A WIDE-ANGLE SEISMIC TRAVERSE THROUGH THE VARISCAN OF SOUTHWEST IRELAND

A wide-angle seismic profile across the western peninsulas of SW Irela nd was performed. This region corresponds to the northernmost Variscan thrust and fold deformation. The dense set of 13 shots and 109 statio ns along the 120 km long profile provides a detailed velocity model of the crust. The seismic velocity model, obtained by forward and invers e modelling, defines a five-layer crust. A sedimentary layer, 5-8 km t hick, is underlain by an upper-crustal layer of variable thickness, wi th a base generally at a depth of 10-12 km. Two midcrustal layers are defined, and a lower-crustal layer below 22 km. The Moho lies at a dep th of 30-32 km. A low-velocity zone, which coincides with a well-defin ed gravity low, is observed in the central part of the region and is m odelled as a Caledonian granite which intruded upper-crustal basement. The granite may have acted as a buffer to northward-directed Variscan thrusting. The Dingle-Dungarvan Line (DDL) marks a major change in se dimentary and crustal velocity and structure. It lies immediately to t he north of the velocity and gravity low, and shows thickness and velo city differences in many of the underlying crustal layers and even in the Moho. This suggests a deep, pre-Variscan control of the structural development of this area. The model is compatible with thin-skinned t ectonics, which terminated at the DDL and which incorporated thrusts i nvolving the sedimentary and upper-crustal layers.