Transcurrent faults and associated cataclasis in Shetland

Abstract

A series of transcurrent faults with northerly trend is well exposed in the cliffs of Shetland. The Walls Boundary Fault which cuts through the middle of Shetland has a continuously changing trend so that it forms a very flattened S. Dextral movement along the fault seems to have created further dextral transcurrent faults, including the Nesting Fault, across the eastern concavity in the trend. These main faults lie within broad zones of cataclasis, subsidiary faulting and local folding. The offsets on the subsidiary faults are very much less than on the main faults, and the crushed rocks have isotropic fabrics. The zones probably arose during faulting from varying local stresses caused by the interaction of the unevennesses on the two sides of the non-planar main faults. Gouge in the Walls Boundary Fault contains analcite. Gouge-like laumontite in subsidiary shears was probably formed by mechanochemical reactions. The occurrence of these minerals and of blastomylonitic scapolite veins near the Walls Boundary Fault may indicate up to three phases of movement taking place between Cretaceous and Devonian times under different depths of overburden. Slices of secondarily cataclastic mylonite occurring along the Walls Boundary Fault are probably relics of the Great Glen Fault now partially cut out by the nearly coincident Walls Boundary Fault.