QUATERNARY SHORTENING RATES ACROSS THE MAIN FRONTAL THRUST ZONE OF THE EASTERN HIMALAYA SUGGEST AN EASTWARD INCREASE IN CONVERGENCE ACROSS THE HIMALAYA

The Main Frontal Thrust zone (MFTZ) is the currently active, orogen-scale thrust system bounding the Himalayan orogen. Understanding its along-strike variation in convergence rate has important implications for determining the mechanisms of Himalayan development. Although the Quaternary shortening rates across the MFTZ have been determined in the western and central Himalaya at ~14 mm/yr and ~20 mm/yr, respectively, no constraint is available on the convergence rate in the eastern Himalaya. Here we report new slip-rate data across the MFTZ from the Bhalukpong area (93°E) of the Arunachal Himalaya. There, the MFTZ comprises the Bhalukpong Thrust (BT) and the Balipara Anticline (BA). The BT places Miocene strata over Quaternary conglomerate, forming a fault scarp with a vertical height of ~ 50 cm. The BA in the BT footwall involves Pliocene and Quaternary fluvial and alluvial strata. In order to constrain the convergence rate across the MFTZ, we conducted detailed topographic surveys using a Total Station and collected key samples for radiocarbon dating. The results indicate that the forelimb terrace surfaces are rotated systematically from a hinge point at the northern edge of the flood plain and the back-limb terraces are all parallel to the active drainages and raised upward without rotation. We quantify the above observations using a new kinematic model for fold growth that invokes simple-shear deformation in the forelimb and fault-parallel translation of the back limb. Using this model and new radiocarbon dates, we estimate the slip rate on the blind thrust below the anticline to be 14.8 ± 0.2 mm/yr and 16.0 ± 0.1 mm/yr, respectively. Adopting the existing OSL ages for the back-limb terraces, we obtained a slip rate of 19.0 ± 4.0 mm/yr for the blind thrust assuming it dips at 30°. The combined radiocarbon dating and observed scarp height suggest a rate of 3.1 ± 0.2 mm/yr on the Bhalukpong thrust. The sum of the two active structures suggests a Quaternary shortening rate of 22.5 ± 4.0 mm/yr across the eastern Himalayan front. This rate is higher than that across the western Himalaya but similar or slightly higher than that across the central Himalaya, indicating an eastward increase in Quaternary convergence across the Himalaya.