Mid crustal thrust tectonic processes: examples from the Dalradian of N.W. Donegal

Abstract

A D2 ductile thrust imbricate stack has been identified within the mid greenschist facies (Appin Group) metasediments of the Breaghy Head area of Co. Donegal. Major stratigraphy parallel tectonic slides detach arrays of subsidiary ductile thrust imbricates, which display patterns of intensifying strain and minor structures generally regarded as being diagnostic of the much broader thrust sense shear zones (tectonic slides) which typify deformation within metamorphic "parts of mountain belts. This commonality of structural associations implies that the. Breaghy Head imbricates and their broader larger scale counterparts must share similar generative and propagative processes. The imbricates have 'shaped' geometries with long bedding parallel flats and shorter 20-30º ramps preserved as hanging wall anticlines, footwall synclines or complex remnant zones of climbing vein arrays. At a number of localities, ramps have escaped direct incorporation into mature thrust profiles and have been preserved within thrust hanging walls, ' frozen' at early on _intermediate stages of development. This has enabled identification of three distinct ramp styles; "Vein array ramps" characterised by vein array complexes, and "fold ramps" & "fabric slip ramps", both hosted by primary F2 folds. These fold hosted ramps can be seen to nucleate or 'result from coallescent propagation of ductile thrust dislocation cells (F5R & PR respectively).The concept of thrust dislocation cells is supported by the presence of D2 extensional flow within the imbricate stack, expressed by shear bands and boudinage. These structures are kinematically and temporally intimate, forming combinant structures at a number of localities. These structures characteristically intensify towards the thrust planes but are never seen to deform them, such that extensional flow is detached at the thrust plane to which it is seen to intensify. The extensional and contractional flow clearly relates spatially and temporally to the generation and movement of individual imbricates and must, therefore, coexist kinematically as this takes place. This can be explained by Theologically focusing (localising) deformation to produce stratigraphy parallel dislocation cells. The development and subsequent propagation of these features produces the observed structural patterns and displacement connectivity via ramp generation to produce mature 'shaped' ductile thrust profiles. Local polyphase fold and fabric histories are seen to be generated during continuum D2 ductile thrusting. These structures are temporally and spatially restricted, chiefly as hanging wall strains produced by local thrust stacking processes (eg. Imbricate back-steepening and culmination extension). Local polyphase sequences are also generated by development of rare backthrusts, buttressing and footwall collapse of ramps and hard band block rotations. These structures are clearly D2 ductile thrust secondary structures, related to local kinematic processes and do not therefore reflect regional polyphase deformation.