Metamorphic rocks in central Tibet: Lateral variations and implications for crustal structure

Insights about lateral variations in the age, composition, and structure of the central Tibetan crust are provided by geologic investigations of metamorphic rocks in the Qiangtang terrane. Previous studies have shown that a tectonic mélange of Triassic age with blueschist- and eclogite-bearing blocks within a greenschist-facies matrix is exposed over an E-W distance of ~600 km in the central Qiangtang terrane. New mapping shows that the mélange extends over a N-S distance of ~150 km, nearly to the trace of the early Mesozoic Jinsha suture in the north. The mélange , exposed structurally beneath Upper Paleozoic to Mesozoic strata in the footwalls of early Mesozoic normal faults, is composed mostly of Paleozoic metasedimentary and crystalline rocks. These findings support the hypothesis that a large part of the central and northern Qiangtang terrane crust is composed of supracrustal rocks. The Duguer Range, <25 km south of the southernmost mélange, exposes high-temperature, low-pressure metasedimentary rocks and ortho-gneisses with 476-471 Ma U-Pb crystallization ages that represent continental basement of the Qiangtang terrane. The boundary at the surface between mélange and Gondwanan basement corresponds closely with abrupt changes in crustal geophysical properties, and allows us to extrapolate this relationship to depth. The resultant crustal structure can be explained by development of major Mesozoic structures documented in surface geology. Antiformal geophysical features imaged in the midcrust are likely inherited from Mesozoic tectonics, suggesting that the crust may not have been as mobile as that required by models invoking large-scale lower-crustal flow beneath the central Tibet Plateau during the Cenozoic India-Asian collision.