NEOGENE EXHUMATION OF THE SIKKIM HIMALAYA FROM ZIRCON (U-TH)/HE THERMOCHRONOLOGY AND 3-D THERMO-KINEMATIC MODELLING

Abstract

Erosion and exhumation of upper crustal material in active orogens, like the Himalaya, result from a combination of both tectonic and surface processes. Although recent studies have well defined the Miocene-Pliocene exhumation history and deformation kinematics along most of the length of the Himalayan arc, the exhumational history and late Tertiary tectonic model of the Sikkim Himalaya are still poorly constrained. In Sikkim, the Indo-Tibetan convergence is accommodated along the Main Himalayan Thrust (MHT), a crustal scale décollement that currently reaches the surface through the Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT), at the toe of the orogenic prism. Erosion in central Sikkim has uncovered a double tectonic window exposing the underlying Lesser Himalayan Sequence (LHS) through the overlying nappe of Greater Himalayan Sequence (GHS). Within the windows, the LHS is structured in a duplex system which is thought to have accommodated significant amounts of horizontal shortening. The goal of this study is to discriminate between tectonic scenarios that might have impacted the exhumational history of the Sikkim Himalaya and determine the relative contribution of steady displacement on the MHT combined (or not) with duplexing in the LHS.

This study adopts a multi-faceted approach involving (U-Th)/He thermochronology on zircon (ZHe) coupled with 3-D thermokinematic modelling. Fifteen rock samples collected along two N-S-trending profiles across the Rangit and Tista Windows yielded cooling ages ranging from 11.87 ± 0.49 Ma to 1.30 ± 0.07 Ma. Approximately 20 - 30 km north of the MBT, the ZHe cooling age distribution shows an abrupt decrease; south of this break cooling ages range from ~12 to ~6 Ma, and north of it, within the double window and beyond, ages are younger than ~4 Ma. This break corresponds roughly to the southern exposure of the LHS units within the windows.

The age dataset was inverted using the thermo-kinematic modelling software Pecube to define the Late Miocene to present deformation and exhumation kinematics of the Sikkim Himalaya. Model scenarios were run both with and without the presence of a duplex. We find that a tectonic scenario only involving steady displacement on the basal décollement (MHT) during the last 12 Ma does not provide a satisfactory fit to the age data, while when combined with localized duplex-driven rock uplift in the LHS, it reproduces the young ages observed in the core of the double window. These results suggest that the MHT is not the only tectonic control on exhumation in Sikkim and that duplexing is a key process in the tectonic evolution of Sikkim since the late Miocene.