Fission track, K-Ar, and Rb-Sr geochronology of Cenozoic and Mesozoic rocks from the Gulf of Alaska area

Steven C. Bergman (Univ. Texas at Dallas), Jeff Corrigan, Robert Swenson, Shari Kelley, Van Mount, and Gerald Van Kooten

Apatite and zircon fission track data constrain the timing of Gulf of Alaska fold and thrust belt development and indicate extremely high Miocene to post-Pliocene (11 to 2-3 Ma) exhumation rates (>1-2.5 km/m.y.) for much of the northern part of the onshore coastal area within ~142-145°W longitude, ~60-60.5°N latitude. An eastward younging of thrust belt activity is observed between Ragged Mtn (11 Ma), the Don Miller Hills (5-6 Ma), and the Kulthieth River area (2-3 Ma) based on apatite FT data. Basement rocks north of the Chugach fault contain reset biotite K-Ar and zircon FT ages reflecting Eocene (~50 Ma) post-metamorphic cooling through 200-300 °C, as well as reset apatite FT ages, together indicating maximum paleotemperatures of >300 °C and Pliocene (~6 Ma) cooling through 120 °C, and further suggesting exhumation was related to thrusting. Partially reset to unreset apatite FT ages are observed in Cenozoic sandstones from along the coast, indicating maximum paleotemperatures of <100°C. Sandstone detrital zircon FT age populations (Miocene to Eocene depositional ages) exhibit complex provenance populations of 30±10, 65±15, and 95±15 Ma ages with ages as old as 160 Ma, suggesting that the Yakutat block was most likely derived from a location adjacent to the Coast Mountains, BC, or possibly as far south as Washington State. Syn-orogenic sandstone detrital zircon FT age populations (Plio-Pleistocene depositional ages) exhibit a complex provenance of 35±10, 60±10 Ma ages, suggesting an Oligocene and Paleocene zircon provenance; apatite FT age populations average 5, 20, and 30 Ma, recording Pliocene, Early Miocene, Oligocene phases of thrust belt activity. Uppermost Poul Creek Formation (coastal outcrops) glauconite Sr isochron ages are 8.2±0.5 Ma that constrain the timing of a phase of Yakutat block accretion to South Alaska at which time sedimentation rates changed from <20 m/m.y to >200 m/m.y. at the onset of the syn-orogenic Yakataga Fm. deposition and a transition from relatively quiet shelf conditions to much more dynamic glaciofluvial/marine conditions. These data have important implications for the tectonic evolution of the region.