Abstract The development of overpressure in continental margins is typically evaluated with hydrogeological models. Such approaches are used to both identify fluid flow patterns and to evaluate the development of high pore pressures within layers with particular physical properties that may promote slope instability. In some instances, these models are defined with sediment properties based on facies characterization and proxy values of porosity; permeability or compressibility are derived from the existing literature as direct measurements are rarely available. This study uses finite-element models to quantify the differences in computed overpressure generated by fine-grained hemipelagic sediments from the Gulf of Cadiz, offshore Martinique and the Gulf of Mexico, and their consequences in terms of submarine slope stability. By comparing our simulation results with in situ pore pressure data measured in the Gulf of Mexico, we demonstrate that physical properties measured on volcanic-influenced hemipelagic sediments underestimate the computed stability of a submarine slope. Physical properties measured on sediments from the study area are key to improving the reliability and accuracy of overpressure models, and when that information is unavailable, literature data from samples with similar lithologies, composition and depositional settings enable better assessment of the overpressure role as a pre-conditioning factor in submarine landslide initiation.

Abstract The Dun Mountain ophiolite and related oceanic-arc rocks (Otama Complex) formed above a westward-dipping subduction zone within Panthalassa, with implications for the emplacement of Cordilleran-type ophiolites and arcs elsewhere. The ophiolite is overlain by the Mid–Late Permian Upukerora Formation (up to 850 m), a predominantly very coarse breccia-conglomerate that mainly accumulated by mass flow. Lesser amounts of sediment accumulated from turbidity currents and as background hemipelagic sediments. The succession unconformably overlies ophiolitic basaltic or, rarely, gabbroic rocks after a regional hiatus. Much of the coarse clastic debris was derived from the underlying ophiolite. However, clasts of plagioclase-phyric basalt, felsic volcanics and quartz-bearing intrusive rocks, including plagiogranite, are over-represented compared to the ophiolite. The evolved igneous material was derived from an incipient oceanic arc (the Otama Complex) that bordered or covered the ophiolite, especially in the south. The coarse clastic material accumulated following the activation of north–south-trending, subaqueous, extensional growth faults within the underlying oceanic crust. Large blocks of mainly basalt, diabase and gabbro were also shed down fault scarps from relatively shallow-water to deeper-water settings. Fault-controlled talus accumulated soon after Mid-Permian docking of the ophiolite and oceanic arc with SE Gondwana to initiate the Mid-Permian–Mid-Triassic Maitai continental margin forearc basin.

ABSTRACT The Shikoku Basin is a back-arc basin located offshore southwest Japan. Sediments within the basin make up a key part of the subduction inputs to the Nankai Trough. A 19 m.y. history of sedimentation has been documented at Sites C0011 and C0012 of the Integrated Ocean Drilling Program (Kumano transect) and Sites 1173 and 1177 of the Ocean Drilling Program (Muroto and Ashizuri transects, respectively). This paper focuses on three noteworthy aspects of that history: (1) the onset of substantial pyroclastic influx, which shifted significantly along the strike length of the margin, from 3.3–3.9 Ma at Sites 1177 and 1173 to 7.6–7.8 Ma at Sites C0011 and C0012; (2) transport of sand by sediment gravity flows, which resulted in three discrete sand bodies during the Miocene (Kyushu, Daiichi Zenisu, and Daini Zenisu submarine fans); and (3) clay mineral assemblages within hemipelagic mudstones, which show systematic reduction of 3 wt% detrital smectite per 1 m.y. decrease in age. Collectively, these temporal and spatial adjustments of lithofacies and sediment composition have important implications for downdip and along-strike projections of frictional, geotechnical, and hydrogeological properties as strata enter the Nankai subduction zone. The stratigraphic positions of smectite-rich Miocene mudstones, for example, should match up with increases in the volume of fluid production by clay dehydration during subduction. The higher-permeability sand bodies (Kyushu and Zenisu submarine fans) should act as preferred conduits for focused fluid flow. The potential for buildup of fluid overpressures should increase above and laterally adjacent to stratigraphic pinch-outs of sand bodies, especially where the aquifers are inclined or confined between basement highs. These three-dimensional complexities set the Nankai-Shikoku system apart from other subduction zones (e.g., Japan Trench, Costa Rica) where inputs consist of comparatively homogeneous pelagic and hemipelagic deposits.

ABSTRACT In subduction zones, fluid-pressure generation in the underthrusting section is of particular interest because it governs the effective stress conditions of the footwall lining the plate interface. Only a few studies have systematically explored the role of lithological heterogeneity of underthrust sediment on the resulting fluid pressure and its distribution. We used a coupled loading and pore-pressure dissipation model with a new compilation of sand properties to investigate the role of such heterogeneity on the drainage state beneath the plate interface in the western Nankai subduction zone offshore Japan, where the incoming sediment sequence hosts numerous sand layers with a total thickness of up to ~210 m within a matrix of hemipelagic mud. Our results show that sand layers act as important conduits for both pressure translation and solute transport from greater depth to the trench and seaward. The simulated pore pressure is mainly controlled by aggregate sand-layer transmissivity, and to second order by sand-layer depth, which affects the ability of fluids to access permeable sands from the surrounding less-permeable mudstone matrix. Modeled sand permeability in the outer subduction system is in the range of previous estimates for décollement zone permeability (10 –13 to 10 –16 m 2 ) and evolves to approximately three orders of magnitude lower permeability in the inner subduction system. The enhanced drainage leads to 15% lower excess pore pressures in models with sands than without sands. Thus, differences in the lithostratigraphy of the subducting sediment should have implications for the mechanical behavior along the Nankai subduction system.

Abstract The Namurian and Westphalian sequences from the onshore well Scaftworth-B2, located in the Gainsborough Trough, central England, have been analysed for whole-rock inorganic geochemical data via inductively coupled plasma optical emission spectrometry (ICP-OES) and mass spectrometry (MS). The changes within key elements, and elemental ratios, results in a chemostratigraphic zonation scheme consisting of eight chemostratigraphic sequences and 13 chemostratigraphic packages, providing the type zonation for the Bowland Shale and overlying formations. Mineralogical data are provided by whole rock X-ray diffraction (XRD) and are used to calibrate the mineral modelling in order to generate a modelled mineral log for the study well. Furthermore, the modelled mineralogy is then used to calculate a relative brittleness for the samples, which can then be collaborated with traditional rock properties data at a later date. Elemental data can also be used to model the relative abundance of detrital quartz and biogenic silica; while total silicon is detected by ICP, biogenic silica is not detected by XRD owing to its amorphous nature. Enrichment factors calculated from the inorganic elemental data suggest that the sediment was deposited in an unrestricted marine setting, which experienced periods of anoxia.