Determination of the seismic parameters and upper crustal structure of the western Midland Valley, Scotland

[摘要] This project was designed to investigate the western part of the Midland Valley of Scotland. A number of seismic profiles had been undertaken to investigate the centre, eastern and southern Midland Valley. The main feature of the western Midland Valley is the wide presence of the Carboniferous Clyde Plateau Lavas and the presence of major NE-SW trending faults. The main profile was chosen to cross such features: the Dusk Water Fault which played a big role in the deposition of Carboniferous rocks and the distribution of the Carboniferous igneous activity, Paisley Ruck, and Gartness Fault. This project consists of 2 seismic refraction lines. Three component seismic data were recorded from quarry blast sources. A short line was intended to study near vertical reflections using three component recording. The line is about 15 km long and was recorded in Ayrshire eastward from Hillhouse quarry. The main line is about 70 km long and extends NNE-SSW in the west of the Midland Valley, crossing major faults which played key roles in the deposition and distribution of Carboniferous rocks. The line crosses mostly Carboniferous sediments and the Clyde Plateau Lavas, crossing the Upper and Lower Old Red Sandstone to the north of the Clyde. It passes through Hillhouse, Loanhead and Sheephill quarries. Frequency and polarization filters were used to improve the data quality. Four layers are recognised in this study for the upper crust from their P-wave velocities. The first layer has a P-wave velocity of 3.5-5.0 km/s and occurs at depths of 0-2 km. This layer is considered as Carboniferous and Upper Old Red Sandstone sediments. The second layer has velocity of 5.0-5.6 km/s and occurs at 1-4 and 0-3 km depth and it shows considerable lateral velocity variation related to the Paisley Ruck. It is interpreted as Lower Old Red Sandstone. This layer is thinned in the area between the Dusk Water Fault and Paisley Ruck and to the east near Troon. The third layer has a P-wave velocity of 5.8 km/s and is restricted to between the Dusk Water Fault and Paisley Ruck. This layer lies at 2.5-3.5 km depth and may represent a Lower ORS volcanic ridge. The fourth layer has a velocity of 5.99-6.08 km/s and its top is at depth 3-4 km. This layer is interpreted as upper crystalline basement. Some faults seen at surface affected the basement: the Dusk Water Fault, Paisley Ruck and Gartness Fault which have a Caledonian NE-SW trend. This suggests that these faults, which controlled Carboniferous basins, may be older fractures in the basement reactivated at that time. The lateral velocity variation of the Lower ORS suggest that the Paisley Ruck represents a strike-slip fault. Previous studies considered that such faults in the Midland Valley sole out with depth above or at top crystalline basement. Deep wide-angle reflections were observed from depths of 8, 12, 15 and 20 km, and are interpreted as being from the boundaries of, and within, a mid-crustal layer.

[发布机构] University:University of Glasgow

[关键词] QE Geology