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| Journal of the Geological Society |  |
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Article |
Geology Department, Imperial college, London SW72BP, UK
The Lewisian rocks in the Scourie-Laxford region, NW Scotland, show variations in shear strains and associated structures on the southern (upper) margin of a zone of intense Laxfordian reworking. The Scourian block was thrust a large distance obliquely northwards over the northern Laxfordian gneisses as part of a Laxfordian episode of crustal thickening. This was followed by crustal extension, during which time the Scourian block moved down to the south. Granite sheets were intruded along the main shear zone in a dilational region at the base of a rotational extensional block.
The Laxfordian contractional and extensional deformation occurred under amphibolite facies conditions and analogies can be made with middle crustal structures inferred from deep seismic data. For example, structures produced during late Laxfordian crustal spreading can be compared to those seen on the BIRPS offshore Scottish data, which formed during Devonian basin development following Caledonian crustal thickening.
The Scourie–Laxford structures give an idea of fault kinematics at the brittle-ductile transition in the middle to lower crust. Steep faults pass down into more gently dipping ductile shears and then to a zone of anastomosing shears which accommodate the strains and rotations at the base of the rigid upper crust. The shears may concentrate along a more planar zone of deformation, forming a detachment or decoupling zone, below which the extension may be variable, depending on the mode of lithospheric stretching.
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