Saneatsu Saito

Consolidation patterns during initiation and evolution of a plate-boundary decollement zone: Northern Barbados accretionary prism

Borehole logs from the northern Barbados accretionary prism show that the plate-boundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a three-dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid-rich conduit extending down the decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch.

Compaction and dewatering processes of the oceanic sediments in the Costa Rica and Barbados subduction zones: estimates from in situ physical property measurements

Abstract During Ocean Drilling Program Legs 170 and 171A, logging-while-drilling (LWD) tools were deployed on the Costa Rica and Barbados subduction margins. High-quality density, resistivity, and natural γ-ray logs were acquired across the decollement zones on both margins. Based on a new method of ‘layer-by-layer’ correlation of the logs, changes in thickness and volume between incoming and subducted or accreted sediments are determined with 15 m resolution and 1% accuracy. The change in sediment thickness and volume generally decreases with depth, however, this change strongly depends on the lithology. Siliceous layers such as diatomaceous and radiolarian clay tend to be fluid-bearing, and the stratigraphic position of such zones is a critical factor in the fate of the subducted sediment section. On the Costa Rica Margin, the sediment section on the Cocos plate is underthrust intact beneath the toe of the Caribbean Plate with no frontal offscraping where a siliceous fluid-bearing zone is present only in the upper part of the section. On the Barbados margin, a layer of radiolarian clay exists, providing a narrow zone of mechanical weakness and anomalously high dewatering in the middle of the sediment section. This layer divides the sediments that are subducted from those that are accreted. Accreted and subducted sediments show different compaction styles. Accreted sediments are characterized by rapid compaction with vertical thickening, whereas subducted sediments are characterized by slow compaction with vertical flattening. The vertical thickening of the accreted sediments is due to horizontal tectonic compaction and contributes to the vertical thickening of the accretionary prism as a whole in the early stages of deformation. Dewatering flux is calculated by the volume change of the sediment sequences across the trench. The dewatering flux computed from the LWD data provides an estimate of the minimum fluid flux in the subduction zone and is significantly greater than the flux estimated from laboratory experiments because of meter-scale fluid conduits which influence the downhole logs but not the centimeter-scale sample measurements.

Acoustic and mechanical properties of Nankai accretionary prism core samples

We studied undeformed sediment and accreted strata recently recovered by Ocean Drilling Program/Integrated Ocean Drilling Program (ODP/IODP) drilling in Nankai Trough convergent margin to unravel the changes in physical properties from initial deposition to incipient deformation. We have derived acoustic (Vp) and mechanical (uniaxial poroelastic compliance, compaction amplitude) properties of samples from various drill sites along the Muroto (ODP 1173) and Kii transects (IODP C0001, C0002, C0006, and C0007) from isotropic loading tests where confining and pore pressure were independently applied. We quantified the dependence of Vp on both effective (Peff) and confining (Pc) pressure, which can be used to correct atmospheric pressure measurements of Vp. Experimental Vp obtained on core samples extrapolated to in situ conditions are slightly higher than logging-derived velocities, which can be attributed either to velocity dispersion or to the effect of large-scale faults and weak zones on waves with longer wavelength. In the high-porosity (30%–60%) tested sediments, velocities are controlled at first order by porosity and not by lithology, which is in agreement with our static measurements of drained framework incompressibility, much smaller than fluid incompressibility. Rather than framework incompressibility, shear modulus is probably the second-order control on Vp, accounting for most of the difference between actual Vp and the prediction by Woods (1941) suspension model. We also quantified the mechanical state of Nankai samples in terms of anisotropy, diagenesis, and consolidation. Both acoustic and mechanical parameters reveal similar values in vertical and horizontal directions, attesting to the very low anisotropy of the tested material. When considering the porous samples of the Upper Shikoku Basin sediments (Site 1173) as examples of diagenetically cemented material, several mechanical and acoustic attributes appeared as reliable experimental indicators of the presence of intergrain cementation. We also detected incipient cementation in samples from IODP Site C0001 (accretionary prism unit). In terms of consolidation, we distinguished two classes of material response (shallow, deformable samples and deep, hardly deformable ones) based on the amount of compaction upon application of a Peff large with respect to the inferred in situ value, with a transition that might be related to a critical porosity.

Magnetic fabric development in the Tertiary Accretionary Complex in the Boso and Miura Peninsulas of central Japan

It has been demonstrated that the anisotropy of magnetic susceptibility (AMS) is a sensitive indicator for distinguishing various processes that had acted during and after the deposition of sediments. Recently, much attention has been focused on proving its usefulness in distinguishing early deformation signatures in sediments which show no visible grain-scale deformation. Here, we present the AMS results of the Neogene Miura Group of Central Japan, which was formed by arc-arc collision tectonics with subdivisions of two contrasting tectonic provinces: the southern province, a highly deformed accretionary prism and slope basin, and the northern province, a gently deformed forearc basin. Samples were collected from 21 sites in both tectonic settings. We obtained from axes orientations three typical fabric domains considered to represent sedimentary, tectonically deformed, and their intermediate types. The shape parameters of AMS also revealed distinctions clearly. Sedimentary fabrics are more foliated and anisotropic. Deformed fabrics show a more prolate shape and are closer to being isotropic. The sedimentary fabrics predominate in the northern forearc basin sequence whereas the intermediate and deformed fabrics occur predominantly in the southern province. The general trend of the shape of the magnetic fabric in studied sediments reveals a clear evolution in the pattern of the isotropic tectonic fabric constructed from foliated sedimentary fabric.

Changes in illite crystallinity within an ancient tectonic boundary thrust caused by thermal, mechanical, and hydrothermal effects: an example from the Nobeoka Thrust, southwest Japan

Illite crystallinity (IC), the full width at half maximum of the illite (001) peak in clay-fraction X-ray diffraction (XRD), is a common geothermometer widely applied to various tectonic settings. Paleotemperature estimation using IC presents methodological ambiguity because IC is not only affected by background temperature but also by mechanical, hydrothermal, and surface weathering effects. To clarify the influences of these effects on IC in the fault zone, we analyzed the IC and the illite 001 peak intensity of continuous borehole core samples from the Nobeoka Thrust, a fossilized tectonic boundary thrust in the Shimanto Belt, the Cretaceous-Paleogene Shimanto accretionary complex in southwest Japan. We also carried out grinding experiments on borehole core samples and sericite standard samples as starting materials and investigated the effect of mechanical comminution on the IC and illite peak intensity of the experimental products. We observed the following: (1) the paleotemperatures of the hanging wall and footwall of the Nobeoka Thrust are estimated to be 288°C to 299°C and 198°C to 249°C, respectively, which are approximately 20°C to 30°C lower than their previously reported temperatures estimated by vitrinite reflectance; (2) the fault core of the Nobeoka Thrust does not exhibit IC decrease; (3) the correlation of IC and illite peak intensity in the hanging wall damage zone were well reproduced by the grinding experiment, suggesting that the effect of mechanical comminution increases toward the fault core and; (4) the abrupt increase in IC value accompanied by high illite peak intensity is explained by hydrothermal alterations including plagioclase breakdown and the formation of white micas. Our results indicate that IC has potential for quantifying the effects of mechanical comminution and hydrothermal alteration within a fault zone.

Redistribution of Sediments by Submarine Landslides on the Eastern Nankai Accretionary Prism

During a recent survey of the Nankai Trough region by JAMSTEC R/V KAIYO, ten piston cores were collected along a NW-SE transect through the Shikoku Basin, Kashinozaki Knoll, and Nankai Trough areas. The purpose was to demonstrate the influence of landslide processes on sediment distribution patterns on an accretionary prism. The Shikoku Basin is a flat abyssal plane covered by ca. 1 m thick hemipelagic mud, and underlain by a ca. 10 cm thick tuffaceous sand corresponding to the Aira-Tn tephra layer (25120 ± 270 yr. B.P.). The sedimentation rates in the Shikoku Basin are 3–4 cm/ky. At least three submarine landslide scars are observable on Kashinozaki Knoll. The abyssal plane surrounding the Kashinozaki Knoll is covered by a characteristic yellowish pumiceous mud intercalated with the hemipelagic mud. Immediately below thelandslide scars, these pumiceous mud layers thicken. Pumiceous mud was likely derived from the flanks of the volcanic Kashinozaki Knoll. These scars and deposits suggest that submarine landslides redistributed material from the knoll to the basin by mass-wasting. More than six trench turbidite beds were observed in a sequence that overlies a submarine landslide deposit at the foot of the accretionary prism within the Nankai Trough. The geomorphology of the axial channel suggests that the flow of turbidite deposits along the axial channel was blocked by this submarine landslide deposit, forcing the flow path and gradient of the channel to re-establish itself. In this event, sediment ponding would occur upstream of the blockage, while the downstream portion of the axial channel would be starved of sediment.

EVOLUTION OF TECTONIC COMPACTION IN THE BARBADOS ACCRETIONARY PRISM : ESTIMATES FROM LOGGING-WHILE-DRILLING

Abstract Resistivity and bulk-density logs acquired while drilling are used to document the evolution of porosity, volumetric loss, and effective stress in the upper 300 m of the Barbados accretionary prism. The computed profiles across a thrust fault enable the separation of pre-, syn-, and post-accretion components; total volume loss is divided into normal consolidation (pre-accretion), tectonic volume loss (syn-accretion), and thrust fault loading (post-accretion) in the footwall. Quantitatively, the tectonic volume loss in the Barbados accretionary prism, estimated from the normal consolidation in a reference section, is as large as the pre-accretion volume loss. The compaction history of the accretionary prism is essentially controlled by the vertical displacement of the thrust and the predicted maximum volume loss throughout the prism may be extrapolated from the volume loss trend in the hanging wall. The porosity and effective stress profile at the present time is consistently larger than its corresponding syn-accretion trend and less than the extrapolated maximum from the hanging wall. The present profile is approximately equal to the average of the two curves. The remaining difference in the accretionary prism above the inferred syn-accretion trend is due to post-accretion compaction. The post-accretion volume loss is a physical feedback process due to the superposition of thrust sheets in an accretionary prism. In the case of multiple thrusts developing in a prism, the porosity and effective stress profiles are saw-toothed and approach the maximum tectonic volume loss gradient with depth. The cumulative post-accretion compaction increases with depth and becomes increasingly greater than the pre- and syn-accretion compaction with thickening of the accretionary prism. Post-accretionary loading is the dominant mechanism of volume loss and dewatering in an accretionary prism during its early stages of growth.

Petrophysically determined lithofacies at the Nankai Trough Accretionary Prism: NanTroSEIZE, IODP Expedition 314

Abstract: Characterizing the physical properties and identifying boundaries within active accretionary prisms is necessary in understanding their behaviour and recent movement. In such unstable conditions core recovery is not always reliable, especially around fault zones. Integrated Ocean Drilling Program (IODP) Expedition 314 was the first stage of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), and used logging-while-drilling (LWD) technology to record continuous physical property data. We use iterative non-hierarchical cluster analysis (INCA) to quantitatively define the characteristics of the slope sediments and sediments within the accretionary prism at Sites C0001 and C0004. A new and detailed log-based lithostratigraphy is developed, and positions of major boundaries, defined by 3D seismic profiles and initial interpretation of log responses, are refined. The results produce clusters that clearly distinguish the slope sediments and characterize formations within the accretionary prism. Boundaries that correlate to the seismic-defined unconformity between the slope sediments and the accretionary prism, and a boundary within the accretionary prism that corresponds to a megasplay fault previously unresolved by log analysis and borehole images, are identified. Our study demonstrates that INCA analysis of LWD data can accurately define boundaries and characterize sediments in environments where core recovery may be incomplete.

Continuous depth profile of the rock strength in the Nankai accretionary prism based on drilling performance parameters

A new method for evaluating the in situ rock strength beneath the seafloor is proposed and applied to the Nankai Trough accretionary prism. The depth-continuous in situ rock strength is a critical parameter for numerous studies in earth science, particularly for seismology and tectonics at plate convergence zones; yet, measurements are limited owing to a lack of drilled cores. Here, we propose a new indicator of strength, the equivalent strength (EST), which is determined only by drilling performance parameters such as drill string rotational torque, bit depth, and string rotational speed. A continuous depth profile of EST was drawn from 0 to 3000 m below the seafloor (mbsf) across the forearc basin and accretionary prism in the Nankai Trough. The EST did not show a significant increase around the forearc basin–accretionary prism boundary, but it did show a clear increase within the prism, ca. below 1500 mbsf. This result may indicate that even the shallow accretionary prism has been strengthened by horizontal compression derived from plate subduction. The EST is a potential parameter to continuously evaluate the in situ rock strength during drilling, and its accuracy of the absolute value can be improved by combining with laboratory drilling experiments.

Stress state and its anomaly observations in the vicinity of a fault in NanTroSEIZE Expedition 322

To better understand the stress state and geological properties within the shallow Shikoku Basin, southwest of Japan, two sites, C0011A and C0011B, were drilled in open-ocean sediments using Logging While Drilling (LWD) and coring, respectively. Resistivity image logging was performed at C0011A from sea floor to 950 m below sea floor (mbsf). At C0011B, the serial coring was obtained in order to determine physical properties from 340 to 880 mbsf. For the LWD images, a notable breakout anomaly was observed at a depth of 615 m. Using resistivity images and a stress polygon, the potential horizontal principal stress azimuth and its magnitude within the 500–750 mbsf section of the C0011A borehole were constrained. Borehole breakout azimuths were observed for the variation by the existence of a fault zone at a depth of 615 mbsf. Out of this fracture zone, the breakout azimuth was located at approximately 109° ± 12°, subparallel to the Nankai Trough convergence vector (300–315°). Our calculations describe a stress drop was determined based on the fracture geometry. A close 90° (73° ± 12°) rotation implied a 100% stress drop, defined as a maximum shear stress drop equal to 1 MPa. The magnitude of the horizontal principal stresses near the fracture stress anomaly ranged between 49 and 52 MPa, and the bearing to the vertical stress (Sv = 52 MPa) was found to be within the normal-faulting stress regime. Low rock strength and a low stress level are necessary to satisfy the observations.