Seismic studies on the Derbyshire dome

Abstract

The Derbyshire Dome is thought to have been a stable uplifted area since at least Lower Carboniferous times. This project is principally concerned with four 30km seismic refraction lines which crossed the limestone outcrop of Derbyshire and N. Staffordshire in order to investigate the Dome's upper crustal structure, using quarry blasts as seismic sources. A time-term analysis of refracted arrival data defined basement structure more complicated than implied by the surface geology. The interpretation of these data was complicated by high (5.6-5.8km/s) velocity refractions from dolomitic horizons within the limestone sequence; the mean overburden velocity was determined to be about 5.2 km/s. The Dome could be divided into two pre-Carboniferous geological units separated approximately by the line of the NNW trending Bonsall Fault. To the north a broadly domal refractor of velocity 5.5-5.55km/s was mapped, and thought to correlate with both the shallow pre-Carboniferous volcanics encountered by the Woo Dale borehole and"the Ordovician shales encountered by the Eyam borehole below 1.8km of limestone. This refractor accordingly deepens beneath the Carboniferous sedimentary basins flanking the Dome. To the south of the Bonsall Fault zone, the Carboniferous was found to be underlain by a refractor of velocity 5.63-5.7km/s, thought to be of Precambrian material similar to the rocks of Charnwood Forest, Leicestershire, some 40km south. By analysing later arrivals, this refractor has been mapped to the north of the Bonsall Fault at a depth of 2.5-3.5km. The shallower Lower Palaeozoic refractor is thought to be no more than 500m thick, and underlain by lower velocity, possibly Cambrian, material. This interpretation is consistent with the Bouguer anomaly map of the region, and sheds light on the structural control of Carboniferous sedimentation. The basement fault dividing the two pre-Carboniferous units is thought to have been active during the Dinantian as the northern unit tilted eastwards