[1] We report a detailed paleomagnetic and rock magnetic study of 17 independent lava flows belonging to the Trans-Mexican Volcanic Belt; 175 oriented samples were collected in the Ceboruco-San Pedro volcanic field. These sites were previously dated by means of a state-of-the-art 40 Ar-39 Ar geochronological method and span from 819 to 2 ka. Rock magnetic experiments, which included continuous susceptibility and hysteresis measurements, point to simple magnetic mineralogy. In most cases, the remanence is carried by Ti-poor titanomagnetite of pseudosingle-domain magnetic structure. Fourteen flows give normal magnetic polarities, while two are reversely magnetized; only one cooling unit yields intermediate paleodirections. The paleodirections of the flow dated at 819 ± 25 ka correspond to a VGP latitude of 18°N. This anomalous field behavior apparently recorded prior to the Matuyama-Brunhes (M-B) reversal may coincide with the transitionally magnetized lavas on La Palma, Canary Islands ( 40 Ar-39 Ar age of 822.2 ± 8.7 ka), and with an event featured in several marine sediment records. Thus this geomagnetic event, defined as M-B precursor, is probably global in extent. Two independent lava flows, dated at 623 ± 91 and 614 ± 16 ka, yield reverse paleodirections. Age uncertainties make it difficult to claim the discovery of a new geomagnetic event. It is possible that these lavas erupted during the worldwide observable Big Lost event ( 40 Ar-39 Ar age of 580.2 ± 7.8 ka), which has probably been longer and more complex than it is generally believed for geomagnetic excursions.

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.

[1] We report a detailed archeomagnetic and micro-Raman spectroscopy investigation on pre-Columbian pottery fragments from Cuanalan (a formative village in the valley of Teotihuacan, central Mexico). Available radiocarbon ages range from 2320 ± 80 to 2060 ± 90 B.P. Continuous low-field susceptibility versus temperature curves performed in air indicate Ti-poor titanomagnetites as magnetization carriers. Few samples, however, show two ferrimagnetic phases with Curie temperatures compatible with both Ti-poor and Ti-rich titanomagnetites. Hysteresis parameter ratios fall essentially in the pseudosingle-domain region, which may indicate a mixture of multidomain and a significant amount of single-domain grains. Mineralogical composition of the Teotihuacan ceramics has been investigated using micro-Raman spectroscopy. Samples are characterized by highly heterogeneous body matrix mineralogy due to the presence of a large variety of minerals with several mineralogical phases. Observed titanomagnetite and magnetite bands shift toward higher wave numbers, confirming a reducing atmosphere and a relatively high temperature (about 800-900°C) during the ceramic production. This definitively indicates the thermoremanent origin of magnetic magnetization. Archeointensity values have been determined from 7 pottery fragments (47 samples) out of 10 (70 samples) analyzed. Anisotropy of thermoremanent magnetization and the cooling rate effect upon thermoremanent magnetization intensity acquisition have been investigated in all the samples. The mean archeointensity values obtained in this study range from 24.2 ± 3.2 to 40.0 ± 1.7 mT, and corresponding virtual axial dipole moments range from 4.8 ± 0.6 to 8.0 ± 0.4 (10 22 A m 2 ). This corresponds to a mean virtual dipole moment value of 5.9 ± 1.1 Â 10 22 A m 2 , which is lower than the present-day field strength and the predicted values by global models and the latest data compilation. However, our data agree well with currently available absolute intensity values from Mesoamerica. The archeointensity values uncorrected for cooling rate and anisotropy are systematically higher than the corrected values. These uncorrected values agree with the CALS7K model, which may be biased by the fact that such corrections were not applied to most of the previous data.

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