Orogens in the Alpine-Himalayan collision zone (AHCZ) exhibit characteristic diffused seismicity compared to the stable continental interiors. Interestingly, they also have a thicker-than-average silica-rich upper crust and total crustal thickness, while their lithosphere thickness is similar to that of stable continental interiors (e.g., Tibet, Zagros). These observations provide a metric for the lithospheric-scale geological inheritance, the role of which we aim to understand in continental lithosphere dynamics over seismic and geologic timescales. To achieve this understanding, we use data-driven modelling to compute the present-day thermomechanical state of the AHCZ lithosphere.
Our results indicate the existence of a critical crustal thickness, which is thermodynamically controlled by the internal energy and chemical composition of the crust and is similar to the global average of continental crust thickness. Orogenic lithospheres with thicknesses above this critical value possess higher potential energy and are weakened by the internal energy from heat-producing elements, whereas continental interior lithospheres with thicknesses close to the critical crustal thickness are stronger. Weaker orogenic lithospheres respond via dissipating this energy in a diffused deformation mode, leading to zones of deformation in contrast to focused deformation at the plate-boundaries. The observed crustal differentiation in the AHCZ could be understood as perturbations to the critical crustal thickness caused by plate-boundary forces. The dynamic evolution of these perturbations indicates that the critical crustal thickness is a stable fixed-point attractor in the evolutionary phase-space.