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

During exhumation of the Brooks Range internal zone, amphibolite-facie s rocks were emplaced atop the blueschist/greenschist facies schist be lt. 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 Ar-40/Ar-39 analysis of hornblende. This contrasts wit h Ar-40/Ar-39 phengite cooling ages from the underlying schist belt, w hich are clearly older (by 17-22 m.y.). Fabrics in both the amphibolit e-facies rocks and schist belt are characterized by repeated cycles of N-vergent crenulation and transposition that was likely associated wi th out-of-sequence ductile thrusting in the internal zone of the Brook s Range orogen. Contractional deformation occurred in an overall envir onment of foreland-directed tectonic transport, broadly synchronous wi th exhumation of the internal zone, and shortening within the thin-ski nned fold and thrust belt. These data are inconsistent with a recently postulated mid-Cretaceous episode of lithospheric extension in northe rn Alaska.