2:45 PM - 3:00 PM
★ [SGL37-03] Formation of forearc basins and its relationship to subduction zone dynamics
Keywords:forearc basin, subduction zone, sediment flux, accretionary prism, sedimentary basin
In this study, I tried to understand formation of forearc basins by means of geological and geomorphological characters of 37 forearc basins with the outer wedges (frontal and middle prisms) in the world. I also measured width (Wbasin) and sediment thickness (Tbasin) of forearc basin, width (Wwedge) and slope angle (α) of outer wedge, slab dip angle under the wedge (β), orthogonal convergent rate of subducting plate (Vorth), thickness of trench fill sediments (Ttrench).
As a result, forearc basins can be divided into 5 types (compressional/extensional accretionary, compressional/extensional strike-slip, and non-accretionary). A character of the compressional accretionary type is landward tilt of the basin due to uplift of outer arc high associated with backthrust or splay fault. Basin formation of this type may be related to self-similar growth or thickening of the outer wedge. In the extensional accretionary type, seaward migration of outer arc high by gravitational force, generating listric normal faults in the basins, can subside the basin. Internal or basal friction may be too weak to keep the taper angle of the wedge. The non-accretionary basins do not have conspicuous outer arc highs, resulting from tectonic erosion-induced subsidence. Strike-slip type shows intermediate characters between accretionary and non-accretionary types, but whose depocenters occasionally migrate parallel to the trench. Some basins show changes from non-accretionary type to accretionary one, and vice versa.
Width of wedge (Wwedge) has a positive correlation with Ttrench/Vorth as a proxy of sediment flux at the trench. Tbasin has a positive correlation to Wbasin for the accretionary type, but is basically constant for the non-accretionary type. Wbasin/Tbasin of the accretionary type is almost constant regardless of Wwedge, but shows a negative correlation to Wwedge for the non-accretionary type. For all types of forearc basins, Tbasin has a positive correlation to Wwedge or Ttrench.
Variations of sediment flux at subduction zones can influence degree of growth/decay of outer wedges, and then, may switch the type of forearc basins. Changes from accretionary to non-accretionary type may cause subsidence of outer arc high because of tectonic erosion, which leads to remove accommodation space and erode the basin strata, and then produces a new basin with large Wbasin/Tbasin. On the other hand, changes from non-accretionary to accretionary type may uplift a part of the basin as an outer arc high, and generate a new basin at the landward side.
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