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.The Nankai Trough accretionary prism is considered an ?end-member? prism accreting a coarse terrigenous sediment section in a setting with structural simplicity, unparalleled resolution by seismic and other geophysical techniques, and large historic earthquakes. It therefore has been the focus of Ocean Drilling Program (ODP) drilling to address several unresolved questions concerning accretionary processes and prism evolution. At six sites cored along two transects across the Nankai Trough accretionary prism during ODP Leg 190, lithostratigraphy and sediment diagenesis vary markedly. For the first time, reference sites at the seaward ends of the two transects defined the stratigraphic framework of the accreting/subducting Shikoku Basin sedimentary section. A thick section of Miocene turbidites and smectite-rich mudstone is present within the subducting section at the Ashizuri site. The turbidites and mudstones are absent in the correlative section at the Muroto site; variations in lithology, mineralogy, and hydrologic properties of the incoming sediments probably contribute to the difference in prism wedge taper between the two transects, while possibly controlling the seismic character of the active plate boundary. The d?collement in both transects is localized within a common stratigraphic unit (?5.9?7 Ma) within the lower Shikoku Basin facies. The d?collement is also a major boundary for both physical and mechanical properties. A broad low-chloride pore water anomaly in the lower Shikoku Basin unit, first identified at Site 808, progressively decreases in magnitude from prism to basin along the Muroto Transect. Physical properties relationships, evidence for mineralogic changes in the sediments, and pore fluid chemistry suggest that the chloride anomaly results primarily from in situ diagenetic reactions in the sediments, possibly augmented by flow of freshened fluid from depth. New constraints on stratigraphy and age of units along more landward parts of the Muroto Transect have dramatically changed our ideas about the tectonic evolution of the prism in this area. Growth of the seaward-most part of the prism took place very rapidly, with 40 km of accretion within the past 2 Myr. This rate is at least 3 times greater than growth rates in a comparable prism.Peer reviewe

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[1] 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.

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

The transformation of smectite-group clay minerals to illite has garnered considerable interest as a potentially important process affecting both the mechanical and hydrologic behavior of subduction zones. Illitization can generate fluid overpressure by release of bound water, and the mineralogical change and associated cementation may increase intrinsic frictional strength while decreasing the sliding stability of faults. Released bound water also contributes to pore water freshening observed in boreholes at numerous margins. Here the authors combine data from Ocean Drilling Program drill sites along two transects at the Nankai subduction zone with numerical models of smectite transformation, to (i) quantify the distribution of smectite transformation and fluid production downdip of the trench; and (ii) evaluate its hydrologic and mechanical implications. High heat flow (ca 180 mW/m 2 ) along the axis of the Kinan Seamount Chain (Muroto transect) initiates clay mineral transformation outboard of the trench, whereas lower heat flow (70-120 mW/m 2 ) 100 km to the SW (Ashizuri transect) results in negligible presubduction diagenesis. As a result, considerably more bound fluid is subducted along the Ashizuri transect; simulated peak fluid sources down-dip of the trench are considerably higher than for the Muroto transect (ca 1.2-1.3 ¥ 10 -14 /s vs ca 6 ¥ 10 -15 /s), and are shifted ca 10 km further from the trench. More generally, sensitivity analysis illustrates that heat flow, taper angle, incoming sediment thickness, and plate convergence rate all systematically affect reaction progress and the distribution of bound water release down-dip of the trench. These shifts in the loci and volume of fluid release are important for constraining fluid flow pathways, and provide insight into the links between clay transformation and fault mechanics.

a b s t r a c tSedimentary deposits in the distal Kumano forearc basin of the Nankai accretionary margin off Kii Peninsula, Japan, have been imaged using three-dimensional (3D) seismic data. The seismic data, along with logging and core data from the Integrated Ocean Drilling Program (IODP) show that the unconformity between the accretionary prism and overlying forearc sediments is time-transgressive. The unconformity at Site C0002 separates 5 Ma prism rocks from 3.65 Ma basin deposits; at Site C0009 it separates 5.6 Ma prism from 3.8 Ma basin sediments. Acoustic reflections in the basal deposits are subparallel to the underlying accretionary prism; the acoustic facies varies in thickness from 50 to 750 m. The mudstone deposits and laterally equivalent turbidites are interpreted as lower trench-slope deposits. The condensed slope sediment (SS) section decreases in age from 3.5 to 1.5 Ma at Site C0002 to 1.5 e0.9 Ma at C0009.Acoustic sequences within the lower forearc basin (LFB) contain higher proportions of silt and sand turbidites and progressively onlap the SS unit along a low-angle discontinuity (KL) in a landward direction. Because of the landward onlap of the LFB unit, the oldest LFB strata at C0002 are older than 1.67 Ma, whereas those at C0009 are younger than~0.9 Ma. Thus, the amount of time missing or characterized by condensed sedimentation across the KL unconformity decreases in duration in the landward direction. The landward-onlapping sequences tilt progressively landward in response to regional uplift along an out-of-sequence thrust (OOST; mega-splay) fault. Regional tilting shifted the basin's depocenter progressively landward, expanding that part of the basin from~10 km in width to >30 km. The onset of sand-silt turbidite deposition in the distal basin began after more accommodation space was created by the uplift of the outer ridge along the splay fault at~1.9 Ma. Conversely, turbidites of the Upper Forearc Basin (UFB) progressively onlap LFB in a seaward direction. Furthermore, the respective thicknesses of the LFB and UFB units switch from the seaward side of the basin (C0002) farther landward (C0009): the LFB unit is > 800 m thick in the seaward region, whereas it is only 200e300 m thick in the landward region; the UFB unit is < 50 m thick in the seaward region, and up to 600 m thick in the landward region. Thus, Kumano Basin responded in both space and time to a complex interplay between tectonics and sedimentation. The stratigraphy records a balance between the effects of prism uplift along the basin's distal edge with the rerouting of channels and canyons along the basin's proximal edge.