Seismological studies of upper-crustal structure in the vicinity of the Girvan-Ballantrae area, SW Scotland

[摘要] The geology of the Midland Valley and the Southern Uplands is outlined with special reference to the Ballantrae ophiolite complex which crops out close to the boundary between them. In this study, several seismic refraction profiles, with different azimuths, were recorded across the complex and the surrounding regions. These lines constitute a network, covering the south-western part of Scotland, for the investigation of the areal extension of the complex at depth.Laboratory velocity measurements were carried out, at confining pressures of up to 200 bars, upon samples of the Ballantrae complex and greywackes of the Northern Belt of the Southern Uplands. Serpen-tinite is found to have the lowest velocity in the Ballantrae complex (circa 4.0 km/sec) and gabbro has velocities as high as 6.3 km/sec at 200 bars. At the same pressure, the greywackes have an average velocity of 5.7 km/sec. These determinations were used to constrain the interpretation of near surface velocities in the study area. High-pressure velocity measurements (up to 5 kbar) on two greywacke samples suggest that some mafic greywackes could have velocities of >6.0 km/sec. The high velocities, previously interpreted as crystalline basement, could be a characteristic of the Northern Belt mafic greywackes.The velocity-depth structure of the Ballantrae complex has been inferred from measurements directly on the complex by applying time-term and wiechert-Herglotz-Bateman (WHB) analysis. The seismic structure below the complex is similar to that of "basement" incorporated in ray-tracing modelling on the wider net of profiles. This implies that the basement may be the same below the complex as around it. Basement is shallowest (1.7 - 2.0 km) under the Ballantrae complex and the Craighead Inlier, with P-wave velocity of 6.0 km/sec, which increases rapidly to 6.35 - 6.40 km/sec at a depth of 6 km. It extends from the Midland Valley underneath the Northern Belt of the Southern Uplands at about 1.8 - 2.3 km and deepens with an unknown dip under the Central and Southern Belts, giving way to thicker overlying Lower Palaeozoic sediments.No conclusive statement on the thickness or extent of the complex can be made. However, a study of recorded S-waves yielded a Poisson's Ratio of 0.31. Combining this result with two-dimensional gravity modelling across the area suggests that the gabbroic components of the Ballantrae complex constitute the basement in the area covering a triangle between Troon (N), Loch Doon (SE) and Portobello (SW). These gabbroic rocks are predicted to be partially hydrated on the basis of a P-wave volocity of only 6.35 - 6.40 km/sec at 6 km depth. The basement may be regionally extensive though the correlation of the seismic refraction line (Colmonell line) where the S-wave velocity is determined with an aeromagnetic anomaly might suggest its restriction to a strip parallel to the Southern Uplands Fault.Travel-tirne delays of about 0.2 sec are associated with the Southern Uplands, Kerse Loch and Stinchar Faults. The geological and geophysical possibilities that can generate such delays are reviewed. From reversed recording on one of the profiles, the delay is attributed to vertical zones of low velocity rocks in the fault zone, postulated to be either serpentinite or sheared, fractured rocks.

[发布机构] University:University of Glasgow;Department:School of Geographical and Earth Sciences

[关键词] QE Geology