Quaternary celestine and gypsum extensional veins in a folded hypersaline lake infill: the Qaidam basin, wes- tern China

The nonmarine, at present hypersaline Qaidam basin of Western China comprises a ca. 8–17 km thick sedimentary fill, which is dominated by Eocene to Recent clastic rocks. These successions were folded due to ongoing India-Eurasia convergence from late Miocene onwards until today. Shortening also resulted in enhanced fluid flow and precipitation of sulphate minerals in subvertical tension gashes, which are mainly filled by celestine and fibrous gypsum. The orientation of subvertical veins is similar over most parts of the western Qaidam basin and indicates NNE-SSW to N-S shortening (present-day coordinates) dissimilar to the NE-motion direction deduced from recent geodetic (GPS) data. In deeper stratigraphic levels of anticlinal cores, tension gashes composed of fibrous gypsum subparallel to bedding demonstrate fluid overpressure during their formation at a shallow structural level, below the conversion temperature of gypsum to anhydrite. We interpret all these veins as near-surface precipitates from sulphate-rich brines 87 86 during an advanced stage of folding. The Sr/ Sr ratios of a synsedimentary celestine layer and a celestine vein at the abandoned Dafengshan mine are 0.711414 and 0.711418, respectively, and are similar to those from Oligocene and Miocene limestones (0.711578 to 0.711679). Although the source area is highly heterogeneous in composition, these data indicate, therefore, in accor87 86 dance with previous observations on clastic rocks, only a very minor change in the average Sr/ Sr ratio of the source region from