THE OCEANIC CRUST OF THE ALEUTIAN BASIN IS A LARGE, TECTONOSTRATIGRAPHIC TERRANE ACCRETED TO THE NORTH AMERICA PLATE--EVIDENCE AND IODP DRILLING TO TEST THIS HYPOTHESIS

INTRODUCTION

The Aleutian Basin of the Bering Sea lies north of the offshore Aleutian Arc. Basin and arc are part of the North America plate. Oceanic crust underlies the basin’s thick (3-10 km) sedimentary fill. Two contrasting hypotheses have been proposed for the origin of this basement—that it is an accreted sector of a potentially far-traveled oceanic plate of Mesozoic age, or that an episode of early Eocene backarc spreading formed the basement in place. Both models call for basin formation at ~50-55 Ma coincident with the birthing of the Aleutian subduction zone (SZ) and initial magmatic growth of the Aleutian Arc. Evidence for the accretionary addition of the basin’s oceanic basement to the North America Cordillera is reviewed below.

PRIMARY EVIDENCE

• Paleomagnetic data attest that the Aleutian Arc formed effectively in place, i.e., it did not migrate to its present offshore position,
• The Aleutian Arc extends offshore directly online with the SW-trending older (Permo-Triassic) continental crust of the Alaska Peninsula,
• The Aleutian Trench and SZ, above which the Aleutian Arc formed, are on-strike continuations of the much older (Triassic at least) Alaska Trench and SZ.

SUPPORTING EVIDENCE:

• Magnetic anomalies formed by backarc spreading typically strike parallel to the trend of remnant and frontal arcs. Contrastingly, the Aleutian Basin is characterized by N-S trending magnetic anomalies striking virtually normal to that of the Aleutian Arc.
• Sub-sea level depth to basinal basement is 9-10 km, when corrected for sediment loading, the depth is that typical of Mesozoic crust.
• Arc-paralleling remnant arcs are characteristic of marginal seas formed by backarc spreading. An arc-paralleling remnant arc is not present in the Aleutian Basin.
• The basin’s Beringian continental margin is potentially the location of a remnant arc. Eocene arc lavas have been recovered from this margin, but its exposed basement rock is mostly Cretaceous and older miogeosynclinal deposits.

IMPLICATIONS AND HYPOTHESIS TESTING

If the accretionary origin is correct, then the oceanic basement of the Aleutian Basin is one of the largest identified Alaskan tectonostratigraphic terrane—and an exotic one at that. Testing of both hypotheses has been proposed by IODP drilling to basement to determine its formative age and latitude.