INTEGRATION OF GEOMETRY, KINEMATICS, BURIAL TIMING, AND EXHUMATION TIMING TO UNDERSTAND 4-D THRUST BELT EVOLUTION: AN EXAMPLE FROM THE HIMALAYAN OROGEN IN BHUTAN (Invited Presentation)

Quantifying records of convergence accommodation in orogenic belts presents significant challenges, as we only see the final snapshot of the deformation geometry. Here, we use a Himalayan case study to demonstrate an approach that integrates thrust belt geometry and related kinematics with the burial and exhumation histories of thrust sheets, in order to constrain a record of shortening rates.

The Himalayan thrust belt in eastern Bhutan consists of foreland thrust sheets of Lesser Himalayan (LH) rocks, a lower duplex of Paleozoic LH rocks, an upper duplex of Proterozoic LH rocks, and a thick sheet of Greater Himalayan (GH) rocks above the Main Central thrust (MCT). MCT emplacement is bracketed between 23 Ma, the youngest prograde metamorphism in its hanging wall, and shearing of 16 Ma leucosomes within the MCT zone. Construction of the upper LH duplex is bracketed between 20 Ma prograde metamorphism in the MCT footwall and 15 Ma prograde metamorphism in underlying Paleozoic LH rocks. GH rocks and Proterozoic LH thrust sheets were exhumed from peak temperatures of 650-750°C and 400-500°C, respectively, and therefore muscovite, zircon, and apatite cooling ages, which record cooling from 350-120°C between 12-4 Ma, are interpreted to represent erosion accompanying passive uplift and translation during deformation of LH rocks further to the foreland. Rapid cooling between 350-180°C in Proterozoic LH thrust sheets likely represents accelerated erosion during passage over a footwall ramp. Thrust sheets of the lower LH duplex were exhumed from peak temperatures of 350-400°C to 180°C between 15-10 Ma, which is interpreted to bracket the majority of duplex construction. Foreland LH thrust sheets cooled from 180-120°C between 10-6 Ma, which is interpreted to date initiation of emplacement.

Integrating these data with incremental shortening magnitudes obtained from cross-section reconstruction defines shortening rates of 20-35 mm/yr between 23-10 Ma, and a slowing to 3-10 mm/yr between 10-0 Ma, interpreted to represent the onset of strain partitioning between the Himalayan thrust belt and Tibetan Plateau. This example illustrates the importance of understanding evolving thrust belt geometry, including the locations of footwall ramps, which provides critical context for relating exhumation and deformation histories.