• Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: multiple topographic development drivers through time

      Terhune, Patrick J.; Benowitz, Jeffrey; Freymueller, Jeffrey; Gillis, Robert (2018-08)
      Intraplate mountain ranges can have polyphase topographic development histories reflecting diverse plate boundary conditions. We apply ⁴⁰Ar/³⁹Ar, apatite fission track (AFT) and apatite (U-Th)/He (AHe) geochronology-thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling and inferred exhumation patterns as proxies for the deformation history of this long-lived intraplate mountain range. High-temperature ⁴⁰Ar/³⁹Ar geochronology on muscovite, biotite and K-feldspar from Jurassic granitoids indicates post-emplacement (~158-125 Ma) cooling and Paleocene (~61 Ma) thermal resetting. ⁴⁰Ar/³⁹Ar whole rock volcanic ages and AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene-Eocene, suggesting that the Range is partially paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ~10 km of the Castle Mountain Fault suggest ~2-3 km of vertical displacement that also contributed to mountain building, likely in response to the inboard progression of the subducted Yakutat microplate. Paleocene-Eocene volcanic and exhumation ages across interior southern Alaska north of the Border Ranges Fault System are similar and show no N-S or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that slab breakoff sub-parallel to the trench and subsequent mantle upwelling drove magmatism, exhumation and rock cooling synchronously across south-central Alaska and played a primary role in the development of the southern Talkeetna Mountains.
    • The taphonomy and phylogenetic relationships of the Talkeetna mountains hadrosaur

      Stack, Kevin P. (2012-05)
      The fossil record of hadrosauroids (Ornithopoda, Hadrosauroidea) from the Albian to Santonian is very sparse, with few described North American and Asian taxa compared to the diverse record of Campanian to Maastrichtian hadrosaurids. In 1994, the partial postcranial remains of a hadrosauriform dinosaur were found in the Matanuska Formation of southern Alaska. The Matanuska Formation is a thick succession of Albian¬to Maastrichtian-aged, dominantly marine, sediments deposited in a forearc basin along the actively accreting western North American margin. The Alaskan specimen is assigned a Turonian age based on molluscan biostratigraphy. The skeleton consists of postcranial elements including cervical, dorsal and caudal vertebrae, a partial pectoral girdle, proximal elements of the forelimbs, a partial pelvic girdle, and representative portions of the hindlimbs. This fossil represents the most complete, single skeleton of a dinosaur known from Alaska, and one of the few skeletal remains recovered outside of the North Slope. It is only the second North American Turonian hadrosauroid described, the other being jeyawati rugoculus from New Mexico. This specimen also represents a new taxon of basal hadrosauroid that can be diagnosed by its unique combination of humeral, filial, and femoral characters. A phylogenetic analysis recovers the new taxon nested within a paraphyletic assemblage of non-hadrosaurid hadrosauroids, being more derived than the North American Cenomanian taxa Eolambia and Protohadros but more basal than stratigraphically younger hadrosauroids from Asia, including Tanius, Bactrosaurus, and Gilmoreosaurus. The temporal and geographic occurrence of the Alaskan taxon provides an important new data point for hypotheses of hadrosauroid biogeography in the Late Cretaceous.