2/20/2013 - Extrusion tectonics in SE Asia: Old problems, new data, future pursuits


12:00 PM - 1:00 PM
Geology 1707

Presented By:
Sinan Akciz - UCLA


A significant part of the post-collisional northward indentation of India into Asia has been accommodated by the Tertiary Gaoligong Shan and Chong Shan shear zones (CSSZ), as well as by the left-lateral displacement along the better known Ailao Shan-Dianchang Shan shear zones that lie farther E in Yunnan, China. Penetrative ductile deformation marked by steep foliation and subhorizontal lineation affects all rock types within both shear zones and kinematic indicators show right-lateral and left-lateral shear along the GSSZ and CSSZ, respectively. . Boudinage structures, indicating both horizontal and vertical stretching, affected compositional bands in mylonitic rocks formed during earlier stages of deformation and leucogranite sills emplaced during later stages of deformation. U/Pb ages on several monazite grains and grain fractions from mylonitic to non mylonitic leucogranites within the GSSZ shear zone range from ca. 80-18 Ma. The youngest ages come from undeformed leucogranites cross-cutting the foliation, which indicate that right-lateral movement ended ~18 Ma, when right-lateral shear between India-Sibumasu switched to the Sagaing fault zone in Burma. High-temperature sinistral strike-slip shearing along the CSSZ was active at ca. 28 Ma, and it terminated before ca.17 Ma, based on U/Pb ages of monazite grains from deformed and foliation cross-cutting leucogranites. The documentation of these structures indicate that crustal fragments in SE extruded as at least two crustal fragments with considerable internal deformation, bounded by three large-scale intra-continental shear zones. The continuation of these strike-slip shear ones to the north, the total amount of offset along them, and the reasons why the motion along them terminated at about 17 Ma and created new structures are some of the questions that remain to be answered.

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