Jh. Mcbride, STRUCTURE OF A CONTINENTAL STRIKE-SLIP-FAULT FROM DEEP SEISMIC-REFLECTION - WALLS BOUNDARY FAULT, NORTHERN BRITISH CALEDONIDES, J GEO R-SOL, 99(B12), 1994, pp. 23985-24005
Reprocessing of four offshore deep reflection profiles across the Wall
s Boundary strike-slip fault, postulated as the northward continuation
of the Great Glen fault over the Shetland platform, provides useful c
onstraints on the lithospheric structure of a major continental transf
orm and some insight into the manner in which strike-slip faults defor
m. The reprocessing was aimed at delineating complex structure and red
ucing noise contamination and consisted of analyses of diffractions an
d amplitude variations and experimentation with seismic migration. The
results demonstrate a highly variable crustal and uppermost mantle st
ructure for the 'Walls Boundary fault along more than 150 km of its le
ngth. Six key observations can be made from this study: (1) as middle
and lower crustal reflections approach the fault, they are truncated b
y, 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) diffract
ions typically are concentrated at the level of the Moho discontinuity
directly beneath the fault; (4) after seismic migration, steeply dipp
ing reflectors are imaged in places directly beneath the fault in the
lower crust; (5) in one location, the Moho reflection is deflected acr
oss the fault into a narrow synform or ''keel'' structure which is exa
ctly bisected by the downward projection of the fault; and (6) in a se
parate location, a prominent upper mantle reflector is truncated acros
s the fault. A vertical or steeply dipping structure is deduced for th
e 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 reflector
s directly beneath the fault that may have been related to a localized
component of transpression and resultant crustal thickening. The pres
ence of diffractions at the level of the Moho discontinuity where the
downward projection of the fault intersects it suggests points of stru
ctural ''roughness'' that may be related to deformation of the Moho by
late motion along the fault. Integrating the interpretation of the re
flection data with previous geological studies implies that the age of
the Moho deflection is post Early Cretaceous but that the dipping str
ucture adjacent to and cut off by the fault is probably Caledonian (Si
lurian-Early Devonian). Although the disparate structure of the crust
on opposite sides of the fault supports geologic interpretations of la
rge-scale displacement, a major step in the Moho discontinuity directl
y beneath the Walls Boundary fault cannot be substantiated from the se
ismic data.