The Tarim–North China Plate Boundary: Evidence from Stratigraphic and Paleontological Records of the Engger Us Structural Belt

The Engger Us Tectonic Belt, located in the Alxa area of Inner Mongolia along the southern margin of the Central Asian Orogenic Belt (CAOB), extends intermittently in the NEE direction for over 800 km. Traditionally, it is recognized as the final collisional suture zone between the Tarim and North China plates, its tectonic affinity and reliability as a plate boundary remain controversial. This paper systematically synthesizes stratigraphic and paleontological data with basement attribute differences of terranes on both sides, to provide comprehensive evidence for defining the Tarim-North China plate boundary. Significant crustal attribute divergence exists across the belt, separating the southern Alxa Block (North China Craton) from the northern Central Asian Orogenic Belt (accretionary). The northern Shalazhashan-Zongnaishan Tectonic Belt, formed by subduction-accretion of the Paleo-Asian Ocean, is characterized by juvenile crust with young basement. Late Paleozoic granites show predominantly positive zircon εHf(t) values and younger model ages. In contrast, the southern Nuoergong-Langshan Tectonic Belt (Alxa Block) consists of ancient continental crust reworked during activation, characterized by negative zircon εHf(t) values in magmatic rocks. Such distinct differences confirm the belt as a boundary between two crustal blocks with independent evolutionary histories, consistent with a plate suture zone. The discovery of Early Permian SSZ-type ophiolites in northern Alxa Youqi indicates formation in a back-arc basin, confirming the belt as a suture and indicating the Paleo-Asian Ocean closure at least until the Early Permian (Zheng et al., 2024). Key evidence comes from ocean plate stratigraphy (OPS), including N-MORB pillow basalts (302±14 Ma), radiolarian cherts, and deep-sea genetic minerals (rhodochrosite, pyrolusite). These represent ancient oceanic crust remnants and deep-sea sediments, confirming the belt as the closure site of the Paleo-Asian Ocean. Paleontological data constrains sedimentary environments and ages. Xie et al. (2014) identified Early to Late Permian radiolarian (Pseudoalbaillella elegans assemblages) in siliceous rocks, confirming these as exotic blocks in ophiolitic mélange. This study extracted well-preserved radiolarian fossils, including the index taxa of the genus Pseudoalbaillella (e.g., P. longtanensis, P. scalprata m. scalprata, P. globosa, P. sakmarensis) and Latentifistularia / Entactinaria groups, that constrains the strata age as Middle-Late Middle Permian. Bu et al. (2013) found Early-Middle Permian Zhesi brachiopod fauna. OPS analysis shows the section is non-Smith type, where local stratigraphic order does not reflect the overall tectonic mélange. In summary, the Engger Us Tectonic Belt preserves a complete OPS sequence and key paleontological fossils, and separates juvenile crust (north) from ancient continental crust (south). The evidence confirms it as the final suture zone between the Tarim and North China plates, formed during the Paleo-Asian Ocean closure that lasted at least until Middle Permian.