Paleomagnetic evidence for timescales of multi-vent basaltic eruptions in Big Pine Volcanic Field, California

Over 280 oriented core samples from basaltic lava flows (47 sites) and one rhyolite dome (2 sites) were collected for paleomagnetic analysis from flows in the Big Pine Volcanic Field (BPVF) in Owens Valley, California. Samples were subjected to standard laboratory demagnetization procedures and statistical analyses, indicating that the 47 sites within basalt flows record normal polarity and the rhyolite dome records reversed polarity. Site mean Virtual Geomagnetic Pole (VGP) calculations were grouped in two different ways to calculate a paleopole and associated angular dispersion value (S) and an Alpha 95 confidence limit (A95) for the entire BPVF. The first calculation treats all 49 sites as individual flows and the second calculation was obtained by grouping the 49 sites into 12 distinct lava flow units based on field relationships. The paleopoles for the respective data sets are: 75.0�N, 166.9�E and 74.2�N, 150.2�E. The angular dispersion value for the data set with all 49 sites is 15.4� with A95 = 3.8�. The second set of data yields an S value of 16.4� and an A95 of 8.2�. The angular dispersion values from both data sets are within the expected range at the site latitude of 37�N (15�-17�), indicating that these data sets adequately sampled lavas that erupted over a long enough period of time to have recorded geomagnetic secular variation (103 to 105 years). The long eruption history implied by these results suggests that the BPVF is polygenetic, supported by geochronologic ages determined by other workers of 1.2 Ma to 11.8 ka. Individual sampling sites were also grouped and examined on the basis of previous geochronologic studies. Angular dispersion values (S) for four dated flow units are less than the expected range given the site latitude, indicating that they all erupted within a relatively short (100 years) period of time and are perhaps monogenetic. The angular dispersion value for a fifth dated flow is 15. 9�, which is within the range of expected values given the site latitude, suggesting that this cone erupted episodically over at least 103 to 105 and is polycyclic. These results support the notion that the BPVF is a polygenetic field composed of individual volcanoes that generally experience monogenetic eruptions.