HR: 0800h
AN: GP21A-09    [Abstracts]
TI: Paleomagnetism of Pleistocene Lava Flows in the Cascade Range Near McKenzie Bridge, Oregon
AU: * McKee, G
EM: gmckee@ups.edu
AF: University of Puget Sound Geology Dept- CMB 1048, 1500 N. Warner, Tacoma, WA 98416-1048, United States
AU: Valentine, M J
EM: mvalentine@ups.edu
AF: University of Puget Sound Geology Dept- CMB 1048, 1500 N. Warner, Tacoma, WA 98416-1048, United States
AB: Paleomagnetic study of a sequence of dozens of basalt flows near Mackenzie Bridge, Oregon, was undertaken to investigate magnetic field behavior during a magnetic reversal. These basalt flows are believed to be of Pleistocene age and exhibit both normal and reversed polarities (Conrey, pers. comm.), suggesting that they may record the Matuyama-Brunhes magnetic reversal of about 780,000 years ago. In 2006, six flows were sampled and examined. Samples from each flow were demagnetized using stepwise alternating field (AF) demagnetization effectively erasing secondary magnetizations. A minimum of two samples from each site was also thermally demagnetized in a shielded oven for comparison with AF results and to help determine magnetic mineralogy. A mean direction for the six flows has a declination of 334.4° and an inclination of 65.8°. The Virtual Geomagnetic Pole (VGP) indicated by these results shows a latitude of 71.9° and a longitude of 169.7°, located in eastern Siberia. Results are similar to results from two studies performed on 24 flows lying atop these 6 flows (LeValley and Valentine, 2005; White and Valentine, 2005). These data suggest three possible conclusions. First, the upper flows of this sequence of basalts erupted during a narrow window of time and do not adequately average paleosecular variation, indicating the pole was stagnant at this location during eruption. Second, that counter clockwise rotation has occurred since formation, affecting declination but not inclination. Finally, that these flows do depict a portion of a reversal, and that the location of VGPs in Siberia is in accord with models for Siberian preferred paths and patches for poles during reversals. It is hoped that examination of numerous flows underlying those already sampled will help resolve which of these possibilities is most plausible.
DE: 1522 Paleomagnetic secular variation
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly