Paper No. 323-4
Presentation Time: 9:45 AM
GEOLOGIC AND GEOMORPHIC EVIDENCE OF INTERMONTANE DEFORMATION AND LATE PLEISTOCENE SURFACE FAULTING ACROSS THE GREAT WESTERN DIVIDE, SOUTHERN SIERRA NEVADA
Field reconnaissance across the Great Western Divide in the southern Sierra Nevada suggests the presence of a previously unidentified, potentially active late Pleistocene fault that extends northwest from the active Kern Canyon fault (KCFZ). The fault extends from the KCFZ at the eastern end of Kaweah Ridge on the southeast, for at least 30 km to Palmer Mountain on the northwest. The 600-m-wide unnamed fault zone (informally: Colby Pass fault, CPF) strikes 320-330, dips 70-80SW, and consists of at least two primary parallel shears with prominent geomorphic expression. Several lines of evidence suggest late Pleistocene SW-down displacement (from south to north): (1) scarps and linear troughs developed on glacial deposits on the northern part of the northern Chagoopa Plateau, (2) an uplifted outlet channel from an unnamed lake along Picket Creek, (3) ponded alluvium in the Kern-Kaweah River valley, (4) displaced moraine crests at the mouth of Milestone Basin, (5) higher peak elevations along the Great Western Divide north of Colby Pass, (6) higher elevation of a glacial trimline along the wall of Cloud Canyon north of the CPF, (7) differences in Cloud Canyon valley morphology north and south of the CPF, and (8) displaced moraine crests comprising Moraine Ridge on the northern side of Cloud Canyon. Topographic profiles of moraine crests along Moraine Ridge show three distinct scarps that are aligned on both smooth- and sharp-crested lateral (Tahoe and Tioga?) moraines, suggesting multiple surface ruptures with SW-down normal displacement. None of these lines of evidence is without an alternative non-tectonic cause, but (1), (2), (4), and (8) are moderately compelling for late Pleistocene displacement; other lines are supportive. The possibility of late Pleistocene activity along the CPF suggests that SW-down intermontane normal faulting branches northwestward from the active KCFZ, lending additional support for late Quaternary internal tectonic deformation of the southern Sierra Nevada. As identified here, the NW-striking CPF is parallel with the SW-down, pre-late Pleistocene Farewell fault in the Little Kern River valley, and may suggest that intermontane deformation west of the KCFZ has occurred along a series of north-stepping normal faults during the late and perhaps middle Quaternary.