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 Scotla nd have revealed two bright continuous reflectors in the continental l ithospheric upper mantle, the dipping Flannan and sub-horizontal W ref lectors. The polarity of reflections from the Flannan has been determi ned from normal-incidence reflection data, using the far-field source wavelet as a starting model. The effective far-field wavelet was calcu lated by adding a receiver ghost and the effects of recording filters, attenuation during transmission and sea bottom multiples to the sourc e wavelet. Reflection polarity was determined in a blind test by compa ring the modelled wavelet with stacks of the Flannan reflection. In ei ght out of ten stacks, the reflection was picked as positive polarity, two out of ten were unknown. To verify this result, the test was repe ated for Moho reflections; in this case nine out of ten stacks were pi cked as positive. The modelling also demonstrated that the Flannan ref lection is apparently from a simple interface; complex interlayering i s not required to explain the waveforms, and they are not consistent w ith reflections from a single thin layer. Seismic data acquired at wid e-angle across the W mantle reflector show sub-crustal high-amplitude arrivals, which can only be explained as post-critical reflections. On ly a high-velocity eclogitic layer, contained within normal mantle, wi th a sharp upper boundary and a diffuse base can explain all our obser vations. 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.