Repeated cataclasis in a reactivated fault zone -- an example from Bengal Basin Margin Fault, Jharkhand, India

Abdul Matin, and Surajit Misra

Fault zones with a complex history may show reactivation in successive faulting events. Successive generations of grain-scale microcracks in reactivated fault zone produce multiphase cataclastic rocks with characteristic microcracks. The optical and cathodoluminescence microscopic study can be fruitfully utilized to analyze the complex deformation history of brittle fault zone rocks developed in response to successive brittle deformation. Within the reactivated Cretaceous Sainthia-Brahmani Fault, cutting the Precambrian Chhotonagpur gneiss and Jurassic sandstone along the western margin of Bengal Basin, three generations of cataclasis can be recognized. The first generation (Ct1) cataclastic rock is infrequently preserved as clasts within the second phase (Ct2) cataclastic rocks. The last phase cataclasis (Ct3) affected both Ct1 and Ct2 cataclastic rocks in a discrete manner producing inhomogeneous network of thin sintered micro-zones of gouge. The cataclastic rocks are formed by progressive cataclasis of granitoid rocks with several pulses of fluid-induced brittle deformation during a history of frictional flow. The repeated cataclasis attest to a prolonged history of repeated failure and reactivation of fault movement in elastico-frictional regime operated in pulses with repeated embrittlement and mechanical failuret. This observation led to the idea of repeated reactivation of the fault, related to the extensional tectonics during early Cretaceous period linked to India’s passage over the Kerguelen and Crozet Hot-Spots presently located in the Indian Ocean. Fracturing and comminution are dominant deformation mechanisms in the formation of cataclastic rocks, and subordinate but evident dissolution and recrystallization of quartz, and mild plastic deformation of quartz, feldspar and biotite are also seen. Dissolution and recrystallization processes during cataclasis, and absence of pseudotachylyte in the fault rocks provides an example of aseismic shear displacement within the brittle shear zone.