Late Miocene-Pleistocene evolution of India-Eurasia convergence partitioning between the Bhutan Himalaya and the Shillong Plateau: New evidences from foreland basin deposits along the Dungsam Chu section, eastern Bhutan

Abstract

The Shillong Plateau is a unique basement‐cored uplift in the foreland of the eastern Himalaya that accommodates part of the India‐Eurasia convergence since the late Miocene. It was uplifted in the late Pliocene to 1600 m, potentially inducing regional climatic perturbations by orographically condensing part of the Indian Summer Monsoon (ISM) precipitations along its southern flank. As such, the eastern Himalaya‐Shillong Plateau‐ISM is suited to investigate effects of tectonics, climate, and erosion in a mountain range‐broken foreland system. This study focuses on a 2200 m thick sedimentary section of the Siwalik Group strategically located in the lee of the Shillong Plateau along the Dungsam Chu at the front of the eastern Bhutan Himalaya. We have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission track dating, combined with sedimentological and palynological analyses. We show that (1) the section was deposited between ~7 and 1 Ma in a marginal marine deltaic transitioning into continental environment after 5 Ma, (2) depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the Shillong Plateau had an unexpected limited impact on the paleoclimate of the Bhutanese foothills, and (3) the diminution of the flexural subsidence in the basin and/or of the detrital input from the range is attributable to a slowdown of the displacement rates along the Main Boundary Thrust in eastern Bhutan during the latest Miocene‐Pleistocene, in response to increasing partitioning of the India‐Eurasia convergence into the active faults bounding the Shillong Plateau.

Timing and mechanism of the rise of the Shillong Plateau in the Himalayan foreland

“…Zircons from eight sandstones from the DC section (see the GSA Data Repository 1 ), magnetostratigraphically dated between 7 and 1 Ma (Coutand et al, 2016), were U-Pb dated using ion-microprobe and inductively coupled plasma-mass spectrometry (ICPMS) techniques (see Data Repository sections 2-6). Samples ≤4.9 Ma show 13%-22% CretaceousCenozoic grains, which we interpret as being derived from the Transhimalaya (Fig.…”

Testing the effects of topography, geometry, and kinematics on modeled thermochronometer cooling ages in the eastern Bhutan Himalaya

“…In Bhutan the MFT emplaces a single thrust sheet of MiocenePliocene Subhimalayan units referred to as the Siwalik Group over modern foreland basin deposits (Gansser, 1983;Long et al, 2011b;Coutand et al, 2016).…”

Formation of a Rain Shadow: O and H Stable Isotope Records in Authigenic Clays From the Siwalik Group in Eastern Bhutan

“…We revised the exclusively lithological subdivision of the Siwaliks by identifying 12 facies and 4 facies associations representative of distinct depositional paleoenvironments. The youngest dated beds at the top of the section are 1.2 Ma old (Coutand et al, 2016). Unit 1 deposits are interpreted as different parts of a river-dominated deltaic system, developed in either a lacustrine or a marine environment between <7.2 and 6.4 Ma.…”

“…Furthermore, we dated Siwalik deposition along the Dungsam Chu section, using magnetostratigraphy constrained by vitrinite reflectance data combined with detrital apatite fission track dating (Figure 3; Coutand et al, 2016), to the latest Miocene and the Pleistocene, between 7.2 Ma and 1 Ma. Paleosols were identified throughout the section (Coutand et al, 2016) and samples for this study were collected from each pedogenically modified horizon. Unit 2 deposits (6.4-5 Ma) represent a river-dominated and storm-dominated deltaic depositional environment, which transitioned after 5 Ma into Unit 3 deposits, a sandy alluvial environment.…”

“…According to the calculated sedimentation rates (Coutand et al, 2016), sediments from the base of the section attained the depth of burial corresponding to 488C by 5.5-6.0 Ma for geothermal gradients of 20 and 308C/km, respectively ( Figure 11). However, because the clay minerals formed or equilibrated with the meteoric water during burial, their isotopic composition was acquired when the sediments reached the corresponding depth, not at their stratigraphic age.…”