Tertiary deformation and metamorphism SE of Tibet -
The folded Tiger-leap décollement of NW Yunnan (China)
Robin LACASSIN *, Urs SCHÄRER #, P. Hervé
LELOUP *, Nicolas ARNAUD&, Paul TAPPONNIER
*, Xiaohan LIU ¦, Liansheng ZHANG #
* Laboratoire de Tectonique, Mécanique de la Lithosphère;
URA 1093 CNRS; IPG-Paris, 4 place Jussieu, 75252 Paris CEDEX 05,
France.
# Laboratoire de Géochronologie; URA 1093 CNRS;
Université Paris 7 and IPG-Paris, 75251 Paris CEDEX 05.
& Université de Clermont-Ferrand; URA 10
CNRS; 5 rue Kessler, 63038 Clermont-Ferrand.
¦ Academia Sinica, Beijing, China.
e-mail address: lacassin@ipgp.jussieu.fr
Abstract:
The Yulong-Haba Xue Shan range, in the Northwestern part of Yunnan
(China) is a large N-S antiform that folds the Paleozoic series
of the Yangzi platform. The upper Yangzi river (Jinsha Jiang)
has cut a 3500m deep valley (Hu Tiao gorge) across this antiform,
thus exposing folded, bedding-parallel, ductile shear-zones (décollements),
with transport towards the SSW (in the present geographical coordinates).
The large finite shear-strain implies tens of transport, pointing
to the regional significance of these décollements. Rb/Sr
radiometric dating of phlogopites that crystallized in marbles
within the foliation planes yields the age of the metamorphic
and deformation event (35.9 ± 0.3 (2s) Ma). The age derives
from an internal Rb-Sr isochron, made on different size fractions
of the same mineral, which provides a novel demonstration of the
feasibility of such plots. Transport on the décollement
and related shortening occurred prior to, or at the onset of,
extrusion of Indochina along the Ailao Shan-Red River shear zone,
80 km west of the Yulong Shan. The 39Ar/40Ar age spectra of K-feldspar
from the core of the Yulong Shan suggest uplift by antiformal
folding around 17 Ma, as Indochina's extrusion came to an end.
We infer that other large-scale Cenozoic décollements such
as that exhumed in the Yulong Shan underlie some of the vast,
folded areas that surround the eastern Himalayan syntaxis. Transport
on such décollements, first towards the South then towards
the East, and folding above them, might have occurred during two
principal shortening phases, whose ages bracket Indochina's escape
towards the SE.