PLATE TECTONIC

Subduction Related Arc System

In the Arc system associated with Subduction, three zones are recognized: the arc trench gap, the arc, the arc rear area.

If we heads towards the landside from the ocean side then a continental margin arc is charcterized by the trench; an accretionary prism with the overlying foreland basins; the volcanic arc with intra arc basins; a fold thrust belt and a retro arc forland basin.

TRENCH:  

Trenches are the resulted surface expression of the subduction of one low density litospheric plate over the other overlying high density plate. sediments get transported along trenches pto 3000 kms for example the Sunda Trench where where the detritus from the himalaya enters the trench on the north from the bengal submarine fans. Trench sediments are dominantly fine grained greywacke with minor pelagic sediments.

ACCRETIONARY PRISM:

These consists of a series of steeply inclined fault bounded wedges. The wedges represent oceanic crust and trench sediments that have been accreted in front of the arc. Individual wedges in the accretionary prism decrease in age as the trench is approached. Accretionary prisms are intensely deformed producing melanges.

ACCRETIONARY PRISM

FOREARC BASIN:

These are marine depositional basins on the trench side of the arcs and they vary in size and abundance with the evolutionary stage of the Arc. Example- Sunda  Arc . Forearc basins overlie the accretionay prism and has a strike length of 700km. Hemipelagic sediments are characterstically found here.

BACK ARC BASIN:

Active back arc basins occur over descending slabs behind arc systems and commonly have high heat flow, relatively thin lithosphere and in many instances, an active ocean ridge which is enlarging the size of the basin. During the early stages of  basin opening, thick epiclastic deposits.

REMNANT ARC:

These are submarine aseismic ridges that are extinct portions of arcs which have been rifted away by the opening of a back arc basin. They are composed chiefly of subaqueous mafic volcanic rocks similar to those formed in submarine arcs. Once isolated by rifting, remnant arcs subside and are blanked progressively by deep water pelagic and biogenic deposits and distal ash showers.

RETRO ARC FORELAND BASINS:

These are formed behind continental margin arc systems and are filled largely with clastic terrigenous sediments derived from a fold thrust belt behind the arc.The characterstic feature of this basin is that the proximal basin margin progressively becomes involved with the propogating fold thrust belt. Sediments shed frm the rising fold thrust belt are eroded and redeposited in the foreland basin, only to be recycled again with the basin ward propogation of this belt.

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