HR: 1330h
AN: GP13B-07    [Abstracts]
TI: Paleomagnetism in Central Oregon Cascade Basalts
AU: Valentine, M J
EM: mvalentine@ups.edu
AF: Geology Department University of Puget Sound, 1500 N. Warner, Tacoma, WA 98416 United States
AB: Twelve Pleistocene-aged basalt flows near McKenzie Bridge in Central Oregon were sampled and the paleomagnetic stratigraphy recorded in the rocks analyzed. The purpose of the study is to examine tectonic crustal motions since eruption of these flows, paleosecular variation during their eruption, and/or a record of a field reversal. The lavas were chosen because they appear to be the right age to record the Matuyama-Brunhes reversal and record both normal and reverse polarity fields (Conrey, pers comm). Four to six samples from each flow were analyzed using stepwise alternating field demagnetization, effecting a neutralization of any secondary magnetism the rock had recorded since its formation. Two samples from each site were heated and demagnetized in a rock oven to give clues to the magnetic mineralogy of the basalt as well as giving another look at the primary thermal remanent magnetization. Thermal demagnetization data suggests the primary source of magnetism is from titanomagnetite. Samples will be analyzed by scanning electron microscope and reflected light microscopy to determine precise magnetic mineralogy. The mean site direction from all twelve flows shows a field with D = 327.1°, I = 59.8°, and a95 = 5.36°. The Virtual Geomagnetic Pole (VGP) from this data lies in Siberia (long. 151.8°, lat. 65.5° N. The expected mean direction for the McKenzie Bridge area, assuming that all measurements of the field over time will eventually result in a magnetic pole resting at the geographical north pole, would have a declination of 0° and an inclination of 63.4°. Several possibilities could account for the declination difference and low-latitude VGP. The first is that the flows sampled do not represent a long enough period of time for the true average geomagnetic field to be measured, and the paleopole was at the resultant VGP position during the eruption of the flows. The second explanation is that the McKenzie Bridge area basalts may have been rotated counter-clockwise since their formation. Finally, the section may record a portion of a reversal. The Siberian location of the VGPs lies along a suggested longitudinal preferred path for poles during reversals (eg. Love, 1998) and in one of Hoffman's (1996) patches. A concurrent study of underlying basalt flows produced similar results (White and Valentine, this volume). Further study of the rest of this section should help determine 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