Journal of the Geological Society, London, Vol. 146, 1989, pp. 261-269,6 figs. Printed in Northern Ireland Reflection seismic evidence for a Moho offset beneath the Walls Boundary strike-slip fault SUSAN McGEARY BIRPS, Bullard Laboratories, Cambridge, CB3 OEZ, UK Present address: Department of Geology, University of Delaware, 101 Penny Hall, Newark, Delaware, 19716 USA Abstract: BIRPS has acquired two deep seismic reflection profiles across the Walls Boundary Fault, a major late Caledonian strike-slip fault that is probably the northern continuation of the Great Glen Fault of Scotland. Where crossed by the SHET deep seismic survey north of the Shetlands, the Walls Boundary Fault appears to be a narrow vertical structure which penetrates the entire crustal thickness and juxtaposes 'crusts of different thicknesses. The seismic data show a Moho offset in travel-time of 1.0-1.5seconds and two high-amplitudediffractions originating at Mohotravel-times. These diffractions are, as yet, unique among seismic data across strike-slip faults and can be used to argue that the Moho is offset in depthby 2-3 km over a lateral distance of less than 6 km. Seismic modelling is used to constrain this interpretation. The preservation of this Moho topography suggests that the structure and rheology of the lower crust of the Shetland Platform was not significantly modified during post-Devonian extension of the North Sea and Atlantic margin. Deep seismic reflection profiling is increasingly being used is exposed as a zone of intensely fractured rock, including to study thenature and geometry atdepth of structures slices of mylonite, which is usually fairly narrow but can be mapped at the surface. Of particular interest are the major up to a kilometre in width (Flinn 1977). The fault can be strike-slip faults found on the continents. Several of these clearly traced on the continental shelf as far south as Fair have been recognized to be the transformboundaries Isle (Fig. 1) and is the likely northern continuation of the between major plates (the San Andreas, Alpine, and Queen transcurrent Great Glen Fault in Scotland (Flinn 1961, 1969; Charlotte faults forexample), while others accommodate McQuillin et al. 1982). stresses within the boundaries of the continents (the Great The Great Glen Fault is the most conspicuous member Glen, North Anatolian,and Garlockfaults). The surface of a set of NE-SW trending wrench faults probably formed expression of these structures is usually well-defined, either in the early Devonian during oblique continental collision in as a single fault zone or as a set of parallel or anastomosing the final stages of the Caledonian orogeny (Watson 1984; faults of measurable width and offset. However, neither the Soper & Hutton 1984; Phillips 1976). The ages, directions deeper geometry of these structures nor the way in which andamounts of movement onthe fault are highly the displacement observed at the surface is accommodated controversial. Palaeomagnetic data have led investigators to at depth within the lithosphere is well understood (Sibson argue for large sinistral displacements ranging from 600 km 1983). in the late Middle Devonian (Storetvedt 1986) to 2000km In August 1984, the British Institutions Reflection during the Carboniferous (Van der Voo & Scotese 1981). Profiling Syndicate (BIRPS) collected 830 km of deep Geological data,on the other hand, suggest an initial multichannel seismic reflection data (the SHET survey) on sinistral displacement of only about 100-200 km before the the continental shelf surrounding the Shetlands, north of end of the Devonian (Smith & Watson 1983), followed by a Scotland (Fig. 1). One of themajor goals of theSHET post-Devonian dextraldisplacement of about 30 km survey was to study thedeep structure of the Walls (Donovan et al. 1976). BoundaryFault (WBF), a major strike-slip fault in the As the northern continuation of the Great Glen Fault, Shetlandsthought to be the northern continuation of the the Walls Boundary Fault is also likely to have experienced Great Glen Fault system in Scotland (Flinn 1961). Two of several episodes of transcurrentmotion both duringand theSHET profiles cross the offshore continuation of the since the end of the Caledonian orogeny. Early transcurrent WBF: UNST north of the Shetlands and FAIR ISLE south motion of the WBF is not directly constrained by Shetland of the Shetlands (Fig. 1). basement geology since the Caledonian metamorphic rocks This paper presents the interesting and somewhat on either side of thefault cannotbe correlated, but the surprising features of these two profiles across the WBF and relative position of the Caledonian front with respect to the shows the results of forward seismic modelling used to Dalradianand Moine equivalents on Shetland suggests constrain theirinterpretation. In particular, this paper 100-200 km of late Caledonian sinistral offset for the WBF investigates the geometry of the WBF at Moho depths and as well (Flinn 1985). Later dextral motion displacing both the bearing of these results on the natureof strike-slip faults the Old Red Sandstone and granitic plutons is estimated to at depth and the rheology of the lower crust. be from 65 km (Flinn 1969; Donovan et al. 1976) to about 95 km (Astin 1982). The timing and nature of the latest movement on either The Walls Boundary Fault theWBF or the Great Glen Fault is again controversial The Walls Boundary Fault is the most important structural (Bacon & Chesher 1974; Bott & Browitt 1975; Flinn 1975; discontinuity in the Shetlands (Flinn 1961; Mykura 1976). It Speight & Mitchell 1979). TheWBF clearly truncates 261 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/2/261/4897852/gsjgs.146.2.0261.pdf by guest on 03 October 2021 262 S. McGEARY Fig. 1. Location map of the SHET deep seismic reflection surveyof BIRPS. Two 15-second profiles, UNST and FAIR ISLE, cross the Walls Boundary strike- slip fault (WBF) north and southof the Shetlands. The WBF is the likely northern continuation of the Great Glen fault (GGF). Line drawing interpreta- tions of each profile are shown in Figs 2 and 3. Land is speckled. Sedimentary basins are dotted. UN Unst island; FI, Fair Isle; FIB, Fair Isle Basin. Permo-Triassic sedimentaryrocks in the West Fair Isle of about 2 km. Beneath the basin and exposed at the sea Basin. Movement onthe Great Glen Fault in the Moray bed to the east is the acoustically transparent metamorphic Firth has affected sediments as young as Lower Cretaceous basement of presumed Caledonian age. (McQuillin et al. 1982; Bacon & Chesher 1974). The latest The most exciting features of the UNST profile lie at movement is therefore post-Early Cretaceous but may have Moho depths within the section (Figs 2 and 4). West of the involved only minor displacement. There is no evidence in WBF, the Moho is interpreted to be the base of a narrow this region east andnorth of Scotlandfor any significant zone of high-amplitude reflections at 9.0-9.5 S TWTT (event Tertiary displacement on the Great Glen Fault or the Walls M1 in Figs 2 and 4). Although there is no nearby refraction Boundary Fault. data to constrain this interpretation, such an interpretation has been justified in other regions by coincident refraction The BIRPS profiles; seismic interpretation and wide-angle reflection data(Barton et al. 1984; Klemperer et al. 1986; Matthews 1986). Immediately east of The SHET survey crosses the WBF both north and southof the WBF, theMoho is not as well defined but can be seen as the Shetlands. Line drawings of the relevant parts of the two a discrete set of horizontal reflections at 10.7 S TWTT (event profiles, UNST and FAIR ISLE, are shown in Figs 2 and 3 M2 in Figs 2 and 4). This abrupt change in reflection time to respectively. Bothsections were recorded to 15 seconds the Moho of about 1.5 S could beproduced either by a two-way travel-time (m)allowing seismic penetration major change in depth to the Moho across the WBF north into the continental crust and mantle to a depth of about of Shetland, a Moho offset, or by a significant difference in 50 km. The data are 30 fold. Below each line drawing is a average crustal velocity across the fault. geological interpretation of the seismic data. The results of The most surprising events in the profile are events D1 thetotal SHET survey will bepresented elsewhere and D2 (Figs 2 and 4). These two events can each be seen to (McGeary 1987). originate at about Moho times and to continue at opposite dips to the base of the section. The slightly curved geometry of events D1 and D2 and the fact that they each collapse to The UNST profile a separate finite area(a ‘point’) upon migration at UNST crosses the WBF 35 km north of the island Unst in reasonable whole crustal velocities suggest that they are the Shetland Isles (Fig. 1)and extendsfrom theFaeroe/ both diffractions, probably from some kind of structure on West Shetland basin to the Viking Graben. Only the 70 km the Moho. The high-amplitudes suggest thatthe seismic section across theWBF is shown in Fig. 2. West of the energy has been focused in some way by this structure. The WBF, Caledonian or Archaean metamorphicbasement is apices of diffractions D1 and D2 are offset in travel-time exposed at the sea bed, and there are few reflections within which makes the diffractions appear asymmetric. This offset the upper crust (Fig. 2). Immediately east of the WBF, and in diffraction apices may be explained in the same way as truncated by it, is a small sedimentary basin, the Sandwick the offset on the Moho, by either Moho structure or velocity Basin, assigned to the Devonian (Hitchen & Ritchie 1987). pulldown.