Constraining the Pattern of Deformation Associated with Low-Displacement Faults in the Cratonic Regions: an Example from the Precambrian Province of Kerala, South India

Surface exposure of a suspected active fault in an intraplate region, in the state of Kerala, south India revealed subtle expressions of brittle deformation in exhumed crystalline bedrock. The deformation zone consists of fracture sets with Small-Scale displacement and slip planes containing gouge; the propagation of displacement at depth being accommodated through fracture planes in the upper levels of the crust. It is spatially coincident with a linear hill scarp, and also the isoseismal elongation of a recent slight earthquake (M=4.3) in this zone. The fault rocks present within the fracture zone indicate cyclic episodes of frictional slips along the fault, which may have led to a progressive increase in the gouge thickness. Major part of the gouge zone along the fault shows alteration at the expense of feldspar and mica. The occurrence of repeated rupture may also imply that the fault zone re-strengthened between each rupture. The structural analysis suggests different generations of fault gouge and the last major episode of frictional sliding may have occurred close to the surface. Available data on the electron spin resonance (ESR) dating of the gouge indicates an age of 430±43 ka, probably representing the last major displacement on this fault Our structural interpretation of the fault rocks suggests that the suitably oriented discrete fault strands, within the Precambrian shear zone can be the source of moderate earthquakes (M≥6.0)bu t slip events are separated by long periods of quiescence. We also suggest possible ways of constraining the nature of activity and the pattern of deformation associated with low-displacement faults that occur within the Precambrian cratons.