REMNANTS IN NORTHEASTERN ALASKA OF PART OF A DEVONIAN COLLISIONAL ZONE FROM THE NORTHERN MARGIN OF LAURENTIA

Rocks lying beneath the regional sub-Mississippian unconformity of the North Slope consist of deformed Proterozoic to Devonian strata whose tectonic affinity is poorly known. In the NE Brooks Range, these rocks include a Neoproterozoic-Late Ordovician carbonate platform sequence exposed in the Sadlerochit and Shublik Mountains in the north and a Neoproterozoic to Early Devonian siliciclastic, volcanic, and chert-rich basinal sequence exposed in the Romanozof Mountains to the south. New structural studies in the Plunge Creek area of the Arctic National Wildlife Refuge reveal that the contact between these successions is a north-vergent thrust fault that places the basinal succession northward onto the carbonate platform succession. Detrital zircon data from Neoproterozoic sandstones of the northern sequence are similar to sandstones derived from both the Precambrian Shield and the Grenvillian orogenic belt of northern Laurentia, whereas the southern sequence yields zircon ages like those shed from the Precambrian Shield. Early Devonian synorogenic strata deposited in wedge-top structural positions contain zircons derived principally from the carbonate platform to the north, but include a subpopulation of 420-450 Ma zircons of possible Caledonian affinity. These relations suggest that pre-Mississippian deformation in NE Alaska composes a truncated part of a collisional belt that reflects the continentward collapse of the outer margin of Laurentia in the Early Devonian.

The Devonian thrust belt verges toward the modern Arctic Ocean, indicating that northern Alaska was probably rifted away from North America by the formation of the Amerasia Basin in the Early Cretaceous. The Devonian thrust belt was probably derived from the western part of the northern margin of the continent where Paleozoic and older carbonate platform rocks can be traced in seismic reflection data to the edge of the continent rather than from the eastern part of this margin where crystalline terranes (e.g., Pearya) are present in outboard positions. This is consistent with the counterclockwise rotational model for opening of the Amerasia Basin and suggests that a broad mobile belt related to the Ellesmerian and(or) Caledonian orogens may once have existed along the northern margin of North America prior to opening of the Amerasia Basin.