Introduction
Cataclastic rocks are commonly formed in the shallower part of the brittle continental crust, above the elastico-frictional to quasiplastic (EF-QP) transition (Sibson, 1977; Snoke et al., 1998). The rocks in the continental crust at shallow crustal depth deform by brittle fracturing and cataclastic flow due to increase of shear strength with confining pressure in the cataclastic domain of deformation by the operation of Coulomb frictional law (Byerlee, 1978). Fault gouges and cataclastic rocks are common products of rock fracturing in the cataclastic domain (e.g. Sibson, 1977; Passchier and Trouw, 2005). Cataclasis is the deformation mechanism to produce cataclastic rocks in fault zone (the term cataclasis is used here following Rutter, 1985 - “crystal structure remains undistorted, but grains or groups of grains become cracked and the fragments may exhibit frictional sliding with respect to one another” and “the process necessarily involves dilatancy and is therefore pressure sensitive”).
Large scale brittle faults at shallow crustal depths are characterized by localized intense deformation in thin zones (Sibson, 1977). Field exposures, hand samples and thin sections show intensely deformed silicified cataclastic rocks are developed in the Bengal Basin margin Sainthia-Brahmani Fault (SBF) in West Bengal and Jharkhand state (Fig. 1). Fault rocks are very well exposed along the SBF for hundreds of meters with thickness reaching up to several meters. We present here an example of repeated cataclasis and meso- and micro-structures of brittle fault rocks. The deformation within a “fault zone” (Sibson, 1977) with prolonged tectonic history involving successive episodes of microfracturing due to fault reactivation during progressive deformation in an elastico-frictional (EF) regime is also discussed. Our observation shows that comminution, dissolution-recrystallization and mild plastic deformation were involved in the generation of cataclastic rocks.
Ductile structures developed in quasi-plastic deformation regime have been extensively investigated and effectively used universally to understand the deformation history and deformation kinematics, however, the use of cataclastic structures in understanding those points is not commonly attempted.
Figure 1. Bengal Basin with Basin Margin fault
(A). Bengal Basin with Basin Margin fault along its western boundary. (B). Disposition of faults in the Bengal Basin including the basin margin Sainthia-Brahmani Fault and study area (marked with box) (after Dasgupta and Mukherjee, 2006, and Dasgupta et al. 2000)