Low-latitude paleosecular variation and the time-averaged field during the late Pliocene and Quaternary - Paleomagnetic study of the Michoacan-Guanajuato volcanic field, Central Mexico

We report paleomagnetic, rock magnetic, and paleointensity studies for 24 volcanic lava flows from the Michoacan-Guanajuato volcanic field (MGVF), erupted between 2.27 Ma to present according to available radiometric ages and historic records. The MGVF located in the central-western sector of the Plio-Quaternary Trans-Mexican volcanic belt is mainly composed by cinder cones and shield volcanoes. Rock magnetic experiments show remanence is carried in most cases by Ti-poor titanomagnetites, resulting from oxy-exsolution of original titanomagnetites during flow cooling. Unblocking temperature spectra and high coercivities point to "small" pseudo-single domain grains for the titanomagnetites. Single component, linear vector plots are obtained after alternating field and thermal demagnetization. Seven flows yield reverse polarity magnetization while sixteen flows are normally magnetized. The overall mean paleodirection obtained (with 14 flows normal and 6 flows reverse) is I=28.4 degrees, D=357.9 degrees, k=21, alpha(95)=7.3 degrees, with a paleomagnetic pole position of P-lat=85.7 degrees, P-long=104.5 degrees, K=27, A(95)=6.4 degrees. The paleodirection is undistinguishable from expected Plio-Quaternary paleodirections derived from reference poles for the North American polar wander curve, and previously reported paleodirections for central Mexico. Paleointensity experiments give high quality results for only twelve samples from two flows with mean values of 7.3 and 8.1 X 10(22) Am-2, which are close to present geomagnetic field intensity. The combination of new MGVF directional results with currently available paleomagnetic data from central Mexico yield angular dispersion estimates of S-F = 15.4 with S-U = 19.6 and S-L = 12.7, which are in agreement with the latitude-dependent PSV model of McFadden et al. (1988, 199\1) for the last 5 Ma, and show no significant inclination anomaly.