Character of different phases of cataclasite
Ct1
It occurs as clasts of Ct1 cataclasis embedded within later phase (Ct2) cataclastic rocks (Fig. 11). This is the oldest identified cataclastic rock consisting of comminuted angular to sub-rounded clasts of quartz and occasionally feldspar (up to 0.2mm in dimension) in a fine dark groundmass (Fig. 11). The fine darker groundmass of Ct1 constitutes >60% of the rock. Major part of the groundmass is aphanitic and consists of dark submicroscopic material composed of secondary silica and iron oxide introduced into the matrix by iron oxide rich solution (Fig. 11). Shapes of the Ct1 clasts are rounded to sub-rounded, rectangular to elliptical to circular and also irregular. The rounded and sub-rounded clasts of Ct1 suggest that the older clasts are abraded by subsequent cataclasis. The proportion of darker substance (iron oxide) in Ct1 is much higher than in Ct2 and that makes it sharp and distinct within Ct2 (Fig. 11). The Ct1 became fairly cohesive due to impregnation of silica and iron oxide and that helps it to take part as hard material in later phase cataclasis. Clasts of remnant Ct1 is not present everywhere because they are possibly destroyed by subsequent cataclasis.
Ct2
In the entire fault rock this phase of cataclasis is most dominant and controls the micro structure of the fault rock. Clasts are dominantly quartz with substantial amount of rock fragments and a few feldspar and muscovite clasts set in a matrix dominantly made of silica and smaller amount of iron oxide (Figs. 4, 7c, 9). The matrix varies in proportion from place to place and the cataclastic rock is protocataclasite to cataclasite proper and rarely ultracataclasite. The proportion of clast to matrix varies from more than 80% to less than 10%. The size of the clast fragments varies from large to quite small (Figs. 4, 12). Matrix is dominantly made up of fine material often submicroscopic with small clasts of quartz and rarely feldspar. The aphanitic part of the matrix contains secondary silica which has percolated into the cataclastic groundmass to form silicified cataclasites. The degree of crushing varies within a short distance. Somewhere large protolith fragments (0.2mm to 10mm in length and 0.15mm to 7mm in width) are present (Fig. 12). In micro-scale large clasts and intact protoliths are penetrated by narrow, continuous to discontinuous, coalescing cataclasite zones of variable thickness forming a network of fractures within protolith (Fig. 8). Clasts of older cataclastic rock (Ct1) are remarkably well preserved at places (Fig. 11).
Figure 12. Relict of rock fragment clast within cataclastic rock
Photomicrograph showing intensely deformed cataclastic rock with remnant of large rock fragment (Rf) with a fewer number of intergranular fractures (If). The matrix (Ct) consists of very fine to medium size angular crystals of quartz and submicroscopic material in cataclastic rock. Crossed polars.
In some Ct2 pressure solution in fine-grained matrix concentrates insoluble opaque material into thin microseams which eventually run together to form a thicker opaque zone. These opaque zones at places show preferred orientation to define a crude foliation. The foliation swings around coarse clasts of quartz and feldspar. Silica veins often form comb structure where quartz crystals grow from the wall towards the centre. The silica veins are fractured by later cataclasis with thin films of gouge along the fractures. These gouge-lined fractures are developed during third phase (Ct3) cataclasis.
Ct3
The last phase of brittle deformation is present as thin, lensoid, isolated patches of gouge and networks of fractures lined with gouge developed on Ct1 and Ct2. At the grain scale, thin open cracks and thin zones of coherent gouge formed, cross-cutting all previous features (Figs. 5 and 10). The cracks are generally healed and stained with iron oxide. The gouge and fractures are developed in the matrix of Ct2 and in the clasts of Ct1 as thin short micro zones and “films” during this phase of cataclasis and thickness of the zones is from up to a few mm to fraction of a mm (Figs. 5, 10). The boundary of Ct3 with Ct2 is sharp to gradational and straight to wavy (Fig. 5) It consists dominantly of aphanitic submicroscopic material impregnated with very fine clasts of quartz (Fig. 5 and 10). Thin pressure solution zones are present as discontinuous seams in gouge zones developed due to further straining of very fine clasts.
The nature of deformation in all the fault rock bands is similar, but not all phases of brittle deformation are equally well preserved in all bands. The last two phases are commonly visible in all the bands but the evidence of earliest phase is rarely preserved. We interpret that all these features are formed at low temperature at shallow crustal depth and associated with the faulting along Bengal Basin margin SBF.