Silurian–Devonian construction of active continental margin in the southwestern Paleo-Asian Ocean witnesses the modification of fluvial system by early land plants

Abstract

In accretionary orogeny, the intra-oceanic subduction-accretion could generate juvenile continental crustal materials and form new continental blocks by their lateral tectonic accretion, and the juvenile continental crust would provide a sialic basement for continental margin arc during the subsequent oceanic-continental subduction-accretion. These two distinct subduction-accretion processes occurred successively in the southwestern part of the Paleo-Asian Ocean during the Neoproterozoic–Late Paleozoic and created a multi-stage evolution of continental crusts in West Junggar. In this study, a Late Silurian landmass in West Junggar is revealed by the detailed geological mapping and comparison of the Late Silurian–Early Devonian angular unconformity in southern and northern West Junggar. This crucial unconformity confines the Middle Silurian termination of Early Paleozoic intra-oceanic subduction-accretion, as evidenced by the pre-Late Silurian emplacement of Cambrian–Ordovician ophiolitic mélanges, the Middle Silurian cessation of Early Paleozoic arc magmatism and the Late Silurian–Early Devonian sedimentary hiatus. The Late Silurian landmass was consequently constructed by amalgamations of island arcs, accretionary complexes and seamounts, and its break-up was marked by the Middle Devonian opening of the Karamay back-arc Ocean in central West Junggar. In contrast with the Early Paleozoic intra-oceanic arc systems, continental margin arcs on the sialic basement of rifted landmass emerged since the Middle Devonian, as suggested by our new chronological data and field observations. We further unravel that the increase in mudstone of Devonian successions on the active continental margin is contemporaneous with the emergence and boom of lycopsid forest in West Junggar, which is in agreement with a global trend in alteration of fluvial systems due to evolution of early land plants. In summary, this paper presents a typical record for two distinguished styles of subduction-accretion systems that developed successively in an accretionary orogeny, and sedimentary successions of the later active continental margin documented the interaction between terrestrial sedimentation and early land plants.