The Totschunda-Denali intersection of Alaska: A low-angle (<25°) strike-slip fault intersection since at least ∼25 Ma

Abstract

Fault intersections are ubiquitous to strike-slip fault systems and can enable significant displacement when slip is transferred between splays over time during earthquake events. If and how these strike-slip fault intersections endure and evolve is an ongoing question in tectonics. The Cretaceous-aged and dextrally active sub-vertically dipping Totschunda and Denali faults of Alaska currently intersect at a low-angle (<25°) and communicate slip as these faults transfer strain inboard from Alaska's southern convergent margin. However, when the Totschunda-Denali low-angle fault intersection formed, and how this junction has responded to far-field stress changes, is unconstrained. We apply geochronology (zircon UPb) and low-temperature thermochronology (zircon and apatite [UTh]/He and apatite fission-track) with inverse thermal modeling to constrain the spatial-temporal cooling history of the crustal blocks around the Totschunda-Denali fault intersection. The block bounded within the apex of the Totschunda and Denali fault intersection (DENT block) began to rapidly cool (>30 °C/Ma) by ∼25 Ma. The DENT block's late Oligocene-early Miocene cooling rate surpasses magnitudes in the palinspasticly restored surrounding blocks (>10 °C/Ma vs ≤ ∼2 °C/Ma). We link the onset of ∼25 Ma DENT block rapid cooling with the probable existence of the fault intersection by this time, as both strands of the fault intersection had to be active and connected to accommodate vertical block extrusion and exhumation. Subsequently, late Miocene-Quaternary rapid cooling (>6 °C/Ma) along the Totschunda fault occurs when this fault becomes the principal slip strand, which is linked to a change in the Pacific-Yakutat plate vector. The vertical extrusion of the DENT block and the switch to increased slip along the Totschunda fault post ∼6 Ma has enabled long-lived (≥25 Ma) geometric stability of this intersection, even during changes in relative convergence, by limiting fault system reorganization. Globally, low-angle (<25°) strike-slip fault intersections are mechanically efficient and may endure for tens of millions of years.