Abstract The Jurassic–Cretaceous Nutzotin, Wrangell Mountains, and Wellesly basins provide an archive of subduction and collisional processes along the southern Alaska convergent margin. This study presents U-Pb ages from each of the three basins, and Hf isotope compositions of detrital zircons from the Nutzotin and Wellesly basins. U-Pb detrital zircon ages from the Upper Jurassic–Lower Cretaceous Nutzotin Mountains sequence in the Nutzotin basin have unimodal populations between 155 and 133 Ma and primarily juvenile Hf isotope compositions. Detrital zircon ages from the Wrangell Mountains basin document unimodal peak ages between 159 and 152 Ma in Upper Jurassic–Lower Cretaceous strata and multimodal peak ages between 196 and 76 Ma for Upper Cretaceous strata. Detrital zircon ages from the Wellesly basin display multimodal peak ages between 216 and 124 Ma and juvenile to evolved Hf compositions. Detrital zircon data from the Wellesly basin are inconsistent with a previous interpretation that suggested the Wellesly and Nutzotin basins are proximal-to-distal equivalents. Our results suggest that Wellesly basin strata are more akin to the Kahiltna basin, which requires that these basins may have been offset ∼380 km along the Denali fault. Our findings from the Wrangell Mountains and Nutzotin basins are consistent with previous stratigraphic interpretations that suggest the two basins formed as a connected retroarc basin system. Integration of our data with previously published data documents a strong provenance and temporal link between depocenters along the southern Alaska convergent margin. Results of our study also have implications for the ongoing discussion concerning the polarity of subduction along the Mesozoic margin of western North America.
Icy debris fans are paraglacial landforms first described in the early 2000's and are largely unexplored. Research to describe the subsurface characteristics is on-going and this contribution aims to describe the GPR profile characteristics with respect to the surface morphologic observations. These landforms are fan shaped in deglaciating alpine regions formed by deposits emerging from bedrock catchments. The deposits mainly consist of ice and minor lithic material from mass flows, including chiefly ice avalanches and subordinate debris flow, slushflow, slush avalanches, and rockfall. We illustrate the GPR signature of recent deposits relative to surface observations and then extend these comparisons to shallow (< approx. 100 ns or ~10 m) characteristics of the GPR profile data. The deeper characteristics of the GPR reflections provide potential depth to bedrock. In addition, contrasting GPR reflections collected on glaciers adjacent to the icy debris fans to those from the fans indicates that there is a significant difference in the ice characteristics. The subsurface observations from GPR profiles are also compared to surface observations (terrestrial laser scanning) of thickness estimates for deposits and estimates of annual volume of deposits to the landforms (photographic). The thickness observations from GPR agree with volume estimates from photographs.
Abstract The collision of oceanic arcs with continental margins is an important mechanism for the growth of continents. Ancient forearc basin strata in collisional orogens provide a record of the upper crustal response to this tectonic process. In south central Alaska, Mesozoic forearc basin strata are exposed in a complete crustal section. U‐Pb detrital zircon geochronology from the forearc basin strata was analyzed within a ~107 Ma stratigraphic framework. The Jurassic strata contain unimodal detrital zircon populations that become progressively younger upsection and range from 175 to 151 Ma. These strata are derived from the active oceanic Talkeetna arc. The Cretaceous strata were deposited above multiple unconformities that collectively represent as much as ~30 Ma of nondeposition and/or erosion in the forearc basin. Erosion in the forearc basin and a general absence of detrital zircon ages between 140 and 120 Ma are interpreted as a hiatus of magmatism triggered by collision of the oceanic arc with the former continental margin. The Cretaceous strata have two main detrital zircon populations: a Cretaceous population ranging from 90 to 68 Ma that becomes progressively younger upsection and a Jurassic population with a broad range of peak ages from 194 to 144 Ma. The Cretaceous population marks the establishment of an active Cretaceous continental arc following the collisional event, and the older population reflects continued erosion of the remnant Jurassic oceanic arc plutons. Our results show that detrital zircon geochronology provides a powerful approach for delineating stages of forearc basin collision and the erosion of multiple magmatic arcs.
Analysis of Upper Cretaceous sedimentary and volcanic strata in the Wrangell Mountains of south-central Alaska provides an opportunity to study the tectonics, depositional systems, and provenance of a forearc basin that developed along an accretionary convergent plate boundary. New data from the 1150 m thick MacColl Ridge Formation indicate that deposition occurred during the Campanian on a coarse-grained submarine fan that was derived from an uplifted allochthonous terrane exposed in the hanging wall of a fault system that separated the forearc basin from the subduction complex. New age controls include palynoflora indicative of a late middle to late Campanian age, and compatible radiometric age determinations of volcanic vitric-crystal tuffs near the top of the formation which have 40Ar/39Ar isochron ages of 79.4 ± 0.7 and 77.9 ± 2.1 Ma. Sedimentological and paleontological data show that sedimentation occurred on the inner portions of a sand- and gravel-rich submarine fan system. Evidence for this interpretation includes dominance of channelized sediment gravity flow deposits, particularly turbidites and debris flows; microflora indicative of open-marine conditions; unidirectional paleocurrent indicators; and syndepositional slump features. The pyroclastic eruptions that formed the vitric-crystal tuffs of the MacColl Ridge Formation are interpreted as products of the Late Cretaceous Kluane magmatic arc that bordered the forearc basin to the north. Sandstone and conglomerate compositional data combined with northward-directed paleocurrent indicators suggest that detritus was derived mainly from igneous rocks of the allochthonous Wrangellia terrane located in the hanging wall of the Border Ranges fault system along the southern margin of the basin. From a regional perspective, deposition of the MacColl Ridge Formation was coeval with the early part of Campanian-Maastrichtian synorogenic sedimentation and contractile deformation documented throughout the northwestern Cordillera.
Abstract The Mesozoic–Cenozoic convergent margin history of southern Alaska has been dominated by arc magmatism, terrane accretion, strike-slip fault systems, and possible spreading-ridge subduction. We apply 40Ar/39Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) geochronology and 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 region’s deformation history and to better delineate the overall tectonic history of southern Alaska. High-temperature 40Ar/39Ar thermochronology on muscovite, biotite, and K-feldspar from Jurassic granitoids indicates postemplacement (ca. 158–125 Ma) cooling and Paleocene (ca. 61 Ma) thermal resetting. 40Ar/39Ar whole-rock volcanic ages and 45 AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene–Eocene, suggesting that the mountain range has a component of 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 and that the Castle Mountain fault also contributed to topographic development in the Talkeetna Mountains, likely in response to the flat-slab subduction of the Yakutat microplate. Paleocene–Eocene volcanic and exhumation-related cooling ages across southern Alaska north of the Border Ranges fault system are similar and show no S-N 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 subparallel to the trench slab breakoff initiated at ca. 60 Ma and led to exhumation, and rock cooling synchronously across south-central Alaska, played a primary role in the development of the southern Talkeetna Mountains, and was potentially followed by a period of southern Alaska transform margin tectonics.
