LATE CRETACEOUS AGE OF THE DOTHAN FORMATION, SW OREGON: IMPLICATIONS FOR EOCENE GROWTH OF THE NORTHERN KLAMATH MOUNTAINS DURING COLLISION AND ACCRETION OF SILETZIA

According to most tectonic models for the northern Klamath Mountains, the last major phase of mountain building occurred during Late Jurassic time and was followed by intrusion of 165-136 Ma post-orogenic stitching plutons. In these models the Dothan Formation of SW Oregon is inferred to be an outboard Cretaceous subduction-accretionary complex that correlates to the Franciscan mélange in northern California, despite prior estimates of a late Jurassic age based on fossil evidence. To explore these issues we dated a total of 630 detrital zircon (DZ) grains from two samples of tightly folded Dothan Fm sandstone located ~6 km apart (across-strike) at 1100-1300 m elevation near the Rogue River northwest of Grants Pass. The samples yielded identical DZ age distributions that we present as a single population (N=597; excludes ages with >10% discordance): (1) a dominant age peak at 90-110 Ma (= northern Sierra Nevada Late Cretaceous arc source); (2) a broad bumpy peak ~140 to 210 Ma (= Klamath Mountains source); and (3) several minor Paleozoic and Precambrian peaks. Maximum Depositional Age (MDA) is 90.4 +/- 1.2 Ma based on the youngest grain cluster overlapping at 2σ (10 grains), or 89.6 +/- 1.2 Ma based on the youngest 3 grains. The DZ age peaks and MDA strongly overlap those of the Late Cretaceous Hornbrook Formation in the eastern Klamath Mts (Surpless and Beverly, 2013; Surpless, 2015), indicating a shared provenance and tectonic history. Two possible interpretations are: (1) the Dothan Formation formed in a Late Cretaceous subduction-accretionary complex as is widely presented in the literature, which would suggest as little as 25 km between forearc basin deposits and the inboard margin of the accretionary wedge; or (2) the Dothan Fm represents fault-bounded fragments of a large Late Cretaceous forearc basin that was later destroyed by imbricate thrusting, tectonic burial, and low-grade metamorphism. In either interpretation these results support findings of previous studies (Wells et al., 2014; Dorsey et al., 2019) that the northern Klamath Mountains in SW Oregon underwent Early Eocene (post-Nevadan) regional shortening, crustal thickening and loading, and growth of a Taiwan-scale orogenic wedge that resulted from collision and accretion of the Siletzia oceanic terrane to the western margin of North America ca. 54-49 Ma.