When continents collide, the arrival of positively buoyant continental crust slows down subduction. This collision often leads to the detachment of earlier subducted oceanic lithosphere, which changes the subsequent dynamics of the orogenic system. Recent studies of continental collision infer that the remaining slab may drive convergence through slab roll-back even after detachment. Here we use two-dimensional visco-elasto-plastic thermo-mechanical models to explore the conditions for post-collisional slab steepening versus shallowing by quantifying the dynamics of continental collision for a wide range of parameters. We monitor the evolution of horizontal mantle drag beneath the overriding plate and vertical slab pull to show that these forces have similar magnitudes and interact continuously with each other. We do not observe slab rollback or steepening after slab detachment within our investigated parameter space. Instead, we observe a two-stage elastic and viscous slab rebound process lasting tens of millions of years, which is associated with slab unbending and eduction that together generate orogenic widening and trench shift towards the foreland. Our parametric studies show that the initial length of the oceanic plate and the stratified lithospheric rheology exert a key control on the orogenic evolution. When correlated with previous studies our results suggest that post-detachment slab rollback may only be possible when minor amounts of continental crust subduct. Among the wide variety of natural scenarios, our modelling applies best to the evolution of the Central European Alps. Furthermore, the mantle drag force may play a more important role in continental dynamics than previously thought. Finally, our study illustrates that dynamic analysis is a useful quantitative framework that also intuitively explains observed model kinematics.

<p>Geological and geophysical observations have highlighted the multi-stage deformation history of the continental lithosphere. Such inherited heterogeneities, observed from microscopic to kilometre-scales, lead to important mechanical weakening for the subsequent development of orogens. This strain-weakening may be frictional (fault gauge, filled veins), ductile (banding, recrystallisation, etc) or caused by changes in grain-size, and largely determines the response of the lithosphere to stresses (Bercovici & Ricard, 2014). Representing the microstructural weakening mechanisms with the relatively low resolution of regional and global numerical modelling studies has been a longstanding challenge. Mechanisms are often grouped into an “effective” plastic strain weakening implementation, where the frictional strength decreases with increasing accumulated strain. Alternatively, materials can be modelled to weaken depending on the local strain-<strong><em>rate</em></strong> (Ruh et al., 2014), which is characteristic for e.g. coseismic frictional weakening of faults. Here we show key differences of strain- vs. strain-rate-dependent faults weakening in terms of orogenic strain propagation patterns in numerical models of a corner collision setting, based on the eastern corner of the India-Eurasia collision. The numerical model I3ELVIS (Gerya & Yuen, 2007) consists of a finite-difference, marker-in-cell method coupled to a diffusion-advection-based finite-difference surface process model, FDSPM (Munch et al., 2022). We highlight key differences between the results of a model with strain-rate-dependent weakening, and a model with conventional strain-dependent weakening based on accumulated strain. The former shows significantly sharper shear zones, as well as a higher number of thrust faults that are relatively evenly spaced, which is more realistic in natural collision zones. </p> <p> </p> <p>Gerya, T. V., & Yuen, D. A. (2007). Robust characteristics method for modelling multiphase visco-elasto-plastic thermo-mechanical problems. <em>Physics of the Earth and Planetary Interiors</em>, <em>163</em>(1), 83–105. https://doi.org/10.1016/j.pepi.2007.04.015</p> <p>Bercovici, D., & Ricard, Y. (2014). Plate tectonics, damage and inheritance. <em>Nature</em>, <em>508</em>(7497), 513–516. https://doi.org/10.1038/nature13072</p> <p>Ruh, J. B., Gerya, T., & Burg, J.-P. (2014). 3D effects of strain vs. Velocity weakening on deformation patterns in accretionary wedges. <em>Tectonophysics</em>, <em>615–616</em>, 122–141. https://doi.org/10.1016/j.tecto.2014.01.003</p> <p>Munch, J., Ueda, K., Schnydrig, S., May, D. A., & Gerya, T. V. (2022). Contrasting influence of sediments vs surface processes on retreating subduction zones dynamics. <em>Tectonophysics</em>, <em>836</em>, 229410. https://doi.org/10.1016/j.tecto.2022.229410</p> <p> </p>

<p>The inherent links between tectonics, surface processes and climatic variations have long since been recognised as the main drivers for the evolution of orogens. Oceanic and continental subduction and collision processes lead to distinct topographic signals. Simultaneously, different climatic forcing factors and denudation rates substantially modify the style of deformation leading to different stress and thermal fields, strain localisation and even deep mantle evolution. An ideal area where the above-mentioned processes and their connections can be studied is the India-Eurasia collision zone.</p><p>Understanding the complex interplay between tectonics, erosion, sediment transportation and deposition requires the coupled application of thermo-mechanical and surface processes modelling techniques. To this aim, we used a 3D coupled numerical modelling approach. The influence of different plate convergence, erosion and sedimentation rates has been tested by the thermo-mechanical code I3ELVIS (Gerya and Yuen, 2007) coupled to the diffusion-advection based (FDSPM) surface processes model.</p><p>We show preliminary results to demonstrate  that the diffusion-advection erosion implementation has significant effects on local and regional mass redistribution and topographic evolution within narrow, curved, high orogens such as the Himalayas and their syntaxes, where erosion is a dominant forcing factor. We also discuss possible implications from different erosion/sedimentation implementations such as DAC (Ueda et al., 2015; Goren et al., 2014) in combination with the reference thermo-mechanical model to analyse changes in orogenic development as a consequence of different erosional processes in more detail.</p><p>References:</p><p>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(1-4), 83-105. <br>Ueda, K., Willett, S. D., Gerya, T., & Ruh, J. (2015). Geomorphological–thermo-mechanical modeling: Application to orogenic wedge dynamics. Tectonophysics, 659, 12-30.<br>Goren, L., Willett, S. D., Herman, F., & Braun, J. (2014). Coupled numerical–analytical approach to landscape evolution modeling. Earth Surface Processes and Landforms, 39(4), 522-545.</p>