SUBDUCTION CHANNEL MODEL FOR ACCRETION, SEDIMENT SUBDUCTION, AND TECTONIC EROSION AT CONVERGENT PLATE MARGINS: EFFECTS OF SUBDUCTING BATHYMETRIC HIGHS

Convergent plate margins display a wide range in tectonic behavior. For example, at Cascadia, a broad accretionary prism has accumulated since the mid-Tertiary. At Guatemala, extensive sediment subduction and significant subduction erosion have occurred. This differing behavior is explainable by the subduction channel model of Shreve and Cloos (1986). The premise of this dynamic model is that the shear zone between the descending plate and overriding block at margins where convergence is faster than a few centimeters per year is analogous to the guide surface and slide block in a slipper bearing and that subducting sediment is analogous to the lubricant. Varied behavior occurs because the overriding block is not rigid but slowly deforms, the buoyancy of subducting sediment affects the flow, sediment (or upwelled melange) can underplate onto the hanging wall, and the sediment supply can vary widely relative to the capacity of the system, and other factors. Sediment supply, the thickness of the incoming sediment layer times the subduction speed, is fundamental. Where convergence is 6 cm/yr, accretionary prisms grow when the supply exceeds 60 km3/km-length whereas subduction erosion occurs where the supply is less than about 15 km3/km-length. The impingement of an elongate bathymetric high (oceanic plateau, spreading ridge) with a trench will cause the forearc region to isostatically uplift, but an even more profound effect will arise from changes in the local sediment supply to the trench axis. Sedimentary mass flows that reach the trench will continue to flow downslope away from the center of the bathymetric high. This will cause the sediment supply to the shear zone to be less in the area of the bathymetric high. The consequence of this is that subduction accretion will lessen where major bathymetric highs are obliquely subducting. Where the decrease in sediment supply is great enough, accretion will cease and subduction erosion may begin. Where subduction erosion was underway, the subduction of a bathymetric high will enhance the process. Margin sections that are sites of subduction accretion will have shear zone thinning with depth whereas those with subduction erosion will have shear zone thickening. This differing behavior has profound implications for arc magma chemistry (Be-10) and sizes of subduction earthquakes.