Stress, strain and crustal flow patterns in a corner collision: insights from coupled 3D numerical models

Large and fast collisional systems such as the Eastern Tibetan-Himalayan orogenic system can have distinct corner structures. Away from the corners, plate convergence is accommodated primarily by convergence-parallel processes such as (continental) subduction, crustal thickening and buckling. Around the corners, oblique and convergence-perpendicular processes become more important, such as strike-slip, transpressional and transtensional faults. The strike of the subduction front itself can also vary in space, as tomographic images show for the case of the Indian slab beneath Tibet and Burma. At the corners themselves, a peculiar syntaxis structure may form which is characterised by effective strain localisation and high rates of exhumation and erosion. However, our understanding of the temporal evolution of orogenic syntaxis formation is still elusive.Here, we use high-resolution, three-dimensional thermomechanical models to investigate principal stress orientations, strain rate patterns and upper versus lower crustal flow patterns within a continental corner collision setting loosely resembling the Eastern Tibetan-Himalayan orogenic system. We use a 1000 x 200 x 1000 (x * y * z) model domain with a permeable lower boundary and a 2 km grid resolution in each dimension. Each grid cell has 8 markers. The models are carried out using I3ELVIS (Gerya and Yuen, 2007) coupled to the surface process model FDSPM (Munch et al., 2022). Our numerical experiments highlight that i) significant lateral variability occurs despite prescribing orthogonal kinematic boundary conditions; ii) a high variability of stress states and deformation styles occur within the modelled orogen and plateau; iii) Lower crust beneath the plateau escapes later than upper crust, but around 3-4 times faster. Lastly, we examine the sensitivity of the model evolution to different degrees of strain weakening, intracrustal layering, and the diffusion coefficient of the surface process model.Gerya, T. V., Yuen, D. A. (2007). Robust characteristics method for modelling multiphase visco-elasto-plastic thermo-mechanical problems. Physics of the Earth and Planetary Interiors, 163(1), 83105. https://doi.org/10.1016/j.pepi.2007.04.015Munch, J., Ueda, K., Schnydrig, S., May, D. A., Gerya, T. V. (2022). Contrasting influence of sediments vs surface processes on retreating subduction zones dynamics. Tectonophysics, 836, 229410. https://doi.org/10.1016/j.tecto.2022.2294