THE ALASKA RANGE SUTURE ZONE: LATEST CRETACEOUS ARC VOLCANISM AND OLIGOCENE STRIKE-SLIP BASIN DEVELOPMENT AND COEVAL DIKE SWARMS

Cretaceous-Oligocene strata and magmatic products within the Alaska Range suture zone provide constraints on the tectonic development of south-central Alaska during the transition from Pacific plate subduction to flat- slab subduction of an oceanic plateau. Colorado Creek basin (CCB) strata crop out in the footwalls of northwest-dipping thrust faults <5 km south of the Denali fault. New geochronologic and mapping data indicate that a northwest-dipping thrust separates previously undated lavas and Oligocene alluvial strata. ⁴⁰Ar/³⁹Ar analyses from five lavas yield a weighted mean average age of ~69 Ma. Lavas were emplaced during deposition of the lower Cantwell Formation, retroarc sedimentary strata exposed <10 km north of the Denali fault with bentonites bearing ~72–~68 Ma U-Pb zircon crystallization ages. CCB lavas range from basalt to trachyte and are geochemically similar to other Upper Cretaceous volcanic/plutonic rocks in the Alaska Range suture zone (magnesian, relatively high-K, high La/Yb). These rocks have a more typical arc affinity compared to Paleocene rocks in the suture zone which are transitional between arc and intraplate characteristics.

U-Pb geochronology of detrital zircons from CCB alluvial strata documents a dominant age population of ~92-~101 Ma (59-82% of ages in two samples) and minor populations of ~67-~71 Ma (0-23%), ~360-~460 Ma (8-10%) and ~1230-~2330 Ma (3-5%). The detrital ages, lithic grain types, and a ~66 Ma volcanic clast indicate erosion chiefly of Cretaceous igneous rocks and sedimentary strata exposed within and north of the suture zone. Sparse ~29 Ma zircons and previously reported palynomorphs support an early Oligocene depositional age.

Newly dated mafic dikes that are oblique to and truncated by the central Denali fault system for >200 km along strike are also ~29 Ma. Together, the dike swarms and alluvial strata indicate early Oligocene strike-slip movement along the Denali fault. Recent thermochronologic studies document rapid and persistent exhumation of the Alaska Range starting ~28 Ma, coincident with and proceeding CCB sedimentation and dike swarm emplacement. These early Oligocene tectono-magmatic events likely reflect a transitional period between Pacific plate subduction and flat-slab subduction of the Yakutat microplate under southern Alaska.