title: Geophysical constraints on upper crustal structure in the Midland Valley of Scotland
Creators: name: Dentith, Michael C.
Publisher: name: University of Glasgow
Year: 1987

description: The Midland Valley Investigation by Seismology (MAVIS) consists of three refraction lines of c. 80 km length across the Carboniferous basins of the Midland Valley of Scotland, Two lines trend approximately east-west (MAVIS I north and south), the latter crossing a major gravity and magnetic anomaly near Bathgate, The third line (MAVIS II) trends north-south, crossing the MAVIS I lines and Bathgate anomaly and extends into Lower Old Red Sandstone outcrop to the north and south of the OchiI and Wilsontown Faults respectively, These data are supplemented by lines recorded using quarry-blast sources, Two profiles trend east-west across the Bathgate anomaly (Sola north and south), A third profile (MAVIS III) trends north-south across the Lothian oil-shaIe fieIds. Three refractors are recognised defining four crustal layers, The first layer has velocities between 3.0 and 5.0 km/s and extends to depths between 0.5 and 3.0 km, This layer is interpreted as the Carboniferous and Upper Old Red Sandstone, The second layer has velocities between 5.3 and 5.9 km/s and occurs between depths of 0.5 to 3.0 km and 4.0 to 6.0 km, This layer is interpreted as the Lower Old Red Sandstone and Lower Palaeozoic, Therefore, the topmost refractor is interpreted as the unconformity between the Upper and Lower Old Red Sandstone mapped at the surface in the Midland Valley, The third layer occurs at between 4.0 to 6.0 and 7.0 and 8.5 km depth with velocities between 6.0 and 6.1 km/s, This layer is interpreted as crystalline basement. The deepest layer occurs at depths greater than 7.0 to 8.5 km and is interpreted as a higher velocity crystalline basement, The interface between the two basement layers may mark the transition from amphibolite to granulite facies metamorphism. Velocity data from layer 1 show Poisson's ratio for this layer to be 0.29 ± 0.06, and the ratio of horizontal to vertical P-wave velocity to be 1.15 +0.12 -0.15. Both values are consistent with the sands tone/Iimestone/shaIe sequence mapped at the surface. The Bathgate gravity anomaly was modelled within the constraints imposed by the seismic data. The results of earlier magnetic modelIing were confirmed, with deep and shallow end members of a series of possible solutions being modelled. The shallow model satisfies the anomaly with a thickened sequence of Carboniferous lavas within the seismic layer 1. The deep model is for a gabbroic intrusion within the crystalline basement extending from 10 to 15 km depth. Gravity modelIing across the OchiI Fault show this fracture to dip to the south. ReIief on the Old Red Sandstone unconformity is found to mirror structures mapped at the surface, whilst the underlying basement is virtually planar. A detachment is postulated between these horizons with the surface structures forming by thin-skinned tectonic processes. The detachment horizon is probably either within the Lower Palaeozoic sequence of seismic layer 2, or at the ductiIity contrast to be anticipated at the interface between Lower Palaeozoic and crystalline rocks. A detailed structural interpretation of the Lothian oil-shale fields suggests there may be several levels of such detachment with the Midland Valley. A model is presented for the development of the Ochil fault as an en echelon fracture resulting from reactivation of a basement lineament.