Timing the Hegenshan Suture in the Central Asian Orogenic Belt: New Paleomagnetic and Geochronological Constraints From Southeastern Mongolia.

The timing of formation of the Hegenshan suture is crucial to understanding the evolution of the Central Asian Orogenic Belt (CAOB). Here we present a well‐dated paleomagnetic pole (103 samples from 15 sites, 36.7°N/29.2°E, A95 = 3.3°) that passed a positive fold test, a reversal test, and a conglomerate test from the ∼250 Ma upper Gunbayn Formation andesites in the southeastern Mongolia Block (MOB). The high‐quality paleomagnetic database may demonstrate that the MOB drifted rapidly southward and collided with the Xilinhot–Songliao Block between 256 and 250 Ma, resulting in the closure of the Hegenshan Ocean at ∼250 Ma. The Hegenshan suture and the Solonker suture were formed almost simultaneously and marked the final closure of the Paleo‐Asian Ocean. Plain Language Summary: The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogenic belt on earth since begin of the Phanerozoic. Since the Late Paleozoic, two prominent sutures, namely the Hegenshan and Solonker sutures, have played crucial roles in the tectonic evolution of the eastern CAOB, as they were associated with the closure of the Paleo‐Asian Ocean (PAO). The timing of the formation of the Hegenshan suture, however, remains ambiguous due to the lack of reliable paleomagnetic data. In this study, we present accurately dated paleomagnetic constraints on the paleogeographic position of the Mongolia Block (MOB), located on the northern side of the Hegenshan suture, at ∼250 Ma. By combining high‐quality Late Permian–Early Triassic paleomagnetic data with geological evidence, we propose a new reconstruction that suggests the near‐simultaneous closure of both the Hegenshan and Solonker oceans around 250 Ma. This significant event marks the final disappearance of the PAO and represents a fundamental stage in the establishment of the tectonic framework of the eastern CAOB at that time. Key Points: We report a well‐dated ∼250 Ma paleomagnetic pole from the Mongolia BlockThe closure of both the Hegenshan and Solonker oceans at ∼250 Ma marked the final disappearance of the Paleo‐Asian Ocean [ABSTRACT FROM AUTHOR]