Crustal structure of the west Bengal basin, India from deep seismic sounding investigations

Abstract

SUMMARY Basement and sub‐basement crustal configuration of parts of the west Bengal basin has been delineated from inverse and forward modelling of refraction and wide‐angle reflection data obtained along two east‐west profiles: (a) Beliator‐Burdwan‐Bangaon (profile‐1) about 180 km long and (b) Gopali‐Tamluk‐Port Canning (profile‐2) about 140 km long. The study has revealed the existence of four groups of prominent refractors (velocities 1.7–2.1 km s−1, 2.7–3.2 km s−1, 3.7–4.2 km s−1, and 4.6–5.3 km s−1) overlying the crystalline basement with a velocity of 5.8–6.2 km s−1. Rajmahal Traps (velocity 4.6–5.3 km s−1, averaging 4.8 km s−1) occur beneath the base of the Tertiary sediments in the middle parts of the profiles. The presence of sub‐Trappean Gondwanas is also indicated from the wide‐angle reflection modelling supported by deep well lithological information. In general, depth to the basement is found to increase from west to east, with a maximum depth of around 10 to 11 km at the extreme eastern parts of the profiles. A basement upwarp, with a sharp flexure west of Bishnupur on profile‐2 in combination with the fault west of Shantipur on profile‐1 possibly brings into focus the regionally extending NNE‐SSW ‘Hinge zone’. A low velocity layer (LVL) of about 2 to 3 km thickness (velocity 5.6–5.8 km s−1) has been inferred in the upper crust in this area. In general the thickness of the upper (including the sedimentary column) and the middle (6.4–6.6 km s−1 velocity) parts of the crust is found to be larger (thickness varying between 22 and 27 km) than the lower crustal column (interval velocity 6.8–7.0 km s−1 with a thickness of 5 to 10 km). The Moho lies at varying depths of 36 to 26 km along profile‐1, with a prominent domal feature of about 40 km width between Satgachia and west of Shantipur. In the middle part of profile‐2 the Moho occurs at depths of 32 to 34 km, with a mild flexure east of Tamluk. Configuration of the Moho indicates that the crust is more akin to continental in nature in the western and middle parts of the basin, whereas significantly thinned continental crust is indicated in the eastern and southeastern parts.

Chapter 2 Active tectonics of Myanmar and the Andaman Sea

“…2.12) is commonly identified as the Eocene palaeoshelf edge, marking the boundary of the thick sediment pile and associated with a sharp increase in depth of the shallow-water Sylhet Limestone (Alam 1989;Alam et al 2003). Deep seismic sounding lines crossing the hinge line confirm that the crust thins sharply here but it is concluded that, close to the hinge line, the evidence is consistent with thinned continental crust rather than with true oceanic material (Kaila et al 1992). Receiver functions sampling the crust immediately to the south of the Shillong Plateau also support this conclusion (Mitra et al 2005).…”

Depth Estimation from Gravity Data Using the Maximum Entropy Method (MEM) and the Multi Taper Method (MTM)

“…The Bengal basin is covered by recent alluvium and bounded by the Indian Peninsula in the west, Lushai-Naga folded belt in the east, the Bay of Bengal in the south and Shilong Plateau in the north (KAILA et al, 1992). The basin has various step faults and thick Tertiary and Mesozoic sediments overlying the basement.…”

“…The gravity data are digitized from the gravity map of the Bengal basin (which was compiled by the NGRI from gravity the data set from the Oil and Natural Gas Corporation, India and MUKHOPADHYAY et al, 1986 at an approximately average gravity station spacing of 1 km) along the seismic profiles, and further data are interpolated at 1-km interval. The deep seismic studies using refraction and wide-angle reflection methods are also carried out along these two profiles (KAILA et al, 1992;MALL et al, 1999;PRASAD et al, 2002). Seismic study maps the sedimentary column thickness, the basement and deep crustal structure.…”

Chipping of cratons and breakup along mobile belts of a supercontinent

“…1(a)), also suggest plume-induced crust-mantle interaction (Tewari, 1998). A high velocity deep crust is inferred beneath the Bengal basin too (Kaila et al, 1992), which also exhibits a gravity high anomaly (∼20 mgal) sub-parallel to 90…”