Thermochronological study of major shear zones around the Eastern Himalayan Syntaxis
Date Issued
2008
Date
2008
Author(s)
Lin, Te-Hsien
Abstract
The Jiali, Gaoligong, and Dien Bien Phu shear zones form the major boundaries for the clockwise rotation zone of crustal materials around the Eastern Himalayan Syntaxis (EHS), regarded as the most significant deformation zones in Southeast Asia. A number of models have been proposed to explain the mechanism of crustal deformation in this region. However, additional geochronological and structural studies from these major shear zones are required to provide more constraints on such models. With the aim of revealing the deformation history of the shear zones and exploring the implications of these findings for the Cenozoic tectonics of Asia, the present study involved a systematic collection and thermochronological analysis of deformed metamorphic and associated plutonic complexes within the Jiali, Gaoligong, and Dien Bien Phu shear zones.eochronological data show that the main phases of deformation along the Jiali and Gaoligong shear zones occurred in the periods of ~22–11 and ~18–13 Ma, respectively. A comparison of the deformation recorded in the Jiali, Gaoligong, and other major shear zones around the EHS reveals that sinistral shearing along the Jiali shear zone was synchronous with that along the Ailao Shan–Red River shear zone, forming a northern boundary to the southeastward extrusion of the Indochina block during the Early Miocene. From the Middle Miocene (~18 Ma), the Jiali shear zone may change to dextral displacement, becoming linked with the dextral Gaoligong shear zone that developed as a consequence of continued northward indentation of the Indian plate into Asia. Since this time, the Jiali and Gaoligong shear zones have been united, defining the southwestern boundary of the EHS during clockwise rotation of the eastward-extruding Tibetan block, as present-day tectonics in southeastern Tibet. The temporal comparison between plausible dextral shearing records within the Jiali and Gaoligong shear zones, and dates of normal faulting in southern Tibet, suggests that lower crustal flow resulting from thickened Tibetan lithosphere and oblique convergence between India and Eurasia may explain the change in deformation style around the EHS at ~18 Ma. Furthermore, the temporal change in regional deformation pattern from southeastward block extrusion to clockwise rotation of crustal fragments may have played an important role in the development of the eastern Himalayan drainage system in the area of the EHS.n NW Vietnam, the NNE–SSW-trending Dien Bien Phu shear zone, which represents part of the eastern boundary for the extruding continental blocks, dextrally offsets the NW–SE-trending Song Ma suture. This study reports the first 40Ar-39Ar geochronological data obtained for metamorphic rocks within the Dien Bien Phu shear zone. 40Ar-39Ar ages of 194 to 212 Ma for mica schists and 158 to 198 Ma for mylonites suggest that the fault was active during the Early Jurassic, making it significantly old and longer-lasting. Temporal correlations indicate that collision between the South China, Indochina, Sibumasu and Simao blocks, representing the late Triassic to early Jurassic Indosinian Orogeny, may have led to the initiation of movement along the shear zone and associated fault systems in the Indochina region. The preservation of a geological record of the Indosinian Orogeny in the Dien Bien Phu shear zone indicates that Cenozoic shearing has not apparently reset the isotopic systems of the metamorphic complex distributed along the fault.
Subjects
Geochronology
Shear Zone
Eastern Himalayan Syntaxis
Tibet
Indochina
Type
thesis
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