Origin and nature of the regional detachment fault in the area of the deep Galicia margin

In the area of the Deep Galicia Margin two multi-channel seismic lines (ISE-1 and ISE-2) have been processed and interpreted in the present study. The main objective was to attempt clarification of the precise nature and origin of the so-called S-reflector. For this purpose a full waveform inversion was carried out for a selected CMP gather besides prestack depth migration and poststack time migration. The results of the full waveform inversion lead to the conclusion that the S-reflector represents an approx. 50m thick low-velocity zone wedged in a stepwise velocity increase between continental crust and mantle. For this zone a compressional wave velocity of 5.2 km/s was determined as compared to 5.45 km/s for the overlying continental crust and 7.0 km/s for the mantle interval below. These p-wave velocities in the uppermost mantle are interpreted to result from a particular serpentinisation history involving two separate stages of seawater influx leading to the development of a detachment fault seismically imaged as the S-reflector.