The effects of plate-bending-related aquifer thickening on temperatures in the Japan Trench subduction zone

Abstract:

Accurate subduction zone thermal models are necessary to understand a wide range of geophysical and geochemical processes, including: metamorphic reaction progress, mantle wedge hydration, and melt generation. For decades, plate convergence rate and subducting plate age and dip have been recognized as basic factors affecting subduction zone temperatures. Recent discoveries highlight the important effects of fluid circulation in oceanic lithosphere on subduction zone temperatures. However, there are contrasting hypotheses for the distribution of the regions within the oceanic lithosphere that host vigorous fluid circulation: one with a constant thickness aquifer extending both seaward and landward of the trench, one with an aquifer that thickens as it approaches the trench (due to bend-related faulting) but assumes vigorous fluid circulation only occurs prior to subduction, and a hybrid that considers both aquifer thickening seaward of the trench and continued circulation in the subducting plate. I examine the effects of bend-related aquifer thickening on temperatures within the Japan Trench subduction zone with a suite of thermal models. Kawada et al. [2014] hypothesize that plate-bending faults offshore northern Japan increase the thickness of the oceanic crustal aquifer, and therefore influence heat redistribution in the system. Existing models explore the effects of aquifer thickening on heat flux seaward of the trench, but they do not examine the effects of this process on temperatures within the subduction zone; additionally, they treat the aquifer seaward of the trench as completely isolated from the aquifer in the subducted plate. Here, I exploit the fact that aquifer thickening from the outer rise to the trench and continued fluid circulation in subducting crust are expected to produce distinct surface heat flux anomalies, in order to constrain the thermal effects of each process. I find the combinations of the amount of aquifer thickening and the degree of continued circulation in the subducting crust that yield results consistent with surface heat flux observations.