Tertiary kinematics of SE Asia deformation.

 

 

 

 

 Beside great strike-slip movements along the ASRR, other Tertiary deformations affected continental SE Asia. Left-lateral strike-slip motion occurred for a total of 300km along the Wang Chao and Three Pagodas faults (Fig. 6); (Le Dain et al., 1984; Tapponnier et al., 1986; Lacassin et al.; 1993; 1997). Such deformation ended on the Wang Chao fault around 30 Ma and the fault zone was reactivated with a dextral/normal motion since 23 Ma (Lacassin et al., 1997). Eastward extrusion of whole Indochina was preceded by extrusion of South Indochina, and left-lateral faults were progressively activated farther to the north. The rates of northward progression of the activation of these left-lateral faults, of the end of such movements, and of their re-activation in the dextral sense, appear to be of the same order and comparable to the India/Eurasia convergence rate (Fig. 22) (Lacassin et al., 1997). The Indian indenter generate left-lateral faults at its front and reactivate these faults in the opposite sense behind him (Lacassin et al., 1997).

Tertiary shortening affected the Indochina and South China blocks on both side of the ASRR. This shortening, compatible with left-lateral shear along the ASRR, is however limited to a few ten of km in the Mesozoic sediments (Leloup et al., 1995, Lacassin et al., 1998). In the South China block, two large antiformal structures, the Yu Long Shan and High Song Chay domes were invistigated to further constrain the age and magnitude of such deformations. In the Yu Long Shan, a flat decollement toward the SW (in present-day frame of reference) occured at 36 Ma (Lacassin et al., 1996). Deformation resumed around 17 Ma with the antiformal folding of the decollement level (Lacassin et al., 1996). This two shortening phases occurred at the beginning and at the end of left-lateral shear along the ASRR. The High Song Chay dome reveals a much longer deformation history starting at more than 400 Ma with the main deformations relating to the Indosinian orogenic event (190 Ma) and with small Tertiary deformations (Roger et al., 2000).
Tertiary deformation au the Indochina and the South China block are one order of magnitude smaller than strike-slip movements along the ASRR.

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