Definitions and images to illustrate geological terms, links to images and website articles

back-arc forearc

Subduction zone associated with trench, accretionary prism, forearc basin, magmatic front, and back-arc basin at spreading axis. Courtesy of zyzzy2Back-arc basins are associated with tensional forces caused by asymmetric seafloor spreading and oceanic trench rollback at some convergent plate boundaries.

Back-arc basins develop where island arcs are split longitudinally, roughly along the line of the magmatic axis, forming a rift that matures to the point of seafloor spreading, thus allowing a new magmatic arc to form on the trenchward side of the basin. This division strands a remnant arc on the side of the basin away from the trench and subduction zone, and the remnant arc shifts away from the arc axis as it reforms.

Most of the sediment that reaches back-arc basins originates in the active magmatic arc. Back-arc basins usually spread for a few tens of millions of years, then spreading ceases, converting the spreading back-arc basin to a fossil back-arc basin or marginal basin.

The Okinawa Trough is a backarc basin lying between Japan and Taiwan, created by extension within the continental lithosphere behind the Ryukyu trench-arc system. The Okinawa Trough is at an early stage of evolution from arc type to backarc activity [s].

Forearc and backarc basins associated with subduction zones and volcanic island arcs. Courtesy USGSForearc basins are sea floor depressions located between subduction zones and their associated volcanic arc.

Forearc basins receive sediments from the adjacent landmass, the island arc system, and trapped oceanic crustal material. Oceanic crustal fragments may be obducted onto the continent as ophiolites complexes during terrane accretion.

The Central Valley of California developed as a forearc during the late Cretaceous and early Paleogene.

[links: images: maps: swUS in the Late Triassic (215 Ma) as the Cordilleran arc developed, a new back arc basin formed behind the McCloud arc (wp, contrasting hypothesis), by early Cretaceous, the Cordilleran arc had converted to a classic continental (Andean-style) arc comprising a fore arc-trench system, fore arc basin, and Andean arc. (The fore arc-trench was site of famous Franciscan mélange). The Great Valley sequence was deposited in the fore arc basin and the Sierra Nevada batholith complex formed within the magmatic arc; Okinawa trench; diagrams: simple diagram of features of an intra-oceanic island arc subduction zone; the Taigonos segment of the Uda-Murgol Island Arc, and Pekulney segment of the island arc, Russia (wp); hypothetical x-c of Great Slave Craton (wp); webpages: Geological History of the western US: swUS: Precambrian, Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Tertiary]

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