Simulated evaluations of hydrothermal anomaly in Muroto Transect, Nankai Trough, Japan

Nankai Trough highlights characteristic hydrothermal anomaly along the contact of Eurasian-Philippine Oceanic Plates. To elucidate geologic features governing the hydrothermal distribution within subduction zones, a series of numerical simulations were conducted with emphasis on subsea topography, basal heat inflow variations with consideration of thermo-kinematic effects generated by plate subduction, distribution of heterogeneous sedimentary facies, and effects of décollement-fault network. Based on the basal heat inflows simulated from the kinematic thermal subduction model (accounting for the subduction of cooled oceanic plate and associated heat redistribution), the thermal conduction was shown to be a dominant control on heat flow regime in the continental crust, with average homogenous horizontal (k_h) and vertical (k_v) facies permeability of 10 ^-20 m² < k_h < 10 ^-18 m² and 10 ^-21m² < k_v < 10^-19 m², respectively. In contrast, thermal conduction was minor for heat flow within the oceanic crust; the increment in permeability caused increase in hydrothermal flow throughout the oceanic crust, eventually leading to the dominance of thermal convection. Elevated heat flow at oceanic crust was reproducible to an extent with combination of basal heat inflow variation and increase in permeability of upper oceanic crust facies. Implementation of the décollement-fault network had noticeably minor effect in comparison. The best fit of the local heat flow anomaly appeared with the implementation of kinematically derived basal heat inflow, elevated upper and lower basement formation facies permeability ((k_h =) 10 ^-10 m² / (k_v =) 10 ^-11 m² and (k_h =) 10 ^-15 m² / (k_v =) 10 ^-15 m², respectively), generating permeability within the commonly accepted range for the upper oceanic crust facies. In summary, it was implied that the hydrothermal anomaly adjacent to the frontal thrust can only be represented by implementation of these factors, which act as dominant control over the hydrothermal distribution.