AN INVERTED METAMORPHIC FIELD GRADIENT IN THE CENTRAL BROOKS RANGE, ALASKA AND IMPLICATIONS FOR EXHUMATION OF HIGH-PRESSURE LOW-TEMPERATURE METAMORPHIC ROCKS

During exhumation of the Brooks Range internal zone, amphibolite-facies rocks were emplaced atop the blueschist/greenschist facies schist belt. The resultant inverted metamorphic field gradient is mappable as a series of isograds encountered as one traverses up structural section. Amphibolite-facies metamorphism occurred at approximately 110 Ma as determined from 40Ar/39Ar analysis of hornblende. This contrasts with 40Ar/39Ar phengite cooling ages from the underlying schist belt, which are clearly older (by 17-22 m.y.). Fabrics in both the amphibolite-facies rocks and schist belt are characterized by repeated cycles of N-vergent crenulation and transposition that was likely associated with out-of-sequence ductile thrusting in the internal zone of the Brooks Range orogen. Contractional deformation occurred in an overall environment of foreland-directed tectonic transport, broadly synchronous with exhumation of the internal zone, and shortening within the thin-skinned fold and thrust belt. These data are inconsistent with a recently postulated mid-Cretaceous episode of lithospheric extension in northern Alaska.