The Effect of Near-Trench Fluid Circulation on Slab Dehydration Depth in the Chile Subduction Zone

Abstract:

Fluids released from subducting slabs affect earthquakes, geochemical recycling, melt generation, and mantle wedge flow. The distribution of this fluid release is controlled by the composition/hydration of the slab entering a subduction zone and the pressure-temperature path that the slab follows. We examine the potential for along-strike changes in the thermal state of the south central Chile subduction zone (36 to 45 °S) to affect the distribution of fluid release from the subducting Nazca Plate. Because the age of the plate entering the subduction zone decreases from ~30 Ma at 36 °S to ~1 Ma at 45 °S, a southward warming of the subduction zone has been hypothesized. We model temperatures in the system, then use results of the thermal models and the thermodynamic calculation code Perple_X to estimate the distribution of dehydration-derived fluid release from the subducting slab. Surface heat flux observations in the region are most consistent with fluid circulation in the high permeability upper oceanic crust redistributing heat. This hydrothermal circulation preferentially cools parts of the system with the youngest subducting lithosphere. For example, relative to simulations with no fluid flow, hydrothermal circulation decreases temperatures in the 45 °S transect by up to 150 °C. Although hydrothermal circulation in the oceanic crust likely ceases by ~50 km landward of the trench, the legacy of its heat redistribution affects slab temperatures and dehydration >100 km farther landward. In the 45 °S transect, using temperatures from a model that includes hydrothermal circulation yields peak slab dehydration centered under the volcanic arc. In contrast, without hydrothermal circulation, peak slab dehydration is predicted at ~70 km seaward of the volcanic arc. For systems with young (<20 Ma) subducting lithosphere, hydrothermal circulation in oceanic crust should be considered in estimating subduction zone temperatures and fluid source distributions.