HR: 1340h
AN: U33A-0029    [Abstracts]
TI: New Tertiary Paleomagnetic Poles at 13 and 30 Ma from Mongolia: Clues on the Inclination Shallowing Problem in Central Asia
AU: * Hankard, F
EM: hankard@ipgp.jussieu.fr
AF: Laboratoire de Pal\'{e}omagn\'{e}tisme, Institut de Physique du Globe de Paris, 4, Place Jussieu, Paris, 75252 France
AU: Cogn\'{e}, J
EM: cogne@ipgp.jussieu.fr
AF: Laboratoire de Pal\'{e}omagn\'{e}tisme, Institut de Physique du Globe de Paris, 4, Place Jussieu, Paris, 75252 France
AU: Kravchinsky, V
EM: vkrav@phys.ualberta.ca
AF: University of Alberta, Physics Department, Edmonton, AB T6G2J1 Canada
AB: Construction of APWPs of different Asian blocks has been particularly active in the last past 20 years, in order to unravel the complex history of the India-Asia collision. The APWPs are valid only inasmuch they represent true primary magnetization. Cenozoic samples from Asia have revealed potential problem of inclination shallowing in sediments. It is therefore important to attempt to estimate paleodirections from lava flows. We present new paleomagnetic results obtained at 24 sampling sites (171 cores) from Tertiary basaltic and trachy-basaltic lava flows collected in 1999 in Taatsyn Gol (TG) region, Mongolia (45.3$^{o}$N, 101.1$^{o}$E). Rock magnetism experiments and thermal demagnetizations allowed us to isolate a high temperature component (HTC) carried by magnetite. HTC directions of the eight sites corresponding to eight distinct flows from TG1 locality (13 Ma) cluster after bedding correction (ks/kg=19.5/10.1), but the fold test remains inconclusive. Sixteen sites from the TG2-3 locality (30 Ma) occur as distinct horizontal flows. Assuming that HTCs from the two localities represent the primary magnetization of the basaltic lava flows, we computed the corresponding Tertiary paleopoles from the two distinct HTC mean directions. These poles lie at $\lambda$=74.3$^{o}$N, $\phi$=161.0$^{o}$E, dp/dm=14.4/17.1 at 13 Ma, and $\lambda$=83.7$^{o}$N, $\phi$=271.3$^{o}$E, dp/dm=3.7/5.0 at 30 Ma. Whereas the pole at 13 Ma is fairly consistent with that of the reference APWP for Eurasia (Besse and Courtillot, 2002), the 30 Ma pole appears far-sided with respect to the corresponding reference pole, arising from a shallowing of paleomagnetic inclination with respect to the predicted one. The discrepancy amounts to 9.8$^{o} \pm 4.2^{o}$ in paleolatitude, which would imply a post Oligocene (30 Ma) convergence of more than 500 km between Mongolia and Siberia. This is unrealistic, as there is no deformed zone north of Mongolia that could have absorbed such motion. Another possibility is that the 30 Ma pole may not have fully averaged paleosecular variation. In order to try and fix this problem, we have performed a complementary sampling of 13 sites (101 cores) of 30Ma basalts from Taatsyn Gol in the summer of 2004; the first results will be presented.
DE: 8110 Continental tectonics--general (0905)
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
SC: Union [U]
MN: 2004 AGU Fall Meeting