POSSIBLE LARGE-SCALE, CENOZOIC, LOW-ANGLE NORMAL FAULT IN THE NAMCHE BARWA AREA, EASTERN HIMALAYAN SYNTAXIS

In the MiLin area, SW of the core of the eastern Himalayan syntaxis at Namche Barwa, structural relationships suggest the presence of an important, previously unrecognized fault with apparent normal-sense displacement of several tens of kilometers. The fault occurs along the southern margin of a NE-striking, NW- dipping zone of quartz-rich, lower amphibolite to greenschist facies metasedimentary rocks of uncertain stratigraphic and metamorphic age. The northern boundary of this zone is the previously mapped Gangdese thrust, which carries in its hanging wall intrusive rocks of the Gangdese batholith and their country rocks. The newly discovered structure along the southern margin places the quartz-rich metasedimentary rocks in its hanging wall over amphibolite facies Triassic(?) flyschoid rocks of the Tibetan Sedimentary sequence. Toward the east, this structure cuts an older low-angle structure that places the flyschoid rocks on high-grade metamorphic rocks of the Greater Himalayan sequence that form the core of the syntaxis at Namche Barwa. Regional relations suggest that this older structure is the eastward continuation of the South Tibetan Detachment (STD), a major fault system that helps define the tectonic architecture of the central Himalaya. Three lines of evidence suggest that the contact at the base of the quartz-rich metasedimentary rocks is a major normal fault: 1) it marks a metamorphic discontinuity, placing lower grade rocks on higher grade rocks; 2) rocks around the contact are strongly mylonitized; and 3) the structure truncates the STD. This fault can be traced eastward into the NW flank, and possibly to the east side, of the Namche Barwa culmination. We interpret the sequence of deformational events to be: 1) southward thrusting of the Gangdese batholithic terrane above the zone of metasedimentary rocks at MiLin; 2) northward normal faulting on the STD; 3) normal faulting, with a northward component, at the base of the MiLin metasedimentary rocks; and 4) formation of the Namche Barwa culmination. Development of the culmination deformed the originally gently northward-dipping tectonic units to form the present concave-south half window framing the culmination by a narrow zone (1-2 km) of lower grade rocks separating two higher-grade terranes.