The Bering Shelf is a broad continental shelf that lies between Alaska and the Soviet Far East. Its southern edge, quiescent today, was a south-facing active margin in Cretaceous time. In late Paleocene to early Eocene time, a north-facing arc-trench complex (the Olyutorsky-Bowers complex) collided with and obducted this active margin in the area of the Soviet Far East. Probably as a result, the Bering active margin was abandoned, and by late Eocene time, the modern Aleutian arc-trench system formed to the south. This tectonic reorganization welded the Olyutorsky-Bowers arc-trench complex, along with a trapped segment of oceanic crust, to the North American plate.
Following this reorganization, in late middle Eocene time, a series of right-lateral strike-slip faults formed along the length of and parallel to the (by now defunct) Beringian active margin. These faults developed in close synchroneity with the 43-Ma change in Kula plate motion described by Engebretson and others (1985). These strike-slip faults formed most of the Tertiary basins and uplifts of the Bering Shelf, among them the Anadyr, Navarin, St. George, and North Aleutian basins, as well as the Black Hills uplift of the Alaska Peninsula. These features are somewhat unusual: with the vast amounts of high-quality, marine seismic-reflection data generated in preparation for the initial round of Outer Continental Shelf (OCS) lease sales in this region, more deep-subsurface structural information is available for these strike-slip basins than for any other similar basins in the published literature. In addition, the wrench faults that created these deep basins ceased activity after only a relatively small amount of slip. As a result, the early structural fabric of each basin is very well preserved, unlike that of many basins along the California margin, where continued rapid motion on the San Andreas system has complicated or obscured early geometry.
Somewhat surprisingly, these seismic data show that the standard pull-apart, rhomb-graben hypothesis is not applicable to the geometry of these strike-slip basins. For example, the pre-basin surface in two Navarin subbasins is very smoothly warped in between and around pairs of en echelon strike-slip faults; large normal faults are not associated with regional subsidence, and there is no rhomb graben, even though the subbasins are nearly 13 km deep. In addition, some basins form adjacent to single strike-slip faults (Amak, North Aleutian basins), rather than the en echelon pair typically assumed necessary; these secondary basins formed outside of a left-stepping pair of en echelon faults associated with the smoothly upwarped Black Hills uplift.
Bering Shelf basins formed in three distinct phases. (1) In late middle Eocene time, a rather ductile deformation phase began in a 300-km-wide simple shear zone along the outer Bering Shelf, marked mainly by en echelon folds and local subaerial erosion, as well as by minor, arcuate, en echelon extension gashes. This belt of en echelon folding extends in the subsurface parallel to the shelf margin for more than 1,400 km between the Alaska Peninsula and Soviet Far East. (2) Shortly thereafter, in late Eocene time, regional-scale strike-slip faults appeared, accompanied by crustal warping between and around en echelon strike-slip faults. This warping produced basin subsidence and uplifts during late Eocene through Miocene time. (3) In Pliocene to Recent time, strike-slip faulting waned and finally ceased; slow regional subsidence and local rebound have occurred.
The lack of pull-apart normal faults at the ends of the basins and the smoothly downwarping subsidence pattern suggest that space at depth for basin subsidence was created by elasto-plastic stretching and thinning rather than by brittle extension. The lack of significant gravity and thermal anomalies indicates that this process is probably mostly intracrustal. Geometric patterns of Navarin basin subsidence are in excellent qualitative agreement with results of elastic dislocation modeling.
The three structural phases of basin formation mentioned above strongly control the lithology of basin strata. For example, early rapid subsidence in the Navarin basin produced a silled, deep-water, anoxic basin with dark shale. As subsidence rates declined, basin sedimentation rates caught up with subsidence, and more laterally extensive, shallow-water strata were deposited.
Norton basin, which lies in the interior of the Bering Shelf, is related to strike-slip motion on the Kaltag fault. Eocene displacement along that fault caused a broad zone of en echelon folds to develop in the adjacent Yukon-Koyukuk basin as well as beneath Norton basin. As in the outer Bering Shelf basins, subsidence in Norton basin accompanied further strike-slip activity and followed erosional truncation of many en echelon folds beneath the basin.
