Direct dating of left-lateral deformation along the Red River shear zone, China and Vietnam
Lisa D. Gilley, a*, T. Mark Harrisona,b,
P.H. Leloupc, F.J. Ryersond, Oscar M. Loveraa, and
Jiang-Hai Wange
aDepartment of Earth and Space Sciences & IGPP, University
of California, Los Angeles,
CA, 90095-1567, U.S.A.
bResearch School of Earth Sciences, The Australian National University, Canberra, A.C.T., 0200, AUSTRALIA
cC.N.R.S., Institut de Physique du Globe de Paris, 75252 Paris 05, FRANCE
dInstitute of Geophysics and Planetary Physics, Lawrence Livermore National. Laboratory, Livermore, CA, 94550, U.S.A.
eGuangzhou Institute of Geochemistry, Chinese Academy of
Sciences, P.O. Box 1131,
Wushan, Guangzhou 510640 Guangdong, CHINA
* Now at the Department of Geology, University of Kansas, 1475 Jayhawk Blvd., Lawrence, KS 66045 (lstockli@ku.edu)
Abstract. Exposures
of high-grade, mid-crustal rocks within the Red River shear zone
(RRSZ), which separates the Indochina and South China blocks,
exhibit clear evidence of left-lateral, ductile deformation.
Assuming the South China Sea represents a pull-apart basin formed
at the southeastern termination of the RRSZ, it has been argued
that seafloor magnetic anomalies constrain the timing of sinistral
slip accommodated by the RRSZ between ~32-17 Ma at a rate of ~4
cm/yr. While 40Ar/39Ar thermochronometry indicates that left-lateral
slip occurred along the RRSZ between 25-17 Ma, the timing of earlier
high-temperature deformation has not been directly constrained.
In situ Th-Pb ion microprobe dating of monazite inclusions
in garnets allows direct assessment of the timing of amphibolite
grade metamorphism and synchronous left-lateral shearing. Results
from northern segments of the RRSZ in Yunnan, China indicate that
synkinematic garnet growth occurred between 34-21 Ma, and are
the first to document late Oligocene metamorphism and left-lateral
shearing. Data from the southern RRSZ within Vietnam are complicated
by Tertiary overprinting of rocks that experienced amphibolite
facies metamorphism during the Indosinian orogeny (~220 Ma).
The period during which sinistral deformation is now constrained
to have occurred along the RRSZ (i.e., 34-17 Ma) is essentially
coincident with spreading of the South China seafloor (32-17 Ma).
This temporal and kinematic link between left-lateral shearing
along the RRSZ and opening of the South China Sea supports the
view that Indochina was extruded from Asia as a block along lithospheric-scale
strike-slip faults.
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Insigths from Tertiary deformation of SE Asia |