Structural Dynamics of the Shelf-Slope Boundary at Active Subduction Zones

ABSTRACT About 40 subduction-zone segments have been identified worldwide on the basis of intermediate-focus earthquakes, calc-alkalic volcanic arcs, and lines of rapid tectonic uplift. The total length of these actively convergent plate boundaries is 57,000 km. Of this length, 42% is of the Japan type, in which the upper plate is relatively stable with respect to the subduction zone; 37% is of the Andes type, in which the upper plate actively overrides the trench; and 21% is of the Himalaya type, in which continental plates or microplates collide with other continental bodies. Subduction zones of both the Japan and Andes types are marked by basement highs at the trench-slope break. Uplift of the crust and upper mantle at the edge of the upper plate causes these basement highs where a relatively low-density prism of accreted oceanic material is emplaced below. The accretionary prism for each cycle of subduction forms within 5 m.y. after a new subduction zone is established, while the megathrust is evolving from an initial dip of about 30° near the zone's seafloor outcrop to a steady-state dip of about 10°. Except during this relatively brief period of accretion, most oceanic sediment at subduction zones is believed to be carried deeply into the lithosphere. Elongate sedimentary basins form on both sides of the uplift at the trench-slope break: forearc basins toward the arc, and trench-slope basins toward the trench. Depending on the balance between sedimentation and tectonic displacement, the topographic shelf-slope boundary may be located anywhere in the forearc basins, approximately to the upper edge of the trench-slope basins. At the lower trench slope, compression usually removes the seawater involved in primary oil migration before thermal maturation of oil precursors can occur. Elsewhere at the active margin, although a low geothermal gradient caused by the subduction of cool oceanic crust delays hydrocarbon maturation, such thermal maturation can nevertheless resume when a normal geothermal gradient is reestablished after continental collision or after the subduction-zone alignment moves to a new position.