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

A rock-magnetic, paleomagnetic and paleointensity study was carried out on 13 Plio-Pleistocene volcanic flows from the Los Tuxtlas volcanic field (Tra ns 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. Th ermomagnetic investigations reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution that probably occ urred during the initial flow cooling. Unblocking temperature spectra and r elatively high coercivity point to 'small' pseudo-single domain magnetic gr ains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Six flows yield reverse polarity magnet ization, five flows are normally magnetized, and one flow shows intermediat e polarity magnetization. Evidence of a strong lightning-produced magnetiza tion overprint was detected for one site. The mean pole position obtained i n this study is Plat = 83.7 degrees, Plong = 178.1 degrees, K = 36, A(95) = 8.1 degrees, N = 10 and the corresponding mean paleodirection is I = 31.3 degrees, D = 352 degrees, k = 37, alpha (95) = 8.2 degrees, which is not si gnificantly 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 m agnetization and relatively weak-with in-site dispersion. Only 21 samples, coming from four individual basaltic lava flows, yielded reliable paleointe nsity estimates with the flow-mean virtual dipole moments (VDM) ranging fro m 6.4 to 9.1 x 10(22) Am-2. Combining the coeval Mexican data with the avai lable comparable quality Pliocene paleointensity results yield a mean VDM o f 6.4 x 10(22) Am-2, which is almost 80% of the present geomagnetic axial d ipole. Reliable paleointensity results for the last 5 Ma are still scarce a nd are of dissimilar quality. Additional high-quality absolute intensity de terminations are needed to better constraint the geomagnetic field strength during the Plio-Pleistocene time.