Wei Dan

The Songpan–Garzê terrane is the largest Triassic remnant flysch basin on Earth and formed as the Palaeo-Tethys Ocean closed during the final amalgamation of the Pangaea supercontinent. However, the origin of the Songpan–Garzê terrane is highly controversial. A synthesis of the tectonic evolution of the Palaeo-Tethys Ocean and its branches surrounding the Songpan–Garzê terrane is presented, which clarifies the nature and relationships among the many Palaeo-Tethys sutures. Provenance analyses suggest that branches of the Palaeo-Tethys near the Songpan–Garzê terrane closed before the Early Triassic. In contrast, the main Palaeo-Tethys Ocean (Longmu Co-Shuanghu) did not close until the beginning of the Late Triassic. This study argues against the Songpan–Garzê terrane being a remnant ocean basin, and proposes that it was a back-arc basin of the main Palaeo-Tethys Ocean. It initially underwent extension by the combined effects of the main Palaeo-Tethys Ocean subduction and the Emeishan mantle plume in the Late Permian, and subsequently developed into a back-arc basin in the Triassic, into which huge turbiditie units were deposited derived from all surrounding terranes and orogens. The final closure of the main Palaeo-Tethys Ocean in the beginning of the Late Triassic and subsequent continent–continent collision led to basin inversion in the Late Triassic.

The drift history of the Lhasa terrane from Gondwana to Asia plays a crucial role in understanding the Tethys evolution and true polar wander (TPW). However, few reliable paleomagnetic results from Jurassic strata are currently available for reconstructing its northward journey. We performed a combined paleomagnetic and geochronological study on Bima Formation strata in the Xigaze area. Combined with previous results from the Sangri area, our results reveal a paleolatitude of 8 ± 4°S at ∼180 Ma for the reference point (29.3°N, 90.3°E). Along with other paleomagnetic results from the Triassic to Cretaceous, our new results suggest that the Lhasa terrane motion accelerated from ∼2 cm/yr during ∼220–180 Ma to ∼17 cm/yr during ∼180–170 Ma. Paleolatitude information of the North Qiangtang terrane and Tethyan Himalaya is calculated from paleopoles that meet five criteria, which include (a) structural control, (b) well‐determined rock age, (c) stepwise demagnetizations, (d) a minimum of 25 specimens or 8 sites are contained, and (e) robust field or reversal tests are provided. Both terranes also show significant acceleration during their northward motion, which may be related to oceanic slab subduction. Thus, all Gondwana‐derived microcontinents seem to share a significant acceleration during their northward motion. In addition, recent paleomagnetic results from volcanic rocks dated at ∼155 Ma subdivide the overall northward motion during ∼170–130 Ma into two stages, which include a southward drift during ∼170–155 Ma followed by northward motion during ∼155–130 Ma. These results support the fast Late Jurassic TPW during a ∼10 Myr time span.

An evaluation of the potential geodynamic connections between the evolution of Paleozoic oceans in NW Gondwana and NE Gondwana is challenging. Until recently, most syntheses emphasized only two Paleozoic oceans (the Proto-Tethys and the Palaeo-Tethys) in the east Tethys realm. However, the discovery of early Paleozoic ophiolites along Palaeo-Tethys sutures located south of Proto-Tethys sutures challenges these traditional views. After a comprehensive review of relevant early Paleozoic tectonomagmatic events, we herein recognize and propose a model for the tectonic evolution of a hitherto unrecognized early Paleozoic ocean, which we call the Proto-Qiangtang Ocean. This ocean was short lived; it opened in the late Cambrian, began to subduct in the Middle Ordovician, and closed diachronously westwards between the Late Ordovician and the middle Silurian. Its closure by middle Silurian time indicates that was a spatially and temporally distinct ocean from the Palaeo-Tethys Ocean. The early tectonic evolution of the Proto-Qiangtang Ocean shares many characteristics with that of the Rheic Ocean. Both opened in the late Cambrian in the back-arc region of the Iapetus–Proto-Tethys Ocean, and the Proto-Qiangtang Ocean is considered to represent the eastern extension of the Rheic Ocean. This correlation has important implications for the Paleozoic tectonic evolution and palaeogeography of northern Gondwana.

Academic Paper published by IAFOR under Creative Commons Attribution 4.0 Attribution Non Commercial International

It is widely thought that oceanic subduction can trigger cratonic keel delamination, but the southwestern Yangtze craton (SYC; southwestern China) lost its lower keel during Cenozoic continental collision. The upper mantle beneath the thinned SYC contains its incompletely delaminated keel, which has high-velocity seismic anomalies. Combining geophysical observations with the geochemistry of Eocene mafic potassic lavas derived from the SYC mantle at different depths, we suggest that the deep (~130 km) delaminated lithosphere was more fertile and dense, with low-forsterite (Fo; molar 100 × Mg/[Mg + Fe] = 91.3) and high-δ¹⁸O (5.9‰) olivine, than the shallow (~55 km) intact lithosphere (Fo = 94.2; δ¹⁸O = 5.2‰), although both were rehydrated and oxidized. The deep keel underwent strong refertilization and densification owing to the addition of Fe-rich basaltic melts during earlier oceanic subduction. Subduction-driven refertilization and subsequent collision-driven cooling caused cratonic keel delamination due to compositional and thermal densification rather than hydration- or oxidation-induced rheological weakening. Our study provides an example of Cenozoic cratonic keel delamination in a collisional orogen and highlights the key roles of compositional and thermal densification in delamination during subduction and collision.

Long‐distance lateral magma propagation may reflect the complex interaction between regional tectonics and crustal uplift. Here, we report observations on the Taxkorgan intrusive complex (TIC) of the Central Pamir. TIC is a 70 km long, 10–18 km wide, north‐northwest–elongated body intruded into mid‐upper crust in parallel with, and close to, the Kongur Shan Extensional System. New zircon U‐Pb dating shows that syenite samples exhibit a clear younging pattern from south to north along the pluton axis within an interval between 11.5 and 10.3 Ma, which is interpreted as lateral magma propagation from a single feeder zone at its northwestern margin. The lateral magma propagation was caused by a combination of crustal rigidity differences, and a topography‐induced pressure gradient that is nearly orthogonal to the least compressive stress. We propose that long‐distance lateral magma propagation plays an important role in plateau expansion, restraining the thickness and elevation of the Pamir Plateau.

The evolution of the western Kunlun‐Pamir region involved the opening and closing of several branches of the Paleo‐Tethys Ocean, although the specific timing of these events is poorly constrained. Here, we present a synthesis of sedimentary, magmatic, and metamorphic records associated from the Mazar‐Kangxiwa suture zone in the western Kunlun‐Pamir that is generally regarded as the main Paleo‐Tethys Ocean suture. These data show that the Paleo‐Tethyan oceanic basin opened at ca. 340 Ma and closed by ca. 250 Ma, and there is no record of a magmatic arc between ca. 300–250 Ma. The absence of a magmatic arc was a result of oceanic crust underthrusting, rather than oceanic subduction, which is consistent with a narrow back‐arc basin. Our study provides an important example of how an oceanic basin opened and closed without oceanic subduction, and highlights a potential mechanism to account for the absence of a magmatic arc.