Shelf-Slope Boundary at Active Subduction Zones: ABSTRACT

Abstract

About 30 subduction-zone segments are identified worldwide by intermediate-focus earthquakes, calc-alkalic volcanic arcs, and lines of rapid mountain uplift. The total length of these active convergent plate boundaries is approximately 58,000 km. Of this length, 46% is of the Japan type, where the upper plate is relatively stable horizontally, 30% is of the Andes type, where the upper plate actively overrides the trench, and 24% is of the Himalaya type, where continental plates or microplates collide with continents. Subduction zones of both the Japan and Andes types are marked by basement highs at the shelf break. Uplift of the crust and upper mantle at the edge of the upper plate causes these highs when a relatively low-density prism of accreted trench sediment is emplaced below. The accretionary prism usually forms within 5 m.y. after a new subduction zone is established, during the time that the megathrust at its sea-floor outcrop is evolving from an initial dip of 30 to 45° to a steady-state dip of about 10°. Except during this relatively brief initial period of accretion, trench sediment at subduction zones normally is carried deeply into the lithosphere. Elongate sedimentary basins form on both sides of the shelf-break uplift, but compression generally removes the fluids from sediment on the lower trench slope before thermal maturation of oil precursors can occur. Elsewhere at the active margin, although the low geothermal gradient caused by subduction of cold oceanic crust delays hydrocarbon maturation, it can resume after continental collision or after subduction-zone realignment. End_of_Article - Last_Page 959------------