ACTIVE FAULTING AND QUATERNARY PALEOHYDROLOGY IN THE TRUCKEE FAULT ZONE, NORTHWESTERN WALKER LANE, CALIFORNIA

I conducted detailed field mapping, aerial photo analysis, and stratigraphic mapping in order to identify active faulting in the northwestern Walker Lane. The Walker Lane is a broad, complex, discontinuous zone of faulting along the eastern Sierra Nevada Mountains that accommodates Pacific-North American transform motion. Geodetic studies indicate that the Sierra Nevada is essentially a rigid block that rotates counterclockwise about an Euler pole located west of the coast of southern California and translates ~12 mm/yr oriented 313° with respect to North America. Air photo analysis, soil coring, and trenching were carried out in Hobart Meadow, located approximately 10 miles north of Truckee, California at a location between the suggestively normal-dextral, northwest striking Truckee Fault Zone (TFZ) and the left lateral, northeast striking Dog Valley Fault Zone. This study may provide evidence for a ~M6 event in the late Pleistocene-early Holocene and aid in identification of seismic sources capable of significant ground rupture.

Hobart Meadow is a long (>500m), linear (<100m wide), meadow striking approximately N20°W with a planar, gentlly sloping (~0.006 m/m to the south) surface (~3m drop from north to south over 500m in length) located at the topographic divide (~1890m) between Sagehen Creek to the north, and Prosser Creek to the south. I hand excavated a small 5m long by 2m deep trench down a ~1m scarp that exposed apparently normal faulted older (Pleistocene?) coarser grained, oxidized, outwash deposits and younger (late Pleistocene or early Holocene?) fine grained, reducing environment, pond deposits. Maximum vertical offset (down to the east) observed was ~1m. The north-striking, near-vertical fault is characterized by a well developed fault gouge with thicker units on the down thrown block (east side). At least four stratigraphic units are truncated by the fault plane with stratigraphic contacts and differential thickness of units across the fault plane suggesting a pre-existing topography in the vicinity of the modern scarp with implications that reoccupation of the fault plane has occurred. These results document apparent normal, down to the east, faulting in the northeastern fringe of the Truckee Fault Zone.