Localization of mid-crustal thrust ramps by metadolerite sheets in the Dalradian of northwest Ireland

Abstract

The Slieve League peninsula in southwest Donegal affords some of the best exposures of the Dalradian Appin/Argyll Group transition available in Ireland. As a result, a refined stratigraphical column is available, allowing resolution of small- (metric-) scale imbrication of parts of the sequence. Large-scale D2 thrust nappes dominate the structure of the region, and the rocks have undergone multiple folding and complex cleavage development. In the Glencolumbkille valley, an imbricate duplex has been mapped in detail. Original intrusive relationships of metadolerite sheets controlled the geometry of subsquent deformation. A major ramp cuts up through the established stratigraphy and is controlled by a pre-D2 metadolerite intrusion. Throughout the region, major and minor ramps are localized by the metadolerite sheets. Within the thrust sheets, analysis of quartz petrofabric and clast fabrics in hangingwall mylonites within the Boulder Bed document kinematics and strain localization within the thrust sheets. Axial ratios in the dolomitic clasts increase up to a maximum of 30 [ratio ] 1 adjacent to the ductile thrusts. However, granite clast axial ratios are less pronounced adjacent to some of the shears, suggesting that strain may have partitioned into the matrix at high strains.