Thrust Sequence in the Western Fold-and-Thrust Belt of the Indo-Burma Range Determined from Fluvial Profile Analysis and Dynamic Landform Modeling

The Indo-Burma Range (IBR) is a forearc fold-and-thrust belt that evolved under oblique convergence between India and the Burma microplate. IBR convergence is understood to be absorbed by north-striking dextral strike -slip faults and a west-propagating fold-and-thrust belt (FTB), however, the FTB spatio-temporal evolution and its seismic potential remain an area of active research. This study combines geomorphic and kinematic analyses to explore simple thrust-propagation scenarios for the IBR outer belt. We use a 30 x 30 m SRTM digital elevation model and investigate four river basins in the outer belt by extracting antecedent tributaries that cross young, NS -striking antiforms and analyzing their fluvial profiles. To help interpret the profiles, we explore first-order surface dynamics models based on four possible scenarios of the FTB evolution, including cases of sequential propagation, partial reactivation, continuous reactivation, and synchronous thrusting. We compare our modeled stream profiles and knickpoint locations to the IBR river profiles. The shapes of the profiles between knickpoints are most similar to model results of sequential propagation and continuous reactivation of older structures. This suggests that as younger antiforms start rising in the west, the eastern folds are still active. The stream profiles of the synchronous thrusting model also demonstrate similar shapes with the IBR tributaries crossing the west-ernmost antiforms. Results from a numerical FTB experiment with a weak detachment overlain by stronger sediment layers predicts cyclic periods of frontal ramp propagation and fault reactivation. Overall, our findings suggest alternating periods of FTB propagation and fault reactivation in the outer belt, implying that earthquakes may alternately rupture to the deformation front and/or along splay thrusts in the wedge. The shapes of ante-cedent stream profiles crossing active FTB may record information about the sequence of ramp propagation and can be studied to inform the kinematic evolution of FTB.