Further constraints for the Plio-Pleistocene geomagnetic field strength: New results from the Los Tuxtlas volcanic field (Mexico)

Abstract

A rock-magnetic, paleomagnetic and paleointensity study was carried out on 13 Plio-Pleistocene volcanic flows from the Los Tuxtlas volcanic field (Trans Mexican Volcanic Belt) in order to obtain some decisive constraints for the geomagnetic field strength during the Plio-Pleistocene time. The age of the volcanic units, which yielded reliable paleointensity estimates, lies between 2.2 and 0.8 Ma according to the available K/Ar radiometric data. Thermomagnetic investigations reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution that probably occurred during the initial flow cooling. Unblocking temperature spectra and relatively high coercivity point to 'small' pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Six flows yield reverse polarity magnetization, five flows are normally magnetized, and one flow shows intermediate polarity magnetization. Evidence of a strong lightning-produced magnetization overprint was detected for one site. The mean pole position obtained in this study is Plat = 83.7• , Plong = 178.1• , N = 10 and the corresponding mean paleodirection is I = 31.3• , which is not significantly different from the expected direction estimated from the North American apparent polar wander path. Thirty-nine samples were pre-selected for Thellier palaeointensity experiments because of their stable remanent magnetization and relatively weak-within-site dispersion. Only 21 samples, coming from four individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 6.4 to 9.1 × 10 22 Am 2 . Combining the coeval Mexican data with the available comparable quality Pliocene paleointensity results yield a mean VDM of 6.4×10 22 Am 2 , which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and are of dissimilar quality. Additional high-quality absolute intensity determinations are needed to better constraint the geomagnetic field strength during the Plio-Pleistocene time.

Variations of the geomagnetic dipole magnitude over the past 400 My

“…The distribution obtained (Fig. 2), to a significant degree, refines the previous data from [4,5,10] about the variations of the VDM in this interval.…”

Paleomagnetic Study of Lavas from the Popocatepetl Volcanic Region, Central Mexico

“…Alva-Valdivia et al (2001) presented paleointensity results for volcanic units in the Los Tuxtlas field, eastern Mexico. K-Ar dating indicates that units studied range from 2.2 Ma to Fig.…”

Geomagnetic field strength during late Miocene: First paleointensity results from Baja California

“…Squares denote to the mean value of several consecutive lava flows (this study, large symbols). For Plio‐Pleistocene time, however, some points represent the results from single cooling units [see Alva‐Valdivia et al , 2001]. …”

“…While procedures for determining the direction of the paleomagnetic field are now more or less standardized, there still exist significant inter‐laboratory differences on how to best obtain reliable estimates of the paleomagnetic field intensity [ Valet and Herrero‐Bervera , 2000]. The Thellier and Thellier technique and all derived approaches, among which Coe et al 's [1978] modification received the largest audience in last years [ Juarez et al , 1998; Selkin and Tauxe , 2000; Alva‐Valdivia et al , 2001; Morales et al , 2001]. This technique was used in present study to determine absolute geomagnetic paleointensity.…”