Oligocene onset of uplift and inversion of the Cascadia forearc basin, southern Oregon Coast Range, USA

Michael H Darin,John M Armentrout,Rebecca J Dorsey

doi:10.1130/g49925.1

Michael H Darin, John M Armentrout + Show 1 more

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https://doi.org/10.1130/g49925.1

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Journal: Geology	Publication Date: Feb 25, 2022
Citations: 3	License type: cc-by

Affiliation: University of Oregon, University of Nevada Reno

Abstract

Abstract An extensive detrital zircon U-Pb data set (n = 6324 dates) from Eocene to Miocene sandstones and modern river sands establishes the onset of arc magmatism and forearc uplift along the Cascadia convergent margin in southwestern Oregon (United States). Middle to late Eocene marine strata in the Coos Bay area were primarily sourced from the Klamath Mountains and coeval Clarno-Challis volcanoes in central Oregon and/or Idaho. Ancestral Cascades arc magmatism initiated at 40 Ma and supplied sediment to a broad forearc basin in western Oregon during late Eocene time. Major reduction of Ancestral Cascades arc (40–12 Ma) and Clarno-Challis (52–40 Ma) zircon in the Tunnel Point Sandstone (ca. 33–30 Ma) records the isolation of the Coos Bay area from the Ancestral Cascades arc due to Oligocene onset of forearc uplift, basin inversion, and emergence of the southern Oregon Coast Range. The Tarheel formation (ca. 18–15 Ma) is characterized by disappearance of Ancestral Cascades arc zircon and a substantial increase in Clarno-Challis zircon recycled from underlying forearc strata. The ~15–20 m.y. delay between subduction initiation (ca. 49–46 Ma) and the onset of forearc uplift (ca. 33–30 Ma) supports insights from thermomechanical models that identify tectonic underplating and thermally activated lower-crustal flow as major drivers of deformation and uplift in active forearc regions.

Highlights

Detrital zircon were separated from 22 consolidated bedrock samples (~3–5 kg each) and three samples of unconsolidated modern river sand (~10–15 kg each) using standard procedures of crushing, sieving, and density and magnetic separation at the Mineral Separation Laboratory at Northern Arizona University by M
We instead follow the recommendations of Vermeesch (2021a) and use his “concordia distance” criterion to evaluate 206Pb/238U vs. 207Pb/235U age discordance for most analyses
In order to produce a minimally filtered dataset with only a nominal 204Pb-correction, the following filter settings were applied for all samples and analyses in AgeCalcML

Summary

Introduction

Detrital zircon were separated from 22 consolidated bedrock samples (~3–5 kg each) and three samples of unconsolidated modern river sand (~10–15 kg each) using standard procedures of crushing, sieving, and density and magnetic separation at the Mineral Separation Laboratory at Northern Arizona University by M. Data reduction was performed using AgeCalcML (version 1.42), an open-source software platform with a MATLAB-based graphical user interface (GUI) developed at the Arizona LaserChron Center by Kurt Sundell ( at Idaho State University).

Results

Conclusion