Crustal assembly of Myanmar: establishing a geo/thermochronologic data base for magmatism, metamorphism, deformation, and exhumation in central and eastern Myanmar
Final Report Abstract
This project aimed to initiate the acquisition of a modern geo/thermochronologic database by dating samples from central and eastern Myanmar; this area straddles plate boundary between India and Asia and is characterized by large-scale dextral wrenching. Regionally, the project addressed the questions how the sutures of the Pamir-Tibet plateau continue to the southeast around the E- Himalayan (Namche Barwa) syntaxis and when deformation, exhumation, and uplift occurred south of the E-Himalayan syntaxis. U/Pb zircon geochronology suggests that all basement units of Myanmar are part of Gondwana. The Shan Plateau—Shan-Thai block of eastern Myanmar lacks Cenozoic and Cretaceous magmatism and its Triassic event links it to the Qiangtang block of Central Tibet; the Jinsha suture lies to the east of aastern Myanmar. Jurassic–Early Cretaceous magmatism ties the Mogok metamorphic belt to the Gangdese belt of Tibet (Lhasa block) and puts the Bangong-Nujiang suture along the eastern margin of the Mogok belt. Late Eocene–Oligocene high-grade metamorphism dominates the Mogok belt. Its ages overlap with Himalayan magmatism and the heat source for its arc-type geothermal gradient may be slab break-off; Tertiary adakitic granitoids provide firsts hints to this interpretation. The Mogok metamorphic belt cooled from high-T metamorphism and associated magmatism over a time-span of >10 Ma to mid-crustal temperatures and within a few Ma to near surface conditions; rapid cooling started at ~17 Ma and terminated before 10 Ma. Pebbles of the Irrawaddy Formation of NE-Myanmar, likely Late Miocene-Pliocene coarse fluvial deposits of the Paleo- Irrawaddy, cooled 5-10 Ma later through 60-100°C than the adjacent Mogok belt rocks. This suggests derivation of these conglomerates from the E-syntaxis region, supporting speculations of a paleo- drainage link with the Yarlung Tsangpo drainage of Tibet. The Mogok belt metamorphism reached -700°C and ~6 kbar at ~35 Ma. Metamorphic conditions are drastically different in the Katha basement of N-central Myanmar. There, pressure-dominated upper greenschist facies occurred prior to ~37 Ma. PTt data, lithology (metapelites and Cambro-Ordovician orthogneisses), and absence of Cretaceous magmatism suggest that the Katha basement is part of the Himalayan Series (Indian plate). The Mogok metamorphic belt rocks experienced ductile deformation in the Oligocene with subhorizontal ~NW-SE extension accompanied by ~NE-SW shortening resulting in folding and constrictional strain geometries; overall, ductile deformation is moderately strong. Low-T ductility and faulting accompanied rapid Miocene cooling. Late faulting, mostly along N-trending dextral strike-slip faults is Late Miocene–Pliocene and related to the Sagaing fault zone that started to be active after 7 Ma. Our ongoing work in Yunnan showed that at least two of the large-scale crustal shear zones that were interpreted to be responsible for large-scale lateral extrusion of the Shan Plateau–Shan-Thai block (the dextral Gaoligong and the sinistral Chongshan shear zone) formed sub-horizontally and were related to the decoupling between upper and middle crust, resulting in southward (present coordinates) flow of the upper crust. Our work in Myanmar allowed us to address flow in the middle crust that is moderate. Combined, our work up to now allows us to speculate that large scale “channel-flow” around the E-Himalayan syntaxis did not occur; the picture that emerges is that of flow of the upper crust, possibly caused by a topographic gradient.
Publications
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2007. Confined fission tracks in ion-irradiated and step-etched prismatic sections of Durango apatite. Chemical Geology, 242, 202?217
Jonckheere, R., Enkelmann, E., Min, M., Trautmann, C. and Ratschbacher L.
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2007. Measurements of confined-fission tracks in ion-irradiated geological samples with low track densities, Nuclear Instruments and Methods in Physics Research B, 259, 943?950
Min Myo, Enkelmann E., Jonckheere R., Trautmann C. and Ratschbacher L.
