SUBDUCTION EROSION UNLIKELY TO TECTONICALLY INCORPORATE UPPER PLATE BLOCKS IN SUBDUCTION COMPLEX MÉLANGES

Subduction erosion, the slicing of the subduction interface into the upper plate, has been proposed for tectonically incorporating upper crustal upper plate rocks into mélanges in subduction complexes. Subduction complex rocks, however, form by accretion, during which the subduction interface cuts into the uppermost part of the downgoing oceanic plate. To incorporate upper plate blocks or slices in a subduction complex by subduction erosion, units removed from the upper plate must have accreted downdip of where the subduction erosion took place. Such a process predicts at least two stages of structures, the first associated with subduction erosion, and the second with accretion at greater depth. The earliest recorded stage of deformation in subduction complexes, with the exception of soft-sediment deformation, is associated with accretion and development of imbricate structures repeating the stratigraphy of the accreting unit at peak burial depth. In more deeply-accreted rocks, accretionary structures developed with growth of syntectonic metamorphic minerals. The lack of the pre-accretionary structures suggests that upper plate blocks and slices are not incorporated into subduction complexes by subduction erosion. The shallower reaches of subduction interfaces, with the exception of a subduction initiation horizon, should have hanging walls composed of previously-accreted subduction complex rocks. The hanging wall of subduction interfaces changes to upper plate rocks at depths too great explain the incorporation of upper crustal upper plate rocks by tectonism (accretion or exhumation faulting). Thus, tectonic incorporation of upper plate blocks into mélanges by either subduction erosion and/or accretion is unlikely. A more likely mechanism for incorporating upper plate rocks into subduction complex mélanges is by trench deposition as olistoliths, followed by subduction and accretion. In contrast to "typical" oceanic subduction complexes, upper plate rocks may be tectonically incorporated into subduction complexes in two types of settings (1) subduction-accretion of island arc rocks prior to terminal arc-arc collision, and (2) subduction-accretion of continental material where subduction nucleated along a hyperextended continental.