ACCRETION AND EROSION AT CONVERGENT MARGINS, CURRENT CONCEPTUAL MODELS AND PRESENT OBSERVATIONS

Theoretical accretionary and erosional end-member models involve some opposite tectonic processes. Observations reveal such contrary processes as material addition and removal, material consolidation and dewatering versus material weakening and hydration. For estimates of mass flux, rates of accretion are constrained using “backstops” as a convenient boundary in time and space. “Backstops” represent a break in the geologic record that is observed along many margins, which we illustrate with an example from Chile, and one from Alaska. Each involves a period of subduction erosion. Long term accretionary storage occurs when the subduction system is overwhelmed by more sediment than can be accommodated by the subduction zone. Erosion and accretion can alternate during the history of a margin or operate simultaneously in front of and behind the backstop with the dominant one controlling net sediment flux Erosion is often associated with the subduction of ocean floor relief and is commonly compared to high friction rasping or sawing. However such processes are lacking in clear seismic images. Rather than strong basement teeth abrading the overlying plate, seismic images commonly indicate disintegration in the backstop area, extensional faulting, and crustal thinning. We present a model of such erosion involving disintegration and collapse tectonics developed from seismic images. Erosional margins can build frontal prisms composed of mass wasting debris when trench sediment is unavailable. Stable frontal prisms have a median width of ~15 km. Few margins remain pure end-member examples for long. This understanding guides our identification of backstops we used in estimating long-term accretionary storage. We infer that the frontal prism can be ephemeral adjusting toward a configuration stable under the prevailing convergence rate, sediment input, and dip of the plate boundary.