Multiphase Topographic and Thermal Histories of the Wallowa and Elkhorn Mountains, Blue Mountains Province, Oregon, USA

Abstract

The Mesozoic and Cenozoic history of Western North America is characterized by terrane accretion, volcanism, and orogenesis. This history complicates the interpretation of paleogeography in northeastern Oregon and adjacent Idaho, where the age of mountainous topography in the Blue Mountains Province is important for validating hypothesized Miocene to present geodynamics. In this study, we analyze the distribution of Columbia River Basalt and low‐temperature apatite and zircon (U‐Th)/He thermochronometry collected from the Wallowa and Bald Mountain Batholiths to refine regional landscape history. These batholiths underly the Wallowa and Elkhorn Mountains respectively, two of the most prominent mountain ranges within the Blue Mountains Province. We find that low‐temperature thermochronometry data from the Bald Mountain and Wallowa Batholiths record distinct thermal histories associated with unroofing and magmatism from the Cretaceous to present. We propose that reheating during intrusion of the Chief Joseph dike swarm led to partial resetting of thermochronometers but did not completely overprint recorded Mesozoic to present thermal histories in the Wallowa Batholith. Using modeled thermal histories and the present‐day distribution of Columbia River Basalt, we conclude that the Wallowa and Elkhorn Mountains have distinct topographic histories. We propose that the Elkhorn Mountains began to form in the Eocene and that the Wallowa Mountains are geologically young, forming as a result of relief generation after the Miocene eruption of Columbia River Basalt.