Paleomagnetic studies in the Jemez Mountains, New Mexico: A progress report on Quaternary volcanic rocks from Valles Caldera VC-2A, Sulphur Springs, and Lower Permian strata in San Diego Canyon and from VC-2B
John W. Geissman and Hope J. Mullally
Paleomagnetic data from Quaternary volcanic rocks intersected in the VC-2A drilling experiment are interpreted to indicate that, despite considerable alteration in an elevated geothermal gradient, the initial, reverse polarity thermoremanent magnetization of intracaldera and pre-caldera ignimbrites is preserved over much of the 1731 ft sampled interval. In all volcanic materials, magnetite is the principal remanence carrier. Overall, the data are consistent with the interpreted stratigraphy, but many rocks below about 800 ft are partially to completely overprinted, due to hydrothermal alteration, by a positive inclination (normal polarity) remanence interpreted to be less than 780 ka (Brunhes normal polarity chron). Below about 1400 ft, NRM intensities decrease by about an order of magnitude, to between 2 and 7 mA/m. Lower Permian Abo and Yeso Formation strata from the VC-2B experiment yield steep, positive inclination magnetizations as the single or dominant component, suggesting that these rocks have been remagnetized during the Brunhes. The mechanism of remagnetization may involve a combination of heating to no greater than 300°C and changes to the population of hematite grains. The same Permian strata exposed in San Diego Canyon, however, do not show evidence of partial remagnetization related to Quaternary magmatism. Of 16 sites fully demagnetized, 12 yield high-precision results providing a grand mean, corrected for gentle tilt of the strata (Declination= 148°, Inclination= -1.4°, α95= 4.6°, k= 88.7; paleomagnetic pole at 44N, 120E, dp = 2.3°, dm = 4.6°), that is displaced in a small, and statistically indistinguishable clockwise sense (R = 2.6°, ΔR = 3.7°) from an expected Early Permian direction. The San Diego Canyon data are interpreted to indicate that this part of the westernmost margin of the Rio Grande rift did not experience appreciable cumulative vertical axis rotation since the Early Permian.
- Geissman, John W.; Mullally, Hope J., 1996, Paleomagnetic studies in the Jemez Mountains, New Mexico: A progress report on Quaternary volcanic rocks from Valles Caldera VC-2A, Sulphur Springs, and Lower Permian strata in San Diego Canyon and from VC-2B, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 115-120.