Flexural subsidence analysis of the Laxmi Basin, Arabian Sea and its tectonic implications

Abstract

Two-dimensional flexural backstripping and thermal modelling (assuming laterally variable stretching) is applied along regional depth-converted interpreted seismic profiles from the Laxmi Basin in the Arabian Sea. Results from reverse post-rift flexural modelling reveal considerable basin-wide subsidence in response to the crustal geodynamics during and after the last extensional phase. Unloading of the stratigraphy allows us to estimate the degree of laterally varying extension, assuming thermal subsidence and pure shear. High degrees of extension in the basin centre predict considerable water depths at the time of rift cessation, consistent with deep drilling data. We suggest that regional extension prior to Paleocene time could have fuelled variable subsidence in the Laxmi Basin but that extension is less than seen in typical oceanic lithosphere. Volcanic loading by the seamounts shortly after extension has flexed the basin and implies an effective elastic thickness (Te) at that time of ∼6 km. Reconstruction of the seamount top near sea level at the end of emplacement indicates no major transient uplift potentially linked to the Deccan mantle plume activity. Backstripping of post-rift sediments from interpreted seismic profiles supports the presence of a hyper-thinned crust underneath the Laxmi Basin, with β factors reaching >7 in the basin centre and ∼3 across much of the basin width. Computations of decompacted sediment accumulation rates in light of new results from IODP Expedition 355 show that basin sedimentation peaked during early–middle Miocene time, possibly coeval with uplift and erosion of the Himalayan–Tibetan Plateau driven by strong summer monsoon rains.

The Indian Subcontinent: Its Tectonics

“…The Paleogene syn-rift sediments are dominated by detrital siliciclastics that eventually gave way to calcareous sediments. The large Eocene submarine masstransport complex at the base of the continental slope (Dailey et al, 2019), listric faults, roll-over anticlines, mud-intrusions, and horst-graben structures (Nair and Pandey, 2018) are consistent with the model of evolution of a passive continental margin passing through the rifting and subsequently drifting phases. The volcanic activity in the Lakshadweep ridge during the Late Palaeocene-Early Eocene times (55-62 Ma) is a contemporary ocean floor volcanism.…”

“…Pande et al (2017Pande et al ( , 2019 inferred that the voluminous volcanism, recorded along this ridge and in the Panikkar-Raman seamounts and the Wadia Guyot along with the coeval volcanism in Mumbai, represents the syn-rift phase related to the Indo-Seychelles separation which occurred between 62.9 Ma and 62.1 Ma; the original (?syn-rift) flexure enabled accumulation of a thick pile of sediments and gave way to incipient subduction along parts of the basin. The average sedimentation rates ranging between 4 cm/ky and 10 cm/ky display a sudden spike of 58 cm/ ky during the Pliocene (Pandey et al, 2018).…”

Evidence for simple volcanic rifting not complex subduction initiation in the Laxmi Basin

“…The shallower than normal oceanic crust depths to basement in Laxmi Basin (2.5-2.7 km sediment unloaded modern depths in the region of the drilling sites 6 compared with 5.2 km for 66 Myr old normal crust 7 ) does not imply presence of subduction-related crust but simply continental crust that has not been extended to the degree required for seafloor spreading to occur (i.e., the crust is thicker than normal oceanic crust and composed of less dense material). Indeed, a rifted origin was proposed by the first two authors of the Pandey et al 1 study based on simple flexural extension models 6 and despite the fact that it is incompatible with the subduction model they concurrently advocate in this second study. This more conventional approach to the origin of LB is moreover predicted by restoration of the flat-topped Raman Seamount at 66 Myr ago to close to sealevel.…”

“…Instead, most evidence supports the Laxmi Basin forming an extensional basin with seaward-dipping reflectors and a normal thermal subsidence history 6 . The age of the basin, which has still not been demonstrated from the drilled volcanic rocks, places it firmly within a wider pre-Deccan volcanic province 4 .…”

Climate–tectonic interactions in the eastern Arabian Sea

“…The tectonics of the Laxmi Basin was investigated by Pandey et al (2018) using two-dimensional post-rift flexural backstripping methods (Kusznir et al 1995). They estimated the degree of extension across the basin, assuming a low flexural rigidity and thermal subsidence following a pure shear extensional model (McKenzie 1978).…”