THE POLARITY OF DEEP SEISMIC REFLECTIONS FROM THE LITHOSPHERIC MANTLE - EVIDENCE FOR A RELICT SUBDUCTION ZONE

BIRPS offshore deep seismic reflection profiles to the north of Scotland have revealed two bright continuous reflectors in the continental lithospheric upper mantle, the dipping Flannan and sub-horizontal W reflectors. The polarity of reflections from the Flannan has been determined from normal-incidence reflection data, using the far-field source wavelet as a starting model. The effective far-field wavelet was calculated by adding a receiver ghost and the effects of recording filters, attenuation during transmission and sea bottom multiples to the source wavelet. Reflection polarity was determined in a blind test by comparing the modelled wavelet with stacks of the Flannan reflection. In eight out of ten stacks, the reflection was picked as positive polarity, two out of ten were unknown. To verify this result, the test was repeated for Moho reflections; in this case nine out of ten stacks were picked as positive. The modelling also demonstrated that the Flannan reflection is apparently from a simple interface; complex interlayering is not required to explain the waveforms, and they are not consistent with reflections from a single thin layer. Seismic data acquired at wide-angle across the W mantle reflector show sub-crustal high-amplitude arrivals, which can only be explained as post-critical reflections. Only a high-velocity eclogitic layer, contained within normal mantle, with a sharp upper boundary and a diffuse base can explain all our observations. We suggest that the Flannan reflector represents the top of a relict oceanic and eclogitic component of a pre-Caledonian subduction zone within the lithospheric mantle.