Rheological Evolution and Strain Partitioning in Orogenic High-Strain Zones: Constraints from Subduction to Collision

Orogenic high-strain zones are fundamental features of the continental lithosphere. These structures accommodate plate convergence and influence the distribution of crustal deformation. However, the specific rheological processes that govern these zones across different tectonic settings remain a subject of investigation. Here, we integrate quantitative data from three distinct systems in China: the Hulin Complex (subduction), the Shangdan Shear Zone (collision), and the Diancang Shan (lateral extrusion). Our goal is to establish a grounded framework for how high-strain zones evolve. Our analysis shows that deformation follows a predictable path influenced by temperature, fluid activity, and strain partitioning.

We identify a consistent relationship between deformation style and kinematic vorticity (W_k). In the deep crust where temperatures exceed 650°C, deformation is characteristically diffuse. Data from the early Shangdan and Diancang Shan complexes indicate that this phase is dominated by pure shear (W_k =0.24–0.41). In these high-temperature regimes, pure shear contributes approximately 65% of the total strain. This mechanism facilitates vertical crustal thickening during the initial stages of plate interaction. As the crust cools to mid-crustal conditions between 300°C and 550°C, a mechanical transition occurs. Strain concentrates into narrow, high-strain paths. This localization coincides with a sharp increase in W_k to 0.53–0.74. This shift demonstrates that simple shear becomes the dominant mode for accommodating plate movement as the orogen matures.

Our integrated dataset suggests that these high-strain zones operate within a “stress window” of 10–50 MPa. Within this range, the crust adjusts its internal fabric to match tectonic driving forces. In subduction systems like the Hulin Complex, rapid slab rollback triggers thermal softening. This process drops differential stress to a minimum of 13–14 MPa and promotes regional extension. In contrast, collisional systems like the Shangdan Shear Zone support higher stresses between 33 and 45 MPa to drive mylonitization. We find that the transition from slow, diffuse flow to rapid, localized shear is non-linear. External factors often trigger this change. In the Shangdan Shear Zone, fluid influx acts as a catalyst. It increases strain rates by two orders of magnitude, from 10⁻¹⁵ to 10⁻¹³ s⁻¹. Similarly, in the Diancang Shan, partial melting helps maintain high strain rates of 10⁻¹² s⁻¹ despite decreasing temperatures.

We conclude that high-strain zones function as dynamic features of the continental crust. They manage deformation by adjusting the ratio of pure shear to simple shear in response to the local thermal and fluid environment. Early-stage diffuse flow accommodates initial convergence through thickening. Later, localized simple shear facilitates lateral extrusion or exhumation. This mechanical flexibility allows the continental lithosphere to endure complex plate cycles. Our findings provide a quantitative framework for predicting how shear zones behave in active orogenic belts. These data bridge the gap between mineral-scale observations and large-scale tectonic processes.