The lower crust and the extension of the continental lithosphere: Kinematic analysis of Birps Deep Seismic Data

Abstract

Deep seismic reflection profiles, recorded in the seas around the United Kingdom, image bands of mantle reflections, interpreted as shear zones, that are not aligned with the basin‐bounding faults in the upper crust. Instead, the lower crust, represented as a zone of sub‐horizontal reflectivity, appears to decouple deformation in the upper crust from that in the mantle. Simple modeling shows that the bulk deformation of the lithosphere, in terms of pure or simple shear, is controlled by the spatial distribution of upper crustal and mantle faults, and that deformation is transferred between the two by sub‐horizontal shear within the lower crust. The simplest form this lower crustal shear can take is a gently inclined zone of simple shear, both transferring deformation between the upper crust and the mantle, and extending the lower crust. This model predicts many of the features of the DRUM profile north of Scotland, including a distortion of the Moho in the vicinity of the Outer Isles Fault that has previously been interpreted as evidence for a Moho offset. In general, the predicted shear strain within the lower crust closely matches its observed reflectivity, so it seems likely that the latter can be related, at least in part, to shearing between the upper crust and the mantle. The model also show that the relative position of upper crustal faults and mantle faults can change during progressive deformation, causing the character of large‐scale deformation of the lithosphere to change with time. This result could have implications for the subsidence behavior of sedimentary basins during progressive extension.