Angela S. Jayko

Surface slip during large Owens Valley earthquakes

Data sets and expanded results contributing to this study are available in the supporting information. The EarthScope Southern and Eastern California Lidar Project (available online at http://opentopo.sdsc.edu) involved data acquisition and processing for the Plate Boundary Observatory (PBO) by NCALM (http://www.ncalm.org). UNAVCO operates the PBO for EarthScope (http://www.earthscope.org), supported by the National Science Foundation (EAR-0350028 and EAR-0732947). Funding for this study was provided by the Southern California Earthquake Center (SCEC) (Project 12140), the Geological Society of America Graduate Student Research fund, the Community Foundation of San Bernardino county, and the Western Washington University Geology Department. We thank G. Seitz, M. Price, and K. Morgan for assistance in the field, and S. Bacon, J. Arrowsmith, R. Weldon, K. Scharer, J. Unruh, C. Madden-Madugo, and D. Haddad for helpful discussions. Constructive reviews by D. Schwartz, R. Briggs, E. Schermer, D. Clark, and one anonymous reviewer substantially improved the paper. We also thank the staff at the UC White Mountain Research Center for facilitating this work. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Holocene and Latest Pleistocene Oblique Dextral Faulting on the Southern Inyo Mountains Fault, Owens Lake Basin, California

The Inyo Mountains fault (imf) is a more or less continuous range-front fault system, with discontinuous late Quaternary activity, at the western base of the Inyo Mountains in Owens Valley, California. The southern section of the imf trends ∼N20°–40° W for at least 12 km at the base of and within the range front near Keeler in Owens Lake basin. The southern imf cuts across a relict early Pliocene alluvial fan complex, which has formed shutter ridges and northeast-facing scarps, and which has dextrally offset, well-developed drainages indicating long-term activity. Numerous fault scarps along the mapped trace are northeast-facing, mountain-side down, and developed in both bedrock and younger alluvium, indicating latest Quaternary activity. Latest Quaternary multiple- and single-event scarps that cut alluvium range in height from 0.5 to 3.0 m. The penultimate event on the southern imf is bracketed between 13,310 and 10,590 cal years b.p., based on radiocarbon dates from faulted alluvium and fissure-fill stratigraphy exposed in a natural wash cut. Evidence of the most recent event is found at many sites along the mapped fault, and, in particular, is seen in an ∼0.5-m northeast-facing scarp and several right-stepping en echelon ∼0.5-m-deep depressions that pond fine sediment on a younger than 13,310 cal years b.p. alluvial fan. A channel that crosses transverse to this scarp is dextrally offset 2.3 ± 0.8 m, providing a poorly constrained oblique slip rate of 0.1–0.3 m/k.y. The identified tectonic geomorphology and sense of displacement demonstrate that the southern imf accommodates predominately dextral slip and should be integrated into kinematic fault models of strain distribution in Owens Valley.

Chronology of tectonic, geomorphic, and volcanic interactions and the tempo of fault slip near Little Lake, California

New geochronologic and geomorphic constraints on the Little Lake fault in the Eastern California shear zone reveal steady, modest rates of dextral slip during and since the mid-to-late Pleistocene. We focus on a suite of offset fluvial landforms in the Pleistocene Owens River channel that formed in response to periodic interaction with nearby basalt flows, thereby recording displacement over multiple time intervals. Overlap between 40 Ar/ 39 Ar ages for the youngest intracanyon basalt flow and 10 Be surface exposure dating of downstream terrace surfaces suggests widespread channel incision during a prominent outburst flood through the Little Lake channel at ca. 64 ka. Older basalt flows flanking the upper and lower canyon margins indicate localization of the Owens River in its current position between 212 ± 14 and 197 ± 11 ka. Coupled with terrestrial light detection and ranging (lidar) and digital topographic measurements of dextral offset, the revised Little Lake chronology indicates average dextral slip rates of at least ∼0.6–0.7 mm/yr and 4 to 10 5 yr. Despite previous geodetic observations of relatively rapid interseismic strain along the Little Lake fault, we find no evidence for sustained temporal fluctuations in slip rates over multiple earthquake cycles. Instead, our results indicate that accelerated fault loading may be transient over much shorter periods (∼10 1 yr) and perhaps indicative of time-dependent seismic hazard associated with Eastern California shear zone faults.

Refining the Southern Extent of the 1872 Owens Valley Earthquake Rupture through Paleoseismic Investigations in the Haiwee Area, Southeastern California

Recent upward revision of the 1872 Owens Valley earthquake from Mw 7.4-7.5 to 7.7-7.9 implies either additional unrecognized rupture length or anoma- louslystronggroundmotionsassociatedwiththisevent.Weinvestigatethefirstpossibility through paleoseismic trenching south of the mapped surface rupture in the Haiwee area, where historical accounts suggest significant surface deformation following the earth- quake. Trenching focused on a prominent north-striking scarp, herein termed the Sage Flat fault, expressed in Pleistocene alluvial fans east of Haiwee Reservoir. Surficial map- ping and ground-based Light Detection and Ranging (lidar) surveying suggest that this faultaccommodateseast-downnormalmotion,andpossiblyacomparableamountofdex- tralslip.Trenchingandluminescencedatingbracketsthetimingofthemostrecentsurface- rupturing earthquake between ∼25:7 and 30.1 ka, and provides evidence for an earlier event predating this time. In combination with scarp profiling, these dates also suggest am aximum rate of normal, dip-slip fault motion up to∼0:1 mm=yr over this period. Although we discovered no evidence for recent surface rupture on the Sage Flat fault, a series of subvertical fractures and fissures cut across young trench stratigraphy, consis- tentwithsecondarydeformationassociatedwithseismicshaking.Assuch,wesuggestthat possible ground disturbance in the Haiwee area during the 1872 event primarily reflected ground shaking or liquefaction-related deformation rather than triggered slip. In addition, we infer a structural and kinematic connection between the Owens Valley fault and oblique-dextral faults north of Lower Cactus Flat in the northwestern Coso Range, rather thanawest-stepintonorthernorwesternRoseValley.Considerationofthesestructuresin the total extent of the Owens Valley fault suggests a length of 140 km, of which at least 113 km ruptured during the 1872 event. Online Material: Procedural details and expanded results from the OSL sample analyses, as well as high-resolution paleoseismic trench logs.

A MACTRID BIVALVE FROM PLEISTOCENE DEPOSITS OF LAKE RUSSELL, MONO BASIN, CALIFORNIA

Rangia des Moulins , 1832 is a small genus of mactrid bivalves that is currently distributed in estuarine waters of the eastern United States, Gulf of Mexico, and Gulf of California (Keen, 1971; Abbott, 1974). (One congener, R. cuneata [Sowerby, 1831], was recently introduced to the Antwerp (Belgium) harbor [Verween et al., 2006].) Although these clams are euryhaline and capable of living in freshwater as adults, they require an estuarine-like salinity regime for successful reproduction and recruitment (Cain, 1973; Hopkins et al., 1974), which has constrained their ability to penetrate the North American continental interior through coastal drainages (Cain, 1974; Swingle and Brand, 1974). The Neogene and Quaternary fossil record of the genus is also restricted to coastal or near-coastal marine-influenced depositional systems, with the exception of Holocene specimens of R. cuneata from two archeological sites in the central United States which were obviously introduced by humans (Baker, 1941; Hill, 1983), and a Pleistocene(?) occurrence of this species from along the Pecos River in New Mexico (more than 800 km from the sea) which has been attributed to transport of Gulf Coast immigrants on waterfowl (Metcalf, 1980; Taylor, 1985). Here we provide fossil evidence that the biogeographic history of this predominantly brackish-coastal genus also includes avian-assisted colonization of a far inland lake in the western United States—Pleistocene Lake Russell, Mono Basin, California (Fig. 1).  Figure 1 —Map showing location of the Mono Basin and other Rangia lecontei (Conrad, 1853) sites (filled circles) The Mono Basin specimens of Rangia described herein were recently discovered by one of us (ASJ) in a sandy deposit closely proximal to beach cobbles, a wave-cut notch and trim line (Fig. 2). This site is in the southeastern portion of the basin near the elevation of …