Paleomagnetic directions from mid-latitude sites in the southern hemisphere (Argentina): Contribution to Time Averaged Field models.

QUIDELLEUR, X.; CARLUT, J.; TCHILINGUIRIAN P.; GERMA, A.; GILLOT, P.Y.

PHYSICS OF THE EARTH AND PLANETARY INTERIORS

ELSEVIER SCIENCE BV

Año: 2009 p. 199 - 209

0031-9201

Back-arc volcanism located to the east of the Andean Cordillerawas sampled in the Argentina provinces of Mendoza and Neuquen for paleomagnetic time average field and paleosecular investigations. The activity ranges from 2Ma to very recent time, with a large variety of products, from basalts to highly differentiated lavas. After removal of sites affected by lightning, those with ˛95 higher than 10◦, and combining of nearby sites displaying close directions, we present new paleomagnetic results from 31 flows units belonging to two volcanic massifs: the Payun Matru and the Cerro Nevado. Previous and new K–Ar age determinations constrain the volcanic activity of these massifs from 300 to 0 ka, and from 1.9 to 0.9 Ma, respectively.Most paleomagnetic samples have NRM intensities between about 1 and 20A/m and depict progressive removal of magnetization components in a consistent fashion during stepwise AF or thermal demagnetization.Nineteen flows yielded a normal direction (declination = 354.8◦, inclination =−53.0◦, ˛95 = 6.8◦)and 12 flows a reverse direction (declination = 181.0◦, inclination = 52.3◦, ˛95 = 5.9◦). The combined data yielded a mean direction (declination = 357.3◦, inclination =−52.8◦, ˛95 = 4.6◦), which is not statistically different from the axial dipole field (g0 1 ) expected at this latitude (36◦S). The angular dispersion of virtual geomagnetic poles calculated from flows with normal directions (ASD = 16.5◦) compares well with the observed value from global datasets for this site latitude, but flows with reverse directions display a surprisingly lowdispersion (ASD = 12.5◦). Since most reverse directionswere sampled fromflows ranging between 1.9 and 0.9 Ma, this can be interpreted as an interval of low paleomagnetic secular variation.Additional data, also with accurate time constraints, are obviously needed to better support this observation. Finally, no convincing evidence for a complex time average field significantly different from the axial dipole can be supported by this study for the last 2 Myr.