HR: 0830h
AN: T21D-0477    [PDF]
TI: New age constraints on the evolution of the Karakorum Fault, West Tibet
AU: * Valli, F
EM: valli@ipgp.jussieu.fr
AF: IPG de Paris, 4 place Jussieu, Paris, 75252 France
AU: Arnaud, N
EM: Nicolas.Arnaud@dstu.univ-montp2.fr
AF: Univ. Joseph Fourier, Place EugŠne Bataillon, Montpellier, 34095 France
AU: Lacassin, R
EM: lacassin@ipgp.jussieu.fr
AF: IPG de Paris, 4 place Jussieu, Paris, 75252 France
AU: Paquette, J
EM: paquette@opgc.univ-bpclermont.fr
AF: Univ. Blaise Pascal, 5, Rue Kessler, Clermont-Ferrand, 63038 France
AU: Leloup, P H
EM: herve.leloup@univ-lyon1.fr
AF: Univ. Claude Bernard, 2 rue Rapha‰l Dubois, Villeurbanne, 69622 France
AU: Li, H
EM: haibing531@sohu.com
AF: Institute of Geology, No. 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Tapponnier, P
EM: tapponnier@ipgp.jussieu.fr
AF: IPG de Paris, 4 place Jussieu, Paris, 75252 France
AU: Guillot, S
EM: Stephane.Guillot@univ-lyon1.fr
AF: Univ. Claude Bernard, 2 rue Rapha‰l Dubois, Villeurbanne, 69622 France
AU: Deloule, E
EM: deloule@crpg.cnrs-nancy.fr
AF: CRPG Nancy, 15 rue Notre Dame des Pauvres, Vandoeuvre, 54501 France
AU: Maheo, G
EM: maheo@gps.caltech.edu
AF: Univ. Claude Bernard, 2 rue Rapha‰l Dubois, Villeurbanne, 69622 France
AU: Xu, Z
EM: zhiqinxu@ccsd.org.cn
AF: Institute of Geology, No. 26 Baiwanzhuang Road, Beijing, 100037 China
AB: Results of detailed mapping and dating of sheared rocks along southern fault-half helps assess the total offset, lifespan, slip-rate and geodynamic importance of the Karakorum Fault (KFZ). South of 33$\deg$N, along SW edge of Tashikang-Gar basin, active right-lateral normal faults, belonging to KFZ, exhume metamorphic and magmatic footwall rocks forming the SE termination of Ladakh-Karakorum range. Close to active fault, gneisses and mylonites are affected by strong ductile dextral shear and inter-layered with leucocratic melt veins sheared to various degrees. Several generations of veins are often observed on a same outcrop, with late, weakly deformed veins cutting highly sheared ones. This implies that anatexy and intrusion were coeval with dextral shearing. Direct dating of leucogranites reveals Oligo-Miocene formation ages. U/Pb thermo-ionization dating on zircons yields concordant ages at 22.7+-0.1Ma. Discordant zircons ages are consistent with a poorly defined lower intercept at 32+-3 Ma, and a Proterozoic (1300+-100 Ma) upper intercept. In situ ion microprobe dating of 17 zircons (24 spots) confirms these results. Four mylonite zircons yield comparable, concordant ages ranging between 20 and 25 Ma in their cores and rims, in good agreement with conventional U/Pb dating. Inherited zircon ages range from Paleozoic to Precambrian (ca.1200 Ma). Some of them recrystallized partially during Tertiary deformation, yielding discordant ages, the youngest being 34+-0.8Ma. Cooling was delayed until 12 to 8Ma, at which time very rapid cooling is recorded by 40Ar/39Ar and fission track data. Purely strike-slip ductile shear was thus already in progress along the fault at 23Ma and possibly earlier (ca.34Ma). A marked kinematic change from purely dextral to dextral-normal motion occurred around or just after 12 Ma. Best offset estimate along this main (northern) branch of KFZ is given by correlation of the ophiolite-bearing melange of Shiquanhe with the Shyok suture zone in Pakistan (minimum offset of 250km). Assuming that this offset accrued in time span of about 23Ma suggests average long-term rates of at least 1 cm/yr. Evidence for strike-slip faulting is also clear within the Ladakh-Karakorum range and along its SW border where mapping shows large-scale boudinage of ophiolite units implying a maximum offset of 400km along a southern branch of KFZ. South of Baer, several active strike-slip branches bound the Kailas-Ponri range to the south and continue eastwards at least up to 82.5E (lake Kunggyu). The southern part of the range is made of steep, fault-bounded slivers with clear field for brittle dextral shear. Our structural study of the area does not support inference that the Karakorum Fault terminates west of Manasarovar lake and merges with the Gurla Mandatha detachment. Instead, both deformation and large-scale geometry concur to show that the Karakorum Fault Zone continues east of 81E as a dextral, transpressive flower structure reactivating the Indus-Tsangpo suture.
DE: 8110 Continental tectonics--general (0905)
DE: 8155 Plate motions--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting