. & Hobson, G. D. (eds.): Petroleum Geology of the Continental Shelf of NW Europe. 67-75. Institute of Petroleum London.

Evans, D. , Kenolty, N., Dobson, M. R. & Whittingham, R. J. 1980: The geology of the Malin Sea. Report of the Institute of Geological Sciences No. 79/15.

Flinn, D. 1961: Continuation of the Great Glen Fault beyond the Moray Firth. Nature 191, 589-591.

Flinn, D. 1969: A geological interpretation of the aeromagnetic maps of the Continental Shelf around Orkney and Shetland. Geological Journal 6, 279-292.

Flinn, D. 1975. Evidence for post-Hercynian transcurrent move­ment of the Great Glen Fault in the Moray Firth. Scottish

Journal of Geology 11, 266-267. Garson, M. S. & Plant, J. 1972: Possible dextral movements on

the Great Glen and Minch Faults in Scotland. Nature of Physical Science 240, 31-35.

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 margins. Journal of the Geological Society 141, 609-620.

Gibbs, A. D. 1987: Development of extensional and mixed mode sedimentary basins. In Coward, M. P., Dewey, J. F. & Hancock, P. L. (eds.): Continental Extensional Tectonics. 640 pp. Geological Society of London Special Publication.

Harding, T. P. 1973: Newport-Inglewood trend, California-an example of wrenching style of deformation. Bulletin of the American Association of Petroleum Geologists 57, 97-116.

Harding, T. P. 1974: Petroleum traps associated with wrench faults. Bulletin of the American Association of Petroleum Geologists 58, 129(}-1304.

Harding, T. P. 1976: Tectonic significance and hydrocarbon trapping consequences of sequential folding synchronous with San Andreas faulting, San Joaquin Valley, California. Bull­etin of the American Association of Petroleum Geologists 60, 356-378.

Harding, T. P. 1983: Divergent wrench fault and negative tlower structure, Andaman Sea. In Bally, A. W. (ed. ) : Seismic Expression of Structural Styles. American Association of Pet­roleum Geologists Special Publication.

Harding, T. P. 1985: Seismic characteristics and identification of negative tlower structures, positive tlower structures and positive structural inversion. Bulletin of the American Associ­ation of Petroleum Geologists 69, 582-600.

Harding, T. P. & Lowell, J. D. 1979: Structural styles, their plate tectonic habitats, and hydrocarbon traps in petroleum provinces. Bulletin of the American Association of Petroleum Geologists 63, 1016-1058.

Holgate, N. 1969: Paleozoic and Tertiary transcurrent move­ments on the Great Glen Fault. Scottish Journal of Geology 5, 97-139.

Kennedy, W. Q. 1946: The Great Glen Fault. QuarterlyJournal of the Geological Society of London 102, 41-72.

Lowell, J. D. 1972: Spitsbergen Tertiary orogenic belt and the Spitsbergen fracture zone. Bulletin of the Geological Society of America 82, 3091-3102.

Mann, P., Hempton, M. R. , Bradley, D. C. & Burke, K. 1983: Development of pull-apart basins. Journal of Geology 91, 529-554.

McQuillin, R. , Barclay, W. & Bacon, M. 1979: An Introduction to Seismic Interpretation. 199 pp. Graham & Trotman.

McQuillin R., Donato, J. A. & Tulstrup, J. 1982: Development

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

of basins in the lnner Moray Firth and North Sea by crustal extension and dextral displacement of the Great GJen Fault. Earth and Planetary Science Letters 60, 127-139.

Sclater, J. G. & Christie, P. A. F. 1980: Continental stretching; an explanation of the post mid-Cretaceous subsidence of the Central North Sea Basin. Journal of Geophysical Research 85, 3711-3739.

Smith, P. J. & Bott, M. H. P. 1975: Structure of the crust beneath Foreland and Caledonian Belt of the North Scottish Shelf region. Geophysical Journal of the Royal Astronomical Society 40, 187-205.

Speight, J. M. & Mitchell, J. G. 1979: the Permo-Carboniferous

TSGS Symposium 1986 369

dyke-swarm of northern Argyll and its bearing on dextral displacement of the Great GJen Fault. Journal of the Geo­logical Society of London 136, 3-11.

Surlyk, F. & Hurst, J. M. 1983: Evolution of the earry Palaeozoic deep-water basin of north Greenland - aulacogen or narrow ocean? Geology 11, 77-81.

Wilcox, R. E. , Harding, T. P. & Seely, D. R. 1973: Basic wrench tectonics. Bulletin of the American Association of Petroleum Geologists 57(1), 74-96.

Woodcock, N. H. & Fischer, M. 1986: Strike-slip duplexes. Journal of Structural Geology 8, 725--735.