Elizabeth J. Screaton

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.

Interactions between deformation and fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan: Results from IODP Expedition 316 Sites C0006 and C0007

Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since ∼0.78–0.436 Ma and accommodated ∼13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out-of-sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained, sand-rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

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

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 decollement 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, λ * , of ∼0.42, where λ * = [(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 λ * = ∼0.47 was estimated, reflecting the additional loading due to recent thickening by the frontal thrust.

Seismic slip propagation to the updip end of plate boundary subduction interface faults: Vitrinite reflectance geothermometry on Integrated Ocean Drilling Program NanTro SEIZE cores

Seismic faulting along subduction-type plate boundaries plays a fundamental role in tsunami genesis. During the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTro SEIZE) Stage 1, the updip ends of plate boundary subduction faults were drilled and cored in the Nankai Trough (offshore Japan), where repeated large earthquakes and tsunamis have occurred, including the A.D. 1944 Tonankai (Mw = 8.1) earthquake. Samples were obtained from the frontal thrust, which connects the deep plate boundary to the seafloor at the toe of the accretionary wedge, and from a megasplay fault that branches from the plate boundary decollement. The toe of the accretionary wedge has classically been considered aseismic, but vitrinite reflectance geothermometry reveals that the two examined fault zones underwent localized temperatures of more than 380 °C. This suggests that frictional heating occurred along these two fault zones, and implies that coseismic slip must have propagated at least one time to the updip end of the megasplay fault and to the toe of the accretionary wedge.

Fluid flow in accretionary prisms: Evidence for focused, time‐variable discharge

Accretionary prisms are wedges of saturated sediment that are subject to intense deformation as a result of lithosphere convergence. Compressive stress and rapid burial of the accreted deposits result in sediment compaction and mineral dehydration. These latter processes, in conjunction with fermentation or thermal maturation of entrained organic matter, yield hydrocarbon-bearing pore fluids that are expelled from the prism. Regional fluxes of heat and a number of dissolved chemical species, most notably carbon, are controlled by the advective expulsion of the pore waters. Numerical modeling, observation and monitoring of flow patterns and rates, and recent in situ hydrogeological tests quantify the conditions that control rates of fluid flow. Dispersed, intergranular flow (10−8 to 10−11 m/s), controlled by the vertical permeability of the prism (10−14 to 10−20 m²), is limited by low-permeability lithologies and seems not to vary much from margin to margin. Focused flow (10−1 to 10−8 m/s) above the decollement is controlled by fault zones or sedimentary intrusions (diapiric structures). At low-fluid pressures, fault zone permeability may be similar to that of adjacent wall rock, but as fluid pressure increases from hydrostatic (λ* = 0) to near lithostatic levels (λ* ≈ 1.0), fault zones dilate, and (fracture) permeability increases by 2–4 orders of magnitude (10−10 to 10−16 m²). Similarly, mud volcanoes and diapirs provide high-permeability fluid conduits to the sediment-water interface. As a result, faults and intrusions become primary flow paths and support surface vents at which syntectonic deposits (carbonate and gas hydrates) accumulate and chemosynthetic organisms cluster. Models of thermal and chemical anomalies and epigenetic deposits indicate that flow is temporally variable. That conclusion has been quantified by extended (1–10 months) seafloor and borehole experiments that measured temperature anomalies associated with flow events. On the Cascadia prism, flow (estimated velocity ∼3 × 10−5 m/s; 950 m/yr), confined to a thrust fault that cuts upward through the prism, has brought thermogenic hydrocarbons from depths >1.5 km to the surface within the last 400 years.

Spatial and temporal evolution of the megasplay fault in the Nankai Trough

The temporal and spatial evolution of a seismogenic megasplay fault in the Kumano area, Nankai Trough (southwest Japan), is revealed by detailed investigation of the three-dimensional structure of the shallow portions of the fault, combined with the results of drilling and dating of cores from Integrated Ocean Drilling Program (IODP) Expedition 316. The ENE striking eastern portion of the splay fault has remained active since the inception of faulting at ∼1.95 Ma. The recent shortening rate is ∼1 m/kyr, which represents ∼1.5%–2.5% of the total plate convergence rate of ∼40–65 m/kyr. The NE striking western portion of the splay fault exhibits a different mode of activity. Early stage activity (before 1.55 Ma) was similar to the eastern portion, but the fault was inactive between 1.55 and 1.24 Ma. The fault was reactivated for a short time at ∼1.24 Ma but again ceased activity after formation of the secondary branch and has been inactive since 1.24 Ma. Cessation of splay fault activity in the western domain after 1.55 Ma may be due to collision with a seamount and resulting bending of the accretionary prism in the splay fault footwall. Continuous activity of the eastern domain of the splay fault after 1.24 Ma may be related to geometrical favorability due to reorientation of the fault after the seamount passed beneath the imbricate thrust zone, leading to initiation of slightly oblique subduction.

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.

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.

Anisotropy of electrical conductivity record of initial strain at the toe of the Nankai accretionary wedge

An approach based on Marchs theory is applied to measurements of the anisotropy of electrical conductivity on samples and is used to quantify initial strain at the toe of the Nankai accretionary wedge. A quantitative determination of strain is possible from simple assumptions: passive reorientation offlat pores forming the porous network and existence of a linear relationship between fabric tensor and electrical conductivity tensor. We show that this simple model correctly accounts for the increase of anisotropy with compaction at a reference site located in the trench (Ocean Drilling Program drill Site 1173). At the toe of the accretionary wedge (Site1174), development of anisotropy in the horizontal plane and concurrent reduction of vertical plane anisotropy are observed. This can be explained by 12%horizontal ductile shortening, occurring after decollement initiation but before slip on imbricate thrust faults. Anisotropy in the underthrust sequence is correctly described by vertical compaction, consistent with decoupled stress states across the decollement. At Site 1174 the magnitude of ductile strain implies at least 75m slip on the decollement. Ductile shortening is associated with porosity loss, implying partly drained conditions above the decollement.

Fluid flow at the toe of convergent margins: interpretation of sharp pore-water geochemical gradients

Abstract Fluid expulsion greatly impacts chemical and mass budgets at subduction zones, particularly if it is focused along faults and stratigraphic conduits. Geochemical anomalies centered at decollement zones, such as pore-water freshening and the presence of thermogenic hydrocarbons, indicate long-distance, focused flow of deeply sourced fluids. The sharp gradient of these anomalies below the decollement zone has been interpreted as evidence for recent pulses of fluid flow, initiating a few to tens of ka. However, this interpretation does not consider that underthrust sediments are moving arcward beneath the decollement zone. In addition, upward flow from the compacting underthrust sediments can modify chemical profiles. Here, we use a simple model that couples fluid flow and solute transport to evaluate these sharp chemical gradients. We find that observed geochemical anomalies at the Northern Barbados and Costa Rican subduction zones can be explained either by recent pulses of flow, or by sustained flow along the decollement zone coupled with modest vertical fluid expulsion from consolidating underthrust sediments. The latter interpretation is consistent with estimates of upward flow rate at Costa Rica based on estimated pore pressure gradients and measured permeabilities within the underthrust sediments. One important implication is that recent pulses of flow along fault conduits may not be required to explain the geochemical anomalies. Furthermore, mixing of locally derived fluids flowing upward from the underthrust sediments and deeply sourced fluids flowing along the decollement zone provides an explanation for the observed changes in pore-water freshening along the decollement at Costa Rica.