Tertiary deformation and metamorphism SE of Tibet: The folded Tiger‐leap décollement of NW Yunnan, China

Abstract

The Yulong‐Haba Xue Shan range, in the northwestern part of Yunnan (China), is a large N‐S antiform that folds the Paleozoic series of the Yangzi platform. The upper Yangzi River (Jinsha Jiang) has cut a 3500 m‐deep valley (Hu Tiao gorge) across this antiform, thus exposing folded, bedding‐parallel, ductile shear zones (décollements), with transport toward the SSW (in the present geographical coordinates). The large finite shear strain implies tens of kilometers of transport, pointing to the regional significance of these décollements. Rb/Sr radiometric dating of phlogopites that crystallized in marbles within the foliation planes yields the age of the metamorphic and deformation event (35.9 ± 0.3 (2σ) Ma). The age derives from an internal Rb‐Sr isochron, made on different size fractions of the same mineral, which provides a novel demonstration of the feasibility of such plots. Transport on the décollement and related shortening occurred prior to, or at the onset of, extrusion of Indochina along the Ailao Shan‐Red River shear zone, ≈80 km west of the Yulong Shan. The 39Ar/40Ar age spectra of K‐feldspar from the core of the Yulong Shan suggest uplift by antiformal folding around 17 Ma, as Indochina's extrusion came to an end. We infer that other large‐scale Cenozoic décollements such as that exhumed in the Yulong Shan underlie some of the vast, folded areas that surround the eastern Himalayan syntaxis. Transport on such décollements, first toward the south and then toward the east, and folding above them, might have occurred during two principal shortening phases, whose ages bracket Indochina's escape toward the SE.