Tertiary diachronic extrusion and deformation of western Indochina: Structural and 40Ar/39Ar evidence from NW Thailand.

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Robin Lacassin, Henri Maluski, P.Hervé Leloup, Paul Tapponnier, Chaiyan Hinthong, Kanchit Siribhakdi, Saengathit Chuaviroj, Adul Charoenravat.

J. Geophys. Res., vol. 102, no B5, May 1997.

Abstract. The Wang Chao and Three Pagodas fault zones cut the western part of the Indochina block and run parallel to the Red River Fault. Evidence of intense ductile leftlateral shear is found in the Lansang gneisses, which form a 5 km wide elongated core along the Wang Chao fault zone. Dating by 40 Ar / 39 Ar shows that such deformation probably terminated around 30.5 Ma. The Wang Chao and Three Pagodas faults offset the north striking lower Mesozoic metamorphic and magmatic belt of northern Thailand. 40 Ar / 39 Ar results suggest that this belt suffered rapid cooling in the Tertiary, probably around 23 Ma. These results imply that the extrusion of the southwestern part of Indochina occurred in the upper Eocene lower Oligocene. It probably induced rifting in some basins of the Gulf of Thailand and in the Malay and Mekong basins. In the OligoMiocene, the continuing penetration of India into Asia culminated with the extrusion of all of Indochina along the Ailao Shan Red River fault. This occurred concurrently with the onset of EW extension more to the south. Plotting in a geographical reference frame the diachronic time spans of movement on left-lateral faults east and southeast of Tibet implies that the northward movement of the Indian indenter successively initiated new strikeslip faults located farther and farther north along its path.

Structural sketch map of NW Indochina   (gif, 165 Ko).

Foliation in Lansang gneisses	Lineation in Lansang gneisses	Brittle-ductile shear zone
Rolling structures	C-S gneisses	Micafish

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