Paleomagnetism of the Pleistocene Tequila Volcanic Field (Western Mexico)

Abstract

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• ,• , 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.

Evidence for geomagnetic excursions recorded in Brunhes and Matuyama Chron lavas from the trans‐Mexican volcanic belt

“…Ceja et al . [] obtained a rather poorly defined result based only on three accepted, but four rejected, samples ( D = 290.5°, I = 14.5°, α 95 = 14.5°, n/N = 3/7, P lat = 24.9°; site TM10 in their publication) and interpreted the direction as intermediate. Using the quoted GPS coordinates, which are identical to those given by Lewis‐Kenedi et al .…”

“…Choosing the correct cutoff angle ( λ cut ) to separate normal PSV from an intermediate/transitional geomagnetic regime is still a matter of debate. Some previous paleomagnetic studies from Mexico [ Petronille et al ., , Ceja et al ., , Conte et al ., ] combined two parameters: (1) a VGP latitude lower than 60° and (2) an angular distance from the mean direction exceeding 30°. We chose a cutoff angle of 40° from the north geographic pole, which corresponds to a VGP latitude of 50°.…”

“…[] and Ceja et al . [] may be associated with geomagnetic excursions and events. Representative demagnetization data of sites that are discussed are shown in Figures and to demonstrate that the ChRM has been reliably determined, even in cases where relatively strong instable components were present, like in sample EN_4.1.…”

“…Ceja et al . [] linked three lavas with intermediate to reversed magnetizations to the Levantine excursion [ Ryan , ; Biswas et al ., ] and the Delta excursion [ Creer et al ., ], respectively.…”

“…[] and Ceja et al . [] by identifying the drill holes, but in other cases, this was not possible using the published coordinates.…”