Contemporaneous closure of the Paleo-Asian Ocean in the Middle-Late Triassic: A synthesis of new evidence and tectonic implications for the final assembly of Pangea

The closure of the Paleo-Asian Ocean (PAO) is crucial for understanding the late Paleozoic–early Mesozoic paleogeography of Proto-Asia and the tectonic configuration of NE Pangea. However, the timing and mechanism for the PAO closure and final amalgamation of the Central Asian Orogenic Belt (CAOB) remain highly controversial. Available studies argue either for a pre-Carboniferous closure or eastward, scissor-style diachronous closure from late Carboniferous to late Permian. This ambiguity is mainly caused by the different criteria applied to evaluate the final closure of oceanic basins and termination of accretionary orogenesis. Here we summarize new evidence including the youngest mélange-bearing accretionary complex and provenance change of syn-orogenic sediments in order to trace the terminal closure of the PAO from Tianshan in the west, through Beishan and Alxa, to Solonker and Changchun-Yanji in the east, a total distance of ∼5000 km.

The Tianshan marks the demise of the western segment of the PAO where the Tarim Craton collided with the Kazakhstan–Yili–Central Tianshan arc. Several ophiolitic mélanges from the Kyrgyz and Chinese Tianshan show the relicts of Permian–Triassic oceanic lithosphere that amalgamated during the Middle-Late Triassic. In Beishan, three ophiolitic mélanges contain Permian igneous rocks and Permian–Triassic sedimentary matrix, suggesting closure of the ocean between ∼245–222 Ma. In the Engger Us mélange of Alxa, a turbidite matrix overlying Permian ocean plate stratigraphy yields a ∼244 Ma maximum depositional age, constraining the time of emplacement of the accretionary complex. To the east, several ophiolitic mélanges in Inner Mongolia contain Permian–Early Triassic gabbro/basalt and middle-late Permian radiolarians, suggesting subduction-accretion continued into the Early Triassic. Along the Changchun–Yanji suture in NE China, several accretionary complexes yield emplacement ages of ∼246–234 Ma. Therefore, the youngest ophiolitic mélanges and accretionary complexes across the whole southern CAOB were formed during the Middle-Late Triassic. In addition, Middle-Late Triassic provenance changes have been identified in syn-orogenic sediments in Tianshan, Beishan, Alxa, Inner Mongolia and NE China. These data are in agreement with Triassic deformation and high-pressure metamorphism, which together indicate that the western, central, and eastern segments of the PAO were closed almost contemporaneously during the Middle-Late Triassic.

To the south, multidisciplinary data suggest that the amalgamation between South China, Sibumasu, Indochina, Qiangtang, and North China took place between ∼230–225 Ma, which led to the closure of the Paleo-Tethys Ocean. Taken together, the Indosinian orogeny leading to the amalgamation of the East Asian blocks marks the final assembly of Pangea. The joining of East Asia in the Late Triassic maximized the land area of Pangea, which potentially fostered the dramatic climate change and formation of the megamonsoon.