HR: 0800h
AN: T31D-0670    [Abstracts]
TI: Miocene Metamorphism and Exhumation of the Granulite-Eclogite Anticlinorium of the Ama Drime Through Normal Faulting Associated to Large Scale E-W Folding
AU: * KALI, E
EM: elisekali@gmail.com
AF: Geoscience Montpellier Universite de Montpellier 2, CNRS, Place Eu. bataillon, MONTPELLIER, 34000, France
AU: ARNAUD, N O
EM: Nicolas.Arnaud@gm.univ-montp2.fr
AF: Geoscience Montpellier Universite de Montpellier 2, CNRS, Place Eu. bataillon, MONTPELLIER, 34000, France
AU: LELOUP, H P
EM: herve.leloup@univ-lyon1.fr
AF: CNRS UMR 5570, Univer. Claude Bernard et ENS Lyon, 2 rue Raphael Dubois, VILLEURBANNE, 69622, France
AU: MAHEO, G
EM: gweltaz.maheo@univ-lyon1.fr
AF: CNRS UMR 5570, Univer. Claude Bernard et ENS Lyon, 2 rue Raphael Dubois, VILLEURBANNE, 69622, France
AU: BOUTONNET, E
EM: emmanuelle.boutonnet@gm.univ-montp2.fr
AF: CNRS UMR 5570, Univer. Claude Bernard et ENS Lyon, 2 rue Raphael Dubois, VILLEURBANNE, 69622, France
AU: LI, H
EM: lihaibing06@yahoo.com.cn
AF: Institute of Geology, Chinese Academny of Geological Sciences, BEIJING, 10003, China
AB: High grade rocks have been exhumed in the core of a foliation dome forming the Ama Drime range west of Dingye graben. This range is bordered on its western and eastern sides by two major presently active normal faults which cut the STDS, the eastern one forming the western border of the Dingye graben. The footwall of this fault consists of sillimanite-garnet bearing gneisses with kyanite relics, locally migmatitic and intruded by deformed and undeformed leucogranites. C/S relations show top to the east ductile normal shearing prior to brittle normal faulting presently active. Petrological study of gneisses via minerals microprobe analysis and pseudosections show minimum pressures of equilibration of 13.5 kbar and temperature of 800°C, followed by decompression to 6kbar and 700°C contemporaneous with the onset of ductile normal shearing and melting. SHRIMP U/Pb dating of monazite from migmatites give middle-miocene ages for the melting event while SIMS U/Pb dating of zircons from orthogneisses from the same unit reveal ages of more than 2 Ga. PT conditions and proterozoic ages indicate that this unit belong to Lower Himalayan Cristalline Series, (LHCS as in Groppo et al., 2007, J Metam. Geol) but also reveal that middle-miocene high grade rocks have been exhumed by the ductile normal fault. The same study was carried out on samples from the hanging-wall of the normal fault which consists of garnet- sillimanite micaschists with staurolite relics, locally intruded by undeformed and deformed tourmaline bearing leucogranites, and are sheared top to North probably in relation with the overlying STDS. U/Pb SIMS dating of zircons on undeformed and deformed leucogranite give middle Miocene ages for the end of the deformation associated with the STDS. Moreoever, Paleozoic inheritage in zircons show that the units located to the east of the active normal fault belong to the High Himalayan Crystallines series. PT paths constrained by garnet isopleths show decompression and reheating from 6 to 4 kbar and 600 to 660°C probably associated with the middle Miocene intrusion of leucogranites. Published and ongoing 40Ar/39Ar dating of ductile fabrics from the normal fault indicates miocene deformation in agreement with petrological evidence inferring that normal faulting was coeval with strong decompression probably through active tectonic unroofing. Regional studies suggest that similar large scale geometries and kinematic associations exist on the western of the Ama Drime range (Groppo et al., 2007) which appears to form a foliation anticlinorium with a core of high pressure-high temperature rocks. The location of these high grade units, far north from the MCT is possibly related with the formation of the N-S Arun anticlinorium. Its formation coeval with the development of ductile normal fault on its flank favoured exhumation and probably uplift of deeply seated metamorphic rocks. In that context ductile normal faulting probably developed on lateral ramps, and played as an accommodation mechanism of compression rather than as a result of extension. The Ama Drime example thus shows that ductile normal faulting may not always be a significant proxy for the onset of presently active extension in southern Tibet.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8104 Continental margins: convergent
DE: 8108 Continental tectonics: compressional
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting