COMBINING KINEMATIC AND NUMERICAL MODELING TO UNDERSTAND THE PROGRESSION FROM DETACHMENT FOLDING TO FAULT-CORED FOLDING; A CASE STUDY FROM THE INDO-BURMAN FOLD-THRUST BELT

The Indo-Burman ranges (IBR), spanning Bangladesh, India and Myanmar, are the planet's largest example of subduction accretion. The IBR absorbs oblique (~70˚) convergence of the ~13-19 km thick Ganges-Brahmaputra Delta (GBD) on the Indian plate with the Burma microplate. The eastward component of convergence, perpendicular to the northerly structural trend of the IBR, is partitioned to the up-dip part of the wedge and absorbed by an ~200 km wide subaerial fold-belt, forming an ideal setting to investigate the tectonic evolution of an active accretionary prism. We present new kinematic models of the IBR constrained by field studies and industry seismic lines. The frontal ~100 km of the IBR is defined by a train of gentle, regularly spaced (~15-20 km) detachment folds that are buried by the GBD. With progressive shortening, the detachment folds transition to fault-cored antiforms, including trishear and kink-band fault propagation folds, and imbricate fans of fault-propagation folds with breakthrough-thrusts. By comparing finite-element numerical modeling experiments from two codes, FLAC and Elfen, we evaluate how the progression from detachment to fault-related folding may reflect a change through time of the mechanical properties of the strata in the foldbelt, including the sediment consolidation state, overpressure, and the accumulation of distributed anelastic strain. By using FLAC to model horizontal shortening of mechanically stratified visco-elasto-plastic sedimentary layers with geologically reasonable physical properties, we successfully simulate gentle viscous folding with dimensions that resemble the buried detachment folds near the front of the IBR. Elfen is capable of applying critical state theory to account for distributed volumetric strain by yielding (e.g., plastic pore collapse and cataclasis) prior to faulting. Using Elfen, we test the hypothesis that strain-hardening by distributed plastic deformation of the initially poorly consolidated sedimentary layers prompts the transition from gentle visco-elastic folding to localized elasto-plastic deformation (faulting). Our results are preliminary and with ongoing studies we will continue to investigate these effects.