The geologic framework of the Chukchi Sea, which lies between Alaska and Siberia north of Bering Strait, was determined by low-frequency sparker profiles, magnetometer profiles, sonobuoy runs, and published gravity and magnetic data. Most of the southern Chukchi Sea is underlain by the Hope basin of Tertiary (and possibly in part Upper Cretaceous) nonmarine and probably marine sedimentary rocks. The basin locally reaches depths of 3 km and its rocks contains gentle folds and high-angle faults. The downwarping that produced the Hope basin continued into late Cenozoic time and, together with Quaternary marine planation, produced the present outlines of the southern Chukchi Sea. The downwarping also must have helped set the stage for the marine invasions that periodically se ered the Bering Land Bridge.

A belt of acoustically incoherent rocks trending northwest from Cape Lisburne 300 km to Herald Shoal is interpreted to be the offshore extension of the pre-Cretaceous rocks of the Brooks Range. Very young sedimentary beds lap against the southwest flank of this belt, indicating that it was emergent during Plio-Pleistocene time. A major northwest-striking thrust-fault zone separates the inferred offshore extension of Brooks Range rocks and structures from acoustically coherent rocks interpreted to represent the offshore extension of Cretaceous rocks in the North Slope's Colville geosyncline. Thrust faults and folds in the Cretaceous rocks near the major fault zone strike northwest and are superimposed across older, east-striking faults and folds that trend into the Chukchi Sea from the North Slope. The folds in both fault and fold systems are of the "wrinkled carpet" type associated with detachment thrust-fault zones. The structural relation of the 2 systems indicates that the great bend in Brooks Range rocks and structures on the Lisburne Peninsula is due to this superposition of an older across a younger fault and fold system, rather than to the oroclinal folding of only one.

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