Localized thermal disturbance caused by subduction of Kyushu-Palau Ridge at western end of Nankai, as revealed from BSR-derived heat flow data

Abstract:

In the Hyuga-nada forearc region off eastern Kyushu, weaker degree of plate coupling is inferred from frequent seismicity and the slow slips inferred from seafloor geodetic data. This exhibits a clear contrast against the strong coupling in the Nankai Trough region to the east. The tectonics around Hyuga-nada is much more complicated; a colder and rough-surface crust (west Philippine Basin; ~60 Ma) is subducting with steeper dip angle at depth, in contrast to the young, smoother Shikoku Basin to the east. Along the old/young boundary, the Kyushu-Palau Ridge (KPR) is obliquely subducting toward N30W since several Ma B.P.

Here we present heat flow data including new ones obtained along JAMSTEC seismic lines to discuss how the heat flow is affected by these tectonic/morphological disturbances. The heat flow marks a sharp boundary across KPR; it is 50-100mW/m2 to the east and 25-40mW/m2. The transition from high to low heat flow occurs in only 20 km across KPR. Higher heat flows of 100 mW/m2 to the east are located near the axis of Nankai Trough, similar to those reported off Muroto. It is attributed to the fluid discharge along decollement. The lack of such high heat flow to the west can be due to the older subducting plate, although the lack of measurements is also possible.

We performed two numerical calculations. First, 3D thermal modeling considers the subduction of old and young crusts (age difference of 20 Ma) neighboring to each other. Due to thermal exchange between the neighboring crusts the width of thermal transition becomes wider with time, but not much difference along subducting direction, probably because of rapid subduction. Calculated heat flow profile is generally in good agreement with the observed heat flow trend. A closer inspection, however, reveals a coincidence between heat flow minimum, location of subducted KPR and location of low-frequency tremors.

We then preformed a simple geomechanical modeling, testing an elastic response of KPR subduction. The result shows a significant increase in compression on the leading side of KPR, which should promote failure of the hanging wall rocks leading to the occurrence of tremors/VLFEs, and the localized heat flow disturbance caused by fluid circulation. These predictions will be tested through IODP drilling proposed by Nakata et al.(2020 JpGU/AGU).