SCIENTIFIC OCEANIC DRILLING (IODP) TO TEST CONTRASTING HYPOTHESES FOR THE ORIGIN OF THE ALEUTIAN SUBDUCTION ZONE, ALEUTIAN ARC, AND BACKARC ALEUTIAN BASIN

INTRODUCTION: The Backarc Aleutian Basin (3000-4000 m) of the Bering Sea Basin lies north of the E-W trending, southward festooning Aleutian Island Arc (AIA). Basin and arc are part of the North American plate (NAP). Oceanic crust underlies the basin’s thick (3->6km) sedimentary fill. Two contrasting hypotheses have been proposed for the origin of its oceanic basement:

(1) basement is a large sector (~500,000 km3) of Mesozoic oceanic plate (Kula/Resurrection?) accreted to NAP during formation of the offshore Aleutian subduction zone (SZ) and arc.

(2) basement formed in-place during an episode of backarc spreading north of the forming AIA.

Both models call for basin formation in the early Eocene (~50-55 Ma) and coincident with the initiation of the offshore Aleutian SZ and arc.

REGIONAL OBSERVATIONS:

• Geomorphically, the AIA is a young (Eocene) westward extension of the SW-NE trending Alaska Peninsula constructed of Permo-Triassic and younger arc crust.
• With respect to the peninsula, paleomagnetic data attest that the AIA formed effectively in place,
• The Aleutian Trench-SZ pair is a western, on-strike continuation of the older (Permo-Triassic) Alaska Trench-SZ pair.
• Magnetically, the Aleutian Basin exhibits a prominent pattern of age-unknown ~N-S-striking spreading anomalies that trend ~normal to that of the AIA.
• The Aleutian Basin is structurally framed by continental crust to the north (Beringian Margin) and arc structures to the west (Shirshov Ridge) and south (Bowers Ridge and AIA).

HYPOTHESIS TESTING: Drilling to basement is technically feasible only at the summits of sediment-buried seamounts. These edifices are ~co-genetic components of the depositionally buried basement surface. The seamounts are moatless and rise ~2-3 km to crest within ~1 km of the basin floor. Samples of basement and overlying sediment will determine edifice age, latitude of formation, and formative setting. IODP proposal 888-Full proposes to acquire this information at three seamounts to determine if basement crust formed before, during, or after formation of the AIA, at what latitude, and depositionally where. This battery of facts will determine if basement is either an accreted sector of Pacific crust or that of a backarc basin, and the tectonic setting for the birthing of the Aleutian SZ.