DEEP-STRUCTURE OF THE VALE OF GLAMORGAN, SOUTH WALES, UK

The deep structure of the Vale of Glamorgan is investigated using rece ntly acquired seismic data. Reflection sections from a commercial seis mic survey in the Vale enable the Carboniferous Limestone, Old Red San dstone and Silurian sequences to be mapped in the subsurface. The Old Red Sandstone sequence thins westwards under the southern part of the Vale, and in the southwest corner of the Vale the base of the Old Red Sandstone is interpreted to overstep across the underlying Silurian se quence in an analogous manner to its regional overstep in west Wales. The Precambrian basement surface is not clearly represented on the sec tions but may coincide with the base of a reflective sequence observed in places along one of the reflection lines. Basement depth estimates from the reflection sections are compared with estimates derived from a new time term analysis of data from long seismic refraction lines. Basement depths remain uncertain to 1-2 km but appear to be greater un der the central part of the Vale than under the western part and along the south coastal zone. A linked system of Variscan forethrusts is tr aced to outcrop in the core of the Cowbridge anticline but none involv es major displacement The Ty'n-y-Nant-Moel Gilau fault system of the S outh Wales coalfield is shown to be a component of the linked fault sy stem and its current net extension is attributed to Mesozoic reactivat ion of a Variscan thrust involving only limited displacement. The indi vidual thrusts appear to connect to a basal thrust that may extend nor thwards into or under the South Wales coalfield. The amount of displac ement along the basal thrust cannot be determined reliably but it may exceed 10 km and involve large scale repetition of Palaeozoic sequence s under the Vale.