STRUCTURAL ANALYSIS OF THE JOHN RIVER REGION, BROOKS RANGE, ALASKA: MID-CRETACEOUS EXTENSIONAL OVERPRINTING AND EXHUMATION OF A JURASSIC-EARLY CRETACEOUS CONTRACTIONAL OROGEN

Preliminary synthesis of new and existing mapping, geo/thermochronology, and microstructural data from the John River transect of the southern Brooks Range provides insight into the history of Mesozoic exhumation of the metamorphic core of the orogen. The study area encompasses the deepest structural domains of the range and fieldwork was completed by expedition river trips to access mapping and sampling objectives over the past three field seasons. High pressures and temperatures span the region between the Tangleblue fault and the southern Schist Belt, reaching a maximum of 12-15 kbar and ~520˚C. Proterozoic rocks are documented across a broad region spanning the Central and Schist belt boundary. Two greenschist-facies metamorphic events are observed, with the youngest fabric (S_D) representing the high-strain foliation and N-S stretching lineations characteristic of the Schist Belt. S_D defines a broad dome in the John River drainage, with shared geometry in adjacent areas along strike. Thermochronological data (U-Pb zircon metamorphic overgrowths, mica ⁴⁰Ar/³⁹Ar, and zircon U-Th/He) constrain a mid-Cretaceous greenschist facies metamorphic event related to development of S_D fabrics. Identification of syn-post tectonic (S_D) faults include the north-dipping Tangleblue fault and several south-dipping faults within and bounding the Schist Belt, demonstrating that the major bounding structures of the metamorphic core are normal faults. Existing data suggests that Mesozoic exhumation of Proterozoic basement and the highest-grade lithologies of the southern Brooks Range are related to a regional mid-Cretaceous extensional event. Vertical shortening, in part by pure shear, and N-S extension formed the S_D fabrics that define a domal structure bound to the north and south by normal faults. Extension overprints prior Jurassic-Early Cretaceous crustal thickening and development of the thrust architecture of the range. Thus, the Brooks Range records both contractional and extensional deformation that contributed to the exhumation of high-pressure/low-temperature rocks and shares key similarities in deformational history to other orogenic belts in the western Cordillera.