Mesozoic orogenesis in the Klamath
Mountains Province (Northern California–southern Oregon, USA)
occurred by tectonic switching: Insights from detrital zircon U-Pb
geochronology of the Condrey Mountain schist

Alan Chapman, Macalester College
Grischuk Jennifer,
Klapper Meghan,
Karlee Taylor,
Rennie DiCarlo,
,
,
,

Abstract

The Klamath Mountains Province of northern California and southern Oregon comprises east-dipping terranes assembled by Paleozoic–Mesozoic subduction along the western margin of North America. Although the province more than doubled in size between the Middle Jurassic and Early Cretaceous through alternating episodes of extension and contraction, the mechanisms driving this growth remain poorly understood. We show that formation of the Condrey Mountain schist (CMS) of the central Klamath Mountains Province spanned this time period and use the archive contained within the CMS as a key to deciphering the Mesozoic tectonics of the Klamath Mountains Province. Igneous rocks from the outer CMS yield U-Pb zircon ages of ca. 175–170 Ma, reflecting the timing of volcanic protolith eruption. A metasedimentary sample from the same unit contains abundant Middle Jurassic zircons and yields a maximum depositional age (MDA) of ca. 170 Ma. Combined with lithologic and thermochronologic constraints, these data indicate that the outer CMS formed in an outboard rift basin before underthrusting beneath the Klamath Mountains during the Late Jurassic Nevadan orogeny. Five metasedimentary samples from the inner CMS yield MDAs ranging from ca. 160 to 130 Ma, with younger ages at deeper structural levels. This inverted age pattern, characteristic of subduction complexes, together with existing K-Ar ages, indicates progressive underplating over more than 10 m.y. Age spectra from both shallow and deep structural levels closely match those of the oldest Franciscan assemblage, the South Fork Mountain schist, suggesting that the inner CMS comprises South Fork Mountain schist slices sequentially underplated beneath the Klamath Mountains. The age, structural position, and location of the CMS directly beneath Jurassic arc rocks, with no intervening mantle, require one or more previously unrecognized episodes of shallow-angle subduction beneath the Klamath Mountains. Emplacement of the deepest CMS coincided with ca. 136 Ma termination of Klamath magmatism, consistent with slab shallowing disrupting asthenospheric flow. Shallow-angle subduction also immediately preceded westward translation of the Klamath Mountains relative to correlative rocks in the northern Sierra Nevada, suggesting that subduction dynamics relocated the province from the arc to the forearc. Collectively, these observations require at least three cycles of extension or rifting followed by shallow-angle underthrusting, supporting a tectonic switching model in which slab steepening and trench retreat alternated with slab shallowing during subduction of buoyant oceanic features. Our ongoing thermochronologic work indicate a strong link between Mesozoic tectonics in the Klamath Mountains Province and Miocene-recent topographic development within the region.

Session

Large-scale tectonics