P-wave anisotropy in the lower lithosphere

Abstract

Long-range controlled source seismic experiments yield detailed information about the velocity depth structure of the lower lithosphere. Between Ireland and Northern Britain three such profiles sample almost the same portion of the subcrustal lithosphere. Three zones with steep velocity gradients have been detected between 30 km and 90 km depth. Both the pattern and velocity of the arrivals are incompatible with isotropy. Preferential alignment of olivine crystals, with an azimuth ca. N25°E for the fast axis, could explain the observations, with the more highly aligned zones occurring in bands, or layers, separated vertically by zones in which the degree of alignment is slight or absent. We suggest that a shear heating mechanism may have played a part in producing these patterns. This deformation is most likely to be of Mesozoic or early Cenozoic age. It is argued that the upper mantle is not necessarily a strain marker for the last major orogenic episode, as recent findings have suggested [1], since it may undergo deformation which decouples from the brittle upper crust and hence is not “transmitted” to the Earth's surface.