Accretionary Prisms

Where present, an accretionary prism forms on the inner wall of an ocean trench. The internal structure and construction of these features have been deduced from seismic reflection profiles and drilling at active subduction zones, and by the study of ancient subduction complexes now exposed on land.

Accretionary prisms develop where trench-fill turbidites (flysch), and some pelagic sediments, are scraped off the descending oceanic plate by the leading edge of the overriding plate, to which they become accreted. The Nankai Trough, located south ofJapan (Fig. 9.20a), illustrates many of the structural, lithologic and hy-drologic attributes of a large, active accretionary prism with a thick sedimentary section (Moore et al., 2001, 2005). Beneath the prism, the plate boundary is defined by a 20- to 30-m-thick, gently dipping fault or shear zone that separates a deformed sedimentary wedge above from a little-deformed section of subducted trench sediment, volcaniclastic rock, and basaltic crust below (Fig. 9.20b). This boundary, or décollement, develops in a weak sedimentary layer, typically a low permeability hemipelagic mud underlying stronger, more permeable trench turbidites. Above the décollement is a fold and thrust belt composed of listric thrust ramps that rise through the stratigraphic section forming imbricated arrays. These faults define wedge-shaped lenses that are internally folded and cleaved. At the base of the imbricate series, the décollement slopes downward toward the volcanic arc where it becomes progressively better developed. Away from the arc, it extends a short distance seaward of the deformation front, which is marked by the first small proto-thrusts and folds located inward from the trench. Farther seaward, the stratigraphic horizon that hosts the décollement is known as the incipient or proto-décollement zone where the incoming sedimentary section is only weakly deformed.

(a) Accretionary forearc

Trench fill

Accretionary

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Trench slope

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basin

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V = fluid vents

Thick forearc basin

Active arc

Trench fill

(b) Non-accretionary forearc

Exposed basement Serpentinite Seamount \ mud volcanoes

Thin forearc basin

Active arc

Exposed basement Serpentinite Seamount \ mud volcanoes

Thin forearc basin

Active arc

Partially serpentinized mantle Lithospheric mantle

Basaltic forearc crust

Gabbroic forearc crust

•( r

Undeformed sediments Deformed sediments

Figure 9.19 Diagrams contrasting the characteristic features of (a) accretionary, and (b) nonaccretionary, convergent margins (redrawn from Stern, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union). V, fluid vents.

Seismic reflection data and the ages of deformed sediments suggest that the youngest faults in accretionary prisms occur at the deformation front and generally become older away from the trench (Moore et al., 2001, 2005) (Fig. 9.20b). As shortening occurs, old thrust wedges gradually move upwards and are rotated toward the arc by the addition of new wedges to the toe of the prism. This process, called frontal accretion, causes older thrusts to become more steeply dipping with time and is responsible for the lateral growth of the prism. Lateral growth requires that the most intense deformation occurs at the oceanward base of the sedimentary pile,

EURASIAN PLATE

Median Tectonic Line^

Honshu

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0 100 km

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PHILLIPPINE SEA PLATE

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S-Mb

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17-12 Ma igneous activity

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Prior ODP/DSDP sites

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ODP Leg 190 sites

Mio-Pliocene package

Pio-Pleistocene package

Recent package

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1S1't7e8 Trench slope break Zone of out-of sequence thrusting Cover sequence site Site

Trench slope 1175 1176 , basin I I Frontal out-of sequence thrust

Imbricate series \ lmbricate series site site ,r . . 808 1174

Frontal thrust

Hemipelagic unit

Trench Site turbidites 1173

Hemipelagic unit

Trench Site turbidites 1173

Seismogenic zone?

Deformation front

Proto-décollement zone

Figure 9.20 (a) Geologic map of the Nankai accretionary prism showing the Leg 190/196 Ocean Drilling Program (ODP, black circles) and previous ODP/Deep Sea Drilling Program (DSDP, open circles) drill sites (modified from Moore et al., 2001, by permission of the American Geophysical Union. Copyright © 2001 American Geophysical Union). CA, Cape Ashizuri; CM, Cape Muroto; Smb, Sanbagawa metamorphic belt; Jp, Jurassic accretionary prism; CSb, Cretaceous Shimanto belt; CzSb, Cenozoic Shimanto belt; S-Mb, Shimanto and Mineoka belts; Ava, Accreted volcanic arc; PTZ, protothrust zone. (b) Generalized cross-section of the Muroto transect showing main structural provinces (modified from Moore et al., 2005, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union).

Seismogenic zone?

Deformation front

Proto-décollement zone

Figure 9.20 (a) Geologic map of the Nankai accretionary prism showing the Leg 190/196 Ocean Drilling Program (ODP, black circles) and previous ODP/Deep Sea Drilling Program (DSDP, open circles) drill sites (modified from Moore et al., 2001, by permission of the American Geophysical Union. Copyright © 2001 American Geophysical Union). CA, Cape Ashizuri; CM, Cape Muroto; Smb, Sanbagawa metamorphic belt; Jp, Jurassic accretionary prism; CSb, Cretaceous Shimanto belt; CzSb, Cenozoic Shimanto belt; S-Mb, Shimanto and Mineoka belts; Ava, Accreted volcanic arc; PTZ, protothrust zone. (b) Generalized cross-section of the Muroto transect showing main structural provinces (modified from Moore et al., 2005, by permission of the American Geophysical Union. Copyright © 2005 American Geophysical Union).

although some older thrusts may remain active during their rotation and some new thrusts may form and cut across older imbricate thrusts. These latter, cross cutting faults are termed out-of-sequence thrusts (Fig. 9.20b) because they do not conform to the common arcward progression of faulting. In addition to frontal accretion, some incoming material is carried downward past the deformation front where it is transferred, or underplated, to the base of the prism by thrust faulting above the décollement. Unlike the off-scraped sediments at the toe, this underplated material may become deeply buried and undergo high pressure metamorphism (Section 9.9). Tectonic underplating, together with internal shortening, thickens the wedge and causes the slope of its upper surface to increase (Konstantinovs-kaia & Malavieille, 2005).

The top of an accretionary prism is defined by a relatively abrupt decrease in slope called the trench slope break. Between this break and the island arc, a forearc basin may develop, which is then filled with sediments derived from erosion of the volcanic arc and its substrate. This basin is a region of tranquil sedimentation where flat-lying units cover the oldest thrust slices in the wedge. Seaward of the forearc basin, on the trench slope, small pockets of sediment also accumulate on top of old thrust slices (Fig. 9.20b). The ages of these old slices, and their distance from the toe of the prism, provide a means of estimating lateral growth rates. For example, drilling at sites 1175 and 1176 in the Nankai prism has shown that trench slope sands unconform-ably overlie thrust slices that may be as young as 1 or 2 Ma (Moore et al., 2001; Underwood et al., 2003). Assuming steady state seaward growth, the distance of these thrust slices from the deformation front implies lateral growth rates as high as 40 km over the last 1 to

2 Myr. In comparison, the Middle America accretionary prism off the coast of Mexico has grown ~23 km in width over the past 10 Myr (Moore et al., 1982) and the eastern Aleutian accretionary prism has grown 20 km in

Erosion of the trench slope and other landward material commonly results in slump deposits and debris flows that can carry material as far as the trench, where it gets offscraped and recycled back into the wedge. At Site 1178 in the Nankai prism, the presence of thrust slices composed of Miocene turbidites indicates that the trench was accumulating large amounts of sediment derived from the erosion of rock exposed on Shikoku Island at that time (Moore et al., 2005). Large (100- to 1000-m-long) blocks of slumped material, called olistostromes, remain semi-coherent during transport. This process provides much of the material that enables accretionary prisms to grow wider (Silver, 2000). Over time, erosion, deformation and sedimentary recycling result in a long-term circulation of material within the wedge (Platt, 1986). Offscraped material first moves down toward the base of the prism and then moves back toward the surface. This pattern results in a general increase in the metamorphic grade of rocks from the trench to the arc such that the oldest, high grade rocks are structurally highest and uplifted with respect to the younger deposits. The processes also may create a chaotic mixture of igneous, sedimentary and metamorphic rock types called a mélange (see also Section 10.6.1). Some of the oldest rock fragments in the mélange may record blueschist or eclogite facies metamorphism, indicating depths of burial of at least 30 km (Section 9.9).

The overall shape of accretionary prisms in profile approximates that of a tapered wedge, where the upper surface slopes in a direction opposite to that of the underlying décollement (Fig. 9.21a). Davis et al. (1983) and Dahlen (1990) showed that this tapered shape is required if the entire wedge moves together and the behavior of the system follows the Mohr-Coulomb fracture criterion (Section 2.10.2). The surface slope (a) is determined by the interplay between resistance to sliding on the décollement and the strength of the rock in the thrust wedge. Both of these latter two factors are strongly influenced by pore fluid pressure (X), the dip of the basal décollement (P), and the weight of the overlying rock (Fig. 9.21b). Tectonic shortening and underplating thicken the wedge, thereby steepening the surface slope. If the surface slope becomes oversteep-ened, then various mechanical adjustments will occur until the slope decreases and a steady state is achieved. These adjustments may involve normal faulting and/or a lengthening of the décollement, and result from the same forces that drive the gravitational collapse of large topographic uplifts (Section 10.4.6). The mechanical behavior of the wedge also is especially sensitive to mass redistribution by surface erosion and deposition (Konstantinovskaia & Malavieille, 2005; Stolar et al., 2006), which change topographic gradients and, at large scales, affect the thermal evolution of the crust (Section 8.6.3).

The results from drilling into active prisms have provided unequivocal evidence of the importance of fluid flow and changes in pore fluid pressure in accretionary prisms. Measurements of porosity, density, resistivity, and other physical characteristics suggest that accreted sediments descend so rapidly that they have no opportunity to dewater before burial (Silver, 2000; Saffer, 2003; Moore et al., 2005). This process, and the low permeabilities that are typical of marine sediments, result in elevated pore pressures that reduce effective stress, lower the shear strength of rock (Section 2.10.2), and allow sliding on the décollement. Episodic fluid flow and the collapse of former flow paths also may allow the décollement to propagate laterally beneath the wedge (Ujiie et al., 2003). These processes explain the generally small taper angles of most accretionary wedges, which can result only if the material within it is very weak and shear stresses on the décollement are very low (Davis et al., 1983; Saffer & Bekins, 2002). High pore fluid pressure also explains

(b)

Figure 9.21 (a) Schematic profile of a Coulomb wedge and (b) theoretical wedge tapers for various pore fluid pressure ratios (l) for submarine accretionary prisms, assuming the pressure at the base is identical to that in the wedge (modified from Davis et al., 1993, by permission of the American Geophysical Union. Copyright © 1993 American Geophysical Union). Boxes in (b) indicate tapers of active wedges. Calculations involved a wedge sediment density of2400kg m~3.

Figure 9.21 (a) Schematic profile of a Coulomb wedge and (b) theoretical wedge tapers for various pore fluid pressure ratios (l) for submarine accretionary prisms, assuming the pressure at the base is identical to that in the wedge (modified from Davis et al., 1993, by permission of the American Geophysical Union. Copyright © 1993 American Geophysical Union). Boxes in (b) indicate tapers of active wedges. Calculations involved a wedge sediment density of2400kg m~3.

many other phenomena that are associated with prisms, including mud volcanoes and diapirs (Westbrook et al., 1984), and the development of unique chemical and biological environments at the leading edge of the prism (Schoonmaker, 1986; Ritger et al., 1987) (Fig. 9.19).

In addition to a mechanism by which pore fluid pressure increases by rapid burial, there also are competing mechanisms that decrease pore fluid pressure within a wedge. Fluids tends to flow along narrow, high permeability channels and exit to the décollement and the seafloor through vertical and lateral conduits (Silver,

2000; Morris & Villinger, 2006). Some of these conduits coincide with thrust faults overlying the décollement zone, whose high fracture permeability allows fluid to escape (Gulick et al., 2004; Tsuji et al., 2006). Fluid escape in this way implies that the décollement zone possesses a lower fluid pressure than its surroundings, a condition that is in apparent conflict with the evidence of high pore fluid pressures in this zone. However, the apparent conflict can be reconciled by models in which the fluid pressure in the décollement zone varies both spatially and temporally within the wedge. The nature of these variations, and their affect on the evolution of

Taper angle |

50 -40 -30 -20 -10 0 Distance arcward from the deformation front (km)

Well drained:

fjr Rapid fluid escape [¿SS Low pore pressures

Wedge steepens

Wedge steepens

Steep stable geometry Strong base

Poorly drained:

Retarded fluid escape Elevated pore pressures

Wedge remains shallow

Shallow stable geometry Weak base

Figure 9.22 (a) Schematic view of a numerical model of fluid flow within an accretionary prism and (b) cross-sections showing relationships between factors influencing accretionary wedge taper angle (modified from Saffer & Bekins, 2002, with permission from the Geological Society of America). Arrows in (a) represent approximate sediment velocities at the deformation front. Shading shows generalized porosity distribution, contours are modeled steady-state pore pressures (l).

the deforming wedge, are greatly influenced by factors such as the convergence rate and the stratigraphy, lithol-ogy, mineralogy, and hydrologic properties of the incoming sediments (Saffer, 2003).

The sensitivity of accretionary prisms to fluctuations in fluid flow and pore fluid pressure has been explored in detail using mechanical and numerical models. By combining a model of groundwater flow with critical taper theory (Fig. 9.22a), Saffer & Bekins (2002) concluded that low permeability, high pore pressure, and rapid convergence rates sustain poorly drained systems and result in shallow tapers, whereas high permeability, low pore pressure, and slow convergence result in well-drained systems and steep taper geometries (Fig. 9.22b). These authors also showed that the stratigraphic thickness and composition of the sediment that is incorporated into the wedge are among the most important factors governing pore fluid pressure in wedges (Saffer & Bekins, 2006). Thick sedimentary sections give rise to large prisms that are able to sustain high pore fluid pressures and low stable taper angles (Fig. 9.23a). The results also suggest that prisms composed mostly of low permeability fine-grained sediment, such as northern Antilles (Barbados) and eastern Nankai (Ashizuri), will exhibit thin taper angles and those characterized by a high proportion of high permeability turbidites, such as Cascadia, Chile, and México, will have steep taper angles (Fig. 9.23b). This sensitivity to the physical properties of accreted and subducted sediment implies that any along-strike variation in sediment lithology or thickness strongly influences the geometry and mechanical behavior of accretionary prisms. Similarly, any variation in incoming sediment thickness or composition over time will force the accretionary complex to readjust until a new dynamic balance is reached.

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0 1000 2000 3000 4000 5000 6000 7000 8000 Incoming sediment thickness (m)

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N. Cascadia: v = 32 mm a 1; t = 2500 m; p = 4°; a = 4°

20 km

20 km

Ashizuri Nankai: v = 40 mm a 1; t = 1050 m; p = 2.6°; a = 1.5°

20 km

N. Antilles: v = 28 mm a"1; t = 700-800 m; p = 2°; a = 1°

20 km

100 80 60 40 20 0 % of incoming section dominated by clay/mud

Figure 9.23 Taper angles of active accretionary prisms plotted as a function of (a) thickness of incoming sediment and (b) lithology where the incoming sediment section has been sampled by drilling (modified from Saffer & Bekins, 2006, by permission of the American Geophysical Union. Copyright © 2006 American Geophysical Union). Horizontal error bars indicate uncertainty in lithology and vertical error bars indicate along-strike variations in taper angle. NA, northern Antilles; SA, southern Antilles; MUR, Nankai Muroto; AL, eastern Aleutians (160°W); EA, eastern Aleutians (148-150°W); CA, central Aleutians (172-176°W); NC, north Cascadia; SC, southern Cascadia; ASH, Nankai Ashizuri; MX, Mexico; JA, Java; CS, central Sumatra; SU, Sunda; CH, Chile; NI, Nicobar; AN, Andaman; LU, Luzon; BU, Burma; MA, Makran. Cross-sections in (b) are from Saffer & Bekins (2002). Plate convergence rate (v), incoming stratigraphic thickness (t), dip of the décollement (ft), and surface slope (a) are indicated.

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