STRUCTURAL AND THERMAL EVOLUTION OF THE SOUTHERN MARGIN OF THE MACLAREN METAMORPHIC BELT, EASTERN ALASKA RANGE, ALASKA

The Alaska Range suture zone, separating the Wrangellia and Yukon-Tanana composite terranes in southern Alaska, is a key crustal domain in post-Jurassic southern Alaska tectonics. Bounded in the north by the Denali fault and in the south by the Broxson Gulch fault, the Maclaren Metamorphic Belt (MMB) is a deeply exhumed section of the Alaska Range suture zone crust. Resolving the structural, burial, and exhumation history for rocks at the southern margin of the MMB shows that the suture zone crust near the Denali fault in the eastern Alaska Range has been a locus of convergent deformation during and after Wrangellia terrane accretion.

We present new mapping, structural data, geochronology, and thermochronology recording prograde metamorphism of the MMB from the late Cretaceous to the Eocene, transitioning to fault controlled exhumation by the Neogene. Detrital U/Pb zircon age spectra show the sedimentary protolith for the MMB received detritus from both Wrangellia and Yukon-Tanana sources until ~88 Ma, after which prograde metamorphism initiated with thrust sheet emplacement. Eocene intrusions locally cross cut and disrupt top-to-the-south outcrop and hand sample scale fabrics in amphibolite facies schists from the MMB. Zircon and apatite (U-Th)/He cooling ages from samples in the hanging wall (north) of the Broxson Gulch fault show cooling from >190ºC through ~70ºC between 20 Ma and 15 Ma. These cooling ages indicate rapid exhumation of schists in the southern portion of the MMB was under way by ~20 Ma. Apatite (U-Th)/He cooling ages from footwall samples within Wrangellia, south of the Broxson Gulch fault, show cooling through ~70º C by ~15 Ma within the Alaska Range and ~20 Ma at the southern margin of the Range. The Broxson Gulch fault in this area is an ~10 m wide zone of cataclastized metamorphic rocks. The metamorphic foliation in the MMB is folded with S-vergent kinematics at hand sample scale within the fault zone. Clay mineralization overprinting the high-grade fabric and strained quartz in the fault zone suggest S-vergent slip on the Broxson Gulch fault persisted through greenschist facies conditions as the MMB was being exhumed. Reactivation of the southern margin of the Alaska Range suture zone as the Broxson Gulch fault and related rapid exhumation of the MMB likely record Yakutat block collision in southern Alaska.