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

Abstract

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 ϳ0.42, 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 * ‫؍‬ ϳ0.47 was estimated, reflecting the additional loading due to recent thickening by the frontal thrust.

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“…Despite inferences of recent activity on the splay fault system, no porosity inversion is observed beneath the splay fault; this contrasts with previous results from the déc-ollement of the Muroto transect (Screaton et al, 2002), in which a clear porosity inversion across the fault likely reflects fluid overpressure. Unlike the Muroto transect décollement, the splay fault system observed at Sites C0004 and C0008 has permeable pathways for dewatering provided by the observed sand and coarse ash layers.…”

Initiation of plate boundary slip in the Nankai Trough off the Muroto peninsula, southwest Japan

“…Furthermore, the fault zone thins from $32 m to $18 m between Sites 1174 and 808 respectively. The difference between porosity patterns of two sites indicates the development of the décollement zone from Site 1174 to Site 808 [Screaton et al, 2002].…”

Deformation and in situ stress in the Nankai Accretionary Prism from resistivity‐at‐bit images, ODP Leg 196

“…[16] A reverse-polarity seismic reflection across the décollement [Moore and Shipley, 1993], reflects an increase in porosity [Screaton et al, 2002]. Waveform modelling suggests variable character of the décollement zone itself with velocities/densities locally lower than overlying strata possibly due to fluid pressure [Moore and Shipley, 1993].…”

“…Coupled with low porosity core material [Taira et al, 1991;Moore et al, 2001b], Bourlange et al [2003] suggest a compacted décollement fault zone with dilatant fractures, although core properties may represent isolated clasts rather than bulk properties. Overpressures suggest that sediment permeability is insufficient to allow fluid expulsion [Screaton et al, 2002]. Long term fluid budgets suggest a zone of transient fluid flow within and/or below the décollement zone [Saffer and Bekins, 1998], but flow is minor relative to likely diffuse flow through the prism.…”