Exhumation Timing in the Oregon Cascade Range Decoupled From Deformation, Magmatic, and Climate Patterns

Abstract

The Cascade Range in the Pacific Northwest, USA, developed as an Eocene to recent volcanic arc along the Pacific/North American ocean-continent subduction zone. The volcanic arc is characterized by temporally and spatially variable magmatism. Crustal deformation that accompanied development of the arc transitions from N-S transpression in the Yakima Fold Belt and Puget lowland (western Washington, northern Oregon) to generally E-W transtension in the south (central and southern Oregon). Orography focuses precipitation along the western flank of the Cascade Range, whereas the eastern flank is relatively arid. We use the unique spatial variation in deformation, magmatic history, and orographic precipitation to investigate the contributions from tectonic and surface processes to rock uplift. We reconstruct the exhumation pathway of plutonic rocks throughout the Cascade Range by exploiting the unique juxtaposition of basalt-capped ridges above valleys exposing exhumed Cenozoic plutons. Multiple bedrock geochronometers and thermochronometers constrain the thermal history in six catchments along the Cascade arc from southern Oregon to southern Washington. U-Pb geochronology defines circa 10-24 Ma pluton crystallization ages. Apatite and zircon (U-Th)/He thermochronology ages range from circa 8-23 Ma. 40 Ar/ 39 Ar geochronology defines circa 5-8 Ma basalt emplacement. Our results reveal spatially and temporally variable shallow exhumation in the southern Washington and Oregon Cascades and that the timing of exhumation is earlier than circa 6-12 Ma exhumation previously reported in the Washington Cascades. Spatially and temporally variable exhumation highlights that crustal deformation plays a significant role in rock uplift in addition to erosional mass removal in concert with orographic precipitation. Plain Language Summary Geoscientists have long wondered whether slow, incremental plate tectonics that shapes our planet have direct impacts on Earth surface processes (like precipitation or erosion). More provocatively, could surface processes actually impact tectonics? Existing studies have provided a range of interpretations but often focus on places where intense precipitation (like the Indian monsoon) and deformation (the Himalaya) change spatially together. This makes it challenging to determine whether tectonics or climate is the main driver. We wanted to investigate this problem along the Cascade Range in Oregon and Washington, where precipitation and deformation patterns do not vary together, allowing us to disentangle the influence of surface and tectonic processes. We use geochronology and thermochronology to determine when rocks were exhumed from deep to shallow levels in the earth. This gave us clues on the timing of rock uplift. We hypothesized that if climate was the main driver, which is similar in the Cascades from north to south, the timing would be synchronized north to south. However, we found that the timing is variable along the Cascades. We conclude that the combination and interaction between tecto...

Drier Winters Drove Cenozoic Open Habitat Expansion in North America

“…However, this hypothesis of Cascades uplift as the OHT driver remains speculative. The uplift history of the Cascades is debated (Bershaw et al, 2019;Kohn & Fremd, 2007;McLean & Bershaw, 2021;Pesek et al, 2020;Reiners et al, 2002;Takeuchi et al, 2010), and while tectonic forcing can explain many aspects of our results, more isotope data (especially clay data) with robust age constraints are needed to rigorously test any links. Improved constraints on the spatial extent of the OHT would also help address the possibility that it was driven by Cascades uplift.…”

“…Other mechanisms that can alter Δδ 18 O clay−carb are unlikely to explain our results. For example, an increase in winter δ 18 O could explain a shift toward higher clay δ 18 O in the west and central domains, but global cooling and Cascades uplift since the OHT (Bershaw et al, 2019;Kohn & Fremd, 2007;McLean & Bershaw, 2021;Pesek et al, 2020;Reiners et al, 2002;Takeuchi et al, 2010;Westerhold et al, 2020;Zachos et al, 2001) should, all else being equal, decrease winter δ 18 O. Colder clay formation temperatures are also consistent with increasing clay δ 18 O and Δδ 18 O clay−carb , but any cooling would likely occur in the winter and summer, causing the same signal in soil carbonate δ 18 O. The OHT itself might decrease soil CO 2 by decreasing productivity, which can affect carbonate δ 18 O by changing the seasonal timing of carbonate formation.…”

Potential encoding of coupling between Milankovitch forcing and Earth's interior processes in the Phanerozoic eustatic sea-level record

“…In addition, at Milankovitch time scales and longer, it was postulated that magmatic variation could cause climate change via volcanic emissions of greenhouse gases, impacting in turn Earth's surface processes including sea-level changes (Sternai et al, 2020). The interplay between tectonics, climate and Earth's surface processes was also suggested to explain the evolution of mountain building (Pesek et al, 2020).…”

“…1). Therefore, low-frequency tectonic cycles and processes (Section 4.4), in response to climatically-driven mass changes on Earth's surface including rock accumulation and erosion (e.g., Calais et al, 2010;Sternai et al, 2016;Pesek et al, 2020), have the potential to explain larger sea-level magnitudes during greenhouse periods (e.g., Vail et al, 1977;Haq et al, 1987;Müller et al, 2008;Conrad, 2013). Grabau (1936).…”

Drier winters drove Cenozoic open habitat expansion in North America

“…For example, an increase in winter δ 18 O could explain a shift toward higher clay δ 18 O in the west and central domains, but global cooling and Cascades uplift since the OHT (Zachos et al, 2001;Reiners et al, 2002;Kohn & Fremd, 2007;Takeuchi et al, 2010;Bershaw et al, 2019;Pesek et al, 2020;Westerhold et increase across the OHT with a scenario where clays and carbonates are recording the same moisture source, even if there is some evaporative offset between the two minerals. We therefore conclude that our ∆δ 18 O clay−carb results require relative seasonal source moisture biases between clays and carbonates.…”