How the Balkhash–West Junggar Remnant Ocean was Lost During the Late Paleozoic in Central Asia

How was the Carboniferous Balkhash–West Junggar remnant ocean filled and closed? Insights from the Well Tacan-1 strata in the Tacheng Basin, NW China

Authors:

Di Li, Dengfa He, Xuefeng Qi and Ningning Zhang

Abstract:

The Balkhash–West Junggar remnant ocean is a conspicuous tectonic feature that had an important effect on the Paleozoic tectonic evolution of the Central Asian orogenic belt. How the Carboniferous Balkhash–West Junggar remnant ocean was filled and closed is poorly understood. In this study, we focus on the integrated tectonic–sedimentary evolution of the Tacheng Basin, which is located in the eastern part of the Balkhash–West Junggar remnant ocean, using high-resolution seismic reflection, well-line, geochronological and geochemical data to establish the filling of the basin by sediments and its evolution. The Carboniferous Balkhash–West Junggar remnant ocean basin was filled with Early Carboniferous deep marine mudstone and interbedded sandstone with minor andesite, basalt and tuff, and Late Carboniferous basalt, andesite, breccia and tuff and shallow marine mudstone and sandstone. The Carboniferous igneous assemblage mainly consists of basalt with high Nb content and (Nb/La)PM values and concomitant magnesian andesites with adakite geochemical characteristics of high Sr and low Y and Yb, suggesting a subduction-related tectonic setting. Zircons in the tuff that underlies the basalts and magnesian andesites were dated to ca. 315 Ma by LA–ICP–MS U–Pb. The identified unconformity between the Carboniferous and Lower Permian from seismic reflection profile in Well Tacan-1 subdivided the Carboniferous–Early Permian strata into two tectonostratigraphic units. The Lower Permian sequences overlap the Carboniferous strata and the facies transition from the submarine Carboniferous to the terrigenous Lower Permian indicates that the Balkhash–West Junggar remnant ocean closed during the Early Permian. These new data suggest that the sedimentary filling of the basin was in response to subduction and accretionary processes and that oceanic subduction was an important mechanism that led to the shrinking, filling and closure of the Balkhash–West Junggar remnant ocean because of compression. Therefore, lateral crust growth through subduction–accretionary processes still played an important role in the Carboniferous construction of western Junggar.