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 Knowledge of rock thermal conductivity is necessary to understand the thermal structure in active seismogenic zones such as the Nankai Trough subduction zone, SW Japan. To estimate in situ thermal conductivity at the oceanic crust surface in the seismogenic zone, we measured the thermal conductivity of a basaltic basement core sample retrieved from subducting oceanic basement at the Nankai Trough Seismogenic Zone Experiment input site C0012 under high temperature (maximum 160 °C) and high pressure (maximum effective pressure 100 MPa), respectively. These conditions correspond to the in situ temperature and pressure at the oceanic crust surface in the updip limit of the Nankai seismogenic zone (~7 km below the seafloor). Thermal conductivity of the oceanic basalt is both temperature and pressure dependent. In contrast to other rock types such as sandstone and granite, for which thermal conductivity decreases with increasing temperature, the thermal conductivity of the oceanic basalt increased with increasing ambient temperature. The thermal conductivity of the basalt also increased with increasing effective pressure; however, the rate of increase was much lower than that for other rocks. These new temperature and pressure effect data for oceanic crust basalt fill a gap in the research. The estimated thermal conductivity of the basalt at in situ temperature and pressure conditions was less than ~2 W m –1 K –1 , although deformation and alteration associated with subduction could decrease pore spaces in the basalt, leading to enhanced thermal conductivity. This value is significantly lower than that typically assumed for thermal structure simulations in the Nankai subduction zone.

ABSTRACT To accurately estimate the coseismic rupture area in a Nankai megathrust earthquake and predict seismic and tsunami hazards, various three-dimensional models of the subducting plate geometry and simple seismic velocity models of the subduction zone in SW Japan have been proposed. However, to ensure consistency among studies, more realistic and reliable standard models must be developed. Here, we use wide-angle ocean-bottom seismographic survey results to develop models of the three-dimensional geometry of the subducting plate and of the three-dimensional P-wave velocity structure around the Nankai Trough. We confirmed the reliability of the proposed models by comparing theoretical first arrivals, calculated from two-dimensional structure models sampled from the three-dimensional model along seismic profile lines, with observed traveltime data. The proposed models are the first to be visible and to attempt to represent the actual seismic velocity structures around the entire Nankai Trough in SW Japan. Although the spatial validity of the three-dimensional velocity structure model could not be strictly evaluated, we confirmed that the differences between hypocenter parameters determined from previously published seismic tomography results and those obtained by using our three-dimensional structure model were sufficiently small (latitude and longitude within ±0.1° and depth within approximately ±5 km). Therefore, our three-dimensional structure model is suitable for use as an initial model for hypocenter determination.

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 Deformation in a subduction zone and the related transition from smectite to illite within the aseismic-seismic transition zone (2–4 km below the seafloor) were analyzed by studying an onland accretionary complex that was previously buried to a depth of just 2–4 km. The early to middle Miocene Hota accretionary complex of central Japan is an excellent example of an accretionary complex that records shallow underthrusting at the updip end of the seismogenic zone. Two types of subduction-related deformation are preserved in the complex: phacoidal deformation (D1) characterized by rhombus-shaped fragments of mudstone with a random fabric and a thin rim of clay minerals with a preferred orientation, similar to the deformation features of the primary décollement zone at the toe of modern accretionary prisms (as revealed by ocean drilling); and block-in-matrix deformation (D2) characterized by an asymmetric S-C foliation with shear bands and an intense shape-preferred orientation of clay minerals, similar to the deformation features of tectonic mélange in ancient, mature décollement zones. D2 is marked by a large reduction in the amount of smectite and a corresponding increase in illite. During D2, the shear zone increased in strength due to the disappearance of weak smectite, which has a low friction coefficient, and due to an increase in the cohesion of sediments associated with a reduction in porosity and the development of a preferred orientation of clay minerals. Such strain hardening represents a fundamental mechanical/chemical change in the properties of sediments immediately before entering the seismogenic zone.

ABSTRACT A 200-m-thick, near-vertical, middle Miocene (ca. 14 Ma), gabbroic sheeted intrusion in the Muroto area of the Shimanto accretionary complex of southwest Japan yields anisotropy of magnetic susceptibility (AMS) showing a magnetic foliation for the minimum axis (K min ) oblique (by ~70°) to the perpendicular of the intrusive contact. Assuming the K min axis represents the paleovertical axis, these data suggest that the gabbroic sheet was not intruded into the host sediments horizontally. Paleomagnetic measurements of the gabbroic intrusion show an in situ mean direction of reversed polarity (declination/inclination [Dec/Inc] = 287°/–65°, α 95 = 3°) that is considerably different from the expected, reversed-polarity dipole-field direction of this region (Dec/Inc = 0°/–56°). A structural analysis combining the paleomagnetic and AMS data led to the determination of a unique pole of rotation, around which the dike can be back-rotated to its initial orientation. The magnitude of rotation necessary for the in situ paleomagnetic direction to be back-rotated to the expected direction is ~60°, which is consistent with the rotation required for the K min axis to be vertical. This consistency can be regarded as independent support for our interpretation of the AMS results and the reliability of the paleomagnetic data. Consequently, we propose that the Muroto gabbro was intruded when the paleo–trench-fill sediments had been tilted landward by ~20°, presumably by accretion, and that the gabbro might have been intruded as a sill-like sheet along a structurally weak zone, possibly part of the frontal thrust plane in the Shimanto accretionary prism.

ABSTRACT Geophysical images and structural cross sections of accretionary wedges are usually aligned orthogonal to the subduction trench axis. These sections often reveal underplated duplexes of subducted oceanic sediment and igneous crust that record trench-normal shortening and wedge thickening facilitated by down-stepping of the décollement. However, this approach may underrecognize trench-parallel strain and the effects of faulting associated with flexure of the downgoing plate. New mapping of a recently exposed transect across a portion of the Marin Headlands terrane, California, United States, documents evidence for structural complexity over short spatio-temporal scales within an underplated system. We documented the geometry, kinematics, vergence, and internal architecture of faults and folds along ~2.5 km of section, and we identified six previously unmapped intraformational imbricate thrusts and 13 high-angle faults that accommodate shortening and flattening of the underthrust section. Thrust faults occur within nearly every lithology without clear preference for any stratigraphic horizon, and fold vergence varies between imbricate sheets by ~10°–40°. In our map area, imbricate bounding thrusts have relatively narrow damage zones (≤5–10 m) and sharp, discrete fault cores and lack veining, in contrast to the wide, highly veined fault zones previously documented in the Marin Headlands terrane. The spacing of imbricate thrusts, combined with paleoconvergence rates, indicates relatively rapid generation of new fault surfaces on ~10–100 k.y. time scales, a process that may contribute to strain hardening and locking within the seismogenic zone. The structural and kinematic complexity documented in the Marin Headlands is an example of the short spatial and temporal scales of heterogeneity that may characterize regions of active underplating. Such features are smaller than the typical spatial resolution of geophysical data from active subduction thrusts and may not be readily resolved, thus highlighting the need for cross-comparison of geophysical data with field analogues when evaluating the kinematic and mechanical processes of underplating.

ABSTRACT Understanding the factors controlling earthquake rupture areas in subduction zones is a fundamental question in geodynamic research. In the Nankai Trough, Japan, three major controlling factors have been proposed so far: surface roughness, such as seamounts, on the subducting Philippine Sea plate, a locally dense and rigid upper plate, and stable or unstable frictional properties of the plate-boundary megathrust with abnormal pore-fluid pressure. The most prominent rupture segment boundary in the Nankai Trough is located off the Kii Peninsula. When the rupture starts and propagates beyond this boundary, the entire megathrust breaks, as in the 1707 Hoei earthquake. When the rupture does not propagate beyond this portion, the rupture area is segmented, as in the 1854 Ansei, 1944 Tonankai, and 1946 Nankai earthquakes. In this case, the boundary works as a barrier. The asperity or barrier depends on the frictional behavior along the megathrust in this area. Thick and dense plutonic rocks are the main component of the upper plate of the Kii Peninsula. Magmatic activity associated with the proto–Izu-Bonin arc collision is speculated to have occurred in the middle Miocene. Thus, we propose that the long-term tectonic hysteresis of the upper plate is the main factor controlling the rupture area segmentation in the Nankai Trough.

ABSTRACT Recent seismic reflection data across the Japan Trench show that frontal accretion involves offscraping sediments on top of horsts and scooping-up sediment from grabens. However, seismic profiling does not illuminate the structure within the accretionary prism, and thus the processes of accretion and prism growth are unknown. Key data from scientific drilling at Integrated Ocean Drilling Program Site C0019 that penetrated the prism in the region of large displacement during the 2011 Tohoku earthquake support a model in which frontal accretion occurs by imbricate thrusting, folding, and stacking of thrust sheets that are composed of semicoherent-sediment strata. Using palinspastic restoration techniques, we conclude that out-of-sequence thrusting and duplex development during the underthrusting of horsts can form and displace hanging-wall ramps along the plate-boundary detachment, which helps to explain the formation of some unexpected tectonostratigraphic relations at C0019, such as the emplacement of a thick section of the youngest sediments at the base of the accretionary prism, and numerous juxtapositions of different-age sediments within the basal plate-boundary fault zone.

ABSTRACT This study examined the thermal history of the Kumano forearc basin, as well as slope basin sediments and the accretionary prism, in the Nankai Trough, offshore Japan, using vitrinite reflectance measurements of sediments collected from Integrated Ocean Drilling Program Sites C0004, C0007, and C0009. We detected a paleothermal anomaly in the Kumano forearc basin at Site C0009 that has a 200-m-wide peak with a maximum temperature of 79 °C, ranging from 1000 to 1200 m below seafloor (mbsf). We concluded that thermal fluid is passing through the peak zone based on a curvilinear paleotemperature structure with a wide peak zone. Estimation of reflectance increase through vitrinite reaction promotion suggests that 80–100 °C thermal fluid had passed within at least 100 k.y., thus causing the anomaly. The thermal fluid upwelling could relate to thrusts and ancient splay fault activity near Site C0009. The thermal anomaly zone in the Kumano forearc basin at Site C0009 coincides with the currently active fluid conduit zone imaged on reflection seismic profiles. These results indicate that massive fluid circulation occurs spatially and temporally through thrusts in the subduction zone.

ABSTRACT We describe and interpret a system of well-preserved normal and reverse faults in the Kayo Formation of the Miocene Shimanto belt, an exhumed accretionary complex exposed on Okinawa Island. The normal and reverse fault systems both strike NE-SW, suggesting systematic horizontal stress variations between compression and extension. Temperature and pressure conditions for the normal and reverse fault systems were estimated from the densities of water in fluid inclusions in the veins along the faults, and previously reported maximum paleotemperature based on values of vitrinite reflectance and illite crystallinity. The fluid inclusion analyses yielded similar estimates for water density in both normal and reverse fault systems. The minimum geothermal gradient was constrained to a narrow range of 40–50 °C/km. These results suggest that the normal and reverse fault systems developed at a similar depth within the seismogenic zone. This can be interpreted as a change between horizontal compression and horizontal extension occurring at a maximum depth of 3.8–7.5 km below the seafloor, assuming lithostatic fluid pressure. This 90° rotation of the principal stress could be controlled by the seismic cycle, as exemplified by the rotation of stresses that occurred after the Tohoku-Oki earthquake.

ABSTRACT The subduction thrust interface represents a zone of concentrated deformation coupled to fluid generation, flow, and escape. Here, we review the internal structure of the megathrust as exposed in exhumed accretionary complexes, and we identify a deformation sequence that develops as material entering the trench is subducted through the seismogenic zone. Initial ductile flow in soft sediment generates dismembered, folded, and boudinaged bedding that is crosscut by later brittle discontinuities. Veins formed along early faults, and filling hydrofractures with the same extension directions as boudins in bedding, attest to fluid-assisted mass transfer during the shallow transition from ductile flow to brittle deformation. In higher-metamorphic-grade rocks, veins crosscut foliations defined by mineral assemblages stable at temperatures beyond those at the base of the seismogenic zone. The veins are, however, themselves ductilely deformed by diffusion and/or dislocation creep, and thus they record fracture and fluid flow at a deeper brittle-to-ductile transition. The results of numerical models and mineral equilibria modeling show that compaction of pore spaces may occur over a wide zone, as underconsolidated sediments carry water under the accretionary prism to the region where the last smectite breaks down at a temperature of ≤150 °C. However, at temperatures above clay stability, no large fluid release occurs until temperatures reach the zone where lawsonite and, subsequently, chlorite break down, i.e., generally in excess of 300 °C. In thermal models and strength calculations along overpressured subduction interfaces, where phyllosilicates form an interconnected network that controls rheology, as is generally observed, the deep brittle-viscous transition—analogous to the base of the seismogenic zone—occurs at temperatures less than 300 °C. We therefore suggest that the seismogenic zone does not produce fluids in significant volumes; however, major fluid release occurs at or near the base of the seismogenic zone. These deep fluids are either trapped, thus enabling embrittlement and features such as episodic tremor and slow slip, or flow updip along a permeable interface. Overall, we highlight fluid production as spatially intermittent, but fluid distribution as controlled also by the permeability of a deforming zone, where secondary porosity is both generated and destroyed, commonly in sync with the generation and movement of fluids.

Abstract This guide provides background information and an itinerary for a two-day field trip in the Northern Apennines leaving from Modena and ending at Riolunato (Modena). The proposed field trip route leads through the Po Valley side of the Northern Apennines, in the Emilia region. The field trip provides opportunities to examine the exposed geological features related to a phase of early-middle Miocene convergence between the European and Adria plates. In particular, outcrops have been selected that exhibit features characterizing deformation in an exhumed plate boundary shear zone, interpreted as an erosive plate boundary shear zone. The sedimentary evolution and deformation of the upper and lower plates are also highlighted.

Abstract This guide provides background information and an itinerary for a one-day field trip leaving Barga Garfagnana (Lucca) and crossing the Alpi Apuane toward Versilia. This field-trip route provides the opportunity to examine structures and strain features that record the underplating and exhumation of the Apuane metamorphic units. Special emphasis will given to the structures produced at the different scales in the Carrara marble, known throughout the world as a highly desirable building stone. The field trip will touch on the role of fluids, fluid-rock interaction, and deformation during underplating and subsequent exhumation of the region along major normal faults.

Abstract The development of theories that link structure, material properties, and fluid flow to the secular variations of the seismic cycle is in its infancy. The importance of this linkage provided the motivation for a March 2012 GSA Penrose conference “Fluid Flow, Material Transfer and Deformation in the Forearcs of Convergent Margins” (Lucca, Italy, in the Northern Apennines). This volume, created for that conference, includes two field trips. One describes a two-day trip in the Ligurides and the other describes a one-day trip in the Alpi Apuane. Together, these field trips address convergent margin processes at a variety of depths within the subduction system.