ARCTIC ALASKA-CHUKOTKA CRUSTAL RESPONSE TO JURASSIC-CRETACEOUS PLATE-MARGIN TECTONICS

A zone of deformation initiated along the southern margin of the Arctic-Alaska Chukotka microplate (AAC) during the Jurassic. As the AAC moved Pacificward, continent-continent collision along its southwestern margin and arc-continent collision along its southeastern margin resulted in pulses of crustal thickening that continued through the Early Cretaceous. The result was growth of an ancestral Brooks Range-Chukotka mountain belt in the orogenic hinterland and basin development in the orogenic foreland.

Three Early Cretaceous pulses of mid-crustal shortening occurred in the hinterland. The first pulse, 140-135 Ma, involved amphibolite-facies metamorphism and north-vergent ductile deformation in the SW part of the hinterland (Chukotka). The second pulse, the most profound of the three, occurred during the Aptian (125-115 Ma). A crustal duplex formed on the orogen’s basal detachment at blueschist facies conditions, suggesting rapid tectonic burial. Greenschist facies metamorphism associated with contractional structures was widespread. The third pulse, Albian in age (108-102 Ma), was manifested as backthrusting in the eastern hinterland (Brooks Range) and E-W contraction in the central hinterland (Seward Peninsula). Pulse 3 coincided with exposure of pulse 1 metamorphic rocks at the surface.

During the Late Cretaceous, the west-central hinterland (Chukotka, Seward Peninsula) was the site of gneiss dome formation and magmatic activity that culminated in development of the Okhotsk-Chukotsk volcanic belt. The eastern part of the hinterland (Brooks Range) experienced no magmatism; brittle extensional structures formed there during this period.

Hinterland events are reflected in patterns of foredeep deposition. Earliest foredeep sequences are mostly Valanginian (pre-136 Ma) and were largely dismembered during subsequent deformation. Sediment influx peaked during the Aptian–Albian, resulting in a regionally extensive clinoform depositional sequence that prograded from west to east. Proximal strata display evidence of syntectonic sedimentation. During the Late Cretaceous, an abrupt reduction in sediment influx and the appearance of volcanic components in sandstones reflect a tectonic reorganization related to flare-up of the Okhotsk-Chukotsk volcanic belt.