EARLY CENOZOIC EVOLUTION OF THE WESTERN HOH XIL BASIN, CENTRAL TIBETAN PLATEAU

This research explores the early Cenozoic evolution of the western Hoh Xil basin, through sedimentary facies analysis, provenance and stable isotope studies to understand the tectonic setting in which the basin formed. Sedimentary sequences in the western Hoh Xil basin include the Kangtuo and Suonahu Formations (Fm). Regional comparisons and our youngest detrital zircon age (30.7±0.6 Ma) indicate that these two formations were deposited during middle Eocene–early Oligocene time. Detrital zircon spectra further suggest that the Suonahu Fm is time-equivalent with the Kangtuo Fm, rather than unconformably overlying the latter as previously interpreted.

The Kangtuo Fm outcrops in the southern part of the western Hoh Xil basin and consists of mainly alluvial fan and braided river facies associations. The Suonahu Fm, distributed widely in the central and northern basin, thins northward and is dominated by delta front, delta plain, marginal lacustrine, and evaporite facies associations. Paleocurrent directions suggest northward flow for the Kangtuo Fm. Detrital zircon data indicate sediments were mainly sourced from the northern Qiangtang terrane. In sum, the source region, paleocurrent directions, fining and thinning northward associated with active shortening in the Qiangtang terrane are consistent with deposition in a flexural basin setting with subsidence controlled by loading of the fold-thrust belt to the south.

Cement carbonates from the Kangtuo Fm have d¹⁸O values ranging between -7 and -11‰. However, samples with similar ages from restricted shortening-related basins to the south of the Hoh Xil basin show more negative values (-11 to -13‰), perhaps indicating higher paleoelevation (e.g., ~ 1 km) than the western Hoh Xil basin in the early Cenozoic, consistent with their formation in a fold-thrust belt setting. The d¹⁸O values of the Suonahu Fm range between -8 and -5‰, more positive than the Kangtuo Fm, but indistinguishable from limestones of the Neogene Wudaoliang Fm. We suggest that the d¹⁸O values of the Kangtuo Fm more closely reflect precipitation signals, whereas the higher values of the lacustrine Suonahu Fm reflect evaporative enrichment of lake water.