RETRODEFORMING THE KLAMATH MOUNTAINS OROCLINE

Large scale oroclinal rotation in the Klamath Mountains (KM) has deflected Mesozoic accreted terranes across southwestern and central Oregon since early Eocene time. The prominent Klamath Mountains Orocline (Mendocino orocline of Carey, 1958; Hamilton & Meyers, 1966) is defined by the concave-east arcuate form of thrust faults, folds, and metamorphic fabrics in Mesozoic to early Cenozoic rocks. Despite the significance of the KM Orocline for understanding late Mesozoic to early Cenozoic paleogeography and Eocene accretion of Siletzia, no modern tectonic reconstruction has unfolded this regional structure. Rock fabrics in the southern KM trend NNW and are not rotated. Fabrics and structures in the northern KM, including thrust faults that cut 55-Ma basalts of the Siletz Terrane, trend NE and are rotated 79° ± 12° clockwise (CW) (Wells et al., 2014); up to half of this rotation may have occurred during accretion of Siletzia 54–49 Ma. The Blue Mts, located along strike NE of the northern KM, comprise similar Mesozoic accreted terranes that have been rotated 60° ± 9° CW since 90 Ma with most rotation 60–45 Ma (Housen, 2018). Thus the Blue Mts and northern KM share a similar rotation history.

We present a new, preliminary tectonic reconstruction that explicitly unfolds the Klamath Mountains Orocline and restores the pre-50 Ma subduction zone in Oregon to a NNW trend (following Wells et al., 2014; Wells, 2022). This model removes ~ 50 km of late Cenozoic E-W extension in SE Oregon and NW Nevada (Colgan et al., 2006), less than the ca. 200 km of Wyld et al. (2006). Unfolding the KM Orocline restores the southern KM and Sierra Nevada about 600 km to the ESE and includes roughly 100% extension in the central Basin-and-Range province, consistent with reconstructions of McQuarrie and Wernicke (2005) and Tikoff et al. (2023). Quesnellia, Stikine, Western Mélange Belt, and other southern B.C. terranes are restored south by various amounts to fill the Columbia Embayment, with large uncertainties. Our preliminary reconstruction also restores paleocurrents of the Early Eocene Umpqua Group and Tyee Formation, which filled a large syn- to post-collision foredeep basin, to ~ westward paleo-transport perpendicular to the restored collisional orogen. This sets the stage to explore tectonic controls on development of the Tyee paleoriver system.