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Porosity loss within the underthrust sediments of the Nankai accretionary complex: Implications for overpressures

¹ Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA
² Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006, USA
³ Département de Géologie, École Normale Supérieure, Paris Cedex 05 75231, France
⁴ Geowissenschaftliche Gemeinschaftsaufgaben-GGA, Stilleweg 2, 30631 Hannover, Germany Leg 190 Shipboard Scientific Party^,*

Subduction complexes provide an opportunity to examine the interactions of deformation and fluid flow in an active setting. Ocean Drilling Program Leg 190 investigated the relationship between deformation, physical properties, and fluid flow in the toe of the Nankai Trough accretionary complex. With three sites (two from Leg 190, one from a previous leg) penetrating the décollement zone at various stages of development along the same transect, it is now possible to examine the change in porosity during rapid loading by trench turbidites and subsequent underthrusting. Results indicate inhibited dewatering and probable overpressure development seaward of the frontal thrust. Comparison of a reference site porosity versus depth curve to data from a site located within the protothrust zone indicates an overpressure ratio, *, of 0.42, where * = [(pore pressure – hydrostatic pressure)/(lithostatic pressure – hydrostatic pressure)]. These overpressures suggest that the hemipelagic sediments have insufficient permeability for fluid escape to keep pace with the rapid loading by turbidite deposition within the trench. At a site 1.75 km farther arcward, an excess pore pressure ratio of * = 0.47 was estimated, reflecting the additional loading due to recent thickening by the frontal thrust.

Key Words: accretionary prism • Nankai Trough • overpressure • compaction • porosity

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