Post 4 Ma initiation of normal faulting in southern Tibet. Constraints from the Kung Co half-graben.
Earth and Planetary Science Letters 256 (2007) 233243
G. Mahéo a, F. Valli b, P. H. Leloup c, R. Lacassin b, N. Arnaud d, J-L. Paquette e, A. Fernandez c, L. Haibing f, K. A. Farley a, P. Tapponnier b
a Division of Geological and Planetary Sciences, California Institute
of Technology, 1200E California Bvd, Pasadena CA91125, USA
b Université de Lyon, Université Claude Bernard
Lyon1, Ecole Normale Supérieure de Lyon, Laboratoire de
Sciences de la Terre,
UMR 5570 CNRS, 2 rue Dubois, Bat géode, la Doua, 69622
Villeurbanne, France
c Institut de Physique du Globe de Paris, 4 place Jussieu, 75252
Paris CEDEX 05, France
d I S T E E M - U S T L, Place Eugène Bataillon, 34095
Montpellier Cedex 5, France
e Laboratoire Magmas et Volcans, UMR 6524 CNRS, Université
Blaise Pascal, rue Kessler, 63036 Clermont Ferrand, France
f Institute of Geology, Chinese Academy of Geological Sciences,
26 Baiwanzhuang Raod, Beijing 100037,PR China
Abstract:
The timing of EW extension of the Tibetan plateau provides
a test of mechanical models of the geodynamic evolution of the
IndiaAsia convergence zone. In this work we focus on the
Kung Co half graben (Southern Tibet, China), bounded by an active
NS normal fault with a minimum vertical offset of 1600 m.
To estimate the onset of normal faulting we combined high and
medium temperature (UPb, Ar/Ar) and low temperature ((UTh)/He)
thermochronometry of the Kung Co pluton, a two-mica granite of
the northern Himalayan granitic belt that outcrop in the footwall
of the fault. Biotite and muscovite Ar/Ar ages , are close from
each other [_16 Ma±0.2 (Ms) and _15±0.4 Ma (Bt)],
which is typical of fast cooling. The zircon and apatite (UTh)/He
ages range from 11.3 to 9.6 Ma and 9.9 to 3.7 Ma respectively.
These He ages are indicative of (1) fast initial cooling, from
11.3 to ~9 Ma, gradually decreasing with time and (2) a high geothermal
gradient (~400 °C/km), close to the surface at ~10 Ma.
The Kung Co pluton was emplaced at about 22 Ma (UPb on zircon)
at less than 10 km depth and 520545 °C. Subsequent to
its shallow emplacement, the pluton underwent fast thermal re-equilibration
ending around 7.5 Ma, followed by a period of slow cooling caused
either by the end of the thermal re-equilibration or by very slow
exhumation (0.020.03 mm/yr) from ~7.5 Ma to at least 4 Ma.
In either case the data suggest that the exhumation rate increased
after 4 Ma. We infer this increase to be related to the initiation
of the Kung Co normal fault. A critical examination of previously
published data show that most ~NS Tibetan normal faults may
have formed less than 5 Ma ago rather than in the Miocene as assumed
by several authors. Such a young age implies that EW extension
is not related to the Neogene South Tibetan magmatism (25 to 8
Ma). Consequently, models relating EW extension to magmatism,
such as convective removal of the lower lithosphere, may be inappropriate.
We rather think that this extension is related with local accommodation
of boundary forces and displacements.
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