Thrust geometries and deep structure of the outer and lesser Himalaya, Kumaon and Garhwal (India): Implications for evolution of the Himalayan fold‐and‐thrust belt

Abstract

The Kumaon‐Garhwal region of the Himalaya lies near the center of the Himalayan fold‐and‐thrust belt. We have drawn two balanced cross sections, 100 km apart, through the Outer and Lesser Himalaya. The cross sections incorporate all the surface, well log, and earthquake seismic data currently available from the region. Two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn. The three dimensional deep structure of the Outer and Lesser Himalaya is interpreted based on the balanced cross sections and the branch line maps. Deep structure of the Higher and Tethyan Himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas. A sequential evolutionary model for the Kumaon Himalaya along the eastern (Pindari) section is proposed. According to this model, the Kumaon Himalaya evolved by an overall forelandward progression of thrusting, with some reactivation along the Munsiari thrust (MT), the Main Boundary thrust (MBT), and the Main Central thrust (MCT). We use structural, stratigraphic, and radiometric criteria to place time constraints on the motion of these thrusts. Earliest motion along the MBT may have occurred in Early‐Middle Paleocene, but the main episode probably started in Late Eocene and may still be continuing. Emplacement of the MT had occurred by Middle‐Late Eocene, whereas the MCT shows activity around 20 Ma, thus exhibiting break back thrusting with respect to the MT. Shortening estimates are obtained from the Pindari section. Minimum shortening in the sedimentary thrust sheets of the Outer and Lesser Himalaya is 161 km or 65%. As a first approximation, we have also restored the crystalline sheets in order to obtain shortening estimates for the entire Himalaya. Minimum shortening for the Himalaya after restoring the MCT sheet varies from 354 (76%) to 421 km (79%). These estimates were further combined with published data from the area between the MCT sheet and the Indus‐Tsangpo Suture Zone (ITSZ). Minimum shortening between the Indo‐Ganga foreland and the ITSZ thus obtained lies in the range 687–754 km or 69–72%. We compare our shortening estimates with those available from the Pakistan and Nepal Himalaya.

Synchrotron FTIR imaging of OH in quartz mylonites

“…Large shear strains accu-mulated over ∼ 150-250 km of displacement along the MCT (e.g., Srivastava and Mitra, 1994;Hodges, 2000;Mitra et al, 2010;Tobgay et al, 2012;Law et al, 2013) and yet quartz OH contents (50-150 ppm) in our MCT samples are far lower than required for water weakening, thereby challenging our understanding of dislocation creep and the role of water in deformation deep in the continental crust. With only a few exceptions, IR spectra of our MCT samples have OH bands of the same character and size as dry natural quartz crystals, which are strong and have not been deformed by dislocation processes in laboratory experiments (e.g., Heard and Carter, 1968;Blacic, 1975;Blacic and Christie, 1984).…”

Insights into the Great <i>Mw</i> 7.9 Nepal Earthquake of 25 April 2015

“…The Indian continental plate penetrates along the base of this thrust plate (i.e. MHT) towards the north below the Himalaya and southern Tibet 5,[23][24][25][26][27][28][29][30][31] . Convergence of the Indian plate varies significantly between ~4.2 and 5.4 cm/yr (refs 32-34).…”

Seismological evidence of simultaneous mountain-building and crust-thickening from the 1999 Taiwan Chi-Chi earthquake (Mw=7.6)

“…Based on the isostasy theory (Airy compensation), the anomaly of crust thickness (crustal root) is not only proportional to, but also significantly larger than that of the topographic relief on the surface (Turcotte and Schubert, 1982). In the cases of such great mountains as the Himalayas, the Alps and the Rockies, it is widely accepted that mountain-building near the surface was primarily caused by a large amount of horizontal shortening taking place along one and/or several major thrusting faults in the upper crust (Molnar, 1988;Schelling and Arita, 1991;Srivastava and Mitra, 1994). However, the dynamic process of involved in crustal deformation in the lower crust of collision zones has still not yet been well understood in that reliable observations in high mountainous areas are generally hard to obtain.…”