HR: 1340h
AN: GP23A-0035    [Abstracts]
TI: Paleomagnetism of Spanish Peaks, Silver Mountain, Associated Dike Swarms and Related Intrusions (South-Central Colorado): Refining the mid-Cenozoic Reference Paleomagnetic Pole
AU: * Muggleton, S
EM: smuggle@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM 87131
AU: Geissman, J
EM: jgeiss@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM 87131
AU: Wawrzyniec, T
EM: tfw@unm.edu
AF: University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM 87131
AB: Exposed in the northernmost Raton Basin, the late Eocene to mid-Miocene Spanish Peaks igneous complex includes numerous discrete intrusions, with considerable temporal, spatial, chemical and thickness variations. The volcanic center includes a northern radial dike swarm (Silver Mountain), a set of N80E trending 10+ km long dikes, radial dikes centered on the Spanish Peaks intrusions, and a set of dikes, sills and stocks emplaced parallel to tilted strata along the eastern Sangre de Cristo Mountains. We have collected intrusions at 138 sites and have completed AF and thermal demagnetization for 95 of the sites, with 49 sites to date yielding highest quality, internally consistent and interpretable results. Of the remaining 46, many need remagnetization circle analysis, some have dispersed directions within-site, others have low within-site dispersion but deviate significantly from typical dipole directions, and a few are interpreted to be lightning-struck. The 36 normal polarity sites yield a mean of (Decl. = 010°, Incl. = 62°, α95 = 3°, k = 55) and 13 reverse polarity sites yield a mean of (Decl. = 183°, Incl. = -53°, α95 = 7°, k = 38). The grand mean for 49 sites is (Decl. = 008°, Incl. = 59°, α95 = 3°, k = 44). Previous work on some of these rocks by Larson and Strangway (JGR, 1969) resulted in a grand mean (Decl. = 351°, Incl. = 6°, α95 = 13°, k = 27) based on only five sites, all of normal polarity. Our data set passes the McFadden reversals test with a B classification. The VGP grand mean is (81.5N / 307.4E, A95 = 5°, K = 20.5, ASD = 17.9). The current dataset deviates from previously reported estimates of a mid-Cenozoic field direction for North America. Permissible, although not verified, explanations for this discrepancy include incomplete sampling of paleosecular variation or a very modest clockwise rotation of the region. Rock magnetic experiments completed on a representative set of the intrusions indicate for the reliable sites that a geologically stable magnetization is dominantly carried by a mixture of multi-domain and single-domain magnetite.
DE: 1522 Paleomagnetic secular variation
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1532 Reference fields: regional, global
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005