Discussion and conclusion

Fault zones with complex and prolonged deformation histories may be reactivated during successive deformation episodes, because they represent distinct weaker inhomogeneities in the country rock of undeformed material. In the reactivated zones, superposition of multiple cataclasis events occurs in an irregular way. Detailed microstructures of the finite product of cataclasis provide an important tool for identifying the superposition of successive events of deformation with the help of morphology and consistent cross-cutting relationships of different generations of microcracks and cataclasis (Ismat and Mitra, 2001).

Pseudotachylite has not been observed in this fault rock. There are no unambiguous criteria which could help in deciding whether a cataclastic rock formed during seismic or aseismic slip (e.g. Cowan, 1999). Energy balance for mechanical cataclasis is less quantified than for pseudotachylite (Fabbri et al., 2000) and the importance of dissolution and recrystallisation processes seen in the fault rocks in SBF suggest deformational increments at aseismic rates. Diffusive mass transfer is typically a slow process acting at strain rates of about 10−14(?)s−1 (Shimizu, 1995; Gratier et al., 1999). Therefore it is most probable that cataclasis of the SBF took place under slow, aseismic rates. Since dissolution-recrystallization is typical of slow aseismic strain rates therefore the cataclastic rocks of the SBF zone provides an example of past aseismic slip and aseismic creep in the fault zone.

Within the SBF zone, three generations of cataclasis events can be recognized in the cataclastic rocks suggesting repeated cataclasis in the evolution of the fault rocks. Microstructural evidences of cataclastic rocks have been efficiently used to evaluate fragmentation processes and progressive history of deformation of cataclastic fault rocks. Prolonged cataclastic deformation often accompanied by repeated fracturing and cementation leads to reduction of grain size (Knipe and Lloyd, 1994). The temporal relation of the ‘mixed’ cataclastic rocks has been determined; however, the length of the different periods in absolute age remains unknown.

Given all this, the role of cataclastic textures and structures in the study of deformation history in elastico-frictional regime may have a significant role in the kinematics of fault movement and therefore, must be better understood to establish a realistic and reliable history.