| Temperature plays a dominant role in many subduction zone processes, including arc magma generation and the distribution of earthquakes. In this thesis, observational constraints on forearc and backarc thermal structure are integrated with numerical models to better understand the thermal consequences of subduction. Forearc thermal models, applied as an example to the Mexico subduction zone, indicate that the forearc is cool, as expected due to the cool subducting slab. The brittle part of this subduction fault extends to depths >30 km where, even though very weak, the fault may generate small but non-negligible frictional heating. The rupture extent of Mexican megathrust earthquakes is consistent with proposed seismogenic limits of 100 and 350°C. A remarkable feature of subduction zones is that, although the subducting plate cools the over-riding crust and produces low temperatures in the forearc, the arc and backarc regions are consistently extremely hot, as indicated by arc volcanism, surface heat flow, seismic velocities, Te , and other observations. At the Cascadia subduction zone, backarc temperatures are ∼1200°C at 50--60 km depth with little variation for 500 km east to the craton. Thermal constraints from other subduction zone backarcs, including those that have had no recent extension, show that they are similarly hot for 100's--1000's of km behind the arc. Local sources of heat (e.g., radiogenic heat production, frictional heating) appear to be small, and mantle flow is invoked to carry heat into the backarc. Thermal-mechanical numerical models for slab-driven corner flow give flow that is strongly focussed into the wedge corner below the arc but low mantle temperatures further into the backarc, inconsistent with observations. It is concluded that slab-driven flow is insufficient to satisfy the heat budget at a subduction zone. Geodetic and geological constraints indicate extremely low mantle viscosities in several backarcs (<1019 Pa s), suggesting that thermal buoyancy may be the primary driving force for flow. High temperatures and hydration of the mantle wedge by the subducting slab may reduce the viscosity, allowing vigourous thermal convection that rapidly carries heat upward from depth into the subduction zone. |
