Transition from frontal accretion to underplating in a part of the Nankai Trough Accretionary Complex off Shikoku (SW Japan) and extensional features on the lower trench slope
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Recent analyses of multichannel seismic data for the Cascadia margin off central Oregon show a systematic, landward increase in velocity (increase in sediment dewatering) within a protothrust zone about 10 km wide seaward of the toe of the accretionary wedge (Cochrane et al., 1994). Here we analyze the mechanical conditions for the transmission of compression from the rear of the accretionary wedge to the area seaward of the frontal thrust to create the protothrust zone and to cause sediment dewatering. The analysis assumes that sediments are elastoplastic (Coulomb) material; i.e., they behave elastically up to a yield limit at which failure occurs. We show that the necessary conditions for the transmission of compression from the accretionary wedge to the seaward area are (1) the presence of incipient decollements seaward of the frontal thrust and (2) the presence of high pore pressures within the incipient decollement. We also show that the magnitude of pore pressure within the incipient decollement may control the width of the protothrust zone. We suggest that the latter determines the location of the next frontal thrust and hence the ramp spacing between the imbricated thrusts within accretionary wedges. Thus pore pressures within incipient decollements may significantly influence the evolution of the internal structures of accretionary wedges.
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Borehole logs from the northern Barbados accretionary prism show that the plateboundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a threedimensional 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. on January 19, 2013 geology.gsapubs.org Downloaded from
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Anisotropy of magnetic susceptibility (AMS) results from sediments spanning the basal decollement of the Barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is strongly overp ressured. In the accretionary prism above the decollement, the minimum AMS axes are subhorizontal and nearly east‐west trending, whereas the maximum AMS axes are nearly north‐south trending, and shallowly inclined. At the top of the decollement, the AMS minimum axes orientations abruptly change to nearly vertical; this orientation is maintained throughout the decollement and in the underthrust sediments below. The AMS orientations in the prism sediments above the decollement are consistent with lateral shortening caused by regional tectonic stress, as the minimum axes generally parallel the convergence vector of the subducting South American Plate, and the maximum axes are trench-parallel. This abrupt change in AMS orientations at the top of the decollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sed iments from the underthrust sediments. The decoupling horizon occurs at the top of the decollement zone, coinciding with the location of flowing, high-pressure fluids. Comparison with magnetic fabrics and susceptibilities of the seaward reference site (Site 672) indicates that the AMS fabrics at Sites 948 and 949 record the orientations of neocrystallized (Ti)magnetite and or (Ti)maghemite, and so reflect decoupling of differential stresses (and perhaps also strains) at the top of the decollement. Fur ther comparisons of susceptibility stratigraphy between sediments just above the lithostratigraphic Unit III/Unit II boundary at Sit es 672 and 948 suggest that the lower portion of the structurally defined decollement at Site 948 may in fact be largely intact. T his suggests that (1) there may be little displacement accommodated by sediments below about 498 mbsf; (2) the deformation structures observed in most of the decollement may have formed via low total strains (but perhaps under high strain rates?); an d (3) the basal decollement of the Barbados prism is a narrow plane (490 -492 mbsf), along which stresses are very effectively decoupled, rather than a thick zone of distributed deformation.
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Accretionary complexes host a variety of fault zones that accommodate plate convergence and internal prism deformation, including the décollement, imbricate thrusts, and out‐of‐sequence thrusts or splays. These faults, especially the décollement and major splay faults, are considered to be candidates for hosting slow slip events and large magnitude earthquakes, but it is not clear what modes of slip should be expected at shallow levels or how they are related to fault rock frictional properties. We conducted laboratory experiments to measure the frictional properties of fault and wall rock from three distinct fault zone systems sampled during Integrated Ocean Drilling Program Expedition 316 and Ocean Drilling Program Leg 190 to the Nankai Trough offshore Japan. These are (1) a major out‐of‐sequence thrust fault, termed the “megasplay” (Site C0004), (2) the frontal thrust zone, a region of diffuse thrust faulting near the trench (Site C0007), and (3) the décollement zone sampled 2 km from the trench (Site 1174). At 25 MPa effective normal stress, at slip rates of 0.03–100 μ m/s, and in the presence of brine as a pore fluid, we observe low friction ( μ ≤ 0.46) for all of the materials we tested; however, the weakest samples ( μ ≤ 0.30) are from the décollement zone. Material from the megasplay fault is significantly weaker than the surrounding wall rocks, a pattern not observed in the frontal thrust and décollement. All samples exhibit primarily velocity‐strengthening frictional behavior, suggesting that earthquakes should not nucleate at these depths. A consistent minimum in the friction rate parameter a‐b at sliding velocities of ∼1–3 μ m/s (∼0.1–0.3 m/d) is observed at all three sites, suggesting that these shallow fault zones may be likely to host slow slip events.
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