LAUSR

Abstract Mid-crustal melting is widely documented under orogenic plateaus. However, the mechanism for its generation and its role in the evolution of orogenic plateaus remain poorly understood. Here we investigate the physical controls and consequences of crustal melting in plateau evolution using 3-D thermo-mechanical modeling method. Our simulations demonstrate that the convergence causes orogenic crust to have more upper crustal compositions and more radiogenic elements. The persistent radioactive and shear heating provides heat source required for in situ partial melting of felsic rocks at mid-crustal depths. Slower convergence rate favors earlier emergence and larger concentration of mid-crustal melt. Radioactive heating accumulated during crustal thickening plays the primary role in generating mid-crustal partial melting, which is consistent with previous 2-D model results. Shear heating promotes crustal melting, but its role is secondary to radioactive heating. Our simulations also show that basal heating produces a broad-area increase of the crust temperature and surface elevation and, to a lesser extent, promotes mid-crustal melting, because convergence-driven heat advection dominates the transport of the newly-added heat. The significant mid-crustal melting dramatically reduces the viscosity of the middle crust and eventually leads to channelized outward flow of the melt-weakened layer. This has the potential to cause mechanical decoupling of the layers above and below the melt channel and the marginal dominance of mid-crustal melting. We suggest that the widespread low velocity or resistivity zones observed in Tibet are caused by interconnected mid-crustal melting, which is an inherent feature of large hot orogenic plateaus.