Seismic Coupling Quantified on Inferred Décollements Beneath the Western Syntaxis of the Himalaya

Abstract

We used episodic GNSS measurements to quantify the present-day velocity field in the northwestern Himalaya from the Himalayan foreland to the Karakoram Range. We report a progressive N-S compressional velocity gradient with two noticeable exceptions: in the Salt Range, where important southward velocities are recorded, and in Nanga Parbat, where an asymmetrical E-W velocity gradient is recorded. A review of Quaternary slip along active thrusts both in and out of sequence allows us to propose a 14 mm/yr shortening rate. This constraint, together with a geometrical model of the Main Himalayan Thrust (MHT), allows us to propose estimations of the slip distributions along the active faults. The lower flat of the MHT is characterized by ductile slip, whereas the coupling increases along the crustal ramp and along the upper flat of the MHT. The basal thrust of the Potwar Plateau and Salt Range presents weak coupling, which is interpreted as the existence of a massive salt layer forming an excellent décollement. In the central part of the frontal Salt Range, the velocities suggest the existence of a southward horizontal flux in the massive salt layer. The simulations also suggest that the velocities recorded in Nanga Parbat can be explained by active westward thrusting along the fault that borders the massif to the west. Simulations suggest that the slip along this fault evolves with depth from 5 mm/yr ductile slip near the MHT to no slip along the upper part of the fault.

Pleistocene–Holoceneout‐of‐sequence faulting along theMedlicott‐WadiaThrust in theNWHimalaya

“…The uplift of the MWT ramp in Jammu sector resulted in the formation of Udhampur piggyback basin (UB) on its hangingwall (Gavillot et al, 2016; Shah & Malik, 2017). The GPS‐derived convergence rate in Jammu sector is 13 ± 1 mm/a (Jouanne et al, 2020 and ref. therein).…”

“…Dey, Thiede, Schildgen, Wittmann, Bookhagen, Scherler, & Strecker, 2016; Lavé & Avouac, 2000; Srivastava, Bhakuni, et al, 2009; Srivastava, Rajak, et al, 2009; Thakur et al, 2014; Vassallo et al, 2015) or even geodetic shortening rate estimates (e.g. Banerjee & Burgmann, 2002; Jouanne et al, 2020; Kundu et al, 2014; Stevens & Avouac, 2015 and references therein) unanimously indicate that most of the ongoing crustal shortening in the western Himalaya is accommodated in the frontal fold‐and‐thrust belt of the Himalaya, known as the Sub‐Himalaya (SH; Figure 1; Yin & Harrison, 2000). The SH is bordered by the Main Boundary Thrust (MBT) in the north and the Himalayan Frontal Thrust (HFT) in the south and it exposes an array of orogen‐parallel faults rooted to a low‐angle basal detachment, known as the Main Himalayan Thrust (MHT; Elliott et al, 2016; Nábělek et al, 2009; Ni & Barazangi, 1984; Figure 1c).…”

A modern pulse of ultrafast exhumation and diachronous crustal melting in the Nanga Parbat Massif

“…Rock uplift in the NPM is accommodated by active faults/shear zones on its eastern and western margins that accommodate E-W contraction indicated by field and geodetic studies ( Fig. 1 ) ( 63 – 67 ): a west-vergent thrust fault on the west (Rakhiot-Liachar thrust) and a steeply dipping east-side down fault on the east (Stak fault). The Stak fault has not been studied in detail; modern slip rates are unknown, although neotectonic displacement is thought to be relatively minor ( Fig.…”

“…Geodetic studies reveal dominantly westward movement of the NPM (relative to the adjacent Kohistan terrane) and an asymmetric present-day velocity field, with the highest velocities recorded on the western margin of the massif ( 65 , 67 ). This suggests that the Rakhiot-Liachar thrust, which slips between 5 and 20 mm/year ( 65 , 67 ), combined with distributed deformation in the core of the massif ( 68 ), is responsible for active uplift of the NPM ( Fig. 1 ) and enables exhumation of hot middle crust ( 66 , 69 ).…”

Neotectonics of the Western Suleiman Fold Belt, Pakistan: Evidence for Bookshelf Faulting

“…The Indian plate is moving at a rate of~35 mm/year and~38 mm/year in the northwestern and northeastern margins, respectively [3]. Most of the present-day compressional tectonics are associated with the frontal thrusts and are well studied in the Central Himalayas in Nepal [4,5], in Northwestern Himalayas in India [6], and across the Salt Range Thrust in Pakistan [7,8]. However, the transpressional tectonics at the western margin of the Indian plate is poorly understood, although important in understanding active plate margin processes and calculating interseismic strain accumulation for an earthquake-prone area.…”