LATE QUATERNARY HISTORY OF LANDSLIDING AND FAULTING IN LAKE TAHOE

High-resolution seismic reflection profiles and a suite of piston cores were analysed to delineate the late Quaternary history of submarine faulting and landsliding in Lake Tahoe. The structure of the Lake Tahoe basin is controlled by the North Tahoe-Incline Village and West Tahoe-Dollar Point fault zones, which cut late Quaternary lake sediments along numerous subparallel faults. The McKinney Bay megaslide scattered very large slide blocks of coherent Pleistocene sedimentary strata throughout the lake; deposited a 70-m-thick chaotic layer; and changed the basin morphology in the Late Pleistocene. Several subsequent generations of moderate to large landslides occur in McKinney Bay and the west Tahoe basin, attesting to several episodes of faulting and landslide activity. The sediments in Lake Tahoe are >300 m thick and are comprised of a 100 to 200 m basal sequence of well-bedded sedimentary strata presumably overlying granitic bedrock. The 70-m-thick chaotic layer is overlain by a 25 to 50 m sediment package whose reflectors can be correlated in profiles throughout the basin. Five 3-m piston cores were recovered; one in the footwall and two in the hanging wall of the North Tahoe fault and two in the deepest part of the lake. The longest core has a basal age of 12,600 14C yrs suggesting that a complete Holocene record is present. All of the cores contain a series of turbidites ranging from 2 to 30 cm thick. These layers can be correlated between cores based on color, lithology, texture, and magnetic properties. Eight distinctive turbidites occur above a 2-4 cm thick ash layer, identified as the 7600-8000 year Tsoyowata ash (pre-Mazama). Radiocarbon ages of macrofossils in the turbidites give an 8000 14C date for stratigraphically younger turbidites, suggesting a major sediment redistribution event about that time, possibly related to post-glacial collapse and landsliding in McKinney Bay. Together, these preliminary data suggest several intervals of landslide activity, perhaps seismically triggered, during the Holocene.