EARLY BROOKIAN OROGENESIS AND THE OPENING OF THE AMERASIA BASIN OF THE ARCTIC: ARE THEY SIMULTANEOUS?

Based on stratigraphic and seismic data sets, many workers have concluded that rifting initiated in the Amerasia Basin in the Jurassic. It is also broadly thought that early Brookian shortening, dated by the syn-orogenic Okpikruak Fm. (latest Jurassic-earliest Cretaceous), took place simultaneously with the southward motion of Arctic Alaska during rotational opening of the Amerasia Basin.

We examine these questions with geological and detrital zircon (DZ) data from the Russian Arctic using a series of maps that highlight 1. Jurassic to earliest Cretaceous belts of shortening and syn-orogenic sediments, 2. distribution of zones of continental extension and 3. regional unconformities. These data suggest that the belts of shortening and the sediments shed from them predate extension and that the bends, displacements and separation of these elements seem most logically linked to the rift opening of the Amerasia Basin.

A post- Early Cretaceous age (younger than ~130-125 Ma) is indicated for the onset of magmatism, extension, uplift and erosion likely linked to the initiation of rifting in the Amerasia Basin. Two stages of extension are suggested: one beginning in the Albian-Aptian and the younger one continuing into the Late Cretaceous: Extension took place in an E-W direction and affected the region between the New Siberian Islands and Pevek (ca.125 Ma to 100 Ma), serving to displace once continuous fold-thrust belts and foreland basin depositional systems. Younger (ca. 100-80 Ma) N-S extension is superimposed on older extension and becomes increasingly younger and focused in the Bering Strait region, where it continues today.

Geologic constraints from the Russian Arctic that document the switch from shortening to extension and time-span of extension-related magmatism, deformation and uplift fit with the known age of pre-rift-related uplift (Barremian) recorded on the Barents Shelf and the eruption of the high Arctic LIP at ~ 124 Ma. A mid-Cretaceous rather than a Jurassic age for the initiation of rift opening of the Amerasia Basin provides a context to examine Aptian-Albian sediment dispersal systems in the Arctic, including the Colville Basin of northern Alaska which most likely was sourced from a region of rift-related topographic uplift in the Arctic.