Paleomagnetism of the Eastern Alkaline Province (Mexico): contribution to the time-averaged field global database and geomagnetic instability time scale

We report a detailed paleomagnetic and rock-magnetic study of 19 independent lava flows belonging to the Eastern Alkaline Province (EAP) in Mexico. In total, 162 oriented samples were collected in four areas (Sierra de Tantima-Alamo, Tlanchinol, Chiconquiaco-Palma Sola and Poza Rica). All sites analyzed in this study ere previously dated by means of the unspiked K-Ar geochronological method (Ferrari et al., J. Volcanol. Geotherm. Res., 146, 284-306, 2005) and span from 14.6 to 1.5 Ma. Rock-magnetic experiments point to simple magnetic mineralogy. In most cases, the remanence is carried by Ti-poor titanomagnetite of pseudo-single-domain grain size. In a few cases, Ti-rich titanomagnetites are responsible for the magnetization. The characteristic paleodirections are successfully isolated for all of the studied units. The mean paleodirection, discarding two intermediate polarity sites, is D=359.5 degrees, I=32.9 degrees, n=17, k=30, alpha(95)=6.7 degrees. This direction is practically undistinguishable from the expected Mio-Pliocene paleodirections, as derived from reference poles for the North American polar wander curve, and is in agreement with the previously reported directions from the western Trans-Mexican Volcanic Belt. This suggests that no major tectonic rotation occurred in studied area since the middle Miocene to Present. The paleosecular variation is estimated through the study of the scatter of virtual aeomagnetic poles, giving S-F=12.7 with S-U = 16.5 and SL = 10.3 (upper and lower limits, respectively). These values are consistent with those predicted by the latitude-dependent variation model of McFadden ct al. (Geophys. Res., 93, 11583-11588, 1991) for the last 5 Myr. Eleven sites yielded reverse magnetic polarity, six are normally magnetized and two lava flows provided apparently intermediate paleodirections. An interesting feature of the paleomagnetic record obtained from EAP volcanics is that two independent lava flows, dated as 2.04 +/- 0.04 and 1.97 +/- 0.04 Ma, respectively, both yield normal paleodirections during the reverse Matuyarna period. These sites were apparently formed during the Reunion geomagnetic event worldwide observed.