FAULT-ZONE WEAKENING PROCESSES ALONG THE REACTIVATED OUTER HEBRIDES FAULT ZONE, SCOTLAND

The Outer Hebrides Fault Zone is a major reactivated structure cutting amphibolite-grade Lewisian basement gneisses in NW Scotland. During a regionally important phase of sinistral strike-slip movements, the in flux of chemically active hydrous fluids along the fault zone was asso ciated with the formation of a network of greenschist-facies phylionit ic shear zones. Later ESE-directed extensional strain was preferential ly focused into these pre-existing zones of weakness. The syn-tectonic alteration of a relatively strong, feldspar/hornblende-dominated load -bearing framework microstructure to an interconnected weak layer micr ostructure of fine-grained, strongly aligned phyllosilicate aggregates leads to the long-term weakening in the fault zone. Comparison with e xperimental data suggests that this produces a shallowing of the frict ional-viscous creep ('brittie-ductile') transition and a substantial r eduction in total crustal strength. Similar processes may account for the apparent weakness of many long-lived fault zones.