The Late Eocene‐Early Miocene Unconformities of the NW Indian Intraplate Basins and Himalayan Foreland: A Record of Tectonics or Mantle Dynamics?

Abstract

A well-developed late Eocene to Miocene unconformity, termed the base Miocene unconformity (BMU), is found throughout the intraplate basins of northwestern India and has previously been ascribed to Himalayan tectonics. This hypothesis is investigated by first describing the nature and age of the BMU in the northwest (NW) Indian intraplate basins and then reconstructing the location of the BMU relative to the Himalayan deformation front at the time it formed. We suggest that formation of the BMU in western India cannot be related to Himalayan tectonic processes associated with plate loading and flexure unless the Indian plate had an elastic thickness of >125 km, which is highly unlikely. Furthermore, the resumption of deposition post unconformity rules out inversion due to compression associated with India-Asia convergence as a cause, as these compressive forces are still present. We note the coeval nature of the unconformity in the NW Indian plate intraplate basins and the Himalayan peripheral foreland basin. If the unconformities of the Himalayan peripheral foreland basin and the NW Indian intraplate basins were formed by a common process, uplift due to circulation in the mantle is the only possible regional-scale mechanism. Such circulation could be the result of the intrinsically time-dependent high-Rayleigh number convection in the mantle, which has resulted in well-documented unconformities elsewhere, or be the result of subducting slab break-off beneath the Himalaya.

Origin of Lower Cretaceous quartzose arenites in northern India and the Indus Basins of Pakistan—The result of provenance composition, weathering or diagenesis?

“…The Cenozoic Era was characterized by faultcontrolled deepening and infilling of the Barmer and Cambay basins with more than 6 km of sediments deposited in continental settings (Dolson et al, 2015;Roy & Jokhar, 2002;Sisodia & Singh, 2000;Tabaei & Singh, 2002;Tripath et al, 2009). During the Oligocene Epoch, the continued collision of the Indian Plate with the Eurasian Plate caused uplift and erosion (Chatterjee et al, 2013;Compton, 2009;Dolson et al, 2015;Najman et al, 2018).…”

Indian plate structural inheritance in the Himalayan foreland basin, Nepal

“…For example, the unit is >700 m thick at its type section in Central Nepal where the top is not exposed, and >1,200 m thick at Swat Khola in western Nepal, where the top is thrust‐truncated (DeCelles et al., 1998; Sakai, 1989). The regional unconformity at its base is interpreted variously as a product of: a peripheral bulge related to the advancing load of the Himalaya (DeCelles et al., 1998), a redistribution of that load (Najman et al., 2004); or of mantle processes such as slab break‐off (Garzanti, 2019; Najman et al., 2018). The Dumri Formation is dominated by trough cross‐stratified and planar sandstone beds that represent channel fills, crevasse splays, and paleosols (DeCelles et al., 1998).…”

Reconciling Geophysical and Petrological Estimates of the Thermal Structure of Southern Tibet

“…One proposed solution has been the removal of large amounts of lithospheric mantle from beneath the plateau, most probably through the break‐off of underthrust Indian lithosphere (DeCelles et al., 2002; Guillot et al., 2003; Replumaz et al., 2010). Over the last decade, increasing evidence, ranging from the identification of a relict “Indian” slab in the deeper mantle beneath Tibet (Replumaz et al., 2010, 2014), the progressive migration and termination of magmatic activity across Tibet (Chung et al., 2005; Webb et al., 2017; Yakovlev et al., 2019), changes in sedimentation in both the Himalayan foreland (Mugnier & Huyghe, 2006), in Northern India (Najman et al., 2018), and within the plateau interior (Carrapa et al., 2014; Leary et al., 2016), and a switch from contraction to extension (Stearns et al., 2013), all support a model in which an initial phase of underthrusting by colder Indian material was followed by a prolonged phase of southward slab retreat back under southern Tibet, ending with at least one break‐off, which was then followed by renewed underthrusting of Indian material beneath the plateau—a model with major implications for the present‐day temperature field beneath the plateau in the lower crust and upper mantle.…”