NEOGENE FAULTING EVENTS RECORDED IN WEAVERVILLE FORMATION GRABENS, SOUTHERNMOST CASCADIA FOREARC

Better understanding of long-term upper plate deformation in the southern Cascadia forearc informs interpretation of tectonic drivers and earthquake hazards at the transition between Cascadia subduction and San Andreas transform faulting. Internal stratigraphy and subsequent deformation of the nonmarine, early Middle Miocene, Weaverville Formation (Tw) record Neogene evolution of the southernmost Cascadia forearc. Data compiled from 420 well completion reports in Neogene grabens in the southern Klamath Mountains, California, provide insight into this tectonic evolution.

Structure contour maps on the base of Tw describe four small (<125 km²), asymmetric, NE-striking grabens and three sets of faults active syn- and post-Tw deposition. The westernmost Hayfork and Reading Creek grabens strike ENE, contain SSE-dipping sediments, and have dominant southern boundary faults (>2.0 and >0.5 km throw, respectively). Coal, found only in basal Tw in the northwestern and western parts of these grabens, records a low-lying, swampy environment at the onset of Tw deposition. SSE bedding dips record tilting away from formerly low-lying coal deposits and toward southern boundary faults.

In contrast, Lowden Ranch and Weaverville grabens strike NE, and have primarily NW-dipping Tw strata and dominant northern boundary faults. The northwest side of the Weaverville graben is the regionally extensive, 20-35° S-dipping La Grange detachment fault (LGF), active pre-, syn- and post-deposition of Tw. The southwest margin of Weaverville graben is a NNW-striking fault that offsets the LGF and Tw strata; it records oblique right-lateral reverse movement. Tw bedding near the fault dips 30-70° NE, forming the west limb of a north-trending syncline.

The grabens and Tw strata record three Neogene faulting events: 1) LGF detachment faulting, 2) multi-stage development of NE-trending grabens, 3) oblique right-lateral reverse faulting and associated folding. These three fault systems crosscut pre-Cenozoic bedrock structure and record Cascadia forearc deformation.