Deep subduction and rapid exhumation: Role of crustal strength and strain weakening in continental subduction and ultrahigh‐pressure rock exhumation

Abstract

The exhumation of crustal ultra‐high‐pressure (UHP) material depends on temporal and spatial variations in its detachment within the subduction channel. This dependence is investigated using numerical models with variable initial crustal strengths, representing a range of initial crustal compositions, and parameterized strain weakening, representing a range of processes that reduce effective crustal viscosity during deformation. Competition between down‐channel shear traction, favoring subduction, and up‐channel buoyancy, favoring exhumation, is expressed as the exhumation number, E, which can vary with time and position along the channel. Exhumed lower strength crust, which resists subduction owing to weak down‐channel traction, records peak conditions <35 kbar and exhumation rates <30 km Ma−1. Higher strength crust is efficiently subducted to UHP depths (E < 1), recording peak pressures >38 kbar. Given sufficient strain weakening, exhumation proceeds at >60 km Ma−1, indicating that buoyancy (E ≫ 1) drives exhumation in these models. In all models, exhuming UHP material forms a deforming ductile plume, with a range of possible structural relationships predicted between exhumed UHP and HP materials.