NEOGENE-QUATERNARY FAULTING IN THE KLAMATH MOUNTAINS PROVINCE, CALIFORNIA AND OREGON: EVIDENCE FROM GEOLOGY AND THERMOCHRONOLOGY

Post-Early Miocene faulting occurred on widely distributed NE- and NW-striking faults within the Klamath Mountains Province (KMP) of the southern Cascadia forearc. Contact relationships, Neogene sediment thicknesses, and thermochronology record i) normal faulting both in Oligocene-Early Miocene and in post early Middle Miocene time, and ii) right-lateral and reverse slip on at least one fault, post early Middle Miocene.

Here, we report evidence for Neogene-Quaternary displacement on the Siskiyou Summit fault, Browns Meadow fault, and Democrat Gulch fault; other Neogene-Quaternary faults include the Eight Dollar fault, Scott Valley fault, and faults bounding the Weaverville Formation in five basins. At the northeastern margin of the KMP, the NE-striking, SE dipping Siskiyou Summit fault records ~6 km of normal separation. Geologic contact relationships show this slip occurred between ~60 and 27 Ma, and new AHe ages of 20.0 ± 5.7 Ma and 23.1 ± 7.7 Ma from the Ashland pluton record post-slip exhumation of footwall rocks to within ~3 km of the surface by ~ 20 Ma. In the south-central KMP, the NW-striking Browns Meadow fault offsets the China Creek pluton with SW-side up displacement. New AHe ages for the China Creek pluton are 39.4 ± 5.4 Ma NE of the fault and 22.8 ± 3.0 Ma SW of the fault, documenting a minimum of 1.5 km of Early Miocene or younger displacement on the fault. In the southern KMP, the NW-striking Democrat Gulch fault forms the western boundary of the Weaverville basin. The base of the Weaverville Fm. is exposed at 905 m.a.s.l. west of the fault and reported in water wells at 710 m.a.s.l. east of the fault, recording 196 m of east-side-down displacement since Weaverville Fm. deposition. A second contact, the surface of the La Grange fault, is offset 1,036 m., east side south, by the Democrat Gulch fault. A syncline in Weaverville Fm. sediments trends parallel to the strike of the Democrat Gulch fault, and bedding dips 70° away from the fault. Together, these observations indicate the Democrat Gulch fault has accommodated reverse and right-lateral displacement. All of these faults cross-cut older structures; they are Neogene to permissibly Quaternary features. Although KMP Neogene faults are moderate in scale, they record a variety of timing, style and geographic distribution of faulting within the southern Cascadia forearc.