Caledonian magmatism and major tectonic structures in the SW highlands of Scotland: implications for ascent, siting and emplacement

Abstract

This thesis describes the emplacement characteristics of four Caledonian plutons, Etive, Glencoe, Rannoch Moor and Strath Ossian, within the SW highlands of Scotland. A brief study of a further two plutons, Ben Nevis and Ballachulish, was carried out to establish whether their emplacement dynamics would provide any additional information on the regional tectonic processes operating at the time of Caledonian plutonism (425-400 ma).The trans-Caledonoid, linear shaped, Strath Ossian pluton was constructed by a process of multiple sheeting along a major mid-crustal shear zone, within a transtensional segment of a NW-SE-trending lineament. However, the Etive, Glencoe, Rannoch Moor, Ben Nevis and Ballachulish complexes, which have more elliptical forms, have been constructed by a process of in situ expansion within an overall sinistral transpressional system; allowing magma to expand predomiantly into the direction of maximum extension imposed by the controlling tectonic structures. A distinct episode of regional sinistral transtension may explain the emplacement of the Etive Dyke Swarm and the high level emplacement phenomenon, the Glencoe Fault Intrusion. Space creation for pluton emplacement at mid-crustal levels has been predominantly achieved by ductile movements along regional shear zones and bulk wall rock shortening by ductile flow. At high crustal levels, brittle translatory mechanisms, such as peripheral ring fault development and subsequent uplift, may provide a major component for space creation. This study has led to the identification of several major Caledonian NE-SW- trending shear zones and faults, which are distinct from an intersecting set of NW-SE- trending pre-Caledonian crustal lineaments which were reactivated during Caledonian orogenesis. The major intrusive phases of the above plutons, are all sited at shear zone or lineament intersections, where transtensional zones allowed and facilitated their ascent. Based on this new mapping and compilation of existing data, models are presented which may not only show that such lineaments actively controlled the siting of Caledonian magmatism in the NE and Northern Highlands, but also in the development of "gross orogenic flow patterns" and associated structures. In northern Scotland it may be possible to sub-divide the underlying basement into several 'distinct tectonic domains', characterised by the orientation of geologically-defined lineaments within the cover. A major deep crustal lineament, the "Ossian-Loch Quoich Line", may have controlled the northern limit of NW-directed underthrusting of a tectonically distinct basement unit (characterised by an E-W 'tectonic fabric') beneath the Dalradian and part of the southern Moine cover sequence.