Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.t22c..01a&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #T22C-01
Mathematics
Logic
1035 Geochronology, 8102 Continental Contractional Orogenic Belts
Scientific paper
While it is widely accepted that strike slip faults in Tibet accommodate a significant part of the tertiary convergence between India and Asia, the true Cenozoic magnitude of the offset is still largely debated. Direct dating of Cenozoic piercing points is the most powerful tool to assess the total offset, but their use is not always possible. Therefore one gets to use older markers although this leads to significant results ONLY at the supreme condition that pre-Cenozoic movement of those markers be accurately known. The Kunlun and Altyn Tagh faults for example form a prominent example of Tibetan presently active fault, but they also constitute geological frontiers between blocks of different geological histories accreted at various times since early Paleozoic. One may thus question how much of the visible offset is indeed Cenozoic. Although deformation facies agree with recent kinematics, multi-geochronological approach indicates a series of events from 280-230 Ma to 120+/-10 Ma. The former may be linked either with suturing of the Qiantang and Kunlun blocks farther to the south, or collision further to the north or east in the Qilian Shan and Bei Shan ranges, while the latter range appears to be growing in importance with ongoing work but is still largely unexplained. Oblique subductions of collision to the north of the Qilian Shan are adequate candidates. Argon loss suggests that deformation was associated to a 250-300° C thermal pulse that lasted 5 to 20 Ma after the onset of movement (Arnaud et al., 2003). Unroofing on all faults occurred much later, around 25 Ma ago when sudden cooling suggests a component of normal faulting (Mock et al., 1999). Strong inheritage was also found along the Ghoza active fault, in central western Tibet. Of course the fact that some of the deformation is much older than the Cretaceous and shares compatible deformation criteria with the present-day deformation leads to false appreciation of the pure Cenozoic offset, potentially concluding to an over or underestimation of the true Tertiary deformation. However these earlier deformation zones can also be used as transcrustal markers to evaluate Cenozoic offset, suggesting for example for the Altyn Tagh fault a minimum offset of 400 km, in agreement with other estimations of post-Jurassic offset (Meyer et al., 1998, Sobel et al., 2001, Ritts et al., 2000). The key to a clear assessment of tertiary movements along the strike slip faults and their true importance in building of the Plateau is thus the very detailed dating of recent, usually cold and/or badly equilibrated, deformation facies. This is complicated by the very nature of the deformed rocks, usually far from the geochronologist standards ! High resolution in situ dating from high to low temperatures with very careful study of the deformation is thus a prerequisite, as well as a complete study of the regional geological history. This is the case for ongoing studies along the Karakorum fault where a continuous deformation from 20 Ma to the present can be characterized with details through time. Arnaud, N., Tapponnier, P., Roger, F., Brunel, M., Schärer, U., Wen, C., and Xu, Z., 2003, J Geophys Res-Solid Earth, v. 108. Meyer, B., Tapponnier, P., Bourjot, L., Métivier, F., Gaudemer, Y., Peltzer, G., Guo Shunmin, and Zhitai, C., 1998, Geophys. J. Int., v. 135, p. 1-47. Mock, C., Arnaud, N.O., and Cantagrel, J., Marie, 1999, Earth and Planetary Science Letters, v. 171, p. pp.107-122. Ritts, B.D., and Biffi, U., 2000, Geological Society of America Bulletin, v. 112, p. pp.61-74. Sobel, E.R., Arnaud, N., Jolivet, M., Ritts, B.D., and Brunel, M., 2001, GSA special publication v. 194, p. 247-267.
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