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.

The crustal structure of the Dead Sea Transform

“…Reflectors in the lowermost crust (25–32 km depth from profile km 55 to 70 in Fig. 11b) that dip away from the suspected fault zone and that are most pronounced east of the AF, might correspond to anisotropic fabrics developing along mylonitic shear zones, similar to the dipping reflectors beneath the Walls Boundary strike‐slip fault in the northern British Caledonides (McBride 1994).…”

“…This is another piece of evidence for the AF cutting through the whole crust. Whereas a bending down of the Moho, or a ‘Moho keel’ has been put forward for some Paleozoic strike‐slip regions in transpressional tectonic regimes (McBride 1994; Stern & McBride 1998), this coupled upward‐downward structure of the Moho might be due to the transtensional character of the DST between the Red Sea and the Dead Sea (see also later discussion). Reflectors in the lowermost crust (25–32 km depth from profile km 55 to 70 in Fig. 11b) that dip away from the suspected fault zone and that are most pronounced east of the AF, might correspond to anisotropic fabrics developing along mylonitic shear zones, similar to the dipping reflectors beneath the Walls Boundary strike‐slip fault in the northern British Caledonides (McBride 1994).…”

“…This is another piece of evidence for the AF cutting through the whole crust. Whereas a bending down of the Moho, or a ‘Moho keel’ has been put forward for some Paleozoic strike‐slip regions in transpressional tectonic regimes (McBride 1994; Stern & McBride 1998), this coupled upward‐downward structure of the Moho might be due to the transtensional character of the DST between the Red Sea and the Dead Sea (see also later discussion).…”

Contrasting seismic characteristics of three major faults in northwestern Canada

“…Strong variations observed in coincident near-surface, tomographic velocity models suggest that alternating blocks of amphibolite to granulite metamorphic-grade rocks and Tertiary volcanic rocks continue from outcrops to at least several kilometre depths, defining a classic "flower" structure over a zone 30 km wide within the upper crust. The disruption of deeper reflections suggests that the fault zone narrows within the mid-crust, then flares to a more than 60-km width near the Moho, but with no direct evidence of inverse flower structure, as observed on some Paleozoic strike-slip faults (McBride 1994;Stern and McBride 1998). Nor is the Tintina fault's seismic reflectivity character like that of well-known active strike-slip faults, such as the 40°inclined ramp geometry of the Alpine fault of New Zealand (Stern et al 1997) or the mid-crustal detachment of the San Andreas fault (Brocher et al 1994;Stern and McBride 1998).…”

A major step in the continental Moho and its geodynamic consequences: the Taranaki-Ruapehu line, New Zealand

“…1), was interpreted as an isostatically compensated step in the Moho of ∼10 km (Stern et al 1987). Steps in the continental Moho are rare, imply high deviatoric stresses and require a special explanation (McBride 1994). Where they have been documented they are often linked to strike slip zones (Doll et al 1996; Z hu & Kanamori 2000; Wilson et al 2004).…”