Collaborative Research: Testing Models for Fluvial Response to Eocene Terrane Collision in Western Oregon Using Integrated Tectono-Stratigraphy and Detrital Provenance Analysis

A major goal of Earth science research is to understand interactions among tectonic and surface processes that control topographic evolution in regions of active mountain building, particularly in zones of terrane-continent collision at convergent plate boundaries. Early Eocene sedimentary rocks in SW Oregon offer an ideal opportunity to test models for the surface response to collision of the oceanic Siletz terrane with western North America, and regional post-collision adjustments, that affected the PNW region between ~ 55 and 40 million years ago.

The events associated with collision of Siletzia are central to understanding the origins of the Coast Ranges in western Oregon and Washington, as well as the regional crustal structure that governs processes active in the modern Cascadia subduction zone. This research applies a novel combination of field and analytical techniques such as isotopic dating of sand grains that are well suited for testing hypotheses in this setting, and the results will be useful for understanding this and other terrane-continent collisional margins around the world.

The study provides a unique opportunity for undergraduate students to conduct place-based research by investigating the deep-time geologic history of Oregon. Student researchers will be recruited from Lane Community College and University of Oregon (UO) to work under the mentorship of PIs and graduate students. This project benefits local communities through cooperation and data sharing among the UO, Oregon State University, and the Oregon Department of Geology and Mineral Industries (DOGAMI).

The study also contributes to the broader impacts of an ongoing NSF-RUI funded study of bedrock evolution in the Klamath Mountains through cross-project fieldwork, conference presentations, and student-focused workshops.

Eocene sedimentary rocks in southwest Oregon preserve an unparalleled archive of the surface response to collisional mountain building and post-collision tectonic reorganization. The syn-collision Umpqua Group (~ 54–49 Ma) rests on basalts of the Siletz terrane and filled a syn-orogenic basin on the north margin of the Klamath Mountains orogen. The post-collision Tyee Group (~ 47–45 Ma) is a thick succession of fluvial, deltaic, and marine turbidite deposits that record rapid progradation of an integrated fluvial-delta-shelf-slope-basin clinoform system during initiation of the modern Cascadia subduction zone.

Hypothesis 1 postulates that the Tyee paleoriver originated in western Idaho, traversed a large low-gradient continent-interior drainage, and flowed through the former collisional orogen to a prograding fluvial-deltaic to offshore marine turbidite system. In Hypothesis 2, the Tyee paleoriver was sourced in the Klamath Mountains, and changes in sand composition record bedrock exhumation, recycling of older sediments, and/or catchment growth.

Because paleocurrent data show unequivocally that the Tyee paleoriver flowed directly out of – not around – the Klamath Mountains, evidence of a large continent-interior catchment for the Tyee paleoriver would imply major post-collision reorganization of the drainage system by headward erosion and stream capture and/or extensional collapse of the former collisional orogen. These hypotheses will be tested through integration of modern provenance tools, detrital thermochronology, and detailed field observations.

Methods include U-Pb dating and Lu-Hf analyses of detrital zircon, 40Ar/39Ar dating of detrital micas, basin subsidence analysis, and detrital thermochronology, all tied to detailed geologic mapping and stratigraphic analysis. The results will generate new insights into the regional surface response to collision-related mountain building and post-collision reorganization associated with accretion of Siletzia to North America.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.