Radiolarian‐based study on the fabric and the formation process of the Early Cretaceous mélange near Zhongba, Yarlung–Tsangpo Suture Zone, southern Tibet

Abstract

The Yarlung–Tsangpo Suture Zone (YTSZ), as the southernmost and youngest among the sutures that subdivides the Tibetan Plateau into several east–west trending blocks, marks where the Neo‐Tethys was consumed as the Indian continent moved northward and collided against the Eurasian continent. Mélanges in the YTSZ represent the remnants of the oceanic plate through subduction and collision. Mélanges are characterized by a highly sheared volcanoclastic or siliceous mudstone matrix including blocks of chert, claystone, and basalt. Detailed radiolarian analyses are conducted on the mélange near Zhongba County. Macroscopic, mesoscopic, and microscopic observations are combined in order to elucidate the relationships among age, lithology, and structure of blocks in the mélange. Reconstructed ocean plate stratigraphy includes Lower Jurassic limestone within the chert sequence accumulated at a depth near the CCD (Unit 2), Upper Jurassic thin‐bedded chert interbedded with claystone deposited in the wide ocean basin (Unit 3), and Lower Cretaceous chert with siliceous mudstone (Units 4 and 5), representing the middle parts of ocean plate stratigraphy. The results highlight the fabric of brecciated chert on mesoscopic scale, which is thought to be due to localized overpressure. The formation of mesoscopic and microscopic block‐in‐matrix fabrics in the mélange is proposed for the chert and siliceous mudstone bearing different extents of consolidation and competence during the progressive deformation of accreted sediments at shallow‐level subduction.

Thematic section: Proceedings of InterRad XV

“…Li et al () investigated the radiolarian‐based ocean plate stratigraphy of the mélange and reconstructed the subduction–accretion process in the western sector of the Yarlung Tsangpo Suture Zone, southern Tibet. The reconstructed ocean plate stratigraphy includes Lower Jurassic limestone within the chert sequence that accumulated at depth near the CCD, Upper Jurassic thin‐bedded chert interbedded with claystone deposited in the wide ocean basin, and Lower Cretaceous chert with siliceous mudstone, representing the middle parts of the ocean plate stratigraphy.…”