Active strike-slip faults and an outer frontal thrust in the Himalayan foreland basin

The Himalayan foreland basin formed by flexure of the Indian Plate below the advancing orogen. Motion on major thrusts within the orogen has resulted in damaging historical seismicity, whereas south of the Main Frontal Thrust (MFT), the foreland basin is typically portrayed as undeformed. Using two-dimensional seismic reflection data from eastern Nepal, we present evidence of recent deformation propagating >37 km south of the MFT. A system of tear faults at a high angle to the orogen is spatially localized above the Munger-Saharsa basement ridge. A blind thrust fault is interpreted in the subsurface, above the sub-Cenozoic unconformity, bounded by two tear faults. Deformation zones beneath the Bhadrapur topographic high record an incipient tectonic wedge or triangle zone. The faults record the subsurface propagation of the Main Himalayan Thrust (MHT) into the foreland basin as an outer frontal thrust, and provide a modern snapshot of the development of tectonic wedges and lateral discontinuities preserved in higher thrust sheets of the Himalaya, and in ancient orogens elsewhere. We estimate a cumulative slip of ∼100 m, accumulated in <0.5 Ma, over a minimum slipped area of ∼780 km2. These observations demonstrate that Himalayan ruptures may pass under the present-day trace of the MFT as blind faults inaccessible to trenching, and that paleoseismic studies may underestimate Holocene convergence.

Foreland basin sediment archives: highlighting their use in documenting deformation of the orogenic hinterland and foreland.

&lt;p&gt;Interrogation of sediment archives allows for documentation of both hinterland and foreland deformation. Examples of their use as an archive of Himalayan foreland deformation include the work of Govin et al. (Geology, 2018) in which determination of the timing of drainage rerouting of the palaeo-Brahmaputra has allowed us to date the timing of surface uplift of the Shillong Plateau, and the work of Najman et al (Tectonics, 2018) in which the presence of the major Paleogene unconformity previously recognised in the Himalayan foreland basin, was shown to extend much further south into the foreland, allowing for a broader range of possible causal mechanisms to be discussed. There are numerous examples of the use of the Himalayan foreland basin sediment record to determine orogenic tectonics, this being a complementary approach to bedrock studies of the orogen. For example, Govin et al. (in review) and Lang et al (GSAB 2016), used detrital mineral lag time studies targeted to the Siwalik Himalayan foreland sediment archive, to demonstrate when the rapid exhumation of the eastern Himalayan syntaxis commenced. Comparison with a similar dataset derived from a more distal sediment archive of the Bengal Fan (Najman et al. GSAB 2019), shows the advantages (as well as disadvantages) in the use of proximal sediment archives.&lt;/p&gt;