Structure of a continental strike‐slip fault from deep seismic reflection: Walls Boundary fault, northern British Caledonides

Abstract

Reprocessing of four offshore deep reflection profiles across the Walls Boundary strike‐slip fault, postulated as the northward continuation of the Great Glen fault over the Shetland platform, provides useful constraints on the lithospheric structure of a major continental transform and some insight into the manner in which strike‐slip faults deform. The reprocessing was aimed at delineating complex structure and reducing noise contamination and consisted of analyses of diffractions and amplitude variations and experimentation with seismic migration. The results demonstrate a highly variable crustal and uppermost mantle structure for the Walls Boundary fault along more than 150 km of its length. Six key observations can be made from this study: (1) as middle and lower crustal reflections approach the fault, they are truncated by, and do not continue across, the downward projection of the fault; (2) trends in reflection structure of the crust and/or uppermost mantle differ markedly from one side of the fault to the other; (3) diffractions typically are concentrated at the level of the Moho discontinuity directly beneath the fault; (4) after seismic migration, steeply dipping reflectors are imaged in places directly beneath the fault in the lower crust; (5) in one location, the Moho reflection is deflected across the fault into a narrow synform or “keel” structure which is exactly bisected by the downward projection of the fault; and (6) in a separate location, a prominent upper mantle reflector is truncated across the fault. A vertical or steeply dipping structure is deduced for the fault in much of the crust and perhaps parts of the uppermost mantle; however, more diverse reflecting structures are interpreted for the lower crust, such as the Moho deflection and steeply dipping reflectors directly beneath the fault that may have been related to a localized component of transpression and resultant crustal thickening. The presence of diffractions at the level of the Moho discontinuity where the downward projection of the fault intersects it suggests points of structural “roughness” that may be related to deformation of the Moho by late motion along the fault. Integrating the interpretation of the reflection data with previous geological studies implies that the age of the Moho deflection is post Early Cretaceous but that the dipping structure adjacent to and cut off by the fault is probably Caledonian (Silurian‐Early Devonian). Although the disparate structure of the crust on opposite sides of the fault supports geologic interpretations of large‐scale displacement, a major step in the Moho discontinuity directly beneath the Walls Boundary fault cannot be substantiated from the seismic data.