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 Ar-40/Ar-39 and nine by the K-40-Ar-40 methods) from high grade metamorphic and foliate d intrusive rocks collected along the Mogok Metamorphic Belt of central Mya nmar, that complete preliminary results we previously published. All 30 age s presented here confirm the Oligocene to Middle Miocene age of the Mogok m etamorphism. 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 respons ible for the cooling of our samples, at a velocity similar to the India rel ative to Indochina velocity at the same period. In addition, intensive fiel d work, along a 800 km long section of the Mogok Metamorphic Belt, has reve aled the numerous ductile structures that resulted from a strong NNW-SSE-tr ending extension. From the analogy between the present-day local extrusion around the eastern Indian syntaxis and the extension observed along the Mog ok Metamorphic Belt, and from the diachronism we document, we suggest that the central part of Myanmar has "recorded" the northward passage of the eas tern Indian syntaxis with respect to Indochina from Oligocene to Middle Mio cene. (C) 2001 Elsevier Science Ltd. All rights reserved.