TECTONIC ASSEMBLY OF SOUTHEASTERN CHINA AND INDOCHINA FROM LATE PALEOZOIC TO PRESENT: INTERACTION BETWEEN DIFFERENT TECTONIC SYSTEMS BEFORE AND DURING DEVELOPMENT OF THE TIBETAN PLATEAU

The Yangtze cratonal unit was sutured to North China in Triassic and to the Songpan Ganze terrane in Early Jurassic, but the nature of its southern boundaries are complex. To the south prior to India/Eurasia collision the basement of Lanping-Simao/Qiangtang unit developed as a Paleotethyan collage of arc, oceanic and continental fragments trending NW/SE extending into North Vietnam. The eastern Yangtze craton formed a generally NE/SW belt of magmatism and deformation by mainly westward subduction of the West Pacific oceanic region. From Late Triassic time the Lanping-Simao/Qiangtang collage basement was overlain by thick Mesozoic/Early Cenozoic mainly nonmarine strata. The eastern Yangtze craton continued development by mainly convergent west-directed subduction from the West Pacific oceanic realm from Triassic to Late Cretaceous at which time it became an extensional region related to West Pacific slab rollback. The Eastern Yangtze belt continued south through Vietnam across the eastern extension of the Lanping-Simao unit and its basement whose position at that time was farther north than present. Early Cenozoic India/Eurasia collision occurred west of the Eastern Himalayan syntaxis, but during early Tibetan Plateau development southeastern parts of Tibet and their eastward continuations were extruded hundreds of kilometers southeast along major transpressional faults; the Lanping-Simao unit was folded, internally rotated and thrust beneath western parts of Yangtze craton that were also translated southward as the deformed Ailao Shan unit. During late Cenozoic parts of SE Tibet were extruded clockwise around the syntaxis into the Burma subduction zone; extrusion faults cut across Early Cenozoic extrusion faults but reactivate some early faults as the right lateral Red River fault zone.

The end product of this evolution is the sharp juxtaposition of different tectonic units in Southeast China and Vietnam; Yangtze units are sharply truncated along the eastern side of the Ailao Shan and the Lanping-Simao unit is sharply truncated along the west side the Ailao Shan. This region shows the complex interaction between pre- and syn-collisional and West Pacific/Indonesian tectonic systems. Unfortunately the original positions of units west of the Red River fault remain undefined.