The Ailao Shan-Red River Shear zone (Yunnan, China), Tertiary transform boundary of Indochina
Philippe Hervé Leloup1, Robin Lacassin1, Paul Tapponnier1,
Urs Schärer2, Zhong Dalaï3, Liu Xiaohan3, Zhang Liangshang2,
Ji Shaocheng3 and Phan Trong Trinh4
1 Laboratoire de tectonique et mécanique de la lithosphère,
C.N.R.S., IPG-Paris, 4 place Jussieu 75252 Paris Cedex O5, France
2 Laboratoire de géochronologie; Université Paris
7 and IPG-Paris, 75252 Paris Cedex O5, France
3 Institute of geology, Academia Sinica, PO Box 634, 100 011 Beijing,
China.
4 Institute of geological Sciences, National center for natural
sciences and technology, Nghia Do, Tuliem, Hanoi, Vietnam
Abstract
The Red-River Fault zone (RRF) is the major geological discontinuity
that separates South China from Indochina. Today it corresponds
to a great right-lateral fault, following for over 900 km the
edges of four narrow (< 20 km wide) high grade gneiss ranges
that together form the Ailao Shan-Red River (ASRR) metamorphic
belt: the Day Nui Con Voi in Vietnam, and the Ailao, Diancang
and Xuelong Shan in Yunnan. The Ailao Shan, longest of those ranges,
is fringed to the south by a stripe of low-grade schists encompassing
ultramafic bodies. The ASRR belt has thus commonly been viewed
as a suture.
A detailed study of the Ailao and Diancang Shan shows that the
gneiss cores of the ranges are composed of strongly foliated and
lineated, mylonitic gneisses. The foliation is usually steep and
the lineation nearly horizontal, both being almost parallel to
the local trend of the gneissic cores. Numerous shear criteria,
including asymmetric tails on porphyroclasts, C-S or C'-S structures,
rolling structures, asymmetric foliation boudinage and asymmetric
quartz <c> axis fabrics, indicate that the gneisses have
undergone intense, progressive left-lateral shear. PT studies
show that left-lateral strain occurred under amphibolite facies
conditions (3-7 kb and 550-780 °C). In both ranges high temperature
shear was coeval with emplacement of leucocratic melts. Such deformed
melts yield U/Pb ages between 22.4 and 26.3 Ma in the in Ailao
Shan and 22.4 and 24.2 Ma in the Diancang Shan, implying shear
in the Lower Miocene. The mylonites in either range rapidly
cooled to 300°C between 22 and 17 Ma, before the end of left-lateral
motion.
The similarity of deformation kinematics, PT conditions, and crystallization
ages in the aligned Ailao and Diancang Shan metamorphic cores,
indicate that they represent two parts of the same Tertiary shear-zone:
the Ailao Shan-Red River (ASRR) shear-zone. Our results thus confirm
the idea that the ASRR belt was the site of major left-lateral
motion, as Indochina was extruded toward the SE as a result of
the India/Asia collision. The absence of metamorphic rocks within
the 80 km-long "Midu gap" between the gneissic cores
of the two ranges results from sinistral dismembering of the shear-zone
by large-scale boudinage followed by uplift and dextral offset
of parts of that zone along the Quaternary Red River fault. Additional
field evidence suggests that the Xuelong Shan in northern Yunnan
and the Day Nui Con Voi in Vietnam are the northward and southward
extensions, respectively, of the ASRR Shear zone, which therefore
reaches a length of nearly 1000 km.
Surface balance restoration of amphibolite boudins trails indicates
layer parallel extension of more than 800% at places where strain
can be measured, suggesting shear strains on the order of 30,
compatible with a minimum offset of 300 km along the ASRR zone.
Various geological markers have been sinistrally offset 500 to
1150 km by the shear-zone. The seafloor spreading kinematics in
the South China Sea are consistent with that sea having formed
as a pull apart basin at the southeast end of the ASRR zone, which
yields a minimum left-lateral offset of 540 km on that zone. Comparison
of Cretaceous magnetic poles for Indochina and South China suggests
up to 1200±500 km of left-lateral slip on the ASRR zone.
Given these data, the Tertiary left-lateral offset on the ASSR
is most probably 700±200 km (figure at which should be
added the later right-lateral offset on the order of few tens
of km).
Those results improve significantly our quantitative understanding
of the finite deformation of Asia under the thrust of the Indian
collision. While being consistent with a two stage extrusion
model, they demonstrate that the great geological discontinuity
that separates Indochina from China results from Cenozoic strike-slip
strain rather than more ancient suturing. Furthermore, they suggest
that this narrow zone acted like a continental transform plate-boundary
in the Oligo-Miocene, governing much of the motion and tectonics
of adjacent regions. 700 and 200 km of left-lateral offset on
the ASRR shear zone and Wang Chao fault zone, respectively, would
imply that the extrusion of Indochina alone accounted for 10 to
25 % of the total shortening of the Asian continent. The geological
youth and degree of exhumation of the ASRR zone make it a worldwide
reference model for large-scale, high temperature, strike-slip
shear in the middle and lower crust.
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