EXTENSIONAL GEOMETRIES AS A RESULT OF REGIONAL-SCALE THRUSTING - TECTONIC SLIDES OF THE DUNLEWY NW DONEGAL AREA, IRELAND

The synmetamorphic ductile dislocations, known in the British Caledoni an literature as 'Tectonic Slides', pose a classical structural proble m. That is, despite being associated with synchronous contractional fo lds and cleavages the low angle dislocations have the effect, in many celebrated cases, of juxtaposing younger over older rocks: a geometry normally associated with extensional rather than contractional deforma tion. Recent models have attempted to demonstrate that this is the res ult of thrust reactivation of original, sedimentary, extensional growt h faults. The Appin Group Dalradian metasediments of the complex and s mall Dunlewy area of NW Donegal, Ireland, contain the following geomet ric elements: (a) an early strike-swing-related stratigraphic facies c hange; (b) a major inter-deformational dolerite sheet; (c) major regio nal recumbent folds and slides; (d) major structures related to the 40 0 Ma sinistral Main Donegal Granite shear zone. This solution to the s tructural geometry reveals that the early mid-crustal (similar to 11 k m depth) D-2 Ardsbeg-Dunlewy Slide is a thrust to the northwest. Its h angingwall contains rocks two-thirds of which are younger than the roc ks of the footwall, together with major recumbent folds, coeval with t he underlying thrust, which face downwards into the thrust in the dire ction of transport. Rather than thrust reactivation of an original ext ensional growth fault, we find that both stratigraphic and structural constraints are satisfied by a double thrusting model, with fault-bend folding onto an upper ramp of an earlier formed but penecontemporaneo us and kinematically linked major fold pair. This solution to the geol ogy also allows us to recognize that the regional (pre-granite) struct ure of the Dalradian of NW Donegal is a series of major D-2 synmetamor phic thrust bounded nappes possibly involving up to 250 km of northwes terly overthrusting.