Combining 2D-Basin and Structural Modeling to Constrain Heat and Mass Transport Along the Muroto Transsect, Nankai Trough, Japan

R. Ondrak¹, C. Gaedicke², and B. Horsfield^1
1GFZ-Potsdam, Germany
²BGR-Hannover, Germany

The Nankai trough located southeast of Shikoku Island, Japan, exhibits a zone of exceptionally high heat flow which is well studied and documented. Wells drilled in nearly undeformed Quaternary and Tertiary sedimentary sequences seawards of the Nankai Trough give access to samples covering a large temperature range over a short depth interval. Using primarily data from different DSDP/ODP cruises and 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 the Nankai trough.

Modeling the temperature and maturation history in the compressive tectonic system of the Nankai trough asked for the integration of 2D-structural and basin modeling. 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 overthrusting 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. Currently we model the effect of fluid flow driven heat transport in order to evaluate its influence on the present day temperature field and heat surface flow distribution. Present day temperatures reach 110°C at a depth of about 1100m below sea floor in 4800m water depth. Present day surface heat flow varies from 120 to 200 mW/m² in the study area close to the proto-thrust zone.

Absolute temperature and temperature variations through time strongly control the evolution and extent of organic and inorganic diagenetic processes during sediment burial. Temperature also most likely limits the occurrence of viable bacterial populations in the subsurface and energy delivered by temperature controlled deep geosphere processes (kinetically controlled early organic diagenesis) may enable life to extend to kilometer depths into the subsurface. In order to understand the processes and interactions taking place at the biosphere:geosphere interface a precise reconstruction of the burial and temperature history of the sedimentary sequence is necessary. 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.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands