Ana Maria Soler-Arechalde

Archaeomagnetic investigation of oriented pre-Columbian lime-plasters from Teotihuacan, Mesoamerica

Results of an archaeomagnetic study of two excavation field seasons at the Xalla and Teopancazco residential areas of the ancient Prehispanic city of Teotihuacan, Central Mexico are reported. One-hundred and fifty three oriented samples of lime-plasters from the two archaeological sites were collected for the study. NRM directions are reasonably well grouped. Alternating field demagnetization shows single or two-component magnetizations. Rock magnetic measurements point to fine-grained titanomagnetites with pseudo-single domain behaviour. Characteristic site mean directions from both sites are correlated to the available palaeosecular variation curve for Mesoamerica. The mean directions obtained from Xalla site point to average dates of 550 AD±25 years that matches with the documented ‘Big Fire’ of Teotihuacan (AD 575). Two consecutive construction levels at Teopancazco were estimated as AD 250–350 and AD 350–425 respectively.

Stratigraphy, K-Ar ages, and magnetostratigraphy of the Acambay graben, central Mexican Volcanic Belt

The Acambay graben is an east-west intraarc tectonic depression in the central sector of the Mexican Volcanic Belt. The graben is 80 km long and 15‐38 km wide, bounded at the south by the Pastores and Venta de Bravo faults, and at the north by the Acambay-Tixmadeje and Epitacio Huerta faults. Two representative sections were measured, one in the northern wall of the graben on the Epitacio Huerta fault (Amealco caldera section), and the other in the southern wall on the Venta de Bravo fault (Tlalpujahua section). These two sections were documented with K-Ar ages and paleomagnetic measurements. The units exposed in the northern section are all volcanic and within a range of 4.7 to 2.2 Ma. The southern section includes metasedimentary rocks that could be Cretaceous (no fossils have been found) and the Miocene-Quaternary volcanic succession. The southern section has a K-Ar age of 4.7 ± 0.2 Ma and reverse polarity for a major ignimbrite of Las Americas Formation. This formation is correlated with the Amealco tuff of the northern section, which is the only map unit widespread enough to be exposed on either side of the Acambay graben. Paleomagnetic studies concentrated on the widespread ignimbrites, such as those within the Amealco and Huichapan tuffs, because of their potential for magnetostratigraphy. The K-Ar and paleomagnetic data are consistent within analytical uncertainties. The Amealco tuff sequence includes three major ignimbrites. The oldest ignimbrite, Amealco I, has a reverse polarity and a K-Ar age of 4.7 Ma; Amealco II has a normal polarity and a K-Ar age of about 4.7 Ma; and Amealco III has a normal polarity and a K-Ar age of about 4.6 Ma. The ignimbrite of the Huichapan tuff has a reverse polarity and a K-Ar age of 3.5 Ma. There is a predominance of reverse polarity for units within the Acambay graben. Magnetostratigraphy is used as another line of evidence for the correlation of Amealco tuff and Las Americas Formation, and proved useful to demonstrate that the Amealco caldera was the source for the Amealco tuff and Las Americas ignimbrites.

Paleomagnetic and rock-magnetic study on volcanic units of the Valsequillo Basin: implications for early human occupation in central Mexico

Alleged human and animal footprints were found within the upper bedding surfaces of the Xalnene volcanic ash layer that outcrops in the Valsequillo Basin, south of Puebla, Mexico (Gonzalez et al, 2005). The ash has been dated at 40 ka by optically stimulated luminescence analysis, thereby providing new evidence that America was colonized earlier than the Clovis culture (about 13.5 Ma). We carried out paleomagnetic and rock magnetic analysis on 18 Xalnene ash block and core samples collected at two distinct localities and 19 standard paleomagnetic cores belonging to nearby monogenetic volcanoes. Our data provide evidence that both the volcanic lava flow and Xalnene ash were emplaced during the Laschamp geomagnetic event spanning from about 45 to 39 ka.

A detailed paleomagnetic and rock-magnetic investigation of the Matuyama-Brunhes geomagnetic reversal recorded in the tephra-paleosol sequence of Tlaxcala (Central Mexico)

Geomagnetic reversals are global phenomena, for about 50 years the paleomagnetists attempted to acquire as many detailed records as possible using the magnetic memory of sediments and lava flows. Yet, transitional field behavior remains poorly characterized largely because of sporadic aspect of volcanic eruptions. In some specific cases, paleosols such as those developed from alluvial or aeolian sediments, may also record the variations of the Geomagnetic Field across the polarity changes. Here, we report a detailed paleomagnetic and rock-magnetic investigation on some radiometrically dated chromic luvisols located in Central Mexico carrying detrital or chemical remanent magnetization. The research was developed in order i) to demonstrate the primary origin of the magnetic remanence and ii) to show that paleosoils are good candidates to provide a high resolution record of the behavior of geomagnetic field during reversals. The lower part of the paleosoil sequence shows a clearly defined reverse polarity magnetization followed by geomagnetically unstable transitional field and ended by normal polarity remanence. Our AMS and rock magnetic data suggest that magnetization is acquired during the initial stage of soil formation in context of active volcanic activity since magnetic fabric is essentially sedimentary and reverse and normal polarity paleodirections are almost antipodal. Titanomagnetites are identified as main magnetic carriers of rock-magnetic measurements including thermomagnetics and hysteresis cycles. We propose that the transition recorded in this study correspond to the B-M boundary, considering the K-Ar datings available at the sequence bottom and that the chromic luvisols are potentially good recorders of the paleosecular variation. The identification of the B-M boundary within the studied sequence has fundamental significance for improving the chronological scale of Tlaxcala paleosol-sedimentary sequence and its correlation with the global proxies.