HR: 1340h
AN: V23B-1438    [Abstracts]
TI: Establishing the Limits of Yellowstone Hotspot Volcanism at the Time of the Steens Mountain Reversal
AU: * Jarboe, N A
EM: njarboe@pmc.ucsc.edu
AF: University of California-Santa Cruz, Earth and Planetary Sciences Department 1156 High St., Santa Cruz, CA 95064, United States
AU: Coe, R S
EM: rcoe@pmc.ucsc.edu
AF: University of California-Santa Cruz, Earth and Planetary Sciences Department 1156 High St., Santa Cruz, CA 95064, United States
AU: Glen, J M
EM: jglen@usgs.gov
AF: US Geological Survey, MS989 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, United States
AB: One of the best records of magnetic field behavior during a geomagnetic polarity reversal is found in the Steens basalts of the Oregon Plateau. Lavas preserving transitional field directions were first discovered near the top of a 1000 m flow-on-flow section at Steens Mountain, Oregon. Other Steens lavas of equivalent age (16.5 Ma) recording transitional field paths consistent with the record at Steens Mtn. are also found at Catlow Peak (70 km to the SSE) and at the Poker Jim Ridge (80 km to the W). These lavas are the earliest eruptions of the Columbia River Basalt Group and field relationships show they are the equivalent to the R0-N0 Imnaha members. Although others have found the Imnaha Basalt to be older than the Steens basalts, our 40Ar/39Ar age of 16.62+-0.28 Ma (2 sigma) from a normally magnetized lava near the top of a reverse to normal (R-N) polarized section of Imnaha Basalt at Squaw Butte, Idaho (225 km to the NE of Steens Mtn.) indicates Steens and Imnaha basalts are coeval. This section is known to have one lava recording a transitional field direction and it is highly likely this flow erupted during the Steens reversal. Therefore, Yellowstone Hotspot Volcanism (YHV) was occurring at locations at least 300km apart during the relatively short 4000 year duration of the Steens reversal. Some authors have suggested extending YHV during the Steens reversal further south. A Steens like series of flows in the Santa Rosa range (125 km to the SE of Steens Mtn.), show a reversal path similar to Steens. Our preliminary 40Ar/39Ar age of 18.09+-0.26 Ma (2 sigma) from a plagioclase separate suggest the lavas are older than the Steens reversal. As the stratigraphy of the section is complicated by many dikes of undetermined age, we have sampled the section to reexamine the reversal path. At Sheep Creek, Nevada (260 km to the SW of Steens Mtn.) a volcanic section with Steens-like basalts record a transitional-to-N geomagnetic polarity change with transitional field behavior similar to Steens. While a date by others (15.58+-.20 Ma, 2 sigma) of whole rock basalt from the bottom lava of the section excludes the possibility that the Steens reversal is found at Sheep Creek, whole rock dating is fraught with problems and leaves open the possibility that eruptions occurred at this distal southern location during the Steens reversal.
DE: 1115 Radioisotope geochronology
DE: 1520 Magnetostratigraphy
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1535 Reversals: process, timescale, magnetostratigraphy
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting