REEXAMINING THE CONCEPT OF THE LONG-TERM, PROGRESSIVE ACCRETIONARY WIDENING OF THE ALASKA FOREARC—DURING THE CENOZOIC IT EVOLVED AS A NARROWING (BY ~70-100 KM) AND THINNING OR EROSIVE MARGIN

INTRODUCTION: The accretionary origin of the Franciscan complex was recognized in the 1970s as were similar rock bodies exposed around the rim of the north Pacific, e.g., the Chugach and Kodiak Fms of Alaska and the Shimanto of Japan. From this understanding it was posited that progressive accretion of material to the submerged forearc worked to widen the arc-trench gap. It was equally widely accepted that the Alaska forearc was an example of a progressively widening arc-trench gap. This supposition was not based on geological and geophysical observations of the submerged Alaska forearc. These data were unavailable until after the mid 1970s.

OBSERVATIONS: In the late 1970s and 1980s multichannel seismic reflection profiles and dredge samples collected by the USGS, MCS and refraction data by academia, and exploration drilling (COST wells) by industry, revealed the structural and tectonic fabric of the submerged Alaska forearc. These findings documented that coastal and island exposures of basement rock of Late Cretaceous and earliest Tertiary age extended seaward beneath the width of the forearc to near the Alaska Trench. Basement rock is the accretionary underplated complex and granite-intruded body of the exhumed Kodiak (and related) Fm. The wave-base unconformity cut across the beveled surface of the accretionary body can be traced seaward to the back of a ~20-30-km-wide frontal prism of offscraped trench sediment compiled along the inner wall of the trench. Dredge samples and stratigraphic continuity traceable to exposed units imply that since the early Eocene the submerged Alaska forearc has been subsiding and, with respect to fixed point on the upper plate, progressively narrowing.

INTERPRETATIONS AND IMPLICATIONS: We ascribe forearc subsidence and narrowing to the tectonic consequences of basal and frontal subduction erosion. During the Cenozoic accretionary processes built a small frontal prism and, based on limited reflection data, a thickening underplate beneath (20-30 km) the coastal area. Since 45-55 Ma accretion has thus dominantly been crustal thickening rather than crustal widening. Sediment subduction to build the posited underplate in turn smoothed the interplate decollement and favored the repeated rupturing of high-magnitude (>Mw8.0) megathrust earthquakes.