TECTONIC IMPLICATIONS OF STRUCTURAL DISCONTINUITIES IN THE HIMALAYAN MID-CRUST

Critical wedge taper and channel flow processes have commonly been characterized as model end-members. Recent studies, however, have shown that is a false dichotomy and that the two convergence accommodation processes are compatible. These studies posit that there is a spatial and temporal relationship between lateral midcrustal flow in the deep orogenic hinterland and wedge taper processes towards the shallower orogenic foreland. Moreover, they outline a transition that occurs between the processes that dominate in the respective domains. This transition is akin to the ‘MCT’ of channel flow models that separate rocks involved in a mid-crustal channel flow above from rocks deformed through wedge taper processes below. While these transitions have been identified in recent literature, little is yet known about their kinematics. Quartz lattice preferred orientation (LPO) data from across the Tama Kosi region of east-central Nepal have been employed to investigate the nature of the transition mapped in that area where it manifests as a tectonometamorphic discontinuity between staurolite grade and kyanite grade metamorphic rocks with distinct metamorphic and deformational histories. The LPO data collected have shed light on not only the staurolite-kyanite discontinuity, but also identified a hitherto unrecognised discontinuity structurally higher within sillimanite grade rocks. These discontinuities represent post-peak metamorphic strain concentrations; elongation directions and asymmetric LPO data are consistent with top-to-the-south thrust-sense shear zones. The development of these discontinuities is interpreted to reflect the expansion of the foreland wedge taper, the translation of rocks metamorphosed as in the deep hinterland into the foreland wedge, or some combination of the two. Moreover, they provide evidence of Miocene imbrication of the Himalayan mid-crust and demonstrate spatial and temporal compatibility between channel flow and wedge taper processes.