Cenozoic structural and metamorphic evolution of the eastern Himalayan syntaxis (Namche Barwa)

Combined geological and geochronological investigations of the eastern Himalayan syntaxis in the Namche Barwa region of Tibet reveal the first-order elements of its Cenozoic tectonic evolution. The syntaxis is characterized by a northeast-plunging antiform and is bounded by two northeast-striking strike-slip shear zones: a left-slip shear zone on the western side and a right-slip shear zone on the eastern side. These strike-slip shear zones are linked by east-west-trending thrusts and served either as (1) a roof thrust to a large duplex system or (2) transfer faults to a south-directed thrust system that accommodated northward indentation of a folded Indian plate. An east-west-trending pop-up structure in the core of the antiform juxtaposes a granulite-bearing complex over sillimanite-bearing gneisses of Gangdese affinity to the north and of Indian affinity to the south. Previous studies suggest that mafic granulites in the complex record at least two episodes of metamorphism at ∼ 800°C: the first at high pressures (14-15 kbar) followed by a second event at 8-10 kbar. Zircons from mafic granulites yield four populations of concordant U-Pb ion microprobe ages. Two groups are at ~ 65 Ma and ~ 160 Ma, and likely crystallized during Andean-type Gangdese magmatism prior to the Indo-Asian collision. A third cluster at ~ 40 Ma exhibits very low Th/U ratios, and is interpreted to have crystallized in the presence of fluids associated with a high-pressure granulite facies metamorphic event during the early stages of the Indo-Asian collision, subsequent to high-pressure metamorphism in the western Himalaya syntaxis between ~ 50 and 43 Ma. A fourth cluster of zircons yields ages between 11 and 25 Ma and Th/U ratios that decrease systematically with decreasing age. We interpret the youngest zircon age ( ~ 11 Ma) to represent the timing of moderate-pressure high-grade metamorphism, with the older ages and higher Th/U ratios being a result of mixing with a restitic igneous component. This interpretation, coupled with a ~ 8 Ma⁴⁰Ar/³⁹Ar age on hornblende from a metadiorite within the core of the antiform, suggests that the Namche Barwa syntaxis has been characterized by rapid cooling and exhumation since at least Late Miocene time. Despite its contrasting structural setting, the Miocene and younger metamorphism and cooling history in the Namche Barwa syntaxis are strikingly similar to those of the Nanga Parbat syntaxis of the western Himalaya.