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Rock-magnetic and paleomagnetic results from the Tepic-Zacoalco rift region (western Mexico) |
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| Authors: | Manuel Calvo-Rathert Bertha Aguilar Reyes Avto Goguitchaichvili José Rosas Elguera Héctor Franco Juan Morales Ruth Soto Ángel Carrancho Hugo Delgado |
| Institution: | 1239. Departamento de Física, Escuela Politécnica Superior, Universidad de Burgos, c/ Francisco de Vitoria, s/n, 09006, Burgos, Spain 2239. Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofísica — Unidad Michoacán, UNAM-Campus Morelia, Morelia, Mexico 3239. CUCEI, Universidad de Guadalajara, Guadalajara, Mexico 4239. Instituto Geológico y Minero de Espa?a, c/ Manuel Lasala 44, 50006, Zaragoza, Spain 5239. Centro Nacional de la Investigación en Evolución Humana, Av. De la Sierra de Atapuerca,s/n, 09002, Burgos, Spain 6239. Departamento de Vulcanología, Instituto de Geofísica, UNAM, Ciudad Universitaria, 04510, México, DF, Mexico |
| Abstract: | A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40Ar/39Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude ? = 81.1°, longitude λ = 94.3°) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed polarity observed in the other site (220 ± 36 ka) very likely provides new evidence for the Pringle Falls excursion or the event recorded in the Mamaku ignimbrite. |
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