The Continential Crust

Topics: Plate tectonics, Subduction, Convergent boundary Pages: 5 (1435 words) Published: March 18, 2013
Accretionary prism

Subduction zone with inset showing structure of accretionary prism An accretionary wedge or accretionary prism is formed from sediments that are accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the downgoing slab of oceanic crust but in some cases includes the erosional products of volcanic island arcs formed on the overriding plate. Contents [hide]

1 Materials within an accretionary wedge
2 Geometry
3 Examples
4 Impacts of accretionary wedges
5 References
[edit]Materials within an accretionary wedge

Accretionary wedges and accreted terranes 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: Ocean-floor basalts – typically seamounts scraped off the subducting plate Pelagic sediments – typically immediately overlying oceanic crust of the subducting plate Trench sediments – typically turbidites that may be derived from: Oceanic, volcanic island arc

Continental volcanic arc and cordilleran orogen
Adjacent continental masses located along strike (such as Barbados). Material transported into the trench by gravity sliding and debris flow from the forearc ridge *:olistostrome) Piggy-back basins, which are small basins located in surface depression on the accretionary prism. Material exposed in the forearc ridge may include fragments of oceanic crust or high pressure *:metamorphic rocks thrust from deeper in the subduction zone. Elevated regions within the ocean basins such as linear island chains, ocean ridges, and small crustal fragments (such as Madagascar or Japan), known as terranes, 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 a mountain-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 the North American continent.[1] [edit]Geometry

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 of mélange, intensely deformed packages of rocks that lack coherent internal layering and coherent internal order.[2] The internal structure of an accretionary wedge is similar to that found in a thin-skinned foreland thrust belt. A series of thrusts verging towards the trench 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 basal decollement and in its interior; this is highly sensitive to pore fluid pressure. This failure will result in a mature wedge that has an equilibrium triangular cross-sectional shape of a critical taper. Once the wedge reaches a critical taper, it will maintain that geometry and grow only into a larger similar triangle.

Accretionary wedge
(USGS Visual Glossary)

Chilean Coast Range between 38°S and 43°S (Bahía Mansa Metamorphic Complex). Calabrian Accretionary Wedge in the Central Mediterranean - The Neogene tectonics of the central Mediterranean are related to the subduction and trench rollback of the Ionian basin under Eurasia, causing the opening of the Liguro‐Provençal and Tyrrhenian back‐arc basins and the formation of the Calabrian accretionary wedge. The Calabrian accretionary wedge is a partially submerged accretionary complex located in the Ionian offshore and laterally bounded by the Apulia and Malta escarpments.[3] The Olympic Mountains located...
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