THE THERMAL EFFECTS OF PLATE-BENDING-RELATED THICKENING OF THE OCEANIC CRUSTAL AQUIFER IN THE JAPAN TRENCH AND NANKAI TROUGH SUBDUCTION ZONES

We examine the potential effects of plate-bending-related normal faulting on heat redistribution by fluid circulation in the Japan Trench and Nankai Trough subduction zones. We construct thermal models for transects across each margin that include an aquifer in the oceanic crust that enhances heat transport. We examine the effects of varying the aquifer thickness resulting from the growth of plate-bending-related normal faults, and varying the extent to which hydrothermal circulation persists within the subducted crust. Bending-related normal faults are hypothesized to enhance vertical heat exchange and inhibit lateral advective heat exchange. Previous thermal models examining aquifer thickening only consider the aquifer seaward of the trench. We find that models most consistent with heat flux data for the Japan Trench include enhanced vertical heat transport, but limited lateral heat transport, in an aquifer that thickens as it approaches and enters the subduction zone. For the Nankai margin, models most consistent with the heat flux data include substantial lateral heat transport in the oceanic crustal aquifer; they do not require enhanced vertical heat transport resulting from aquifer thickening. The greater effect of aquifer thickening at the Japan Trench (relative to the Nankai margin) may result from the subducting plate having a greater degree of curvature than that at the Nankai margin. The large offsets of normal faults in the Pacific Plate approaching and entering the Japan Trench may disrupt lateral advective heat exchange in the aquifer, but permit enhanced vertical heat exchange between the lower and upper oceanic crust. The Nankai margin is substantially hotter than the Japan Trench. As a result, the temperature gradient between the incoming and subducted aquifer is larger at the Nankai margin. The large lateral temperature gradient supports vigorous lateral heat exchange. The thermal effects of aquifer thickening are modest relative to the large lateral advective heat redistribution in hot subduction zones. However, for cold subduction zones with high degree of curvature in the subducting plate, aquifer thickening can be an important thermal process.