Variable shortening rates in the eastern Himalayan thrust belt, Bhutan: Insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions

Abstract

We present data on the burial, displacement and exhumation history of the Himalayan fold‐thrust belt in eastern Bhutan. These data document the magnitude and timing of displacement of large, discrete structures and highlight temporal variability in shortening rates. Eight new40Ar/39Ar ages from white mica, 32 new zircon (U‐Th)/He ages, 7 new apatite fission track ages, and 1 new U‐Pb zircon (LA‐MC‐ICP‐MS) metamorphic rim growth age are combined with published cooling ages and deformation temperatures, and incremental shortening magnitudes from restorations of two published balanced cross sections, to illustrate the kinematic and temporal development of the Bhutan thrust belt. Integrating these data from ∼23 Ma to the present illustrates rapid horizontal shortening rates (28–35 mm/yr) between 23–20 Ma and 15–10 Ma, separated by more moderate rates (10–23 mm/yr). Shortening rates decrease significantly to 7–10 mm/yr (and possibly as low as 3–4 mm/yr) from 10 to 0 Ma. This decrease is interpreted to represent the onset of strain partitioning in the eastern part of the Himalayan‐Tibetan orogenic system, between shortening in the Bhutan thrust belt, uplift of the Shillong Plateau, and deformation and outward growth of the northern and eastern Tibetan Plateau. Within estimated error, horizontal shortening rates during emplacement of the Main Central thrust sheet and during construction of the upper Lesser Himalayan duplex approached India‐Asia tectonic velocities. Thus, for periods of time between ∼23–20 Ma and ∼15–10 Ma, the Bhutan thrust belt may have absorbed nearly all India‐Asian convergence at this longitude.

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

“…In addition to residence time and thermal conditions experienced by the rocks affecting argon loss, they suggested that a ±2 Ma age dispersion would be expected due to diffusive differences caused by grain size variations. MAr ages from eastern Bhutan postdate the age of south-directed shear on the MCT in this region (Stüwe and Foster, 2001;Grujic et al, 2002;Daniel et al, 2003;Kellett et al, 2009;Long et al, 2012). Thus, we interpret the age range of these four MAr samples as the window of permissible exhumation-induced cooling through the modeled closure temperatures of white mica (Ehlers, 2005;Braun, 2003).…”

“…North of the MCT, ZHe cooling ages are limited to two samples, one from the structurally higher Greater Himalaya and one from Tethyan rocks at the western edge of the Sakteng Klippe. These samples recorded cooling ages of 7.4 ± 1.6 and 7.1 ± 0.3 Ma, respectively (Coutand et al, 2014;Long et al, 2012).…”

“…In order to maintain structural context along the cross section, sample locations are projected onto the cross section along structure (i.e., in the direction of the fault trend, maintaining distance from structures). The 40 Ar / 39 Ar values of muscovite (MAr) and apatite fission track (AFT) data used are from previous studies and presented with 2σ analytical error (Table S1; Stüwe and Foster, 2001;Grujic et al, 2006;Long et al, 2012;Coutand et al, 2014). Previously published zircon (UTh) / He (ZHe) data are determined from the mean age of replicates (typically three grains) and presented in this study with a 2σ error that encompasses the range in measured ages .…”

“…3c, no. 2). Late KT is modeled with 45 km of out-of-sequence thrusting after the development of the upper Lesser Himalayan duplex, similar to the proposed model of sequential deformation by Long et al (2012;Fig. 3c, no.…”

“…These ages were interpreted by Long et al (2012) to indicate structural uplift, exhumation, and cooling of Lesser Himalayan rocks through the zircon (U-Th) / He closure temperature at ∼ 11.5-9.5 Ma. Measured ZHe ages in the Kuru Chu are 1-2 Myr younger than the ZHe ages along the Trashigang section between 15 and 30 km from the MFT Figs.…”