ACTIVE TECTONISM SINCE 8 MA IN THE SIERRA NEVADA – BASIN AND RANGE TRANSITION ZONE, LASSEN COUNTY, CA

Late Miocene fluvial, fluvial-deltaic, and lacustrine sediments of the Hallelujah Formation record the depositional and deformational history in the Long Valley, CA (Hallelujah Junction) area. Deposition of Hallelujah Formation started before 8.0 Ma and continued into the Blancan (5.03 – 2.68) Ma. Age control for the Hallelujah Formation is based on tephrochronology and mammalian fossils. Granitic boulder conglomerates within the section are monolithologic and immature. They record significant periods of active tectonism during deposition.

The Long Valley section dips west and is bounded by faults to both the east and west. The Diamond Mountain fault, on the west, is a north-striking, east-dipping normal fault. The eastern basin boundary is an unnamed north-striking fault with dextral-normal displacement. This suggests that post-depositional deformation occurred in a transtensional setting.

Gravity profiles across the basin constrain the thickness of the Neogene section and the offset on basin-bounding faults. Modeling based on gravity and borehole data suggests a total stratigraphic thickness of ~3500 meters. The Diamond Mountain fault is modeled to have a total displacement of up to 2500 meters. The unnamed fault bounding Petersen Mountain shows at least 500 meters of west-side-down offset. This indicates as much as 550 meters of section could be present below the 8 Ma tephrochronology date within the lower member of the Hallelujah Formation.

The tectonic record in the Neogene sedimentary rocks in Long Valley differs from nearby Neogene basins in the Sierra Nevada - Basin and Range transition zone. Boulder sheets within the section reveal 2-3 distinct periods of active tectonism between 8.0 and (5.03 – 2.68) Ma. Correlation of paleocurrent data with volumetric distribution of the boulder sheets suggests a northwest source direction for boulders in the upper section. Boulder sheets in the lower section depict an ambiguous east-west source direction. This suggests geographic distribution of faulting with time. Although Long Valley appears to be a west-tilted half graben, there is significant faulting along its eastern edge as well, and this faulting includes both strike-slip and dip-slip components.