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Detailed Reference Information |
Alva-Valdivia, L.M., Goguitchaichvili, A. and Urrutia-Fucugauchi, J. (2001). Further constraints for the Plio-Pleistocene geomagnetic field strength: New results from the Los Tuxtlas volcanic field (Mexico). Earth Planets and Space 53(9): 873-881. doi: 10.5636/eps.53.873. |
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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 degrees, Plong = 178.1 degrees, K = 36, A (sub 95) = 8.1 degrees, N = 10 and the corresponding mean paleodirection is I = 31.3 degrees, D = 352 degrees, k = 37, alpha (sub 95) = 8.2 degrees, 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 9.1X10 (super 22) Am (super 2). Combining the coeval Mexican data with the available comparable quality Pliocene paleointensity results yield a mean VDM of 6.4X10 (super 22) Am (super 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. |
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Abstract |
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Table 1 |
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Table 2 |
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Table 3 |
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Demagnetization |
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Susceptibility Curves |
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Hysteresis Measurements |
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Paleointensity |
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Keywords
basalts, Cenozoic, dipole moment, glasses, igneous rocks, Los Tuxlas, volcanic field, magnetic field, magnetic intensity, magnetic, properties, magnetization, marine environment, Mexican volcanic belt, Mexico, Neogene, oxides, paleomagnetism, Pleistocene, Pliocene, pole, positions, Quaternary, submarine environment, Tertiary, titanomagnetite, volcanic rocks, 18, Solid-earth geophysics |
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Earth Planets and Space |
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