Active strike-slip faults and an outer frontal thrust in the Himalayan foreland basin Michael J. Duvalla, John W. F. Waldrona,1, Laurent Godinb, and Yani Najmanc aDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G2E3, Canada; bDepartment of Geological Sciences and Geological Engineering, Queen’s University, Kingston ON K7L 3N6, Canada; and cLancaster Environment Centre, Lancaster University, LA1 4YQ Lancaster, United Kingdom Edited by Lisa Tauxe, University of California San Diego, La Jolla, CA, and approved June 11, 2020 (received for review February 2, 2020) The Himalayan foreland basin formed by flexure of the Indian Plate unconformably on Proterozoic mobile belts, sedimentary basins, below the advancing orogen. Motion on major thrusts within the and an Archean craton, exposed along the southern edge of the orogen has resulted in damaging historical seismicity, whereas south basin. The stratigraphy of the basin is known from drilling and of the Main Frontal Thrust (MFT), the foreland basin is typically from outcrop in the sub-Himalaya and Lesser Himalaya (22). portrayed as undeformed. Using two-dimensional seismic reflection The basin fill is divided by an Oligocene disconformity in the sub- data from eastern Nepal, we present evidence of recent deformation Himalaya (23, 24), below which a thin (>90 m) Paleogene suc- propagating >37 km south of the MFT. A system of tear faults at a cession is dominated by marine mudstone (25). The overlying high angle to the orogen is spatially localized above the Munger- Miocene to Quaternary rocks are fluvial deposits that filled the Saharsa basement ridge. A blind thrust fault is interpreted in the subsiding basin (4). This package comprises the Siwalik Group subsurface, above the sub-Cenozoic unconformity, bounded by two and the thinner, underlying, Dumri Formation. Seismic reflec- tear faults. Deformation zones beneath the Bhadrapur topographic tions, corresponding approximately to the lithological bound- high record an incipient tectonic wedge or triangle zone. The faults aries between the lower, middle, and upper Siwalik Group, record the subsurface propagation of the Main Himalayan Thrust identified in the log of the Biratnagar-1 well (Fig. 2), are traced (MHT) into the foreland basin as an outer frontal thrust, and provide through two-dimensional (2D) industry seismic data; the well is a modern snapshot of the development of tectonic wedges and lat- not deep enough to allow us to pick the Oligocene disconformity, eral discontinuities preserved in higher thrust sheets of the Himalaya, but the deeper, angular sub-Cenozoic unconformity, represent- and in ancient orogens elsewhere. We estimate a cumulative slip of ing the base of the foreland basin deposits, was traced. Locally EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES ∼100 m, accumulated in <0.5 Ma, over a minimum slipped area of we identified a deeper horizon marking the top of unstratified ∼780 km2. These observations demonstrate that Himalayan ruptures acoustic basement. Regional variations in the thickness of the may pass under the present-day trace of the MFT as blind faults in- Siwalik Group are controlled by a series of basement ridges (11), accessible to trenching, and that paleoseismic studies may transverse to the orogen, of which the easternmost, Munger– underestimate Holocene convergence. Saharsa ridge, underlies the area of this study (Fig. 1). The – Himalaya | thrust fault | foreland basin | seismicity Munger Saharsa ridge, like other basement ridges beneath the basin, appears to be defined by NE-SW–striking faults (11) that bound Proterozoic to Paleozoic grabens beneath the foreland he Himalayan orogen, the Earth’s highest mountain range, is > – basin and are locally sources of earthquakes at depths 30 km Ta product of ongoing continent continent collision between (e.g., ref. 26). Rare events well to the south of the foreland basin India and Asia (Fig. 1). The orogen is subdivided into longitu- show normal-sense focal mechanisms (Fig. 1) but close to the dinally continuous lithotectonic domains, bounded by continent- scale faults (1, 2). The southernmost fault, the Main Frontal Significance Thrust (MFT), separates the Himalayan foreland basin, typically regarded as undeformed, from the sub-Himalaya, composed of thrusted and folded foreland basin sedimentary rocks (3, 4). We The Himalayan mountain belt results from continuing conver- show that a previously unknown blind thrust and a series of gence between the Indian Plate and Asia. Damaging earth- strike-slip tear faults propagate southward into the Himalayan quakes occur on major thrust faults north of the Main Frontal foreland basin up to 37 km south of the MFT in eastern Nepal, Thrust (MFT). To the south, the Ganga foreland basin is typi- forming an isolated topographic feature, the Bhadrapur High cally described as undeformed. We show that active thrust and strike-slip faults, with accumulated slip up to ∼100 m, pass that rises ∼60 m above the surrounding plain (Fig. 2). We esti- under the trace of the MFT into the foreland basin in eastern mate slip along this incipient thrust system, and discuss the im- Nepal, leading to propagation of deformation at least ∼37 km plications for the development of structure in the Himalaya and into the foreland basin beneath the densely populated Ganga for its seismicity. plain. The development of these faults at the active thrust North of the MFT, the sub-Himalaya shows lateral changes in front helps to explain structures preserved in higher thrust structure along strike, resulting in variations in thrust vergence sheets of the Himalaya, and in ancient mountain belts and the preservation of piggyback basins (15). Farther north, the elsewhere. Main Boundary Thrust (MBT) and Main Central Thrust (MCT) bound the Lesser and Greater Himalaya, respectively, in which a Author contributions: J.W.F.W., L.G., and Y.N. designed research; M.J.D. performed re- series of along-strike culminations and depressions locally lead search; M.J.D., J.W.F.W., and Y.N. analyzed data; and M.J.D., J.W.F.W., and L.G. wrote to the preservation of fenster and klippen (16). These major the paper. thrusts root at depth on the Main Himalayan Thrust (MHT), a The authors declare no competing interest. crustal-scale detachment (17–20) above autochthonous Indian This article is a PNAS Direct Submission. basement. Lateral segmentation is also evident in the episodic Published under the PNAS license. occurrence of seismic slip on the major thrust faults (21). 1To whom correspondence may be addressed. Email: [email protected]. South of the MFT, the Ganga Basin (Fig. 1) is the Himalayan This article contains supporting information online at https://www.pnas.org/lookup/suppl/ foreland basin (Fig. 1A) in Nepal and northern India. The basin doi:10.1073/pnas.2001979117/-/DCSupplemental. is filled (4) by 3 to >7 km of sedimentary rock that rests First published July 13, 2020. www.pnas.org/cgi/doi/10.1073/pnas.2001979117 PNAS | July 28, 2020 | vol. 117 | no. 30 | 17615–17621 Downloaded by guest on September 24, 2021 STD Tethyan Himalaya Shillong Plateau gneissic complex A MCT Greater Himalaya Chotanagpur gneissic complex Major city Lesser Himalaya MBT Satpura mobile belt Basement fault Indus MFT Sub-Himalaya Bundelkhand Craton Seismic survey Basin Quaternary sediments Nepal Aravalli-Delhi fold belt 22 Late Cretaceous volcanic rocks Focal mechanism with depth (km) Gondwanan sedimentary rocks Other units Ganga Basin Proterozoic Vindhyan succession Basement ridges Well location 28 India B 74° MFT 80° ITSZ 15 10 Bengal MCT 17 21 10 Basin 15 12 15 13 16 15 STD 15 N 12 15 15 22 15 15 34 10 19 15 New Delhi 33 28° 21 12 12 MBT 46 12 20 Delhi-Haridwar Ridge 44 Kathmandu 21 MFT 35 Faizabad Block 10 Ridge Munger- Saharsa Ridge 24° 200 km 38 Kolkata Fig. 1. Location maps. (A) Generalized map of the Himalaya showing principal Neogene basins. Image credit: Google Earth. © 2019 TerraMetrics, map data © 2019 used with permission. (B) Map of northern India, Nepal, and adjacent areas, from published sources (2, 5–10) and US Geological Survey public data. Approximate traces of basement ridges after Godin and Harris (11). ITSZ: Indus-Tsangpo Suture Zone; STD: South Tibet Detachment. Focal mechanisms and depths (kilometers) are derived from the Global Centroid Moment Tensor Project (12, 13) shown for seismic events with moment magnitude Mw > 5.5 during the period 1976–2019, plotted with the aid of GMT software (14). Box encloses area of Fig. 4C. MFT, and beneath the Himalaya, these faults appear to be vertical faults are typically inferred at discontinuities in the reactivated in sinistral strike-slip (5, 26–29). poorly coherent reflections. Folds may be differentiated as Faults and folds within the overlying sedimentary package of contractional or extensional, based on the upwarp or downwarp the Ganga Basin are uncommon, with the result that most strata of mapped horizons relative to their regional structural level. lie flat and undisturbed. The foreland basin is thus commonly Correlated between profiles, the faults are traced up to 37 km, presented as undeformed, despite the occurrence of earthquakes showing two main strike orientations: ∼NNE-SSW and ∼NNW- (30), river migration patterns that indicate active tectonic con- SSE. However, the faults show distinct bends when traced along trols (31), and enigmatic topographic features such as the Bha- their strike (Fig. 2) that coincide with changes between con- drapur high in southeast Nepal (Fig. 2), which is identified as a traction and extension. We therefore interpret them as strike-slip doubly plunging anticline in existing geologic maps (32). faults. The transitions from contraction to extension in the ad- jacent damage zones (Fig. 2) allow us to identify bends in the Results fault traces as restraining or releasing, and therefore to charac- Using 2D seismic reflection data provided by Cairn Energy, we terize the faults as sinistral or dextral (Fig.