PRELIMINARY PALEOMAGNETIC AND ROCK MAGNETIC DATA FROM THE SPANISH PEAKS, SILVER MOUNTAIN, ASSOCIATED DIKE SWARMS AND RELATED INTRUSIONS (SOUTH-CENTRAL COLORADO)

Exposed in the northernmost Raton Basin, east of the Culebra Range in the Sangre de Cristo Mountains, south-central Colorado, the Spanish Peaks, Silver Mountain, associated dike swarms and other intrusions intrude sub-horizontal Paleogene strata in the Raton Basin. West Spanish Peak and East Spanish Peak are the largest exposed stocks of the late Eocene to early Miocene volcanic center (4153 meters and 3866 meters elevation respectively). Other intrusions include two radial dike swarms (one focused on Silver Mountain, the other, larger swarm centered on East Spanish Peak), a set of sub-parallel N80E trending dikes, and a set of “independent” dikes, sills and stocks emplaced parallel to strata of the Sangre de Cristo Mountains.

We have collected intrusions at 47 sites and completed AF and pilot thermal demagnetization on 37 of the sites. Normal polarity dominates the magnetizations characteristic of this preliminary data set. The grand mean (Decl.=004, Incl.=62, á95=6o, k=29) for 21 sites includes three reverse polarity site means. Previous work on some of these rocks (Larson and Strangway,1969) resulted in a grand mean (Decl.=351, Incl.=63, á95=13o, k=27) from only five sites all of normal polarity. The large number of discrete intrusions in this area promises to yield a robust and high-quality data set that, with a complete analysis, will result in a refined mid-Cenozoic paleopole for the North American craton.

Magnetic fabric data have been obtained from 19 of the sites using anisotropy of magnetic susceptibility (AMS) methods. The range in the degree of anisotropy is 0.7% to 2.6%, with an average of 1.6%. Magnetic fabrics, assumed to be a proxy for magma flow, are varied. Five well-grouped sites have K1 directions (axis of maximum susceptibility) that are distributed with regard to both declination and inclination, lying in the plane of the dike. Other sites either show a dominantly horizontal K1 direction with a distribution of declinations or a bimodal distribution of K1 populations. The nine bimodal sites have K1 directions with distributed declinations but with inclinations grouped either vertically or horizontally, also lying in the plane of the dike. This complex relationship between magnetic fabrics and magma flow is the focus of ongoing rock magnetic work.