The South Tianshan Orogenic Belt is located between the Tarim Plate and Yili-Central Tianshan Plate. The tectonic evolution of the South Tianshan Ocean in the Paleozoic has long been debated. Litho-geochemical features of the major and trace elements of the Silurian sandstones from Akeqi area, northwestern margin of the Tarim Plate of Xinjiang,show that Chondrite-normalized REE distribution patterns of the samples are all right skewed, with high(La/Yb)_N value, low δEu value, and negative Ce anomaly. The maturity of the sandstones from the Middle to Top Silurian Tataertage Formation is higher than the samples from the Lower Silurian Kepingtage Formation. The provenance and tectonic setting of the sandstones were evaluated using ternary diagrams that highlight their geochemical characteristics. The results suggest that the provenance of the Akeqi area gradually transformed from active provenance to stable provenance. Some sediments from the Kepingtage Formation are sourced from active continental margin and continental island arc, indicating felsic igneous provenance; Sediments from the Tataertage Formation were sourced from quartzite sediment provenance of mature continent. Combined with widely distributed parallel unconformities between the Ordovician and the Lower Silurian, it can be inferred that the Akeqi area was located in an active continental margin, the South Tianshan Ocean subducted southward beneath the Tarim Plate shortly during the Late Ordovician, which provided the active provenance for the study area. From the Early Silurian, as southwards subduction ended, the tectonic setting of Akeqi area transformed from active continental margin to stable passive continental margin. Sediments from active and stable provenances gradually decreased and increased, respectively. During the deposition of Middle to Top Silurian succession, tectonic setting of the Akeqi area completely transformed to stable passive continental margin, so that sediments were exclusively sourced from stable provenances. In conclusion, this study proposes that bidirectional subduction was developed during the evolution of the South Tianshan Ocean within the Paleozoic.

Based on the core materials collected from the Borehole PGZ01,which is located at Yufa Township of Daxing district in southern Beijing plain area and has a depth of 420m,the analyses of lithostrotigraphy,magnetostratigraphy,AMS ¹⁴C dating,Ostracoda and sporopollen assemblages were carried out. The results show that: (1)The boundaries of Matuyama/Gauss polarity chron and Brunhes/Matuyama polarity chron appear at the depth of 280m and 78.35m, respectively. It suggests that the bottom strata of this borehole correspond to Gauss normal polarity zone at the bottom. Considering that the boundary of Matuyama/Gauss polarity chron at the depth of 280m is the lower boundary of Quaternary system. It suggests that the debris flow and alluvial fan or possible palaeo-Yongding River had arrived at the southern part of Beijing plain area before Quaternary. Moreover, thick-layer brownish red clay which is considered as the upper mark lithostratigraphic unit of the Pliocene occurred at the Olduvai subzone. (2)Integrated with the AMS ¹⁴C dating,the present study tentatively correlate the stratal oxidation-reduction status above the depth of 60m to correlate to cores from palaeomonsoonal-sensitive coastal plain environment,the boundary of the Upper Pleistocene and Holocene was determined to be at 56.60m and 18.95m, respectively. (3)The characteristic sediments deposited since MIS5 have a good correlation with core sediments from coastal plain environment,and show evidence of the influence of Prehistoric Flood of 8-4ka. The present paper provided important age control for subdividion and correlation of Quaternary stratigraphy,construction of palaeogeography,fault activities and tectonic history of the westnorthern margin region of Lang(fang)-Gu(an) Depression,and understanding of the environmental effect of palaeomonsoon change since the Late Pleistocene.