Indian plate structural inheritance in the Himalayan foreland basin, Nepal

Abstract

The Himalaya, the Earth's largest active orogen, produces a deep but relatively unexplored foreland basin by loading the Indian Plate. Newly available two‐dimensional seismic data (ca. 5,180 line km) spanning 900 km of the Nepali lowlands allow mapping and interpretation of several regional subsurface markers in two‐way‐travel time and estimated depth. Isopach maps for the major intervals allow us to interpret the interplay between basement structure, flexure, and faulting within the Ganga Basin. The Indian continental lithosphere beneath the foreland basin contains basement ridges oriented at high angles to the thrust belt. These basement structural highs and intervening depressions, tens to hundreds of kilometres wide, influenced deposition of the Precambrian Vindhyan strata and overlying Paleozoic to Mesozoic successions. The overlying Miocene to Quaternary foreland basin shows along‐strike thickness variations across the basement features. Because the foreland basin sediments were mainly deposited in an alluvial plain close to sea‐level, accommodation, and therefore thickness, was predominantly controlled by subsidence of the Indian Plate, providing evidence that the basement features controlled foreland basin development. Subsidence varied in time and space during Neogene basin development. When combined with flexural modelling, these observations imply that the subsidence history of the basin was controlled by inherited lateral variations in the flexural rigidity of the Indian Plate, as it was translated northward beneath the Himalayan Orogen. Basement features continue to play a role in higher levels of the thrust belt, showing that basement features in a down‐going plate may produce non‐cylindrical structures throughout orogen development.

Coupled Rapid Erosion and Foreland Sedimentation Control Orogenic Wedge Kinematics in the Himalayan Thrust Belt of Central Nepal

“… (a) Generalized geological map of Nepal (after DeCelles et al., 2020) showing tectonostratigraphic units and major faults, and highlighting location of Chitwan wedge‐top basin. Also shown are contours of depth to basement beneath the foreland basin (after Duvall et al., 2018; Raiverman et al., 1983). Geology is depicted only for Nepal, and only the depth of basement is depicted for northern India.…”

“…The oldest stratigraphic unit in the hanging wall of the Main Frontal thrust is the lower member of the Siwalik Group; this implies that the frontal part of the thrust belt is detached at the top of the lower Miocene Dumri Formation in the subsurface. South of the Main Frontal thrust lies the actively subsiding Himalayan foreland basin, beneath which Indian basement has been flexed downward ∼5–6.5 km below sea level directly adjacent to the Main Frontal thrust (Figure 2; Burbank et al., 1996; Duvall et al., 2018; Lyon‐Caen & Molnar, 1985; Raiverman et al., 1983). Geomorphic and reflection seismic evidence exists for local, active blind thrusting in the subsurface south of the Main Frontal thrust (e.g., Bhadrapur thrust in Figure 2; Almeida et al., 2018; Delcaillau, 1997; Duvall et al., 2020).…”

“…Sedimentation in the foreland basin of southern Nepal and northern India is documented by reflection seismic data and sparse hydrocarbon exploratory wells (Duvall et al., 2018, 2020; Karunakaran & Ranga Rao, 1976; Raiverman et al., 1983; Rao, 1973). Depth to basement is greater than 6 km in the most proximal parts of the foredeep (Figure 2a).…”

“…Depth to basement decreases southward over a distance of 200 km to ∼1,000 m, consistent with deposition in a northward down‐flexing foreland basin system (Lyon‐Caen & Molnar, 1985). Second‐order complexities in the depth contours are interpreted as related to intra‐ or sub‐basinal faults (Duvall et al., 2018; Raiverman et al., 1983). A large southward bulging salient in the basement depth contours exists in the region directly south of the Chitwan salient, with a maximum depth >6,000 m, roughly 2,000 m deeper than in adjacent areas to the east and west (Figure 2a).…”

“…A large southward bulging salient in the basement depth contours exists in the region directly south of the Chitwan salient, with a maximum depth >6,000 m, roughly 2,000 m deeper than in adjacent areas to the east and west (Figure 2a). At least 4,500 m of this depth consists of Miocene‐Pleistocene Siwalik Group foredeep facies (Duvall et al., 2018; Friedenreich et al., 1994). Unfortunately, the ages of these deposits are not well resolved, although some authors have reported age picks on seismic profiles (Duvall et al., 2020).…”