Palaeosecular variation in Central Mexico over the last 30 000 years: the record from lavas

Abstract

S U M M A R Y 13 lava flows of known age (ages from I4C dating), which have been erupted in the last 30000 years, have been studied to determine the palaeosecular variation of the geomagnetic field in Central Mexico. Samples were taken from two different monogenetic volcanic fields: the Michoacan-Guanajuato volcanic field (six sites) and the Chichinautzin Formation (seven sites), both part of the Transmexican Volcanic Belt. The lavas were studied in detail using rock magnetic methods (magnetic susceptibility at room temperature, low-temperature susceptibility behaviour, hysteresis loops, Curie temperatures), combined with reflected light microscopy, in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals. The magnetic carriers are titanomagnetites, which show differing degrees of high-temperature deuteric oxidation, and seem to be predominantly pseudo-single domain (PSD), though in many cases are probably a mixture of domain states. Mean palaeomagnetic directions and palaeointensity values using Shaw and Thellier techniques were obtained using several specimens from each flow. Our data seem to indicate a sharp easterly swing in declination about 5000 years ago, which is also observed in lake sediments from Central Mexico. The calculated values of the virtual dipole moment (VDM) range from 3.1 to 14.9 x A m2. Our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr, with a peak at about 9000 years BP. These are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field.

Comprehensive paleomagnetic study of a succession of Holocene olivine-basalt flow: Xitle Volcano (Mexico) revisited

“…Previous PI results of Xitle lava flows Gonzalez et al, 1997;Nagata et al, 1965;Morales-Contreras, 1995;Morales et al, 2003;and UrrutiaFucugauchi, 1996) found very similar mean values to our mean PI estimate (Fig. 11, Table 4).…”

“…(5) Earlier PI studies of the Xitle lava flow disclose intensely varying mean paleointensities (Table 4), ranging from 58 to 77 μT Gonzalez et al, 1997;Nagata et al, 1965;MoralesContreras, 1995;Morales et al, 2003;UrrutiaFucugauchi, 1996). All of these studies were performed on single or several lava flows exposed at dis-tinct places over the flanks of the volcano.…”

Archaeomagnetic intensity results from California and Ecuador: evaluation of regional data

“…In theory, some thermal alteration from this heating can be corrected for [29,30]. In practice, results generally tend to agree with those of the Thellier method, but in several cases have been reported to show more scatter [31,32].…”

Plio-pleistocene paleomagnetic record from the Michoacán-Guanajuato Monogenetic Volcanic Field (Western Mexico)

“…These directions, are practically indistinguishable from the expected Plio-Quaternary paleodirections, as derived from the reference poles for the North American polar wander curve (Besse and Courtillot, 2002) and is in agreement with previous studies and compiled data from the MGVF (Tables 2 and 3, Fig. 7) considering all previously reported studies (Gonzalez at al., 1997;Bohnel and Molina, 2002;Panfil, 1999;Michalk et al, 2010;Maciel Peña et al, 2009). This suggests that no major tectonic deformation occurred in the studied area since the Late Pliocene to the present.…”

“…Table 2. Flow-mean paleodirections of characteristic remanence, location and available isotopic age determinations (1 - Gonzalez et al, 1997, 2 -Bohnel and Molina, 2002, 3 -Panfil, 1999, 4 -Michalk et al, 2010, 5 -Maciel Peña et al, 2009 The mean paleodirection obtained in this study is inclination I = 28.8°, declination D = 354.9°, Fisherian parameters k = 28,  95 = 7.3°, N = 15, which corresponds ( Fig. 6) to the mean paleomagnetic pole position P lat = 83.9°, P long = 321.6°, K = 34, A 95 = 6.6°.…”