GEODYNAMICS OF STRAIN PARTITIONING FACILITATED BY MECHANICAL HETEROGENEITY AND OBLIQUE CONVERGENCE IN THE ARCUATE HIMALAYA

Continuous exposure of major shear zones and rock units along the strike of the Himalaya suggests along-strike similarity in tectonic architecture. However, significant along-strike complexity is also exhibited by many non-cylindrical features in the western and central Himalaya. These observations include a supradetachment basin in the hinterland; a metamorphic core complex that accommodates wedge-parallel extension; along-strike thickness variation in the high-grade metamorphic core of the orogen; a transtensional fault system that cuts across the wedge; distributed strike-slip sense and normal sense brittle faults that accommodate wedge-parallel movement; and a deviation from an arc-shaped active uplift front. Here we use 3D mechanical numerical models to show that these features can be explained by strain partitioning caused by oblique convergence in an arcuate orogen with along-strike mechanical heterogeneity in the detachment. A set of 3D mechanical numerical models with varying mechanical properties of the material in the wedge, rear shear zone, and basal shear zone reveal that the distribution of strain in the wedge is most sensitive to the basal shear zone. The modeling results suggest that a strong segment in the basal shear zone can facilitate the development of an embayment in the active uplift front and enhanced thickening to the hinterland-ward of it and that the oblique convergence with a gradient in obliquity along the strike can facilitate the development of the wedge-parallel lengthening features. The combination of these conditions is the cause of complex geological features such as metamorphic core complex with thickened footwall and transtensional fault system that obliquely cuts the wedge along one limb of the embayment. We also compare different parts of our model with other convergent wedges in similar tectonic settings to illustrate its applicability to other regions. The first-order consistency highlights the significance of 3D strain partitioning in obliquely convergent boundaries and along-strike mechanical heterogeneity in a megathrust in generating regional-scale geological features.