Accretional and Collisional Processes of the Lhasa Terrane towards the Formation of the Tibetan Plateau

This chapter evaluates the different scenarios of subduction polarity, closure timing, and geodynamic processes related to the accretion of the Lhasa Terrane based on abundant geological and geochemical observations. The accretion of the Lhasa Terrane to the southern Tibetan Plateau involves the closure of the Shiquanhe-Nam Tso Ocean between Central and Northern Lhasa subterranes and the Bangong-Nujiang Tethys between the Lhasa and Qiangtang terranes. The closure of the Bangong-Nujiang Tethys is clear, whereas the evolution of the Shiquanhe-Nam Tso Ocean remains debated and requires further investigation. The tectonic affinity of the Shiquanhe-Nam Tso mélange belt has been proposed to represent an ophiolite nappe or an imbricate of the Bangong-Nujiang suture, the south branch or back-arc basin of the Bangong-Nujiang Tethys, and an embryonic oceanic basin within the Lhasa Terrane. A Jurassic-Early Cretaceous arc-trench system, termed Beila-Nagqu in the eastern segment and Rutog-Gerze in the western segment, has been documented in the Southern Qiangtang subterrane, resulting from the northward subduction of the Bangong-Nujiang Tethys. Several distinct observations, e.g., ophiolitic formation, angular unconformities, and rock association of adakites and ocean island basalt-type rocks, as well as magmatic gap, likely reflect a series of geodynamic events including subparallel mid-ocean ridge-trench collision (ca. 170–160 Ma), slab window formation (ca. 160–148 Ma), and subsequent subduction termination (ca. 148–125 Ma). The Early Cretaceous Baingoin-Yanhu magmatic arc identified in the Northern Lhasa subterrane cannot be attributed to the low-angle northward subduction of the Neo-Tethys but related to the southward subduction of the Bangong-Nujiang Tethys. It is most likely that the Bangong-Nujiang Tethys experienced a divergent double-sided subduction before ca. 140 Ma followed by a soft collision between opposing arcs that may have occurred at ca. 140–130 Ma. A magmatic flare-up (ca. 120–110 Ma) following a short magmatic lull (ca. 130–120 Ma) in both the Southern Qiangtang and Northern Lhasa subterranes likely recorded the detachment and break-off of the Bangong-Nujiang Tethyan oceanic slab.