EXPLORING THE NOTION THAT CRUSTAL THINNING CAUSED BY SUBDUCTION EROSION FORMED THE PROMINENT DEEP-WATER FOREARC BASIN OF THE ALEUTIAN TERRACE

SCHOLL, David William, Geophysics, Stanford, Stanford, CA 94305, [email protected], RYAN, Holly F., U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, and VON HUENE, Roland, Geology, Univ of California Davis, Department of Geology, University of California, Davis, CA 95616

INTRODUCTION: A widely accepted mechanism for the formation of deep-water forearc basins is the creation of a depression inboard of where an accretionary prism has been added to the seaward edge of a slab of accreted ocean crust. Because the prominent Aleutian forearc basin (AFB) is underlain by the Eocene basement rock of the Aleutian arc massif, we explore the notion that the basin's structural relief was substantially created in situ by basal subduction erosion. THE AFB. The Aleutian Ridge (arc) is fronted by a wide (~50 km), laterally continuous (1500-2000 km), and bathymetrically conspicuous platform--the Aleutian Terrace. The terrace overlies the structural relief of the deep-water (4-5 km) AFB, which cradles a fill of late Cenozoic deposits as thick as 2-3 km. The fill accumulated above an older pre-basinal sequence of slope deposits (~0.5-1.0 km thick) resting on the igneous basement of the arc massif. Seaward of the AFB, the lower landward trench slope is constructed of a 30-40-km wide frontal prism of presumably mostly offscraped trench floor deposits of late Cenozoic age. SUBDUCTION EROSION. Basal subduction erosion thins the forearc crust of a convergent margin by processes of tectonic erosion. Subduction erosion detaches crustal rock from the base of the upper plate and transports this material toward the mantle in the subduction channel (~1 km thick) separating the two plates. Observations that subduction erosion has thinned Aleutian crust include (1) the landward migration of the volcanic front (~30 km since ~34 Ma and 15 km since ~12 Ma), and (2), a deeply (1-1.5 km) subsided and seaward tilted shelf edge of late Neogene age bordering the AFB. We speculate that during the past 5-7 Myr underthrusting beneath the forearc of a sub-horizontal slab covered by a ~1-km-thick layer of subducted trench sediment enhanced subduction erosion and tectonically created most of the structural relief of the AFB. TESTING THE IDEA. If subduction erosion formed the AFB, drilling (or dredging) will document that its basement is arc massif overlying by a basal section of terrigenous deposits that accumulated in a much shallower--even shoreline-- setting. If the basement is a slab of accreted and thus originally deeply submerged sector of oceanic crust, then the deposition depth of the basin's basal beds will demonstrate this.