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

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 new ⁴⁰Ar/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.