Initiation of subduction zones: implications for arc evolution and ophiolite development

Publisher Website
Google Scholar
Abstract
Summary: The initial location of trenches with respect to continental margins is a critical factor in the nature of the forearc region and its subsequent evolution, as well as for the interpretation of ophiolites. Although recent studies of ancient margins and highly evolved arcs suggest that wide strips of oceanic crust are commonly trapped between the trench and volcanic arc, review of young active arcs does not support this conclusion. Neogene trenches have formed predominantly very close to the crustal interface between oceanic crust and continental or older island arc crust. A few arcs have developed along transform zones, but none can be identified as having formed by breakage within an oceanic plate. Apparently the high strength of normal oceanic lithosphere causes the initial fracture to utilize pre-existing zones of weakness or pre-stressing, such as the downbowed and fractured crustal interfaces along passive margins. There seems to be no physical basis for the initial rupture to form smooth, large radius arcs, thereby trapping oceanic crust in re-entrants. Moreover, oceanic crust or ophiolites originating adjacent to a continental or arc margin ought to betray that heritage by a high terrigenous or volcaniclastic component in their overlying sediment. Large ophiolite sheets that can be interpreted as forearc slabs may develop in areas with relatively unusual plate geometries, as where oceanic spreading zones cut diagonally across the forearc. Disrupted ophiolites could, in many cases, represent slices of oceanic crust and upper mantle accreted to and responsible for the growth of oceanic arcs. Evidence for accreted slices in the Mariana arc and the range of forearc geometries among oceanic arcs allow construction of an evolutionary sequence in which an initial steep and narrow trench shape is differentiated into upper and lower sections by intermittent accretion of oceanic crustal slices. As a result, the slope apron on the upper section rotates arcward and can rise to form a basin as is displayed in the southern Middle America arc.