Variations in transitional magnetic field geometries during the Matayuma-Brunhes reversal: Data from the Tatara-San Pedro volcanic complex, Chilean Andes

Two facets of paleomagnetism are discussed as part of this study; the use of paleomagnetic records to improve our understanding of magnetic field reversals, and the use of paleosecular variation in volcanology studies. A detailed volcanic record of the Matuyama-Brunhes (MBR) reversal (780 ka) suggests that the reversing magnetic field remains dipolar throughout the transition. This record, taken from the Tatara-San Pedro Volcanic Complex located in the Andean region (36$\sp\circ$S, 71$\sp\circ$W) of central Chile, is recorded in 29 andesite flows. The record is exceptionally well constrained temporally and geochemically as well as paleomagnetically. The lavas record a quick change from reversed polarity to a period of transitional stability, with virtual geomagnetic poles (VGP) forming a cluster centered in Australia, followed by a quick swing to normal polarity. The Chilean data demonstrate that the dipole assumes an intermediate subequatorial position before completing the transition. VGP data from the four other available MBR lava records are largely coincident with the Chilean grouping and further strengthen the suggestion of a dipolar Matuyama-Brunhes reversal field geometry. The integration of paleomagnetism with geochemistry offers volcanologists an efficient and independent correlation tool for studying geochemical trends and growth morphology in volcanic centers. The Tatara-San Pedro Volcanic Complex, a Quaternary arc volcanic center in the Chilean Andes (Singer et al., in press), serves as an excellent model of complex stratocone evolution and lends itself well to a demonstration of this technique. Three fundamental relationships between paleomagnetic and geochemical data are demonstrated within the TSPVC: an absolute correlation, a paleomagnetic dependent correlation, and a geochemical dependent correlation. These correlations provide a degree of temporal insight into the rates of magma productivity that would be difficult to gain through conventional radiometric dating techniques