BASIN STRUCTURE AND TERTIARY MAGMATISM ON SKYE, NW SCOTLAND

The emplacement of igneous material into upper crustal rocks of sedime ntary basins is likely to be strongly controlled by the geometry of th e 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 h alf-graben related to SE-dipping normal faults. These pre-volcanic, Me sozoic structures are traced near the igneous complexes using geologic al relationships preserved unconformably beneath the widespread basalt ic lava fields. The unconformity represents a period of Cretaceous upl ift and denudation of the basin and its hanks, entirely pre-dating the Tertiary volcanism of NW Scotland. This unconformity seals stratigrap hically the major basin faults, preserving field relationships that pe rmit the tracing of these faults in the country rocks to the Tertiary intrusions. The major Camasunary fault is separated from the Raasay fa ult via a series of minor graben, linked by a series of steep, NW-SE-t rending faults that transfered Mesozoic displacements between the prin cipal fault strands. A broad range of igneous material of various comp ositions was intruded into part of the Mesozoic Sea of Hebrides basins and their flanks during Palaeocene times. Different emplacement style s and different structural controls are found. The major gabbroic cent res do not appear to be controlled by upper crustal structures, having been emplaced into the footwalls of major faults. However, minor synm agmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions, thereby facil itating emplacement. In contrast, the granitic melts have been emplace d as sheets and domed into the sediments and overlying lava pile, reac tivating 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 th e intrusions. The sill levels and their transgressive forms are strong ly related to inferred Mesozoic basin structures. The major fold struc tures of Tertiary age in southern Skye are interpreted as accommodatin g granitic emplacement rather than crustal shortening. The NW-SE Mesoz oic transfer fault trend appears to have strongly influenced the segme ntation of the granite domes. These interpretations are illustrated us ing field relationships mapped in the vicinity of the Coire Uaigneich granophyre. It is concluded that although the higher parts of the basi n faults were reactivated to facilitate the doming of granitic intrusi ons, the deeper levels of the Mesozoic faults show no evidence of subs tantial reactivation.