EXPLORING THE HYPOTHESIS THAT SUBDUCTION EROSION FORMED THE PROMINENT ALEUTIAN TERRACE AND ITS UNDERLYING DEEP WATER FOREARC BASIN

INTRODUCTION: A widely cited model links the formation of deep-water forearc basins to the outward growth of an accretionary prism along the seaward and uplifted edge of a slab of ocean crust that is abandoned in the forearc when a new oceanic subduction (SZ) forms. Because the structurally prominent Aleutian forearc basin (AFB) formed ~50 Myr after the initiation of the Aleutian SZ, we explore the notion that the AFB was substantially created by basal subduction erosion.

THE AFB. The Aleutian Ridge (arc) is fronted by a wide (~50 km), laterally continuous (1500-2000 km), and bathymetrically prominent platform--the Aleutian Terrace. The terrace overlies the deep-water (4-5 km) AFB, which contains a 2-3-km thick fill of latest Miocene and younger sedimentary deposits overlying an older pre-basinal sedimentary sequence (~0.5 km thick) and an underlying basement that in part or in whole consists 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.

SUBDUCTION EROSION. Basal subduction erosion thins the forearc crust by processes of tectonic erosion that detach rock from the upper plate and transports this material toward the mantle. Evidence for subduction erosion gained by drilling and geophysical studies of SZ margins includes (1) rapid (~0.2-0.8 km/Myr) and substantial (3-5 km) forearc subsidence, and (2) long-term (>10-15 Myr) landward migration of the arc magmatic front. Observations that subduction erosion has thinned Aleutian crust include (1) the landward migration of the volcanic front (~30 km since ~34 Ma and 20 km since ~12 Ma), and, bordering the AFB, (2), a deeply (1-1.5 km) subsided and seaward tilted shelf edge of late Neogene age. We speculate that during the past 5-6 Myr underthrusting beneath the forearc of a nearly horizontal slab covered by a ~1-km-thick layer of subducted trench sediment enhanced subduction erosion and created most of the structural relief of the AFB.

TESTING THE IDEA. If subduction erosion formed the AFB, drilling (or dredging) will establish that its basement is overlain by slope deposits that deepen with time. If the basement is a residual slab of trapped oceanic crust, then the overlying sedimentary body will shallow with time.