Tectonic Underplating and Dismemberment of the Maclaren-Kluane Schist Records Late Cretaceous Terrane Accretion Polarity and ~480 km of Post-52 Ma Dextral Displacement on the Denali Fault

Abstract

Terrane accretion introduces irregular geometry and allochthonous material to obliquely convergent margins, which create opportunities to quantify strike-slip displacement along otherwise margin-parallel fault systems. We present new bedrock geologic mapping and U-Pb and 40Ar/39Ar geochronology from the Alaska Range suture zone in the eastern Alaska Range, which confirm a long-hypothesized correlation between the Maclaren Glacier metamorphic belt (Alaska, USA) and the Kluane metamorphic assemblage (Yukon Territory, Canada) across the right-lateral Denali fault. The new data inform a palinspastic reconstruction showing that the dissected metamorphic belts and associated plutons record ~480 km of dextral displacement along the Denali fault since ca. 52 Ma. Before strike-slip separation, the Maclaren-Kluane schist formed by west-vergent forearc underplating in the waning stage of the ca. 100–90 Ma arc built upon the Yukon-Tanana terrane. The prograde structural and metamorphic evolution of the Maclaren-Kluane schist records the final collision of the Wrangellia composite terrane at ca. 75–65 Ma along a set of east-dipping thrust shear zones, which we infer to record the polarity of the Late Cretaceous plate boundary between the composite terrane and North America. Paleogene extension partially exhumed the schists to the upper crust and may be a consequence of regionally distributed strike-slip faulting at that time. Localization of the modern Denali fault after ca. 52 Ma dismembered the schists and four neighboring belts of plutonic, metasedimentary, and volcanic rocks. The transition to Yakutat oblique flat slab subduction at ca. 30–25 Ma marks the onset of transpressional deformation in the Denali fault system, which reactivated Late Cretaceous collisional structures bounding the Maclaren schist. Neogene reactivation of the Totschunda fault reduced strike-slip motion on the Denali fault east of the Denali-Totschunda intersection and continues to transfer residual plate boundary slip onto the Denali fault west of the intersection. Key outcomes of our synthesis include: (a) Much of the ~480 km of displacement on the Denali fault accumulated after strike-slip on the neighboring Tintina and Border Ranges fault systems had largely shut down; (b) The modern Denali fault system should not be grouped with strike-slip faults credited with large-scale margin-parallel transport of Cordilleran terranes in the Cretaceous. Instead, a poorly understood proto-Denali fault system may be a candidate for large-scale Cretaceous translation; and (c) the longevity (≥33 Myr) of the highly localized Denali fault master strand (≤1 km wide) implies that it occupies a major mechanical boundary that penetrates the lithosphere.