Structures Related to the Emplacement of Shallow-Level Intrusions

A systematic view of the vast nomenclature used to describe the structures of shallow-level intrusions is presented here. Structures are organised in four main groups, according to logical breaks in the timing of magma emplacement, independent of the scales of features: (1) Intrusion-related structures, formed as the magma is making space and then develops into its intrusion shape; (2) Magmatic flow-related structures, developed as magma moves with suspended crystals that are free to rotate; (3) Solid-state, flow-related structures that formed in portions of the intrusions affected by continuing flow of nearby magma, therefore considered to have a syn-magmatic, non-tectonic origin; (4) Thermal and fragmental structures, related to creation of space and impact on host materials. This scheme appears as a rational organisation, helpful in describing and interpreting the large variety of structures observed in shallow-level intrusions

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 Caledonian literature as 'Tectonic Slides', pose a classical structural problem. That is, despite being associated with synchronous contractional folds 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 deformation. Recent models have attempted to demonstrate that this is the result of thrust reactivation of original, sedimentary, extensional growth faults.The Appin Group Dalradian metasediments of the complex and small Dunlewy area of NW Donegal, Ireland, contain the following geometric elements: (a) an early strike-swing-related stratigraphic facies change; (b) a major inter-deformational dolerite sheet; (c) major regional recumbent folds and slides; (d) major structures related to the 400 Ma sinistral Main Donegal Granite shear zone. This solution to the structural geometry reveals that the early mid-crustal (similar to 11 km depth) D-2 Ardsbeg-Dunlewy Slide is a thrust to the northwest. Its hangingwall contains rocks two-thirds of which are younger than the rocks of the footwall, together with major recumbent folds, coeval with the underlying thrust, which face downwards into the thrust in the direction of transport. Rather than thrust reactivation of an original extensional 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 penecontemporaneous and kinematically linked major fold pair.This solution to the geology also allows us to recognize that the regional (pre-granite) structure of the Dalradian of NW Donegal is a series of major D-2 synmetamorphic thrust bounded nappes possibly involving up to 250 km of northwesterly overthrusting.</p