A reappraisal of the Sibson-Scholz fault zone model: The nature of the frictional to viscous ("brittle-ductile") transition along a long-lived, crustal-scale fault, Outer Hebrides, Scotland

The widely cited Sibson-Scholz conceptual fault zone model suggests that se ismically active, upper crustal brittle faults pass downward across a predo minantly thermally controlled transition at 10-15 km depth into ductile she ar zones in which deformation occurs by aseimic viscous creep. The crustal- scale Outer Hebrides Fault Zone (OHFZ) in NW Scotland has been described as the type example of such a continental fault zone. It cuts Precambrian bas ement gneisses and is deeply exhumed, allowing direct study of the deformat ion products and processes that occur across a wide range of crustal depths . A number of fault rock assemblages are recognized to have formed during a long-lived displacement history lasting in excess of 1000 Myr. During Cale donian movements that are recognized along much of the 190 km onshore fault trace, brittle, cataclasite-bearing faults in the west of the OHFZ are une quivocally overprinted to the east by a younger fabric related to a network of ductile shear zones. Field observations and regional geochronological d ata demonstrate that there is no evidence for reheating of the fault zone d ue to thrust-related crustal thickening or shear heating. Microstructural o bservations show that the onset of viscous deformation was related to a maj or influx of hydrous fluids. This led to retrogression, with the widespread development of new fine-grained phyllosilicate-bearing-fault rocks ("phyll onites"), and the onset of fluid-assisted, grain size-sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phy llonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllo-nitization had previously occurred at an e arlier stage and that this process is possible over a wide temperature (dep th) range within crustal-scale faults. Our data provide an observational ba sis for recent theoretical and experimental studies which suggest that crus tal-scale faults containing interconnected networks of phyllosilicate-beari ng fault rocks will be characterized by long-term relative weakness and sha llow (similar to5 km) frictional-viscous transition zones. Similar processe s acting at depth may provide an explanation for the apparent weakness of p resently active structures such as the San Andreas Fault.