Deformation and Fluid Flow Processes in the Nankai Trough Accretionary Prism

Abstract

Ocean Drilling Program Legs 190 and 196 were a two-part program to study deformation and fluid flow in the central Nankai Trough off Shikoku Island. During Leg 190 two reference sites were drilled outboard of the trench (Sites 1173 and 1178), one site into the protothrust zone (Site 1174), two sites into a trench slope basin above a major outof-sequence thrust (Sites 1175, 1176), and one site into an older portion of the accretionary prism. During Leg 196, Sites 1173 and 808 (drilled through the frontal thrust during Leg 131) were revisited, employing logging while drilling and installing two Advanced Circulation Obviation Retrofit Kits (ACORKs). Our reference sites defined the stratigraphic framework and physical properties baseline of the accreting/ subducting Shikoku Basin sedimentary section. The proto-thrust and frontal thrust sites documented the dewatering and deformation processes at the toe of the accretionary prism. Porosity comparisons between Sites 1173 and 808 suggest that elevated fluid pressures occur beneath the décollement at Site 808. Initial measurements from the ACORK at Site 808 indicate a pressure pulse apparently from the décollement. Negative chloride anomalies at Sites 1174 and 808 could be due to fluid flow from deeper in the prism, but active smectite dehydration could also be responsible for the anomalies. Resistivity imaging of the frontal thrust shows borehole breakouts with principal stress orientations consistent with core-scale structures and plate convergence

Molecular monitoring of culturable bacteria from deep-sea sediment of the Nankai Trough, Leg 190 Ocean Drilling Program

“…This strain is a strictly anaerobic chemoorganotroph or autotroph with slow growth rate. Accumulation of fresh electron acceptors such as acetate produced by acetogenic bacterium in deep sediment of Nankai Trough may constitute available energy sources in sediment deposited over 3 Ma [34] for development of bacterial populations. The presence of acetogenic bacteria at similar temperatures has been reported in other deep sub-seafloor environments [53] and is consistent with the isolation of our strain LT18.…”

Elemental and Isotopic Chloride Geochemistry and Fluid Flow in the Nankai Trough

“…Sites 1173 and 1174 were drilled during ODP Leg 190 and Site 808 was drilled during ODP Leg 131. The geothermal gradient is high in this region, approximately 0.12°C/m, due to the relatively young age of the subducting plate [Moore et al, 2000].…”

“…[5] Pore waters were collected and chloride concentrations, Cl isotope ratios, sediment porosites and formation factors were determined utilizing previously established methods [Manheim, 1970;Manheim and Sayles, 1974;Mageniem et al, 1994;Moore et al, 2000]. Data and further details on the methods are available at www.gso.uri.edu/$spivack/grl2002.…”

Marinilactibacillus piezotolerans sp. nov., a novel marine lactic acid bacterium isolated from deep sub-seafloor sediment of the Nankai Trough

“…All details of the environmental conditions, sampling and sub-sampling procedures have been previously reported (Cragg et al, 1992a, b;Parkes et al, 1995;Moore et al, 2001;Toffin et al, 2004a).…”