HR: 11:20h
AN: T12D-05    [Abstracts]
TI: Middle Miocene exhumation of the Laji Shan, Northeastern Tibetan Plateau
AU: * Dupont-Nivet, G
EM: gdn@geo.uu.nl
AF: Paleomagnetic Laboratory "Fort Hoofddijk", Faculty of Geosciences, Utrecht University, Budapestlaan 17, Utrecht, 3584 CD, Netherlands
AU: Andriessen, P
EM: paul.andriessen@falw.vu.nl
AF: Department of Isotope Geochemistry, Faculty of Earth- and Life Sciences (FALW), Vrije Universiteit Amsterdam, De Boelelaan 1085, Amsterdam, 1080 HV, Netherlands
AU: Juez-Larre, J
EM: juej@geo.vu.nl
AF: Department of Isotope Geochemistry, Faculty of Earth- and Life Sciences (FALW), Vrije Universiteit Amsterdam, De Boelelaan 1085, Amsterdam, 1080 HV, Netherlands
AU: Foeken, J
EM: j.foeken@suerc.gla.ac.uk
AF: Scottish Universities Environmental Research Centre (SUERC), ankine Avenue, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, United Kingdom
AU: Jiang, X
EM: jxwphd@sohu.com
AF: Guangzhou Institute of Geochemistry, Chinese Academy of Science, P.O. Box 1131, Guangzhou, 510640, China
AB: New data from northeastern Tibet uplift and associated basin tectonic evolution provides constraints on the mechanics of plateau growth and tectonic/climate interactions related to the aridification of central Asia and the intensification of the east Asian and Indian monsoons. Evolution of the large Longzhong Basin - including the Xining, Lanzhou, Linxia, Gonghe and Guide sub-basins - is well-constrained by recent advances in magnetostratigraphic dating of climatic and tectonic records. In particular, aridification is associated to global cooling at the Eocene-Oligocene transtion 34 yrs ago (see Hoorn et al., this meeting, Session T17), and tectonic activity is associated to widespread clockwise rotations of paleomagnetic directions at ca. 41 Ma and 17-11 Ma (see Dai et al., this meeting, Session GP12). However, poorly constrained thermal histories in adjacent fault- bounded ranges make linkages between tectonic, exhumation/sedimentation and climate tentative. We present here preliminary apatite (U-Th)/He and Fission track (FT) ages from the Laji Shan, a mountain range presently separating the Guide-Gonghe basins from the Xining-Linxia-Lanzhou basins. Sampling was performed at three elevation profiles (MD, LJ and TG;) in plutonic rocks from 2068 m to 4114 m. (U-Th)/He results from the lower elevation profile MD transect indicate fast exhumation between 14 Ma to 8 Ma, while the higher elevation profiles LJ and TG results indicate widespread ages from 40 to 110 Ma typical of partial retention zone. Obtained apatite FT dates of the MD elevation profile range from 16 to 35 Ma and all samples have long Mean Tracks Lengths (MTL) indicating fast cooling through the partial annealing zone for all samples. There is a clear correlation between elevation and FT date. The data indicate that a fossil partial zone has been uplifted. The sample at the lowest elevation (2068 m) with an apatite FT age of 16.2 Ma has the highest MTL suggesting that rock uplift of some 6 km has taken place since 16 My ago, assuming a geothermal gradient of 25°C/km. Obtained apatite FT datesof the LJ elevation profile cluster around 60 ± 10 Ma and do not differ much over the entire sampled elevation range. MTL are shorter for samples from the high elevation part of the profile, compared to longer MTL from the lowest samples. This is interpreted in reflecting a fossil partial annealing zone that has been uplifted since 55 My ago, as indicated by the sample with the longest MTL of 14 μ and FT age of 52.4 Ma. The total amount of rock uplift since that time amount to 6-7 km (assuming 25°C/km). Obtained apatite FT dates from the TG elevation profile cluster around 130 ± 10 Ma. The FT data reveal a fast cooling of the rocks around 160-170 Ma ago and all samples seem to have passes through the partial annealing zone in a similar way, suggesting a continuous exhumation of the rocks. Taken together these data suggest that significant midlle Miocene (ca. 15 Ma) unroofing in the Laji Shan belt can be associated to compartmentalization of the large Longzhong basin with tectonic rotation and onset of sedimentation in the Gonghe-Guide basins.
UR: http://www.geo.uu.nl/~forth/people/Guillaume
DE: 1100 GEOCHRONOLOGY
DE: 1140 Thermochronology
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 8100 TECTONOPHYSICS
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: 2007 Fall Meeting