Modeling Heat and Mass Transfer Along the Muroto Transsect, Nankai Trough, Combining 2D-Basin and Structural Modeling and its Importance for the Deep Biosphere

Ondrak, Robert¹, Christoph Gaedicke², Brian Horsfield³ (1) GeoForschungsZentrum, Potsdam, Germany (2) Federal Institute for Geosciences and Natural Resources (BGR), Germany, Hannover, Germany (3) GeoForschungsZentrum-Potsdam, Potsdam, Germany

The Nankai trough located southeast of Shikoku Island, Japan, exhibits a zone of exceptionally high heat. Using the results of various heat flow studies we have modeled the temperature history along the Muroto Transsect reaching from the tip of the thrust zone out into nearly undeformed Quaternary and Tertiary sediments seawards of Nankai trough. We used two balanced cross-sections defining the sections before and after overthrusting as input for 2d-basin modeling. We can show that rapid burial and over thrusting during the Quaternary is not sufficient to explain measured maturity of organic material in the sediments. The observed maturity levels are reached only in combination with very high heat flow during the phase of rapid burial. Today's heat flow distribution is not purely conductive but modified by hydrothermal convection. This is in agreement with previous studies observing a large scatter in heat flow which can only be explained by transient effects due to hydrothermal convection in the compressional regime of the Nankai accretionary prism. Present day temperatures reach 110°C at a depth of about 1100m below sea floor in 5800m water depth. The model predicts that generation proceeds at 5200-5600m depth, depending on well location. We used published and laboratory determined kinetic parameters for total hydrocarbon generation and carbon dioxide generation, in order to predict the timing of generation of these substances in time and space. An overlap was observed between generation zones and subsurface microbial communities, pointing to bio-geo coupling feeding the deep biosphere.