Tectonic control on asymmetric structural evolution and exhumation in the Southern Alps of New Zealand

Tectonics and climate are major drivers of exhumation in the Southern Alps of New Zealand (SANZ). The region exhibits strong orographic precipitation and asymmetric exhumation, with many studies attributing the latter to orographic precipitation. Here, we use a coupled model of tectonics, surface processes, and orographic precipitation to examine their interactions in the SANZ. In contrast to previous work, we do not prescribe a velocity discontinuity at the base of the crust but instead fully resolve the dynamic evolution of the mantle lithosphere. We model two contrasting precipitation scenarios, where winds are either aligned with or oppose the subduction direction. Both scenarios yield similar structures and exhumation patterns that match geological and geomorphological observations from the SANZ. These results indicate that, regardless of wind direction, a steady-state orogen always has a major thrust fault dipping opposite to the subduction direction, with exhumation focused on the retro-side. Our models suggest that the mantle lithospheric structure of the orogen impacts crustal convergence patterns such that the sensitivity of orogenic processes to climatic asymmetries is reduced. This implies that tectonics primarily governs structural evolution and asymmetric exhumation in small orogens while variability in erosion due to orographic precipitation is a secondary process.