EVOLUTION OF MULTI-ARC ACCRETION SYSTEMS: A CASE STUDY FROM THE SOUTHERN BEISHAN OROGEN, CENTRAL ASIAN OROGENIC BELT, NW CHINA

Abstract

The Central Asian Orogenic Belt (CAOB) is characterized as an accretionary orogen, evolved for ca. 800 m.y from late Mesoproterozoic to early Mesozoic. Its terminal collision and accretion generally commenced in the west and terminated in the east, creating the Tien Shan suture and Solonker suture, respectively. The Beishan Orogen is a southern-central segment of the Central Asian Orogenic Belt, located in Northwest China. It connects the two sutures and contains some of the youngest ophiolitic complexes, arc terranes, and sedimentary basins in the CAOB. Hence, investigations of the southern Beishan Orogen can contribute to correlating the geology on both sides and establishing an integrated tectonic history of the Paleo-Asian Ocean’s closure. Through detailed mapping at 1:25000 scale in southern Beishan, five distinctive tectono-stratigraphic packages have been identified, from south to north, including the Shiyaozi Complex, the Baidunzi Complex, the Ganquan Complex, the Liuyuan Complex, and a cover sequence forming the Heishankou Basin. The Baidunzi Complex comprises a mixture of sheared and transposed metasedimentary and syn-kinematic meta-plutonic rocks, ranging from greenschist to amphibolite facies. The metasedimentary sequences contain Carboniferous-Permian marble and meta-calcareous clastic rocks, which were intruded by syn-tectonic arc magmatic rocks (Baidunzi Intrusive Suite). Collectively these rocks represent a continental arc setting. A massive sheet of bedded quartzite in this complex, displaying no age signatures younger than 1.0 Ga, stands as the most probable candidate for Precambrian basement, with an inferred affinity to the Tarim Craton. The Baidunzi Complex was overthrusted by the Ganquan Complex, which primarily consists of two sequences separated by unconformities and thrust faults. The lower sequence exhibits minimal reworking and lacks continent-derived materials in its stratigraphy. U-Pb zircon dating of this sequence yielded ages ranging from 295 to 285 Ma, without zircon inheritance. In contrast, the upper sequence is dominated by epiclastic sedimentary rocks, structurally and/or unconformably overlying rocks of the lower sequence. Both sequences display geochemical characteristics typical of island arcs, with an increase of εHf values over time. They correlate temporally and tectonically with the adjacent back-arc ophiolitic rocks within the Liuyuan Complex, an ophiolite mélange formed in a backarc setting. Together they represent an arc-backarc system, with arc-trench migration leading to the progressive incorporation of juvenile components in the source. A cover sequence forming a Permian foreland basin, with typical fold-and-thrust style deformation, developed unconformably on the Ganquan and Liuyuan Complexes. From bottom to top, it consists of black shale, an unconformably overlying flysch-like sedimentary unit, and a syn-orogenic conglomerate with clasts of rocks from all the underlying sequences. The sedimentary record and structural analyses of this basin suggest south-verging deformation, associated with a collisional event involving the Ganquan arc-Liuyuan Backarc system over the Baidunzi Complex. Subsequently, this thrust-and-fold belt experienced a phase of north-verging deformation, aligning with the amalgamation of the Shiyaozi Complex from the southernmost region toward the Baidunzi Complex. The overall field relationships record a transition from a passive to an active margin (Baidunzi Complex), formation and accretion of a relatively juvenile arc, and collision during the Permian.