Detrital zircon geochronology along a structural transect across the Kahiltna assemblage in the western Alaska Range: Implications for emplacement of the Alexander-Wrangellia-Peninsular terrane against North America

Abstract

The Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular terrane to the south and the Farewell and other pericratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard pericratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the Alexander-Wrangellia-Peninsular terrane and mid- to Late Cretaceous arc-related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at ca. 81 Ma. They were deformed together ca. 80 Ma by southeast-verging fold-and-thrust–style deformation interpreted to represent final accretion of the Alexander-Wrangellia-Peninsular terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the back-arc flank of an active, south-facing island-arc system (Alexander-Wrangellia-Peninsular terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench–Nankai Trough on the east side of central Japan.

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

“…Thus, they interpreted the titanite age to record metamorphism associated with northwest-directed thrusting and folding of the Farewell terrane and older components of the Kahiltna assemblage to the north. In contrast, cleavage and bedding are moderately northwest dipping across much of the outcrop belt to the south, suggesting tight, south-vergent folds (Box et al, 2019). South-vergent deformation affected younger parts of the Kahiltna assemblage that have depositional ages younger than ca.…”

“…Layered strata of the Dillinger and Mystic subterranes are deformed into a northwest-vergent, fold-and-thrust belt that is truncated on the north by the Denali fault. The age of folding and thrust faulting is not completely understood but is interpreted to predate final collision and translation of the Peninsular and Wrangellia terranes to the south (Box et al, 2019).…”

“…The Kahiltna assemblage is made up of generally fine grained siliciclastic rocks deposited in a marine environment, and Kalbas et al (2007) estimated a minimum stratigraphic thickness of 5.5 km. Petrographic observations and detrital zircon data indicate that sediment was derived from distinctive source terranes on the north and south (i.e., Farewell and YTA versus Wrangellia and Peninsular) but that the two depocenters remained completely separated until the Late Cretaceous (Kalbas et al, 2007;Hults et al, 2013;Box et al, 2019).…”

“…Slaty cleavage is locally developed in parts of the outcrop belt, and multiple cleavages are observed in some areas. In the northwestern part of the study area, kilometer-scale, northwest-vergent folds and thrust faults are formed in the older parts of the Kahiltna assemblage (Dalzell petrofacies unit; Box et al, 2019) and underlying rocks of the Farewell terrane. Box et al (2019) reported a metamorphic titanite U-Pb age of 96.8 ± 2.3 Ma from a mafic dike crosscutting deformed rocks of the Farewell terrane, which is younger than the youngest depositional ages of the Dalzell unit as constrained by detrital zircon.…”

“…In the northwestern part of the study area, kilometer-scale, northwest-vergent folds and thrust faults are formed in the older parts of the Kahiltna assemblage (Dalzell petrofacies unit; Box et al, 2019) and underlying rocks of the Farewell terrane. Box et al (2019) reported a metamorphic titanite U-Pb age of 96.8 ± 2.3 Ma from a mafic dike crosscutting deformed rocks of the Farewell terrane, which is younger than the youngest depositional ages of the Dalzell unit as constrained by detrital zircon. Thus, they interpreted the titanite age to record metamorphism associated with northwest-directed thrusting and folding of the Farewell terrane and older components of the Kahiltna assemblage to the north.…”