Paleomagnetism of the Pleistocene Tequila Volcanic Field (Western Mexico)

This paper presents new paleomagnetic results from 24 independent cooling units in Tequila area (western Trans-Mexican Volcanic Belt). These units were recently dated by means of state-of-the-art 40 Ar- 39 Ar method (Lewis-Kenedy et al. , 2005) and span from 1130 to 150 ka. The characteristic paleodirections are successfully isolated for 20 cooling units. The mean paleodirection, discarding intermediate polarity sites, is I = 29.6°, D = 359.2°, k = 26, α 95 = 7.1°, n = 17, which corresponds to the mean paleomagnetic pole position P lat = 85.8°, P long = 84.3°, K = 27.5, A 95 = 6.9°. These directions are practically undistinguishable from the expected Plestocene paleodirections, as derived from reference poles for the North American polar wander curve and in agreement with previously reported directions from western Trans-Mexican Volcanic Belt. This suggests that no major tectonic deformation occurred in studied area since early-middle Plestocene to present. The paleosecular variation is estimated trough the study of the scatter of virtual geomagnetic poles giving S F = 15.4 with S U = 19.9 and S L = 12.5 (upper and lower limits respectively). These values are consistent with those predicted by the latitude-dependent variation model of McFadden et al. (1991) for the last 5 Myr. The interesting feature of the paleomagnetic record obtained here is the occurrence of an intermediate polarity at 671 ± 13 ka which may correspond the worldwide observed Delta excursion at about 680–690 ka. This gives the volcanic evidence of this event. Two independent lava flows dated as 362 ±13 and 354 ±5 ka respectively, yield transitional paleodirections as well, probably corresponding to the Levantine excursion.