EOCENE ACTIVITY ON THE WESTERN SIERRA FAULT SYSTEM AND ITS ROLE INCISING KINGS CANYON, CALIFORNIA

New and published apatite (U/Th)-He and apatite ⁴He/³He data from along the Kings River canyon, California allow us to rediscover a west-down normal fault on the western slope of the central Sierra Nevada. This is one of a series of scarps initially discovered by Hake (1928) which we call the western Sierra fault system, which has been neglected or dismissed (Wahrhaftig, 1965) in the literature ever since. Combining field observations with apatite (U/Th)-He data, we infer a fault trace 30 kilometers long, and constrain the offset across this fault to be roughly a kilometer. Thermal modeling of apatite ⁴He/³He data documents a pulse of footwall cooling near the fault and upstream in the footwall at circa 45-40 Ma which we infer to be the timing of a kilometer-scale incision pulse resulting from the fault activity. This pulse accounts for much of the relief present in the modern Kings Canyon. In the context of published data from the subsurface of the Sacramento and San Joaquin Valleys, rediscovery of the western Sierra fault system suggests an Eocene tectonic regime dominated by extension, uplift, and internal structural deformation of the Sierra Nevada and proximal Great Valley forearc. Our data, and this Eocene tectonic regime, helps integrate previously published datasets regarding the Late Cretaceous and early Cenozoic evolution of the Sierra Nevada which have previously been interpreted as mutually exclusive.