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The morphology, development, composition, and structure of submarine fans reflect five controlling factors. (1) Grain-size distribution--sandy debris results in radial fans, whereas mud-dominated debris promotes elongate fans. (2) Eustacy--during high sea-level stands, fan growth is curtailed as sediment, especially coarse sediment, is trapped in paralic settings, whereas during low stands, fan growth is accelerated owing to direct access of sediment to canyon heads and the flushing of debris from shelf areas. (3) Basin architecture--confined or unconfined. (4) Penecontemporaneous tectonics from seismic triggering to migrating source terranes. (5) Postdepositional tectonics that bias or alter lithofacies preservation--bias in sediment subduction results in the preferentia loss of distal facies.
Some examples that demonstrate these five controlling factors include the following. (1a) The Navy and Arguello fans of the California continental borderland are sandy, radial fans, whereas (1b) the Bengal and Indus fans are muddy elongate fans. (2) The Eocene Farello fan, southern California borderland, is composed of thick intervals of sand and conglomerate lithofacies, corresponding to low sea level stands, and thin fine-grained intervals, corresponding to high sea level stands. (3) Relatively unconfined fans are most typical of passive or trailing continental margins, e.g., the Bengal, Amazon, and Mississippi fans, although even here diapir fields, seamounts, and fracture zones offer local impediments. In contrast, fans in confined basins are characteristic of convergent or transf rm continental margins, e.g., borderland, forearc, and backarc basins, although passive margins may also have confined basins. The Laurentian fan, for example, is blocked by the Kelvin Seamounts. (4) Large infrequent earthquakes (one to several per millenium) may trigger large depositional events, e.g., the 1929 Grand Banks shock. Shingling of depositional units occurs in pull-apart basins of strike-slip fault systems; such sequences may have stratigraphic thicknesses (not vertical accumulations) of 10 km/m.y. (33,000 ft/m.y.). Examples include the Neogene Ridge basin in California and the Cretaceous Izumi basin in Japan. Where a canyon head lies seaward of a transform fault, a large fan develops with multiple sources, e.g., the Monterey and Delgada fan systems. Where a canyon lies landw rd of a transform boundary, multiple small fans of one source terrane are strung out along a continental margin, e.g., the Baranoff fans of southeastern Alaska. (5) Inferred subduction complexes, e.g., the Torlesse of New Zealand, are composed principally of thousands of meters of tectonically thickened (thrust faulted) thick-bedded sandstone lithofacies. This composition may be due to decoupling of the more distal, fine-grained facies from the proximal facies and its transportation into the mantle along with the underlying oceanic crust.
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