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Department of Geology, University of Glasgow, Glasgow, G12 8QQ
Normal incidence seismic reflection profiles often show strong reflectivity—indicating sub-horizontal layering— in the deep continental crust, below poorly reflective upper crust. An example from the WINCH profile across the Caledonides of NW Britain shows necking of the lower crustal reflective layer near Islay. Combining the seismic data with gravity modelling constrains the lower crustal density to be about 3.1 Mg m–3 and the P-wave velocity to be about 7.3 km s–1. This layer can be correlated with the deepest crustal layer of the parallel LISPB refraction profile where the layer has a similar P-wave velocity, Poisson’s ratio of 0.25 and electrical conductivity of more than 0.003 S m–1. These data suggest that a) the lower crust is heterogeneous, b) its mean composition is like that of very basic igneous rocks, and c) hydrated minerals and possibly free water are significant components. Aqueous fluids are shown to permeate deep crustal rocks even more effectively than they do sedimentary rocks and a mechanism of water capture in the lower crust is presented. Layering may result from interleaving of mantle and crust, fabric re-orientation due to shearing or, most likely, by intrusive underplating. Reflectivity may be enhanced by layer-variable water content.
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