CARBONIFEROUS FORMATION OF THE MOTIANLING EXTENSIONAL DOME (SW CHINA): STRUCTURAL AND THERMOCHRONOLOGICAL CONSTRAINTS

Extensional gneiss domes of the South China Block record contraction, extension, decompression and partial melting of middle or/and upper crust, the condition of detachment faulting and tectonic evolution of Southeast Asia. Here, we outline the tectonothermal evolution of Motianling Dome in this region using structural analysis and 40Ar/39Ar thermochronology. The Motianling Dome located at the southernmost of the Jiangnan fold belt is cored by a granitoid orthogneiss that is covered by low-grade metasedimentary rocks. The dome records at least three deformation events, here termed D1, D2, and D3. D1 deformation is represented by top-to-northwest ductile-brittle normal faulting system and SE-dipping ductile shear zones with mylonitic foliation and greenschist metamorphism, indicating the formation of the dome during NW–SE extension. The subsequent D2 and D3 deformations are marked by Late Mesozoic top-to-the-NW thrusting, and Cenozoic NE–SW trending normal faults, respectively. The timing of deformation and cooling path are obtained by mica 40Ar/39Ar thermochronology. New muscovite 40Ar/39Ar thermochronology suggests that the dome was initially produced in deep tectonic level at ca.416 Ma by ductile shear zones. Biotite 40Ar/39Ar thermochronology from granitic mylonite of ductile shear zones yields concordant spectra with 356.0 ± 3.1 Ma cooling age. Integrated with previous studies, the initial D1 deformation is interpreted to formed and exhumed the dome, and then it was finally exhumed by the Cenozoic brittle faults to surface. Therefore, the formation and evolution of the Motianling Dome record two periods of exhumation in the South China Block: (1) post - Caledonian orogenic collapse at ~416-356 Ma; and (2) the Cenozoic regional extensional tectonics at ~45 Ma to form the present-day Jiangnan fold belt.