HIMALAYAN THRUST BELT PROPAGATION INTO ITS LOW-ALPHA FORELAND: RESPONSE TO ALONG-STRIKE VARIATIONS IN EROSION, SEDIMENTATION, AND FLEXURAL SUBSIDENCE

Most of Earth’s large orogens, including the Himalaya, consist of tapered (wedge-shaped in cross-section) thrust belts that are shortened and thickened by tectonic stresses near convergent plate boundaries. Critical taper models (in the broadest sense, including Mohr-Coulomb, perfectly plastic, and viscous formulations) have been applied successfully to explain large-scale kinematics in numerous thrust belts. An apparent paradox, however, is presented by the fact that orogenic wedges routinely propagate into foreland regions where surface slope (α) is extremely low—perhaps even negative. This seemingly counterintuitive reality exists because critical taper is possible at α ≤ 0, which is commonly the case at the fronts of thrust belts where they transition into foreland basins. Very low (but sufficient) values of α are commonly observed in thrust belts riding on overpressured and salt-rich basal detachments, but these conditions do not apply to most of the Himalayan thrust belt. Because foreland α is perpetually very low, foreland taper is dominated by basal slope (β), which is controlled by flexural rigidity of underthrusting Indian lithosphere. β increases exponentially as a given spot in the foreland approaches the thrust front, and any increase in α owing to sedimentation can drive initial taper of the foreland basin to a critical or supercritical value. These principles are employed to analyze the kinematic history of the Himalayan orogenic wedge in central Nepal over the past ca. 10 Ma. The plan-view shape of the thrust belt in this region includes a strongly reentrant Main Boundary thrust (MBT) and a nearly mirror-image salient on the Main Frontal thrust (MFT), suggesting a recent change from erosion-dominated subcritical behavior to critical-supercritical behavior. Flexurally increasing β, coupled with localized high-volume sedimentation linked to a large erosional bight in the thrust belt (which probably retarded propagation of the MBT), allowed for increased initial taper and MFT activation. Thus, the thrust belt experiences spurts of forward propagation along ca. 150-200 km-long strike segments that may be erosionally modulated. Numerical models combining orogenic-wedge mechanics with syntectonic sedimentation support this hypothesis.