Abstract
AbstractWe 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°, D=357.9°, k=21, α95=7.3°, with a paleomagnetic pole position ofPlat=85.7°,Plong= 104.5°, K=27, A95=6.4°. 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 × 1022Am2, 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 ofSF= 15.4 withSU= 19.6 andSL= 12.7, which are in agreement with the latitude-dependent PSV model of McFaddenet al.(1988, 1991) for the last 5 Ma, and show no significant inclination anomaly.
Highlights
Study of the spatial and temporal variations of the Earth’s magnetic field and the nature of the time-averaged field have remained central topics for paleomagnetic research
Main Results and Discussion We consider the characteristic paleomagnetic directions determined for the Michoacan-Guanajuato volcanics to be of primary origin
This is supported by the occurrence of both normal and reversed polarities and absence of secondary magnetizations with linear vector plots after thermal and alternating field (AF) demagnetization
Summary
Study of the spatial and temporal variations of the Earth’s magnetic field and the nature of the time-averaged field have remained central topics for paleomagnetic research. TMVB is a 20–150 km wide, 1,000 km long and about 1000–2000 m high volcanic plateau, which roughly extends from the Pacific Ocean to the Gulf of Mexico; it includes high stratovolcanoes (e.g. Popocatepetl, Colima, Pico de Orizaba), shield volcanoes (e.g., Cerro Paracho, Cerro Culiacan, Cerro Yahuarato), monogenetic cinder cone fields (e.g., MGVF, Chichinautzin) and silicic calderas (e.g., Amealco). Six cinder cones have been dated by the 14C method yielding an age between 3,800–29,000 yr BP (Hasenaka and Carmichael, 1985), and other 71 cones seem to correspond to the same time interval (probably within the last 40,000 yr BP) judging from the degree of erosion These young volcanoes only occur in the southern half of the volcanic field (Hasenaka and Carmichael, 1985, 1987). Few samples show two different thermomagnetic phases during heating
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