THE SIERRA NEVADA FRONTAL FAULT SYSTEM: KINEMATICS AND ASSOCIATED LANDSCAPE EVOLUTION

The Sierra Nevada Frontal Fault system (FFS) bounds the western Basin and Range province and defines the eastern margin of the Sierra Nevada microplate. The FFS's role in the uplift of the Sierra Nevada, however, remains an unquantified parameter. We present new data that constrains temporal and spatial development of the FFS in the Feather River region of the northern Sierra Nevada, CA. We interpret the high-relief, short wavelength modern landscape as a signature of relatively youthful rock uplift in the northern Sierra that has been accommodated by the FFS; we also posit that uplift, incision and FFS vertical separation have propagated northward during the Late Cenozoic. Previous work by Stock (2005) concludes that rapid incision began at ~3 Ma in the southern Sierra and slowed to a near halt at ~1.5 Ma. In the northern Sierra, south of the North Fork Feather River, unique piercing points of the contact between overlying Miocene-Pliocene Mehrten Formation and an underlying paleochannel were vertically separated across the FFS by more than 700 m between 5 Ma and ~1.2 Ma. Similarly, the North Fork Feather River has incised 713 m below the paleochannel-bottom since ~5 Ma. That relation suggests the FFS plays a critical role in rock uplift of the Sierra Nevada, in addition to its roles of accommodating Basin and Range extension and NW motion of the Sierra Nevada microplate. Farther north, volcanic terraces in the North Fork Feather River and upland-capping 2.8 Ma Yana volcanics reveal incision in the northernmost Sierra that began after 2.8 Ma and reached maximum at 1.2 Ma; incision has slowed very little into the present. In this region, vertical separation across the FFS is more difficult to constrain but is markedly less than that found farther south; however, stream incision has outpaced measurable vertical separation of Late Cenozoic volcanics. Vertical separation rates south of the North Fork Feather River (0.14 mm/yr) are an order of magnitude greater than preliminary estimates from north of the river (~0.06 mm/yr). We thus conclude that the northernmost FFS is an incipient fault zone that has only recently began to accommodate earlier uplift.