TIMING OF LATE PLEISTOCENE VOLCANISM AT BIG PINE VOLCANIC FIELD: INSIGHTS FROM VOLCANIC STRATIGRAPHY, COSMOGENIC 36CL DATING, AND PALEOMAGNETISM

The Big Pine volcanic field (BPVF) is one of several Quaternary volcanic fields located along the tectonically active Owens Valley of eastern California. Previous geochronology establishes an ~1.3 Ma history of volcanism, but the ages and distribution of volcanic products associated with the youngest eruptions are poorly resolved. To delimit the timing and products of the youngest volcanism at BPVF, we combine field mapping, cosmogenic ³⁶Cl dating, and paleomagnetism of lavas and pyroclastic deposits in the area surrounding the town of Aberdeen where basaltic lavas with well-preserved flow structures and morphology are concentrated. Mapping and petrography reveal that ~ fifteen vents and six principal flow fields occur in the Aberdeen area. Discrete basalt units are distinguishable using phenocryst-xenolith composition and mode, and overlapping flow margins establish the relative ages of most of the flows. For ³⁶Cl dating, samples were taken from pahoehoe flow surfaces, near-vent tumuli, and the tops of meter-scale accretionary lava balls. Using the scaling model of Desilets and Zreda (2003) and assuming erosion rates of <3 mm/ka, ³⁶Cl ages for lava flows from the top, middle, and bottom of the volcanic stratigraphy are ca. 15 ka, 25 ka, and 40 ka, suggesting multiple episodes of late Pleistocene volcanism. These ages agree with cosmogenic ³He and ⁴⁰Ar/³⁹Ar ages from previous studies, and are relatively insensitive to choice of scaling model or erosion rate. Samples from the three stratigraphically youngest flows, which cover much of the area south of Taboose Creek and north of Division Creek, yield ³⁶Cl ages of ca. 15 ka. This youngest episode of volcanism erupted olivine to plagioclase-pyroxene phyric basalt from at least four vents along the Sierran front and separated by up to 5 km. Paleomagnetic directions for the lavas are consistent with the distinct episodes of volcanism implied by the ³⁶Cl ages. The youngest lavas yield nearly indistinguishable paleomagnetic directions, suggesting that the youngest episode may have erupted compositionally diverse basalt over decadal to centurial timescales. The young ³⁶Cl ages and high concentration of vents relative to other areas of BPVF indicate that late Pleistocene volcanism has migrated to the west-central portion of the volcanic field.