2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 324-3
Presentation Time: 9:30 AM


TROP, Jeffrey M., Dept. of Geology, Bucknell University, 701 Moore Avenue, Lewisburg, PA 17837, BENOWITZ, Jeff, Geophysical Institute and Geochronology Laboratory, University of Alaska Fairbanks, Fairbanks, AK 99775, COLE, Ronald B., Department of Geology, Allegheny College, 520 N. Main St, Box 37A, Meadville, PA 16335 and O'SULLIVAN, Paul, Apatite to Zircon, Inc, 1075 Matson Road, Viola, ID 83872-9709

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.