Neogene extension in the southern Cascadia forearc: insights from provenance of the Tertiary Weaverville Formation

The Klamath Mountain Province (KMP) in the southern Cascadia forearc records a history of Paleozoic-Mesozoic subduction accretion and Jurassic-Cretaceous magmatism. However, much less is known about Tertiary tectonic deformation and sedimentary deposition. Remnants of the Weaverville Formation (Tw) preserved within four geographically isolated structural basins in the southern KMP provide geologic evidence for crustal extension during Tertiary time.

In general, the Tw records a transition of sedimentary facies from a basal lacustrine to upper fluvial unit. Based on palynological studies, the Tw maintains a loosely constrained depositional age of Early to early Middle Miocene (Barnett, 1989). However, the Tw is poorly correlated across the modern basins it is preserved in and it is unclear how genetically alike these basins were at the time of deposition.

We present detrital zircon U-Pb ages from the fluvial section of the Tw from two basins, Lowden Ranch and Hyampom. The Lowden Ranch Basin (n=50) yields a dominant peak age of ~135 Ma, and a smaller peak at ~400 Ma. The Hyampom Basin (n=156) yields bimodal peak ages of ~140 and ~166 Ma, and a smaller peak at ~400 Ma. Additionally, peak ages were selected from each sample for Hf isotopic analysis.

U-Pb zircon ages and ɛHf values of the Lowden Ranch and Hyampom Basins can be explained entirely by KMP sources. The 135-140 Ma ages are likely derived from the granodioritic and tonalite-trondhjemite-granodiorite suites, the 166 Ma ages are likely derived from the Wooley Creek Suite, and the 400 Ma ages are likely derived from plutons of the Eastern Klamath Terrane. The absence of ages from other regional sources implies that the KMP experienced locally elevated topography which prevented input from outside sources during Tw deposition in the Early to Middle Miocene.

The presence of small Neogene intermontane basins in the southern KMP is consistent with previous palynology and current paleontology studies. Our identification of diatoms of the genera Pinnularia and Hantzschia from mud lenses in the Lowden Ranch Basin indicate a freshwater terrestrial environment. Cumulatively, these lines of evidence provide a distinct and compelling picture of Neogene paleotopography in Southern Cascadia that has not been recognized elsewhere in the forearc.