Diachronous cooling along the Mogok Metamorphic Belt (Shan scarp, Myanmar): the trace of the northward migration of the Indian syntaxis

We present new observations and new mineral ages (eight by the 40Ar/39Ar and nine by the K-40-40Ar methods) from high grade metamorphic and foliated intrusive rocks collected along the Mogok Metamorphic Belt of central Myanmar, that complete preliminary results we previously published. All 30 ages presented here confirm the Oligocene to Middle Miocene age of the Mogok metamorphism. Moreover, the ages are young from the Gulf of Martaban in the south to the China-Myanmar border in the north, showing a clear diachronism. These results confirm a northward migration of the tectonic event responsible for the cooling of our samples, at a velocity similar to the India relative to Indochina velocity at the same period. In addition, intensive field work, along a 800 km long section of the Mogok Metamorphic Belt, has revealed the numerous ductile structures that resulted from a strong NNW-SSE-trending extension. From the analogy between the present-day local extrusion around the eastern Indian syntaxis and the extension observed along the Mogok Metamorphic Belt, and from the diachronism we document, we suggest that the central part of Myanmar has "recorded" the northward passage of the eastern Indian syntaxis with respect to Indochina from Oligocene to Middle Miocene. (C) 2001 Elsevier Science Ltd. All rights reserved.