DIAGENETIC REACTION PROGRESS CONSTRAINS THE THERMAL HISTORY OF THE NANKAI MARGIN

The percentage of illite in illite/smectite mixed-layer clay provides an important constraint on the thermal history of the Nankai Trough subduction zone. Integrated Ocean Drilling Program Site C0002 has penetrated to 3058 meters below seafloor (mbsf) into the inner accretionary prism of the Nankai margin. Clay mineral assemblages for sediment samples and cuttings from this site have been computed from X-ray diffraction analyses. The percentage of illite in illite/smectite mixed-layer clay increases with depth, from initial detrital constituents with ~30% illite in sediment <2000 mbsf to ~60% at ~3000 mbsf. Over the same interval, smectite in bulk mudstones decreases to ~5 wt%, and illite increases to ~30 wt%. We generate thermal models for the subduction zone, and use the modeled temperatures to predict the amount and distribution of smectite-to-illite reaction progress. The new thermal models include transient changes in plate convergence rate and the age (and thus, heat content) of the oceanic lithosphere entering the subduction zone, based on current models for the tectonic history for the margin, with Miocene subduction of the Pacific plate followed by the incoming Philippine Sea plate. Additionally, the thermal models include an aquifer in the oceanic crust that enhances heat transport via a high Nusselt number proxy for hydrothermal circulation. The temporal changes in convergence rate and plate age result in higher modeled subduction zone temperatures than predicted using solely the present-day rate and subducting-plate age. Hydrothermal circulation redistributes heat from the subducted crust to the oceanic crust near the trench. Models most consistent with surface heat flux observations include both the effects of hydrothermal circulation and temporal changes in convergence rate and plate age. The modeled depth distribution of clay mineral assemblages is most consistent to the observations in simulations with ~850 meters of burial of the sediment (due to the rapid filling of the Kumano forearc basin by turbidites) over the last ~1.6 million years.