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Long Valley caldera, California, formed during the cataclysmic Pleistocene eruption of the Bishop Tuff. Previous stratigraphic and petrologic studies of this eruption deciphered an intriguing pattern of vent migration, thought to mirror the lateral propagation ("unzipping") of magma-tapping ring fractures during caldera collapse. From scaled analog models, we show that this unzipping pattern was intrinsically related to the high plan-view ellipticity of the precollapse magma chamber roof. We also provide a first-order kinematic explanation for the systematic location of initial elliptical roof failure and for the lateral propagation of highly elliptical ring fractures. [ABSTRACT FROM AUTHOR]

An array of east-trending ridges 1–2 m high and up to 2 km long occurs on the Tahoe City shelf, a submerged wave-cut bench <15 m deep in the northwest sector of the lake. The shelf is just north of the amphitheater of the giant subaqueous 10 km³ McKinney Bay landslide, which originated on the west wall of Lake Tahoe. Images from a submersible camera show that the ridges are composed of loose piles of boulders and cobbles that lie directly on poorly consolidated, fine-bedded lake beds deposited in an ancestral Lake Tahoe. Dredge hauls from landslide distal blocks, as well as from the walls of the reentrant of the landslide, recovered similar lake sediments. The McKinney Bay landslide generated strong currents, which rearranged previous glacial-derived debris into giant ripples creating the boulder ridges. The uncollapsed part of the sediment bench, including the Tahoe City shelf, poses a hazard because it may fail again, producing a landslide and damaging waves. [ABSTRACT FROM AUTHOR]

The Golden Bear dike in the Sierra Nevada and the Coso dikes in the Coso Range crop out on opposite sides of Owens Valley, California, and strike roughly perpendicular to it. Neither dike reappears along strike across the valley. New data demonstrate that the dike sets are ca. 83 Ma in age, share nearly identical mineralogy and petrography, and intrude similar wall rocks including distinctive 102 Ma leucogranite. Dike bulk-chemical and Sr and Nd isotope compositions are nearly indistinguishable. These data suggest that the dike sets were originally continuous and were offset dextrally by ∼65 km. This displacement estimate is consistent with other recent estimates of total slip across Owens Valley. If faulting began during the Pliocene, the average slip rate was significantly faster than the current rate. Alternatively, motion could have been episodic and have begun as early as the Late Cretaceous. [ABSTRACT FROM AUTHOR]

Correlation of continental shelf-slope stratigraphy in Santa Monica Bay (southern California) with Ocean Drilling Program records for nearby slope-basin sites has illuminated the timing and scale of terrigenous sediment dispersal on margin since the Last Glacial Maximum (LGM). Marine flooding surfaces preserved in a transgressive sequence on the Santa Monica Shelf provide a key link between base-level elevation and sediment transport across shelf. Sediment-accumulation rates at slope-basin sites were maximal ca. 15-10 ka, well after the LGM, decreased during the 12-9 ka transition from fluvial-estuarine to fully marine conditions on the shelf, and decelerated throughout the Holocene to 30%-75% of their values at the LGM. The deceleration is interpreted to manifest a landward shift in the margin depocenter with the onset of transgressive sedimentation beginning when sea level surmounted the shelf edge ca. 13 ka, as predicted by sequence-stratigraphic models. However, the records make clear that factors other than base level modulated slope-basin accumulation rates during the deglaciation. [ABSTRACT FROM AUTHOR]

The sediments of Santa Barbara Basin, a valuable source of information about the history of the California Current, show cyclic deposition in three properties here investigated: varve thickness, total organic carbon (TOC) accumulation, and fish-scale abundance. We analyzed the record since A.D. 1350 for varves, A.D. 1120 for TOC, and A.D. 1200 for fish scales. Prominent periods are 169 yr, 93 yr, 51.4 yr, 17.7 yr (varve thickness), 124 yr, 56 yr, 37.4 yr (TOC flux), and 103 yr, 128 yr (fish-scale abundance). A surprising number of the varve and TOC cycles can be interpreted as deriving from the common tidal cycles, i.e., the 4.425 yr perigee period (describing the changing distance between Earth and Moon) and the 18.6 yr lunar nodal period (which pertains to the lineup of Earth, Moon, and Sun). We suggest that the record of the basin is partly a result of pulsed sedimentation aligned with tides. These findings support the hypothesis that a considerable portion of sediment destined for deposition in the center of the basin goes through a phase of shallow storage on the shelf where it can be remobilized through storms and tides. Mechanisms calling on tidally pulsed productivity or tidally influenced climate change are not excluded, but are considered less attractive than remobilization. Fish-scale cycles may reflect the influence of solar forcing. [ABSTRACT FROM AUTHOR]