Comparison of frictional strength and velocity dependence between fault zones in the Nankai accretionary complex
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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.Keywords:
Accretionary wedge
Décollement
Thrust fault
Echelon formation
Accretionary wedge
Anticline
Thrust fault
Echelon formation
Outcrop
Trough (economics)
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Décollement
Prism
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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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Décollement
Wedge (geometry)
Thrust fault
Continental Margin
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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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Prism
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Accretionary wedge
Décollement
Prism
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To reveal evolution of the plate boundary décollement of the Nankai accretionary prism, we used 3D seismic reflection data off the Kii peninsula. We observe that geometry of the oceanic crust surface strongly influences on the décollement. Furthermore the geometry of crust surface was controlled by the displacement along the thrusts within oceanic crust; the offset at the dip of the thrust is ~1 km. These thrusts within ocean crust should be still active because their locations are consistent with the hypocenters of the 2004 earthquake off the Kii peninsula. We observe the décollement transition at the landward side of the elevated basement, and two décollement horizons exist. This transition of the décollement could originate underplating and induce prism thickening. On the seismic profile, several discontinuous reflections are observed above the décollement seaward of the elevated basement, and imbricate thrusts seem to sole down into the discontinuous reflections but do not extend down to the basal décollement. Furthermore, the accretionary prism is thickened on the seaward side of the mega-splay fault and the thickening cannot be explained only by increasing the thrust angle. From these observations, we interpret that the accretionary wedge has thickened by underplating on the seaward side of the mega-splay fault. Therefore, the crustal elevation due to thrust displacements within the oceanic crust is important in the underplating processes in our survey area.
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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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The tectonic and deformation processes active during transfer of material in accretionary prisms are the results of a complex interplay of different factors. Among those, as demonstrated by studies on modern accretionary prisms, the mutual dependence between fluid circulation and sediment budget seems to play a crucial role in influencing the morphology and dynamics of both prisms and decollement zone.
In this work we report a detailed structural study of two examples of fossil prisms characterized by different sediment budgets, and representative of different accretionary processes: the Franciscan Complex (FC, N-California) and the Internal Ligurian Units (ULI, N-Apennines). Two fluid circulation models are proposed and their comparison suggests that sediment budget controls prism hydrogeology and tectonics.
The analyses of the two analogues have shown that, despite a different structural style, in both examples deformation is strongly controlled by repeated injection of overpressured fluids during underthrusting and accretion. Hydrofracturing occurs through dilatant fractures sub-parallel to the decollement zone, favored also by the wedge regional stress field. Fluid injection is transient, is associated with variation of local stress field at the decollement, and cyclical variation of permeability and cohesion states of rocks and sediments, allowing cyclical variation of deformation mechanisms and crosscutting veining episodes.
The sediment budget at trenches controls the fluid pathways, which are located along the sedimentary layers of the turbiditic deposits in the ULI. Moreover, the presence of thick sequences of sediments, which undergo modification while moving to depth, allows the deformation to be strongly influenced by diagenetic processes. While deformation mechanisms, hydrofracturing and veining are essentially “shear zone (i.e. tectonically)-controlled” in the FC, in the ULI they are syn-diagenetic, and change when mechanical and diagenetic conditions in the turbiditic sequence, such as lithification state and competency contrasts, change.
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Echelon formation
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Wedge (geometry)
Thrust fault
Décollement
Prism
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Abstract We conducted a 3‐D seismic inversion study to investigate spatial variations of physical properties of the décollement zone (DZ) and protodécollement zone (PDZ) under the northern Barbados accretionary prism. Significant spatial variations of physical properties were observed in the PDZ seaward of the thrust front from the inversion data. The density generally increases southward with a few localized low‐density patches. A lower density commonly corresponds to a thicker PDZ, suggesting that the paleomorphology may at least partially control the variations of the physical properties. Similar low‐density patches were also found in the DZ. These features may be inherited from those of the PDZ and enhanced after subduction through localized arrested consolidation. Under the prism toe, the density of the DZ increases landward. This trend may mainly result from shear‐induced consolidation of the DZ but may also be related to landward increasing tectonic loading. Significant north–south differences in density and, thus, porosity and strength of the PDZ, are observed and these differences may continue into the DZ. A stronger DZ is likely responsible for a larger prism taper observed in the southern area of the prism toe. The larger taper, thus more horizontal shortening, coupled with a thinner sediment sheet above the PDZ in the southern area, may cause a relative retreat of the thrust front and a pronounced change in strike of the sequence thrusts south of seismic Line 690. The north–south differences may ultimately have originated in the approach of a structurally higher segment of the Tiburon Rise. The Tiburon Rise affects regional morphology and, thus, it controls the sedimentation and physical properties of the PDZ. It may also control sediment accumulation above the PDZ. Therefore, the sedimentational change induced by the structural high of the Tiburon Rise, in turn, resulted in structural change of the prism in the southern area.
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PDZ domain
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