Diachronous Initiation of Transtension Along the Ailao Shan-Red River Shear Zone, Yunnan and Vietnam
T. Mark Harrison, P.H. Leloup
Department of Earth and Space Sciences and Institute of Geophysics
and Planetary Physics
University of California, Los Angeles
Los Angeles, CA 90024, U.S.A.
F.J. Ryerson
Institute of Geophysics and Planetary Physics
Lawrence Livermore National Laboratory
Livermore, CA 94550, U.S.A.
Paul Tapponnier, Urs Schärer, and R. Lacassin
Laboratoire de Tectonique, Mécanique de la Lithosphère
Institut de Physique du Globe de Paris
75252 Paris Cedex 05, FRANCE
Chen Wenji
State Seismological Bureau
Beijing, 100029, People's Republic of China,
ABSTRACT
The Red River Shear Zone (RRSZ) extends from eastern Tibet to
the South China Sea and is comprised of four elongated massifs.
It plays a central role in the hypothesis that strike-slip extrusion
of Indo-China accommodated a significant portion of Indo-Asian
convergence immediately following onset of collision. The massifs
of the RRSZ contain high-grade metamorphic rocks that were plastically
deformed in a left-lateral sense during the mid-Tertiary. 40Ar/39Ar
analysis of K-feldspars from ten transects perpendicular to the
NW-SE strike of the Ailao Shan, the longest massif in the chain,
yield a pattern of ages consistent with diachronous initiation
of transtensional faulting along the RRSZ. The normal component
of this faulting occurs along the eastern boundary of the shear
zone, the locus of the active Range Front fault. Numerical modeling
of isotopically-derived cooling histories yields a dip-slip rate
of ~6 mm/a. A 400-km long segment of the Ailao Shan reveals a
pattern of ages that increase smoothly from 17 Ma in the northwest
Ailao Shan to 25 Ma in Vietnam. This relationship indicates that
the onset of extension was diachronous, beginning in the southeast
and propagating northwest at a rate of ~4.5 cm/a. This rate and
the spatial and temporal variability of extension are consistent
with predictions, based on Oligocene-Early Miocene magnetic anomalies
from the South China Sea, that the RRSZ experienced strike-slip
rates of ~4 cm/a and that its present position does not represent
a small circle about the Indochina/South China poles of rotation.
As a consequence, a strike-perpendicular component of compression
would have existed in the northern portion of the belt, and an
increasing component of strike-perpendicular extension to the
southeast across the Ailao Shan. The general shallowing of foliation
from northwest to southeast across the Ailao Shan is consistent
with this prediction. Although consistent with predictions of
the extrusion hypothesis, the pattern of thermochronology across
the Ailao Shan contrasts with a model of rotation of rigid crustal
block
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Insigths from Tertiary deformation of SE Asia. |