Michael B. Underwood

New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190

Moore, G. F., Taira, A., Klaus, A., Becker, L., Boeckel, B., Cragg, B. A., Dean, A., Fergusson, C. L., Henry, P., Hirano, S., Hisamitsu, T. et al. (2001). New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems, 2, Article No: 2001GC000166.

Compositional and fluid pressure controls on the state of stress on the Nankai subduction thrust: A weak plate boundary

Abstract We show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the Nankai subduction plate boundary off southwest Japan. Ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients ( μ r ) at stress levels between 1.0 and 40 MPa. The direct shear μ r values for purified smectite are ∼0.14±0.02, for illite ∼0.25±0.01, and for chlorite 0.26±0.02 (for point load velocities of 0.0001 mm/s). These clay minerals dominate the Nankai subduction decollement zone. Illite (plus quartz) is mechanically important in the altered incoming Muroto section and the predicted decollement μ r should lie between 0.2 and 0.32. This low residual strength, together with elevated fluid pressure, limits shear stresses to below ∼4 MPa within the frontal ∼50 km of the subduction system, consistent with the low wedge taper in this region. A higher wedge taper off the Ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region. Our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in μ r are predicted to occur as a result of variations in smectite and total clay content. These variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite. However, our data suggest the low μ r values of the clay-rich decollement still limit shear stresses to between ∼17 and 29 MPa within the frontal ∼50 km of the wedge, consistent with other estimates of plate boundary weakness. Indeed, we propose that it should be expected that subduction plate boundaries like Nankai will be weak because of the intrinsic presence of clay-rich faults and moderate fluid overpressures. Our data do not support the hypothesis that the smectite-to-illite reaction directly controls the onset of seismogenic behavior deep in the Nankai system because there is already a mechanical dominance of illite (rather than smectite) in the shallow decollement zone, and we find all the clay phases tend to velocity strengthen. However, temperature-activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up-dip limit of the seismogenic zone.

Sediment deformation and hydrogeology of the Nankai Trough accretionary prism: Synthesis of shipboard results of ODP Leg 131

The main objective of Leg 131 was to provide data on the deformational processes and associated hydrogeology of the Nankai prism toe. Drilling succeeded, for the first time in the history of ocean drilling, in penetrating the complete sedimentary sequence to basaltic basement, reaching 1327 mbsf (metres below seafloor) with good core recovery (55%). Excellent correlation of the lithology and structure, including the frontal thrust and the decollement, with seismic reflection images was also determined. Bedding dips, faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to NW-SE shortening, as expected from the plate tectonic convergence vector. Below the decollement, no significant deformation features were observed, indicating that the decollement is a sharp discontinuity in stress transmission. Physical properties data show major discontinuities at the decollement, notably an increase in porosity below the later. This may indicate excess pore pressure in the subducted section and decollement zone. A less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature. Attempts to make downhole measurements were severely hampered by unstable hole conditions, but useful constraints have been placed on the thermal regime, and some calibration of laboratory physical properties toin-situ conditions has been provided, andin-situ stress and pore pressure were measured in the uppermost sediments. Evidence of channelized fluid flows is inconclusive. No sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found. Thermal measurements are not significantly different from those predicted by a purely conductive heat flow model. A signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events. We conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site.

Abnormal fluid pressures and fault-zone dilation in the Barbados accretionary prism: Evidence from logging while drilling

Logs collected while drilling measured density in situ, through the accretionary prism and decollement zone of the northern Barbados Ridge. Consolidation tests relate void ratio (derived from density) to effective stress and predict a fluid pressure profile, assuming that the upper 100 m of the prism is at a hydrostatic pressure gradient. The calculated fluid pressure curve rises to >90% of lithostatic below thrusts in the prism, presumably due to the increase in overburden and lateral tectonic loading. Thin (0.5–2.0 m) intervals of anomalously low density and resistivity in the logs through the basal decollement zone suggest dilation and perhaps hydrofracturing. A peak in hydraulic head in the upper half of the decollement zone requires lateral influx of fluid, a conclusion consistent with previous geochemical studies. Although the calculated fluid-pressure profile is model dependent, its inherent character ties to major structural features.

Slumping and mass transport deposition in the Nankai fore arc: Evidence from IODP drilling and 3-D reflection seismic data

Multiple lines of evidence exist for a range of sediment mass movement processes within the shallow megasplay fault zone (MSFZ) area and the adjacent slope basin in the outer fore arc of the Nankai subduction zone, Japan. Diagnostic features observed in three-dimensional reflection seismic data and in cores of the Integrated Ocean Drilling Program (IODP) document a multifarious mass movement history spanning ∼2.87 million years. Various modes and scales of sediment remobilization can be related to the different morphotectonic settings in which they occurred. From this evidence, we decipher the tectonic control on slumping and mass transport deposition in the Nankai fore arc. Three periods of intensified mass wasting coincided with pulses of enhanced activity on the splay fault: (1) an initial phase of juvenile out-of-sequence thrusting ∼1.95 to 1.7 Ma, (2) a reactivation phase between ∼1.55 and 1.24 Ma, and (3) at about 1 Ma, during a phase of uplift of the fore-arc high and motion along the MSFZ. We suggest that slope oversteepening, extensional stress regimes, and lateral transmission of fluid overpressures may have preconditioned the slope sediments to fail. Individual mass-wasting events may have been triggered by dynamic loading from earthquake waves and/or transient pulses of pore pressure along the splay fault. Overall, our results provide insights into the complicated interplay between tectonic and submarine mass movement processes. We demonstrate that detailed knowledge about the spatial and temporal distribution of submarine mass movements can be integrated into a holistic reconstruction of tectonostratigraphic evolution of accretionary margins.

Diagenesis, sediment strength, and pore collapse in sediment approaching the Nankai Trough subduction zone

A minor amount of opal cement inhibits consolidation of sediment approaching the Nankai Trough subduction zone at Ocean Drilling Program Sites 1173 and 1177. Secondary and backscattered electron images of sediments from Site 1173 reveal a low-density, silica phase (opal-CT) coating grain contacts. The grain-coating cement is more widespread in the upper Shikoku Basin facies than in the lower Shikoku Basin facies. Numerical models of opal-CT content display increases with depth through the cemented upper Shikoku Basin section. Once temperature increases above ∼55 °C, the rate of opal-CT dissolution outpaces precipitation, the cement can no longer support the overburden, and the open framework of the sediment begins to collapse. Cementation followed by cement failure is consistent with observed anomalies in porosity, seismic velocities, and shear rigidity. Porosity is anomalously high and nearly constant near the base of the upper Shikoku Basin facies, whereas seismic velocity increases with depth in the same interval. Across the boundary between the upper Shikoku Basin facies and the lower Shikoku Basin facies, there are step decreases in porosity from ∼60% to ∼45%, P-wave velocity from ∼1800 m/s to ∼1650 m/s, and S-wave velocity from ∼550 m/s to ∼300 m/s. Similar cementation and porosity collapse may be important in other locations where heating of hemipelagic deposits, with minor amounts of opal, is sufficient to trigger opal diagenesis.

Upslope flow of turbidity currents: A comparison among field observations, theory, and laboratory models

Field observations from the marine realm indicate that turbidity currents are capable of transferring detritus considerable distances upslope onto bathymetric highs. Upslope deposition occurs because of the combined effects of flow thickness, tilting of the cross-flow surface slope, and a vertical shift in the flows center of gravity. Numerical analysis and laboratory experiments indicate that the maximum run-up elevation of a subcritical turbidity current is approximately equal to 1.53 times the flow thickness.

Sedimentary facies associations within subduction complexes

Summary Sedimentation patterns within modern subduction zones are complex and variable, and do not necessarily follow models of submarine fan sedimentation. Environmental reconstructions within ancient subduction complexes should follow modern analogues as closely as possible and consider several criteria, including turbidite facies associations, vertical depositional cycles, regional palaeocurrent patterns, and in many cases, structural style and sandstone petrology. Important variables in trench sedimentation include the volume and texture of sediment entering the trench and the distribution of major sediment sources, especially large submarine canyons. Sediment transport in the trench is commonly longitudinal, although locally, such as at the mouth of a submarine canyon, flow may be at a high angle to the continental margin. Patterns of trench-slope sedimentation depend largely upon the topography of the slope. In general, coarse sediment is either trapped behind tectonic ridges (within slope basins) or bypasses the slope via submarine canyons. Background sedimentation is dominated by hemipelagic settling. Current directions are commonly at a high angle to the margin, but longitudinal flow may occur within large elongate slope basins. Major facies associations include submarine-canyon, slope, mature-slope-basin, and immature-slope-basin.

Sedimentary and Tectonic Evolution of a Trench-Slope Basin in the Nankai Subduction Zone of Southwest Japan

Leg 190 of the Ocean Drilling Program yielded discoveries about the early stages of tectonic and sedimentary evolution of a trench-slope basin in the Nankai subduction zone of southwest Japan. Lithofacies character, biostratigraphy, and seismic-reflection data show that the basins architecture was constructed during the early Quaternary by frontal offscraping of coarse-grained trench-wedge deposits. Clast types in muddy gravel beds indicate that one of the trenchs polymictic sources was enriched in low-grade metasedimentary rocks. Outcrops of the Shimanto Belt on the island of Shikoku contain comparable lithologic assemblages, so we suggest that some of the turbidity currents and debris flows were funneled from that source through a transverse canyon-channel system. Offscraped trench deposits are mildly deformed and nearly flat lying beneath the slope basin. Bedding within the basin laps onto a hanging-wall anticline that formed above a major out-of-sequence thrust fault. Rapid uplift brought the substrate above the calcite compensation depth soon after the basin was created. The sediment delivery system probably was rerouted during subduction of the Kinan seamounts, thereby isolating the juvenile basin from coarse sediment influx. As a consequence, the upper 200 meters of basin fill consist of nannofossil-rich hemipelagic mud with sparse beds of volcanic ash and thin silty turbidites. Intervals of stratal disruption are also common, and the soft-sediment folding resulted from north- to northeast-directed gravitational failure. The Nankai accretionary prism has grown 40 km in width during the past 1 My, and the slope basin is already filled to its sill point on the seaward side. The stratigraphy displays an upward fining and thinning trend, in contrast to the upward coarsening and thickening mega-sequences depicted by some conceptual models for slope basins.

Open ocean to trench turbidity-current flow in the Nankai Trough: Flow collapse and reflection

The Ocean Drilling Program Leg 131, which drilled in the Nankai Trough in the western Pacific Ocean, has shown for the first time that turbidity currents are deflected and reflected against the trench slopes, such that in the trench outer slope there is a predominant pattern of currents, carrying terrigenous sediments, apparently coming from the open-ocean Shikoku Basin directly toward the trench landward-slope. The purpose of this paper is to summarize the evidence for flow rebound at ODP Leg 131 Site 808, and to show that these results have important implications for the interpretation of paleocurrents in ancient linear turbidite systems.