Andrzej Zelazniewicz, Instytut Nauk Geologicznych PAN (Poland)
Izabella Nowak, Instytut Nauk Geologicznych PAN (Poland)
Trong Hoa Tran, Vietnamese Academy of Sciences and Technology (Vietnam)
Alexander Larionov, CIR, All Russian Geological Research Institute (Russian Federation)
The Red River Fault Zone (RRFZ) is a still active crustal feature which separates the South China and Indochina blocks in SE Asia. In NW Vietnam, it exposes strongly zonally sheared, high-grade metamorphic rocks of the Day Nui Con Voi Massif (MCV). Both high-temperature metamorphism and ductile shearing of these rocks were attributed to left-lateral slip at 35?20 Ma (Leloup et al., 2001) and subsequent doming (Anczkiewicz et al., 2007). However, Searle (2006) argued that the metamorphism was earlier and largely unrelated to shearing, whereas Zelazniewicz et al. (2005) reported pegmatite veins dated at 77-69 Ma which intruded into earlier sheared gneisses due to dextral-lateral slip. Our further detailed studies on the MCV high-grade paragneisses, mica schists and subordinate metabasites intruded by several sets of granite, leucogranite and pegmatite veins show that intrusive episodes occurred at 152-135 Ma, 80-70 Ma, 60-40 Ma, and 30-25 Ma intervals (SHRIMP data on zircons). Multiple ductile deformation was zonally accomplished in consecutively changing tectonic regimes along the RRFZ, namely: dextral wrenching, sinistral wrenching associated with vertical folding, dextral transpression associated with subhorizontal folding, sinistral transtension accompanied by faulting, the two latter events being related to uplift. Since the Miocene, the presently exposed MCV rocks have only been deformed in a brittle manner.
Three groups of metabasites have been discernible of which two younger geochemically indicate suprasubduction settings. Geochemical characteristics of the leucocratic veins also point to a suprasubduction setting. An interpretation of the geochemistry seems to suggest that necessary heat might have come from an astenospheric mantle wedge which could be related to the subduction of the Pacific plate under the SE Asia plate since the Late Jurassic. Our isotopic data indicate that crustal domains which occur presently on either side of the MCV (walls of the RRFZ) received significant magmatic accretions: (1) in the SW wall, during the Neoproterozoic (780-740 Ma), Mesozoic (250 Ma and 150-70 Ma) and Palaeogene (35 Ma), and (2) in the NE wall, during the Silurian (440-420 Ma) and the Triassic (258-243 Ma). The RRFZ appears to be a long-lived fault zone rejuvenated during multiple collisions in SE Asia in the Triassic and during the India-Asia collision in Palaeogene times.
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Leloup. P.H. et al. (2001). J. Geoph. Res., 106: 6683-6732.
Searle, M.P. (2006). J. Geol. Soc. Lond., 163: 1025-1036.
Zelazniewicz, A. et al. (2005). GeoLines, 19: 121-122.