Variscan structure of the inner Bristol Channel, UK

Abstract

Major Variscan structures under the Bristol Channel are concealed beneath a Mesozoic and Quaternary cover. Mapping of reflectors on offshore seismic sections reveals two major WNW–ESE-striking thrusts within the underlying Palaeozoic sequence. The more southerly, with a moderate SSW dip, has previously been identified as the Bristol Channel Thrust. Another thrust, the previously undescribed Gravel Margin Thrust, has been identified in the eastern part of the inner Bristol Channel and lies in the footwall of the Bristol Channel Thrust. It has a similar strike but steeper dip and extends to anticipated Precambrian basement depths. The Bristol Channel and Gravel Margin Thrusts together juxtaposed the contrasting Palaeozoic successions of South Wales and SW England before Mesozoic reactivation. Reflection packages identified on the offshore seismic sections are tentatively correlated with specific onshore Palaeozoic sequences. Interpretation of the offshore seismic sections in conjunction with geological mapping along the north Devon coast shows that the Bristol Channel and Gravel Margin Thrusts have orientations and geometries typical of several mesoscale thrusts observed and measured at Foreland Point, north Devon. Along-strike structural changes beneath the inner Bristol Channel seen on seismic sections provide evidence, additional to earlier refraction and structural studies, for the offshore continuation of the NW–SE-trending Watchet–Cothelstone–Hatch Fault and its linkage with the previously described Central Bristol Channel Fault Zone. The Bristol Channel and Gravel Margin Thrusts are interpreted to be offset from the Cannington Park Thrust in north Somerset by a 14–16 km Variscan dextral strike-slip displacement across the Watchet–Cothelstone–Hatch Fault. The Bristol Channel Gravel Margin Thrust and the Cannington Park Thrust are interpreted as syngenetic structures linked by their lateral ramp, the Watchet–Cothelstone–Hatch Fault. Using all the above information, the overall structure of the Variscides of the inner Bristol Channel is re-evaluated and a new tectonic model is presented. The structures discussed in this paper, at a scale portrayed by commercial seismic data and published geological maps, may represent merely the tips of linked thrusts that have previously been mapped down to mid-crustal depths on BIRPS seismic sections from the South West Approaches Traverse. We show that enough data now exist to resolve outstanding problems of the concealed Variscan structure in the inner Bristol Channel, providing local structural detail that can be incorporated into the wider tectonic framework of NW Europe.