Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: Insights into a potentially alternating convergent and transform plate margin

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.

Raising the Resurrection plate from an unfolded-slab plate tectonic reconstruction of northwestern North America since early Cenozoic time

“…Similar inboard evidence of ridge-trench intersection has been reported in Alaska (Cole et al, 2006). How-ever, recent thermochronology data have been used to suggest a margin-parallel slab breakoff event instead (Terhune et al, 2019). If "Mobilist" models are correct, significant spatiotemporal uncertainty exists in interpreting upper plate geologic constraints for ridge-trench intersection models ( Fig.…”

Partitioning Pervasive Detrital Geochronologic Age Distributions in the Southern Alaskan Forearc

“…Combined U-Pb and fission track double-dating of detrital zircon from modern rivers in the Talkeetna and Chugach Mountains reveals a preponderance of unreset, magmatic zircon grains with Jurassic and Late Cretaceous ages, suggesting limited amounts of exhumation of those regions during Cenozoic time (Enkelmann et al, 2019). Furthermore, Terhune et al (2019) suggest that significant paleotopography in the Talkeetna Mountains was created by Paleocene time based on 40 Ar/ 39 Ar, apatite fission track, and apatite (U-Th)/He cooling ages, but the region has not experienced much exhumation since then. Minor and localized Miocene cooling ages there are geographically restricted to proximity with the active Castle Mountain fault.…”

“…During Paleocene and Eocene time (ca. 62-50 Ma), either a spreading ridge that subducted from west to east across the entire southern margin, or a slab break-off event following final suturing of the Insular terranes, resulted in a hiatus in arc magmatism, emplacement of slab-window igneous rocks in the forearc and accretionary prism regions, and high-temperature/low-pressure metamorphism of accretionary prism strata Haeussler et al, 2003;Sisson et al, 2003;Cole et al, 2006;Terhune et al, 2019;Trop et al, 2019). In addition, previous detrital geochronologic and thermochronologic data from the Alaskan forearc basin demonstrate that after this event retro-arc sources become predominant over more proximal arc sources (Finzel et al, 2016).…”

“…Either subduction of oceanic crust with normal slab dip or a period of transform tectonics following slab break-off briefly occurred after spreading-ridge subduction until the late Eocene or early Oligocene (∼35 Ma) when subduction of the Yakutat microplate initiated shallow subduction along the outboard margin of south-central Alaska and continues to the present day (Finzel et al, 2011(Finzel et al, , 2015Arkle et al, 2013;Terhune et al, 2019;Trop et al, 2019). The Yakutat microplate is an ∼11-30 km thick, wedge-shaped oceanic plateau that is subducting at a dip angle of 11-16 • , decreasing from west to east, to near the modern coastline (Ferris et al, 2003;Eberhart-Phillips et al, 2006;Worthington et al, 2008Worthington et al, , 2012Christeson et al, 2010;Bauer et al, 2014).…”

“…By the Oligocene, however, the dominant sediment source area was again the Chugach Mountains with smaller contributions from the central Alaska Range and Yukon Tanana Uplands. Initiation of shallow subduction of the Yakutat microplate is inferred to have triggered uplift and exhumation in the Chugach Mountains (Arkle et al, 2013;Ferguson et al, 2015) and across the entire Alaska Range, which is also recorded in synorogenic Oligocene strata in the central Alaska Range (Benowitz et al, 2011(Benowitz et al, , 2012Fitzgerald et al, 2014;Riccio et al, 2014;Lease et al, 2016;Enkelmann et al, 2019;Terhune et al, 2019;Trop et al, 2019).…”