Anaccretionary wedge oraccretionary prism forms fromsedimentsaccreted onto the non-subductingtectonic plate at aconvergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the downgoingslab ofoceanic crust, but in some cases the wedge includes the erosional products of volcanicisland arcs formed on the overriding plate.
Anaccretionary complex is a current (in modern use) or former accretionary wedge. Accretionary complexes are typically made up of a mix ofturbidites of terrestrial material,basalts from theocean floor, andpelagic andhemipelagicsediments. For example, most of thegeological basement ofJapan is made up of accretionary complexes.[1]
Accretionary wedges and accretedterranes are not equivalent to tectonic plates, but rather are associated with tectonic plates and accrete as a result of tectonic collision. Materials incorporated in accretionary wedges include:
Elevated regions within theocean basins such as linear island chains,ocean ridges, and smallcrustal fragments (such as Madagascar or Japan), known asterranes, are transported toward the subduction zone and accreted to the continental margin. Since the Late Devonian and Early Carboniferous periods, some 360 million years ago, subduction beneath the western margin of North America has resulted in several collisions with terranes, each producing amountain-building event. The piecemeal addition of these accreted terranes has added an average of 600 km (370 mi) in width along the western margin of theNorth American continent.[2]
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The topographic expression of the accretionary wedge forms a lip, which may dam basins of accumulated materials that, otherwise, would be transported into the trench from the overriding plate. Accretionary wedges are the home ofmélange, intensely deformed packages of rocks that lack coherent internal layering and coherent internal order.[3]
The internal structure of an accretionary wedge is similar to that found in a thin-skinned forelandthrust belt. A series of thrustsverging towards thetrench are formed with the youngest most outboard structures progressively uplifting the older more inboard thrusts.
The shape of the wedge is determined by how readily the wedge will fail along its basaldecollement and in its interior; this is highly sensitive topore fluid pressure. This failure will result in a mature wedge that has an equilibrium triangular cross-sectional shape of acritical taper. Once the wedge reaches a critical taper, it will maintain that geometry and grow only into a largersimilar triangle.
The small sections of oceanic crust that are thrust over the overriding plate are said to beobducted. Where this occurs, rare slices of ocean crust, known asophiolites, are preserved on land. They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on land. A classic example is theCoast Range ophiolite of California, which is one of the most extensive ophiolite terranes in North America. This oceanic crust likely formed during the middleJurassic Period, roughly 170 million years ago, in an extensional regime within either a back-arc or a forearc basin. It was later accreted to the continental margin of Laurasia.[4]
Longitudinal sedimentary tapering of pre-orogenic sediments correlates strongly with curvature of the submarine frontal accretionary belt in theSouth China Sea margin, suggesting that pre-orogenic sediment thickness is the major control on the geometry of frontal structures. The preexisting South China Sea slope that lies obliquely in front of the advancing accretionary wedge has impeded the advancing of frontal folds resulting in a successive termination of folds against and alongstrike of the South China Sea slope. The existence of the South China Sea slope also leads the strike of impinging folds with NNW-trend to turn more sharply to a NE-strike, parallel to strike of the South China Sea slope. Analysis shows that the pre-orogenic mechanical/crustal heterogeneities and seafloor morphology exert strong controls on the thrust-belt development in the incipient Taiwanarc-continent collision zone.[5]
In accretionary wedges, seismicity activating superimposed thrusts may drive methane and oil upraising from the upper crust.[6]
Mechanical models that treataccretionary complexes as critically tapered wedges of sediment demonstrate that pore pressure controls their taper angle by modifying basal and internal shear strength. Results from some studies show that pore pressure in accretionary wedges can be viewed as a dynamically maintained response to factors which drive pore pressure (source terms) and those that limit flow (permeability and drainage path length). Sediment permeability and incoming sediment thickness are the most important factors, whereas fault permeability and the partitioning of sediment have a small effect. In one such study, it was found that as sediment permeability is increased, pore pressure decreases from near-lithostatic to hydrostatic values and allows stable taper angles to increase from ~2.5° to 8°–12.5°. With increased sediment thickness (from 100–8,000 m (330–26,250 ft)), increased pore pressure drives a decrease in stable taper angle from 8.4°–12.5° to <2.5–5°. In general, low-permeability and thick incoming sediment sustain high pore pressures consistent with shallowly tapered geometry, whereas high-permeability and thin incoming sediment should result in steep geometry. Active margins characterized by a significant proportion of fine-grained sediment within the incoming section, such as northernAntilles and easternNankai, exhibit thin taper angles, whereas those characterized by a higher proportion of sandy turbidites, such asCascadia,Chile, andMexico, have steep taper angles. Observations from active margins also indicate a strong trend of decreasing taper angle (from >15° to <4°) with increased sediment thickness (from <1 to 7 km).[7]
Rapid tectonic loading of wet sediment in accretionary wedges is likely to cause the fluid pressure to rise until it is sufficient to cause dilatant fracturing. Dewatering of sediment that has been underthrust and accreted beneath the wedge can produce a large steady supply of such highly overpressured fluid. Dilatant fracturing will create escape routes, so the fluid pressure is likely to be buffered at the value required for the transition between shear and oblique tensile (dilatant) fracture, which is slightly in excess of the load pressure if the maximum compression is nearly horizontal. This in turn buffers the strength of the wedge at the cohesive strength, which is not pressure-dependent, and will not vary greatly throughout the wedge. Near the wedge front the strength is likely to be that of the cohesion on existing thrust faults in the wedge. The shear resistance on the base of the wedge will also be fairly constant and related to the cohesive strength of the weak sediment layer that acts as the basal detachment. These assumptions allow the application of a simple plastic continuum model, which successfully predicts the observed gently convex taper of accretionary wedges.[8]
Pelayo and Weins have postulated that sometsunami events have resulted from rupture through the sedimentary rock along the basal decollement of an accretionary wedge.[9]
Backthrusting of the rear of the accretionary wedge, arcward over the rocks of the forearc basin, is a common aspect of accretionary tectonics. An older assumption thatbackstops of accretionary wedges dip back toward the arc, and that accreted material is emplaced below such backstops, is contradicted by observations from many active forearcs that indicate (1) backthrusting is common, (2) forearc basins are nearly ubiquitous associates of accretionary wedges, and (3) forearc basement, where imaged, appears to diverge from the sedimentary package, dipping under the wedge while the overlying sediments are often lifted up against it. Backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported byshear stress along the backthrust.[10]
