Azimuthal seismic anthropology in a zone exhumed continental mantle, west Iberia margin

Abstract

P-wave azimuthal seismic anisotropy of the uppermost mantle has been shown to indicate directions of extension, both recent and fossil, in oceanic and continental settings. We have determined the P-wave anisotropy of a zone of exhumed continental mantle beneath the southern Iberia Abyssal Plain, as a possible indication of the initial direction of separation of Iberia from North America in a region where there are no magnetic lineations or fracture zone lineations adjacent to the margin with which to constrain this direction. Over 10 000 airgun shots were recorded by 11 ocean-bottom seismometers/hydrophones from which 24 900 arrivals that had traversed unaltered/weakly serpentinized upper mantle were picked. Complete 360° azimuthal coverage was attained between the shots and receivers for ranges up to 55 km. P-wave raypaths through the zone of exhumed continental mantle were ray-traced through a velocity model that incorporated the 3-D relief of the seabed and acoustic basement. Time residuals with respect to this model were fitted using an azimuthally dependent function. Small anisotropy (≲ 7 per cent) with a fast direction of 143° was determined to exist between 3.1 and 6.7 km below the top of the basement. Although the fast direction suggests that the post-breakup mantle stretching between Iberia and North America was northwest-southeast, this hypothesis is hard to reconcile with other observations. Instead, we conclude that motion along concave-downward faults during the exhumation of the upper mantle could have been sufficient to change an initially roughly east-west margin-normal fast direction to the 143° azimuth observed today. The degree of serpentinization was estimated to decrease from ~20 to 0 per cent between 3 and 7 km depth into the basement in the zone of exhumed continental mantle. The degree of upper-mantle anisotropy is similar to that observed in other oceanic and continental settings where similar degrees of serpentinization are inferred from mean P-wave velocities.