Basin Structure and Tertiary Magmatism on Skye, NW Scotland
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Journal of the Geological Sociery, London, Vol. 151, 1994, pp. 931-944, 11 figs. Printed in Northern Ireland Basin structure and Tertiary magmatism on Skye, NW Scotland R. W. H.BUTLER’ & D. H. W. HUTTON* ‘Department of Earth Sciences, The University, Leeds LS2 9JT, UK 2Department of Geological Sciences, The University, Science Site, South Road, Durham DH1 3LE, UK Abstract: The emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre-existing basin structures. These controls are investigated using examples from the Tertiary igneous complexes of Skye, part of the Sea of Hebrides basin of NW Scotland. The basin consists of an array of half-graben related to SE-dipping normal faults. These pre-volcanic,Mesozoic structures aretraced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields. The unconformity represents a period of Cretaceous uplift and denudation of the basin and its flanks, entirely pre-dating the Tertiary volcanism of NW Scotland. This unconformity seals stratigraphically the major basin faults, preserving field relationships that permit the tracing of these faults in the country rocks to the Tertiary intrusions. The major Camasunary fault is separated from the Raasay fault via a series of minor graben, linked by a series of steep, NW-SE-trending faults that transfered Mesozoic displace- ments between the principal fault strands. A broad range of igneous material of various compositions was intruded into part of the Mesozoic Sea of Hebrides basins and their flanks during Palaeocene times. Different emplacement styles and different structural controls are found. The major gabbroic centres do not appear to be controlled by upper crustal structures, having been emplaced into the footwalls of major faults. However, minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions, thereby facilitating emplacement. In contrast, the granitic melts have been emplaced as sheets and domed intothe sediments and overlying lava pile, reactivating segments of the basin fault network. Doming occurred from an array of sills, the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions. The sill levels and their transgressive forms are strongly related to inferred Mesozoic basin structures. The major fold structures of Tertiary age in southern Skye are interpreted as accommodating granitic emplacement rather than crustal shortening. The NW-SE Mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes. These interpretations are illustrated using field relationships mapped in the vicinity of the Coire Uaigneich granophyre. It is concluded that althoughthe higher parts of the basinfaults werereactivated to facilitate thedoming of granitic intrusions, the deeper levels of the Mesozoic faults show no evidence of substantial reactivation. The extension of continental lithosphere which controls the may drive basin-wide advective systems in formation waters, development of sedimentary basins is associated commonly influencingburial diagenesis and consequentlymodifying with syntectonic igneous activity. In recent years, mantle- carrierbed, reservoir and seal characteristics. These meltingmodels have emphasized the importance of processes andphenomema willbeinfluenced by the asthenosphericplumes which elevatetemperatures at the duration and sites of magmatism within the lithosphere. Of base of the lithosphere (McKenzie & Bickle 1988; White & these it is the site of magmatism and its structural controls McKenzie1989). Plumes may promotesurface uplift that we wish to address, with special reference to the upper (Griffiths et al. 1989), increase the geothermal gradient and crust. generate substantial volumes of basaltic magma (McKenzie Despite the importance of igneousrocks forthe & Bickle1988). If plumes coincidewith regions of geological evolution of some sedimentary basins, there has lithospheric extension this magma may be emplaced into the been little attention directed at the structureof such igneous crust. This may have a profound effect on basin geometry, complexes. Since the pioneering work of Anderson (1936), by increasingcrustal volume through intrusion. Normal studies of the structural geology of igneousrocks, faults generally createsedimentary basins.Syn-tectonic particularlylinking intrusion mechanisms and tectonics, magma may fill space otherwise available to accommodate have largely been directed at examples exhumed from the sedimentsthrough eruption of lava piles. It mayalso fill middle and lower crust wheredeformation hasbeen intra-crustal ‘pull-apart’ sites along faults thus reducing the dominantlyductile (e.g. Hutton 1982,1988; Hollister & depth of overlyingbasins. The advective heattransfer Crawford 1986; Brun et al. 1990; McCaffrey 1992). In these associated with emplaced basalticmagma may promote deep settings,the interplay between tectonic structures, melting of the continental crust and will certainly modify the tectonicand magmabuoyancy forces andthe interplay rheology of the lithosphere, its strength and the manner in between magma and country-rockrheologies provide the which it responds to tectonicboundary conditions and key controls on the styles and sites of intrusion. However, gravitational loads. In addition to these geometric aspects, within most sedimentary basins, the close proximity of the magmatismwill strongly affect thethermal structure of Earth’ssurface, as a free surface of stressrelease and sedimentary basins, thereby influencing hydrocarbon matu- deformation, is critical. Deformation of this surface caused ration(e.g. England et al. 1993). Thethermal anomalies bymagma spacecreation in thenear subsurfacethrough 93 1 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/6/931/4890044/gsjgs.151.6.0931.pdf by guest on 26 September 2021 932 BUTLER H. W. R. & D. H. W. HUTTON processes such as cauldron subsidence and doming is widely Scotland, with the greatest amalgam of centres occurring on documentedfrom modem volcanic centres.thisIn Skye (Fig. 1).Recent reviews of theTertiary geology of contribution we investigate the extent to which pre-existing Skye are provided by Bell & Hams (1986) andEmeleus basinstructures control the siting and geometry of large (1991). The igneousrocks arerepresented by asuite of magma chambersthat represent sub-volcanic complexes. plateau lavas into which are intruded a,layered complex of Our examples come fromthe British Tertiary Volcanic basicand ultrabasic composition (collectively formingthe Province of NW Scotland, especially the Isle of Skye. Cuillin complex) and a later groupof granitic intrusions (the Red Hills complex). Most of these units both cut and are themselves intruded by the major NW-SE-trending swarm Geological background of, dominantly basaltic, dykes. Thus there is a well-known The igneous centres of the Isle of Skye are classic ground, igneous ‘stratigraphy’: our conclusions will not modify it. not only for British geology but also for the development of TheHebridean province collectively experiencedpeak igneous studies in general. The classification of major and igneous activity from c. 63 to 59 Ma (Mussett et al. 1988). minorintrusions and their importance in building up On Skye, theplateau lavas were erupteddominantly at volcanic edifices owes much to the seminal studies of Harker about 59 Ma with the youngest Red Hills granite dated at (1904), Richey (1932), Bailey (1947) and others. These and 53.5 Ma (Dickin & Exley 1981). These rocks were emplaced earlierstudies (e.g. Geikie 1897) identifieda number of into the Sea of Hebrides basin which had experienced a long largeigneous complexes along the western coast of history of relatively minor subsidencethroughout late Fig. 1. Simplified map of the geology of the Sea of Hebrides area, onshore and offshore NW Scotland (modified after BGS 1986u, b).Mesozoic sediments, wide stipple; Oligocene sediments, fine stipple; Palaeocene lavas, horizontal ruling; principal igneous centres, dashes. Pre-Mesozoic rocks are unornamented. Mesozoic faults (‘lolly-pop’ symbolson downthrown side): CF, Camasunary Fault; RF, Raasay Fault; AF, Apple- cross Fault; KF, Kishorn Fault; MF, Minch Fault. Boxed area shows Fig. 4. Section line of Fig. 2b is indicated (b-b’). Location of critical outcrops of Raasay fault at Balachuirn is indicated (f). Inset: British Isles, location map Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/6/931/4890044/gsjgs.151.6.0931.pdf by guest on 26 September 2021 B ASIN STRUCTURE AND MAGMATISM,STRUCTURE ANDBASIN SKYE 933 Palaeozoicand Mesozoic times with activefaulting thatthe dyke swarm hasno geometric relationship to continuing intothe Cenozoic(Peach et al. 1910; Hallam previousstructures but they do suggest thatthe major 1983; Morton 1987). centres are spatially connected with pre-existing faults; this Since the early studies, much of the igneous research has spatial association is also recognized by Emeleus (1991). been of a geochemical nature. This has suggested that at Ourpurpose here is toexamine the relationships least the later portions of the magmatism were derived from between upper crustal structures and magmatic activity as thelowermost lithosphere andupper asthenosphere indicatedprimarily by surfacegeology, not