PRELIMINARY GEOLOGICAL MAP OF THE JOHN RIVER DRAINAGE, SOUTHERN BROOKS RANGE, ALASKA

New geologic mapping has been completed in the remote and enigmatic region of the John River drainage of the southern Brooks Range, Alaska. Mapping advances a refined geologic framework for answering questions about stratigraphic and structural relationships of rocks of the Central and Schist belts. Mapping was completed at 1:63,000 scale and defines an area of ~ 2,625 km² in the west-central portion of the Wiseman 1:250,000 quadrangle. A preliminary map and cross section have been constructed, in addition to petrographic and microstructural analysis of 195 thin sections, collectively representing a comprehensive mapping transect across the metamorphic core of the orogen. Identification of a major north-dipping normal fault within the Central Belt, informally named the Tangleblue fault, juxtaposes low-greenschist facies Devonian rocks to the north relative to upper greenschist facies Proterozoic-Devonian rocks to the south. Mesoproterozoic-Neoproterozoic rocks are documented across a broad region spanning the Central and Schist belt boundary and intruded by 642 ± 2 Ma leucogranite. The southern part of the Schist Belt and the normal fault bound Phyllite Belt to the south contains Devonian and younger (?) protoliths. A major south-dipping normal fault, termed the Bullrun fault, has been newly identified within the central Schist Belt based on the juxtaposition of different lithologies, distinctive topographic expression, and omission of metamorphic section. Two greenschist-facies metamorphic events are observed in the Central and Schist belts, with the youngest fabric (S_D) representing the well-developed, high-strain foliation that defines the Schist Belt. S_D forms a broad dome in the John River drainage and identification of syn-post tectonic (S_D) normal faults with similar geometries include the north-dipping Tangleblue fault and several south dipping faults within and bounding the Schist Belt. Outcrop scale kinematic indicators, microstructural analysis, prior temperature and pressure data, and omission of both metamorphic and structural section support kinematic interpretations that the major bounding faults across the metamorphic core of the Brooks Range are mid-Cretaceous normal faults.