Comment on "Onset timing of left-lateral movement along the Ailao Shan - Red River shear zone: 40Ar/39Ar dating constraint from the Nam Dinh area, northeastern Vietnam"
P.H. Leloup1*, R. Lacassin1, P. Tapponnier1, T.M. Harrison2.
1 Laboratoire de Tectonique, Mécanique de la Lithosphère,
UMR7573 CNRS, Institut de Physique du Globe de Paris, 4 Place
Jussieu, 75252 Paris CEDEX 05, France.
* corresponding author: leloup@ipgp.jussieu.fr, tel (33) 1 44
27 24 37, fax (33) 1 44 27 24 40.
2 W.M. Keck Center for isotope geochemistry, Dept. of Earth and
Space Sciences and IGPP, University of California at Los Angeles,
405 Hilgard avenue, CA 90024, Los Angeles, USA.
Wang et al. claim to have dated the onset of left-lateral
shear along the Ailao Shan - Red River shear zone (ASRR) (Leloup
et al., 1995; Tapponnier et al., 1990) at 27 Ma ago, thus 3 Ma
after the initiation of sea floor spreading in the South China
Sea (Wang et al., 2000) We show below that their argument is fundamentally
flawed and that motion along the ASRR started at least a few Myr
earlier than seafloor spreading.
Wang et al. (2000) present biotite, muscovite and K-feldspar
40Ar/39Ar data from the southernmost exposure of the ASRR near
Nam Dinh (Harrison et al., 1996; Leloup et al., 1996). From these
data, they propose that rapid cooling from 400°C to 150°C
occurred in this part of the shear zone, between 27 and 25 Ma
(Fig. 6 of Wang et al., 2000). They then suggest. (1) that there
is a correlation between rapid cooling ages, mainly constrained
by K-feldspar data, and distance along strike in the Nam Dinh
and DayNuiConVoi (DNCV) range (Fig. 8 of Wang et al., 2000; Wang
et al., 1998); (2) that this correlation reflects a cooling diachronism
comparable to that described elsewhere along the Ailao Shan by
Harrison et al. (1996; 1994) and Leloup et al. (1994); (3) that
the extrapolation of this correlation to a postulated southeast
termination of the ASRR, which they locate 50 km away from Nam
Dinh, dates the onset of left-lateral shear along the ASRR at
27.5 Ma; and (4) that this onset age is younger than the oldest
magnetic anomaly in the South China Sea, implying that seafloor
spreading was not caused by left-lateral shear along the ASRR
as suggested by Briais (1993), Tapponnier et al. (1986) and Leloup
et al. (1995). We do not dispute the data presented by Wang et
al. (2000) and their interpretation of rapid cooling at Nam Dinh
around 27Ma. However we show below that their interpretation of
this data is wrong, which invalidates all their final conclusions.
We write this comment to warn readers of their erroneous reasoning
and conclusions concerning the time of motion and cooling history
of the ASRR.
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