The structure and kinematics of the ox mountains, western Ireland: a mid-crustal transcurrent shear-zone

Abstract

The stratigraphy, structure and kinematics of the central and southwest Ox Mountains, a major mid-crustal shear zone, are described. The tectono-stratigraphic sequence has been established using a combination of structural and stratigraphic techniques and lithostratigraphic correlation with southern Donegal (the closest area of autl ochonous Dalradian). These data suggest that the Tawneyshane Formation is the equivalent of the Port Askaig Tillite and that the metasediments of the southwest and central Ox Mountains Succession represent part of the Argyll Group of the Middle Dalradian. The coarser grained rocks of the Cloonygowan Formation are tentatively correlated with the turbiditic sediments of the Southern Highland Group on the basis of lithostratigraphic similarity. Structural analysis indicates that the Ox Mountains Succession has experienced a similar structural history to the Cloonygowan Formation although at a deeper structural level. Four distinct kinematic episodes are represented. Initial fold and fabric development (D1-D2, pre 478 Ma) was followed by sinistral transcurrent deformation, synkinematic intrusion of the Ox Mountains Granodiorite (478±12 Ma) and development of a braided system of high strain zones and tectonic slides (D3, Arenig-Llanvirn). The tectonic contact with the granulite facies metasediments of the northeast Ox Mountains is identified as a D3 tectonic slide, which dips gently to the south. This basement-cover interface strongly influences the structural geometry of the central and southwest Ox Mountains. These data suggest that at middle-lower crustal levels strike slip fault zones may be expressed as a series of high strain zones that converge both laterally and vertically. The geometry and kinematic history of the Ox Mountains is consistent with the interpretation of the inlier as the root of a major mid-crustal transpression zone. Following the cessation of transcurrent activity uplift began throughout the inlier, which is reflected by decreasing metamorphic grade and the development of conjugate folds (D4). This was succeeded by renewed sinistral transcurrent deformation of the Lough Easky and Lough Talt Adamellites (401±33 Ma), (D5, Early Devonian).Evidence is presented that the Ox Mountains form part of the northwestern side wall of the Highland Boundary Fault Zone. Deeper levels of this structure are exposed in Ireland than in Scotland. Structural data suggest that in Ireland this structure is expressed as a major mid- crustal transpression zone that developed in response to large sinistral displacements during Arenig-Llanvirn times, followed by smaller sinistral displacements during the Early Devonian. This protracted history of sinistral transcurrent deformation can be related to terrane accretion events along the Highland Boundary Fault Zone and provides information on its early kinematic history not available in the remainder of the British Isles.