The Red River zone is the major morphologic
and geological discontinuity in East Asia. It stretches for more
than 1000km from Tibet to the Hanoi basin and separates the Indochina
and South China continental blocks (Figs. 1, 2 and 6).
This zone is the result of at least two successive phases of
deformation (e.g. Tapponnier et al., 1986).
The most recent phase corresponds to right-lateral/normal movements
along the Red River fault zone (e.g., Allen et al., 1984; Tapponnier
and Molnar, 1977; Leloup et al., 1993, 1995; Replumaz et al,
2001) (Fig.7).
Total right lateral offset has been estimated between) and 60
kilometres (Leloup et al., 1995), whereas the most probable value
appears to be 25 km (Replumaz et al, 2001). Such right-lateral/normal
motion began 5 (Leloup et al., 1995) to 13 Ma (Bergman et al.,
1997) ago, and is partly responsible for the denudation of an
older shear zone: the Ailao Shan - Red River (ASRR).
Whereas the existence of the ASRR has long been denied (e.g.
Cobbold and Davy, 1988; Dewey et al., 1989; Houseman and England,
1993) or minimised (e.g. Rangin et al., 1995), our results have
proven its importance in SE Asia tectonics. The ASRR is composed
of four narrow (< 20 km wide), high-grade gneiss ranges (Fig.
8):
the DayNuiConVoi in Northern Vietnam, and the Ailao Shan, Diancang
Shan and Xuelong Shan in the Chinese province of Yunnan. Strongly
foliated and lineated mylonitic gneisses compose the cores of
these metamorphic ranges (Tapponnier et al., 1990, Lacassin et
al., 1993, Leloup et al., 1993, Leloup et al., 1995, Leloup et
al., 2001). The mylonitic foliation is generally steep and bears
a horizontal stretching lineation (Figs. 8,
9, 10,
11,
12),
both being almost parallel to the local trend of the gneissic
cores (Figs. 8,
10),
In some places, foliation dips more gently, but lineations are
always horizontal and parallel to the trend of the belt. Numerous
shear criteria indicate that the gneisses have undergone intense,
progressive left-lateral shear (Fig. 13).
