Page 282 - Global Tectonics

SUBDUCTION ZONES 267

The top of an accretionary prism is defined by a metamorphism, indicating depths of burial of at least
relatively abrupt decrease in slope called the trench slope 30 km (Section 9.9).
break. Between this break and the island arc, a forearc The overall shape of accretionary prisms in profi le
basin may develop, which is then filled with sediments approximates that of a tapered wedge, where the upper

derived from erosion of the volcanic arc and its sub- surface slopes in a direction opposite to that of the
strate. This basin is a region of tranquil sedimentation underlying décollement (Fig. 9.21a). Davis et al. (1983)

where flat-lying units cover the oldest thrust slices in and Dahlen (1990) showed that this tapered shape is
the wedge. Seaward of the forearc basin, on the trench required if the entire wedge moves together and the
slope, small pockets of sediment also accumulate on behavior of the system follows the Mohr–Coulomb
top of old thrust slices (Fig. 9.20b). The ages of these fracture criterion (Section 2.10.2). The surface slope (α)
old slices, and their distance from the toe of the prism, is determined by the interplay between resistance to
provide a means of estimating lateral growth rates. For sliding on the décollement and the strength of the rock
example, drilling at sites 1175 and 1176 in the Nankai in the thrust wedge. Both of these latter two factors are

prism has shown that trench slope sands unconform- strongly influenced by pore fluid pressure (λ), the dip
ably overlie thrust slices that may be as young as 1 or of the basal décollement (β), and the weight of the
2 Ma (Moore et al., 2001; Underwood et al., 2003). overlying rock (Fig. 9.21b). Tectonic shortening and
Assuming steady state seaward growth, the distance of underplating thicken the wedge, thereby steepening the
these thrust slices from the deformation front implies surface slope. If the surface slope becomes oversteep-
lateral growth rates as high as 40 km over the last 1 to ened, then various mechanical adjustments will occur
2 Myr. In comparison, the Middle America accretionary until the slope decreases and a steady state is achieved.
prism off the coast of Mexico has grown ∼23 km in These adjustments may involve normal faulting and/or
width over the past 10 Myr (Moore et al., 1982) and the a lengthening of the décollement, and result from the
eastern Aleutian accretionary prism has grown 20 km in same forces that drive the gravitational collapse of large
3 Myr (von Huene et al., 1998). topographic uplifts (Section 10.4.6). The mechanical
Erosion of the trench slope and other landward behavior of the wedge also is especially sensitive to
material commonly results in slump deposits and debris mass redistribution by surface erosion and deposition

flows that can carry material as far as the trench, where (Konstantinovskaia & Malavieille, 2005; Stolar et al.,
it gets offscraped and recycled back into the wedge. 2006), which change topographic gradients and, at large
At Site 1178 in the Nankai prism, the presence of scales, affect the thermal evolution of the crust (Section
thrust slices composed of Miocene turbidites indicates 8.6.3).
that the trench was accumulating large amounts of The results from drilling into active prisms have
sediment derived from the erosion of rock exposed on provided unequivocal evidence of the importance

Shikoku Island at that time (Moore et al., 2005). Large of fl uid flow and changes in pore fluid pressure in
(100- to 1000-m-long) blocks of slumped material, accretionary prisms. Measurements of porosity, density,
called olistostromes, remain semi-coherent during resistivity, and other physical characteristics suggest
transport. This process provides much of the material that accreted sediments descend so rapidly that they
that enables accretionary prisms to grow wider have no opportunity to dewater before burial (Silver,
(Silver, 2000). Over time, erosion, deformation and 2000; Saffer, 2003; Moore et al., 2005). This process,
sedimentary recycling result in a long-term circulation and the low permeabilities that are typical of marine
of material within the wedge (Platt, 1986). Offscraped sediments, result in elevated pore pressures that reduce

material first moves down toward the base of the prism effective stress, lower the shear strength of rock (Section
and then moves back toward the surface. This pattern 2.10.2), and allow sliding on the décollement. Episodic

results in a general increase in the metamorphic grade fl uid flow and the collapse of former fl ow paths
of rocks from the trench to the arc such that the oldest, also may allow the décollement to propagate laterally
high grade rocks are structurally highest and uplifted beneath the wedge (Ujiie et al., 2003). These
with respect to the younger deposits. The processes processes explain the generally small taper angles of
also may create a chaotic mixture of igneous, sedimen- most accretionary wedges, which can result only if the
tary and metamorphic rock types called a mélange (see material within it is very weak and shear stresses on
also Section 10.6.1). Some of the oldest rock fragments the décollement are very low (Davis et al., 1983; Saffer
in the mélange may record blueschist or eclogite facies & Bekins, 2002). High pore fl uid pressure also explains