Physical properties of Lewisian rocks: implications for deep crustal structure

J. Hall
Geological Society, London, Special Publications, 27, 185-192, 1 January 1987, https://doi.org/10.1144/GSL.SP.1987.027.01.16

Summary

Data on the densities, seismic velocities, magnetization, and permeability of Lewisian rocks are summarized. These properties have been used to estimate the shallow-depth extension of exposed boundaries within the Lewisian, and between the Lewisian and other rocks; and also to predict downward changes in porosity and electrical conductivity. Geophysical surveys of the deep crust can be interpreted in terms of subsurface distributions of Lewisian, or Lewisian-like, rocks.

Among the more significant conclusions relevant to the interpretation of surface structure of the Lewisian is that the Kylesku Group rocks of the central belt in Sutherland may be isolated from deeper crustal rocks of similar composition which are characterized by P-wave velocities of 6.4 km s−1 and over. Such rocks may form the mid crust below NW Scotland, shallowing north of the mainland possibly with a sharp discontinuity upwards into more acid basement rocks. West of the mainland direct evidence of the 6.4 km s−1 rocks is missing; they appear to be absent from the upper crust and buried rather deeply, perhaps with a transitional top, in the middle or lower crust.

Granulite-facies rocks occurring in the upper crust west of the mainland are likely to be more acid than average Kylesku Group rocks. The layered basic rocks of the central belt provide an analogue of the deepest crust in NW Britain.

Permeability/porosity studies allow estimates of electrical conductivity to be made in an attempt to explain the high conductivity of the lower crust in terms of trapped saline fluids. Porosities in the deep crust may be high enough to account for deep crustal seismic reflections in areas where porous layers are sub-horizontal.

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