Evidence for shear zones in the lower crust offshore Britain

Abstract

Lower crustal reflections seen on British Institutions Reflection Profiling Syndicate (BIRPS) deep seismic reflection data, recorded offshore the United Kingdom, are commonly short, convex‐up, and crosscutting. Consequently, the lower crust should not be described as layered, as much of the reflecting structure is not fully resolved and may be quite complex. Moreover, the lower crustal reflective zone (LCRZ) is not a uniformly reflective layer, but contains reflective bands that wrap around nonreflective (“transparent”) zones. Because transparent zones probably correspond to a lack of a reflecting fabric, understanding why these features are unreflective may help constrain the cause of the reflectivity of the lower crust. Although transparent zones may have a variety of explanations (e.g., large shear pods or frozen magma bodies), the geometry of reflective bands and transparent zones is perhaps best explained by the geometry of shear zones and low‐strain lozenges predicted both by combining theoretical lithospheric strength profiles with slip‐line field theory and by drawing analogies with structures developed in highly tectonised high‐grade metamorphic terrains. This interpretation implies that lower crustal reflections may be from shear zones and also has implications for the way the lower crust deforms. In particular, extension may be accommodated by the relative movement of low‐strain lozenges along intervening zones of simple shear. The lower crust may be stretched during extension, but it must also transfer deformation between the upper crust and the mantle. The observation on BIRPS data of mantle reflections, interpreted as mantle shear zones, implies that deformation in the mantle is localised. However, the manue reflections are never colinear with faults in the upper crust, requiring, at least locally, a component of sub‐horizontal bulk strain within the lower crust, which may greatly increase the amount of shearing therein and hence perhaps may increase the reflectivity of the LCRZ.