Timing, quantification and tectonic modelling of Pliocene–Quaternary movements in the NW Himalaya: evidence from fission track dating

Abstract

Variable exhumation rates, deduced from the Pliocene–Quaternary FT zircon–apatite ages from the Himalayan Metamorphic Belt (HMB) of the NW Himalaya along the Sutlej Valley in Himachal Pradesh, have been modelled in the tectonic framework of fast exhumed Lesser Himalayan windows, which caused lateral extensional sliding of the metamorphic nappe cover along the well-known Main Central Thrust (MCT) and differential movements along thrust zones as well. In the northern belt of the Higher Himalayan Crystallines (HHC), two distinct clusters of the FT apatite ages have been deciphered: apatite ages having a weighted mean of 4.9±0.2 Ma (1 σ) in basal parts on the hanging wall of the MCT, and 1.49±0.07 Ma (1 σ) in the hanging wall of a newly, recognized NE, dipping Chaura thrust further north. Fast exhumation of the Chaura thrust hanging wall has been inferred at a rate of 4.82±0.55 mm/yr from the zircon–apatite cogenetic pairs during 1.54 Ma and 0.97 Ma, and 2.01±0.35 mm/yr since 1.49 Ma. In comparison, its foot wall has been exhumed at a much slower rate of 0.61±0.10 mm/yr since 4.9 Ma. The overlying Vaikrita Thrust zone rocks reveal an exhumation rate of 1.98±0.34 mm/yr from 2.70±0.40 Ma to 1.31±0.22 Ma and 2.29±0.66 mm/yr since 1.31±0.22 Ma. Using these data, a vertical displacement of ca. 2.08±0.68 km has been calculated along the Chaura thrust between 4.9 and 1.50 Ma on an average rate of 0.6 mm/yr. It is of the order of 1.18 km from 2.70 Ma to 1.54 Ma along the Vaikrita Thrust, and 0.78 mm/yr from 1.31 Ma to 0.97 Ma, and has behaved as an extensional normal fault during these periods. Tectonic modelling of the exhumation rates in the NW Himalaya reveals fastest uplifting Himalayan domes and windows like the Nanga Parbat in Pakistan, Suru and Chisoti domes in Zanskar and Kishwar–Kulu–Rampur Window axis in SE Kashmir and Himachal Pradesh during Pliocene–Quaternary. These windows appear to have caused lateral extensional sliding of the Himalayan metamorphic nappes in the lower parts. The middle parts of the HHC belt have witnessed both overthrusting and extensional faulting due to complex and variable exhumation patterns within the hanging and foot walls of the MCT and Vaikrita Thrust along the Sutlej Valley, thus causing movement of upthrust crustal wedge between the extensional ones. Thus, FT zircon–apatite ages provide evidence for the presence of a number of crustal wedges having distinct tectonothermal history within the HHC.

Introduction

Northern parts of the extensively remobilized Himalayan Metamorphic Belt (HMB) due to continental collision tectonics between the northerly moving Indian Plate and the Eurasian Plate since Early Cenozoic (∼55 Ma) constitute the highest uplifted and fastest eroded terrain of the Himalaya [1], [2], [3], [4], [5]. It has been modelled to have evolved within a 15–20 km thick, northeast-dipping intracontinental ductile shear zone due to intense crustal shortening [3], [6], [7], and has been thrust southwestward over the Lesser Himalayan sedimentary sequence along the Main Central Thrust (MCT) and its various splays, e.g. the Jutogh/Vaikrita thrusts (Fig. 1).

Geochronological data from the NW Himalaya during the past two decades have demonstrated that the Higher Himalayan Crystallines (HHC) have undergone fastest and episodic exhumation during the Cenozoic after the collision tectonics; the main Neo-Himalayan phase (∼25–18 Ma) has caused rapid uplift, tectonic denudation and deposition of colossal molassic sediments in the Sub-Himalayan foreland basin and the maga fans within the Indian Ocean [8]. Another major uplift phase during the Late Pliocene–Quaternary appears to be mainly responsible for the present-day Himalayan relief [9], and links the ongoing crustal deformation of the Himalaya with the Early Cenozoic geodynamics.

Fission track dating of zircon and apatite has recently been applied to numerous geological problems covering low-temperature tectonothermal evolution, and calculation of variable cooling and exhumation rates across numerous fault zones, e.g. Tectonic Median Line [10], the Insubric fault line in Alps [11], the Main Mantle Thrust [12], [13], and many others. However, the application of FT technique to calculate the amount of displacement along such fault zones is limited [14]. The main objective of this paper is to document and interpret accelerated exhumation of the HHC from the FT ages along the Sutlej Valley, Himachal Pradesh as well as to estimate the displacements along the thrust zone, and to provide tectonothermal models for similar phenomena elsewhere in the NW Himalaya.