Evidence for shear zones in the lower crust offshore Britain
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Colleges, School and Institutes
- Cornell University
Lower crustal reflections seen on British Institutions Reflecting 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. 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 lithosphere strength profiles with slip-line field theory and by drawing analogies with structures developed in high tectonised high-grade metamorphic terrains. The observation on BIRPS data of mantle reflections, interpreted as mantle shear zones, implies that deformation in the mantle is localised. However, the mantle 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.
|Number of pages||17|
|Publication status||Published - 1 Dec 1988|