The dalradian of central Donegal: An example of polyphase mid-crustal thrusting

Abstract

The geometry and kinematic development of a major syn-metamorphic slide zone, the Central Donegal slide, is described from the Dalradian Succession of Central Donegal, Ireland. The earliest manifestation of this structure is a large scale reversal in the direction of younging and stratigraphy across the slide plane, a feature that probably developed at the lower greenschist facies of metamorphism. Kinematic indicators associated with this early deformation have been destroyed by later higher metamorphic grade overprinting, but by correlation with other early structures in Donegal, and with reference to sub-basin development in South and Central Donegal it was probably a Grampian thrust with North West directed transport. This tectonic package was then redeformed at lower amphibolite facies by a kilometric scale shear zone associated with slides and large scale sheath folds. Kinematic indicators show that the sheath folds 'intrude' downwards to the south east although originally they are regarded as having faced upwards to the south east. Following this phase of deformation the Dalradian stratigraphy of Central and South Donegal was largely uninverted and probably lay on the upper normal limb (and represents a deeper level) of the upward facing Glenelly Anticline, a major fold to the east in the Sperrin Mountains which is equated with the Tay Nappe. The zone was then reworked during another (mid-amphibolite facies) event during which large scale south facing and verging folds were formed: the Ballybofey Antiformal complex. Earlier formed sheath folds were refolded, slides reactivated and new slides formed. This phase of deformation is associated with major oblique dextral overthrusting, which with increasing metamorphic grade eventually carries the entire Dalradian cover sequence over Moine - like Proterozoic basement 30 km to the south along the Lough Derg slide. This second amphibolite facies metamorphism can be directly associated with the large scale South East directed overthrusting of the entire Dalradian stratigraphy, and related to crustal thickening and loading via the development of a nappe pile towards the south.