Post-breakup deformations in the Bay of Bengal: Response of crustal strata to the sediment load

Abstract

Passive continental margins are tectonically inactive, but a few of them including the East India Passive Margin (EIPM) show evidences for post-breakup deformations. This unusual process prompted us to investigate the post-rift deformations on EIPM and adjacent deep-water region for understanding the possible mechanisms. Seismic reflection images of the Bay of Bengal reveal a post-rift deformation with a manifestation of extensional faults in Krishna-Godavari (K-G) basin and on flanks of the 85�E Ridge. In K-G basin, one of the rift-related faults reactivated during the Early Miocene time (~16�Ma), while on flanks of the 85�E Ridge new normal faults originated at about 6.8�Ma. From detailed analysis of fault throws, it is observed that the fault in K-G basin recorded a cumulative throw of about 900�m between the basement and Early Miocene horizon (~16�Ma), later the fault was reactivated at 6.8 Ma and continued the activity progressively until 0.3 Ma before cessation. The fault system on the margin spatially extends for about 300 km between offshore extensions of the Pranahita-Godavari graben and Nagavali-Vamshadhara shear zone. The faults on 85�E Ridge, initiated at 6.8 Ma and continued until 0.8 Ma, have cumulative throws of about 60 and 110 m on western and eastern flanks of the ridge, respectively. Back-stripping analysis of the fault from the K-G basin discloses two distinct phases of subsidence history: (i) during the first phase (120-23�Ma) the basement subsided at a rate of 46-18�m/Myr due to thermal cooling of the lithosphere, (ii) during the second phase (23�Ma-Present) rapid subsidence rate (69.56�m/Myr) of basement is noticed as a consequence of deposition of copious amounts of Bengal Fan sediments. The thick sedimentary strata exerted vertical load on underlying heterogeneous crust that led to build excessive internal stress and release through weak zones (lying at intersecting planes of heterogeneous crustal blocks). The stress, thus released through fault planes has caused the deformation of crust as well as overlying sedimentary strata.