THE LITHOSPHERE BELOW THE ROCKALL TROUGH - WIDE-ANGLE SEISMIC EVIDENCE FOR EXTENSIVE SERPENTINISATION

The longer-range data from wide-angle seismic reflection experiments a long an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the Rockall Trough in the N orth Atlantic. Two subcrustal P- and S-wave reflections are observed. The first defines a layer 3-10 km thick below the Moho where P-wave ve locities vary from 7.5 to 7.8 km/s. V-p/V-s ratios increase within thi s layer from 1.80 in the north to 1.83 in the south of the basin along the axial profile. The second reflection from approximately 34 km dep th identifies a layer 15-20 km thick, with a V-p velocity of approxima tely 8.1 km/s and a V-p/V-s ratio of 1.73. These values are typical fo r normal mantle peridotites. Both P- and S-wave velocities and V-p/V-s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below th e Moho. About 15% volume alteration of the parent mantle peridotite is required to produce the observed seismic properties. This degree of a lteration accounts for a systematic deficit in total tectonic subsiden ce when compared with that predicted from the variation in bulk crusta l stretching along the axis of the basin. Syntectonic cooling occurred during differential Iithospheric stretching, as the upper to mid-crus t became more extended over a narrower region than the mantle lithosph ere. This served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle/ductile tr ansition zone in the crust migrated downwards into the mantle lithosph ere. The resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle.