Cretaceous to Oligocene magmatic and tectonic evolution of the western Alaska Range: Insights from U-Pb and 40Ar/39Ar geochronology

Abstract

New U-Pb and 40Ar/39Ar ages integrated with geologic mapping and observations across the western Alaska Range constrain the distribution and tectonic setting of Cretaceous to Oligocene magmatism along an evolving accretionary plate margin in south-central Alaska. These rocks were emplaced across basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Triassic and Jurassic plutonic and volcanic rocks of the Peninsular terrane, and Jurassic and Cretaceous siliciclastic strata of the Kahiltna assemblage. Plutonic rocks of different ages also host economic mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related rare-earth elements. The oldest intrusive suites were emplaced ca. 104–80 Ma into the Peninsular terrane only prior to final accretion. Deformation of the northern Kahiltna succession and underlying Farewell terrane occurred at ca. 97 Ma, and more widespread deformation ca. 80 Ma involved south-vergent folding and thrusting of the Kahiltna assemblage that records collisional accretion of the Peninsular-Wrangellia terrane and juxtaposition of sediment wedges formed on the inboard and outboard terranes. More widespread magmatism ca. 75–55 Ma occurred in two general pulses, each having distinct styles of localized deformation. Circa 75–65 Ma plutons were emplaced in a transpressional setting and stitch the accreted Peninsular and Wrangellia terranes to the Farewell terrane. Circa 65–55 Ma magmatism occurred across the entire range and extends for more than 200 km inboard from the inferred position of the continental margin. The Paleocene plutonic suite generally reflects shallower emplacement depths relative to older suites and is associated with more abundant andesitic to rhyolitic volcanic rocks. Deformation ca. 58–56 Ma was concentrated along two high-strain zones, the most prominent of which is 1 km wide, strikes east-northeast, and accommodated dextral oblique motion. Emplacement of widespread intermediate to mafic dikes ca. 59–51 Ma occurred before a notable magmatic lull from ca. 51–44 Ma reflecting a late Paleocene to early Eocene slab window. Magmatism resumed ca. 44 Ma, recording the transition from slab window to renewed subduction that formed the Aleutian-Meshik arc to the southwest. In the western Alaska Range, Eocene magmatism included emplacement of the elongate north-south Merrill Pass pluton and large volumes of ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, a latest Eocene to Oligocene magmatic pulse involved emplacement of a compositionally variable but spatially concentrated suite of magmas ranging from gabbro to peralkaline granite ca. 35–26 Ma, followed by waning magmatism that coincided with initiation of Yakutat shallow-slab subduction. Cretaceous to Oligocene magmatism throughout the western Alaska Range collectively records terrane accretion, translation, and integration together with evolving subduction dynamics that have shaped the southern Alaska margin since the middle Mesozoic.

Continental shelves as detrital mixers: U–Pb and Lu–Hf detrital zircon provenance of the Pleistocene–Holocene Bering Sea and its margins

“…Middle Cretaceous zircon (age peaks at 108 and 94 Ma) are most likely sourced from plutonic rocks within multiple mid‐Cretaceous magmatic belts (80–110 Ma) that extend from the southern Brooks Range to the Alaska Range, into the Yukon River interior, and potentially as far west as eastern Russia (Hart et al, 2004; Plafker & Berg, 1994). Cenozoic age peaks, particularly near 38 and 65 Ma, are likely derived from plutons in the central and western Alaska Range (Benowitz et al, 2011; Csejtey et al, 1992; Jones et al, 2014; Lease et al, 2016).…”

Large-scale, crustal-block vertical extrusion between the Hines Creek and Denali faults coeval with slip localization on the Denali fault since ca. 45 Ma, Hayes Range, Alaska, USA

“…30 Ma due to the initiation of Yakutat flat slab subduction (Figs. 1 and 13) (Lease et al, 2016;Jones et al, 2021;Regan et al, 2021).…”

“…30 Ma initiation of the Wrangell arc (Brueseke et al, 2019;Berkelhammer et al, 2019) was coeval with the cessation of plutonism in the Alaska Range arc (Fig. 3; Jones et al, 2021;Regan et al, in 2021) and at ca. 25 Ma was followed by the cessation of Alaska Range dike emplacement .…”

“…25 Ma was followed by the cessation of Alaska Range dike emplacement . These magmatic events and hiatuses are interpreted to reflect the arrival of the Yakutat flat slab under southern Alaska (Trop et al, 2019;Jones et al, 2021). Another major plate boundary change occurred at 25 Ma, when the Pacific plate vector relative to North America underwent counter-clockwise rotation and increased in velocity (Jicha et al, 2018), resulting in a high magnitude of oblique convergence across the Denali fault system as southern Alaska moved to the northwest (Fig.…”

Detrital zircon geochronology of modern river sediment in south-central Alaska: Provenance, magmatic, and tectonic insights into the Mesozoic and Cenozoic development of the southern Alaska convergent margin

“…It is also important to note that detrital zircons best reflect episodes of felsic magmatism and, therefore, are not an accurate recorder of major mafic and intermediate magmatic episodes (e.g., Martínez Ardila et al, 2019). Another important consideration in evaluating magmatic episodes in southern Alaska is that well documented magmatic cessation events are associated with processes such as terrane accretion, spreading ridge subduction, and oceanic plateau subduction (Bradley et al, 2003;Finzel et al, 2011;Finzel and Ridgway, 2017;Stevens Goddard et al, 2018;Jones et al, 2021).…”