RAPID, LATE OLIGOCENE EXHUMATION IN THE NORTHEAST KLAMATH MOUNTAINS, CALIFORNIA AND OREGON

Neogene unroofing of the northeastern margin of the Klamath Mountains Province of the southern Cascadia forearc included at least 4 km of exhumation since late Oligocene time, constrained by apatite (U-Th)/He (AHe) dates from the Late Jurassic Ashland pluton, vitrinite reflectance values in overlying Late Cretaceous Hornbrook Formation, and thicknesses and burial histories of Cenozoic sedimentary units in the region.

Exhumation of the Late Jurassic Ashland pluton is chronicled by three Early Cretaceous zircon (U-Th)/He (ZHe) dates (123-113 Ma) and three early-to-middle Miocene AHe dates (20-18, 17-15, 14-15 Ma). The ZHe dates reflect cooling associated with an initial unroofing that began soon after pluton emplacement and the AHe dates record more recent exhumational cooling.

Deposition of Late Cretaceous Hornbrook Formation on the Ashland pluton indicates the pluton had been exhumed to the surface prior to burial by ~ 4 km of Late Cretaceous and Paleogene strata. This stratigraphic history, which is corroborated by maturation of vitrinite (R₀ generally ~ 0.50 - 0.60, with a range of 0.40 - 0.83) sampled from the Ditch Creek Siltstone, Rocky Gulch Sandstone, and Blue Gulch Mudstone, the three upper members of the Hornbrook Formation, requires several km of rock exhumation since Late Oliogocene (~ 30 Ma).

Together, the AHe dates for the pluton and vitrinite reflectance values for overlying sediments act as a > 2 km vertical section constraining exhumation histories. Our thermal-tectonic modeling indicates that an ~ 5 m.y. interval of rapid exhumation (3.5 km at ~ 0.7 mm/yr) beginning ~ 28 Ma is needed to match the vertical age distribution for AHe samples within the pluton. The interval of rapid exhumation was followed by slower exhumation throughout the Miocene. This thermal history is compatible with the observed values of vitrinite R₀.

These results imply a major tectonic event in the northeastern Klamath Mountains Province in the Neogene and provide context for both timing and geographic extent of Neogene unroofing reported in the Condrey Mountain area ~50 km SW of the Ashland pluton.