HR: 16:20h
AN: V14A-02    [Abstracts]
TI: Exhumation of UHP rocks: Elastic Deformation or Shearing Creep?
AU: * Yang, W
EM: yangwencai@a-1.net.cn
AB: Since 1967, Chinese geophysicists have carried out deep seismic reflection, magnetotellulic, geothermal and potential-field observations in Sulu area where the Chinese continental scientific drilling site is located. In order to perfect interpretation of these data, we have measured physical properties of main rocks in the area at room pressure/temperature laboratories. Geophysical data reveal both external and internal structures along the UHP belt, including the Triassic collisional and subduction structures, occurrence of UHP and HP rocks in the upper crust, eclogite occurrence and magnetization history, multi-cycle ductile shear zones, etc. These features are useful to understand processes of the UHP metamorphic dynamics. The crustal structures in the area are characterized by following facts. (a) Some seismic signatures of Triassic collisional and subduction structures are remained in the current crust, but most seismic signatures illustrate extensional structures formed after the Triassic event. (b) The UHP rock slices only appear within the upper crust and are surrounded by non-UHP rocks, the seismic patterns of the middle and lower crust are typically extensional. (c) Multiple superimposed ductile shear zones densely develop in the crust and coexist with UHP rocks. (d) In the upper crust, UHP and HP rock slumps are coexist and superimposed each other with contact zones mostly the shearing faults. (e) The strike-dip of eclogite bodies is different from the shearing faults. (f) There are two groups of eclogites that have different permanent magnetization directions and magnetization history, possibly implying rotation of the UHP rock slumps. The exhumation dynamics of the UHP rocks can hardly explained by theories based on elastic deformation, which could hardly result the crustal structures mentioned above. On the contrary, the exhumation of the UHP rocks in the Sulu area might strongly relate to rheological creeps, especially complex shearing creep motions occurred in the lithosphere and upper mantle in the late Triassic. As a matter of fact, evidences of viscous flow can be seen not only in the UHP rock samples and cores, but also in the lithospheric structures, such as the ductile shearing faults. Though we have not measured the shear creep compliance of the UHP rocks, we know that these rocks have high yield strength and high fracture energy. However, their enclosing rocks, such as gneisses or most of lower-crust rocks, could act in shearing creep manners with a considerable rate under high temperature and pressure, pushing the UHP rock slices exhumed onto the upper crust.
DE: 8102 Continental contractional orogenic belts
DE: 8159 Rheology--crust and lithosphere
DE: 7218 Lithosphere and upper mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting