HR: 1330h
AN: GP13B-06    [Abstracts]
TI: Paleomagnetism of Basalt Flows near McKenzie Bridge, Oregon
AU: Valentine, M J
EM: mvalentine@ups.edu
AF: Geology Department University of Puget Sound, 1500 N.Warner, Tacoma, WA 98416 United States
AB: A section of twelve Quaternary basalt flows near McKenzie Bridge, Oregon was sampled in July 2004 for paleomagnetic analysis. The thick sequence was chosen for study because field surveys suggest it is the proper age to potentially record the Matuyama-Brunhes transition, and field measurements with a fluxgate magnetometer indicated the presence of both normal and reversely magnetized units (Conrey, pers. comm.). At each site, six to ten oriented cores were collected and analyzed to determine the mean magnetic direction and Virtual Geomagnetic Pole (VGP) position. Standard alternating field demagnetization to 999 oe and thermal demagnetization to 680° C were performed to remove secondary magnetization and get clues to magnetic mineralogy. Eleven of the twelve sites yielded VGPs that cluster in Eastern Siberia. The mean inclination and declination of these 11 sites are 59.5° and 328.3° with a95 = 2.95° . The remaining flow yielded radically different directions from the other sites (D = 278.8°, I = 8.7° ) and a VGP just north of Australia (lat. = 9.4° S long. = 145.0° ). Further investigation of this site needs to be done in order to ensure that the outcrop is in place and yielding a reliable magnetic direction. Changes in intensity during thermal demagnetizations suggest titano-magnetite as the main carrier of magnetic signal. SEM and optical examination will be used to confirm magnetic mineralogy. Assuming a Geocentric Axial Dipole (GAD) field, the expected mean direction for the McKenzie Bridge area is D = 0°, I = 63.4° . The difference in the expected and mean directions for the 11 sites can be explained in several ways. The area may have undergone counterclockwise rotation since these lavas were erupted, or the units sampled may not represent a long enough time period to adequately average paleosecular variation. The low-latitude VGP of the other site suggests an intriguing possibility; that the section does record a reversal. The VGP for this site and VGPs for the other 11 sites fall along one of the preferred longitudinal VGP paths for reversals suggested by several researchers (eg. Love, 1998) and in Hoffman's (1996) patches. Paleomagnetic analyses of 12 flows sitting atop the section studied here reveal similar Siberian VGP locations (LeValley and Valentine, this volume). Further study of flows lying below both of these should clarify the cause of the directional anomaly.
DE: 1520 Magnetostratigraphy
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly