Heat Flow and Thermal Source of the Xi’an Depression, Weihe Basin, Central China

The Xi’an Depression of the Weihe Basin, located in the transition zone where the North China, Qinling and Yangtze plates collide with each other, is an important area of geothermal energy utilization in China. Studies of heat flow and thermal sources are of great significance to the exploration and development of geothermal resources in this area. In this paper, six temperature logs boreholes, and 14 thermal conductivity samples have been used to study the geothermal gradient and terrestrial heat flow in the area. The results show that the geothermal gradients of Xi’an Depression range from 20.8 C/km to 49.1 C/km, with an average of 31.7 ± 5.0 C/km. The calculated heat flow is 59.4–88.6 mW/m2, and the average value is 71.0 ± 6.3 mW/m2, which indicates a high thermal background in the area. The high anomalous zones are near the Lintong-Chang’an Fault zone in the southeast, the Weihe Fault in the north, and the Fenghe Fault in the central Xi’an Depression. These deep and large faults not only control the formation of the Xi’an Depression but also provide an important channel for the circulation of groundwater and affect the characteristics of the shallow geothermal distribution. The temperature of the Moho in the Xi’an Depression ranges from 600 to 700°C, and the thermal lithosphere thickness is about 90–100 km. The characteristics of lithospheric thermal structure in Xi’an Depression indicate that the higher thermal background in the study area is related to lithospheric extension and thinning and asthenosphere thermal material upwelling.

Scientific drilling workshop on the Weihe Basin Drilling Project (WBDP): Cenozoic tectonic–monsoon interactions

The Weihe Basin, enclosed by the Chinese Loess Plateau to the north and the Qinling Mountains to the south, is an outstanding, world-class continental site for obtaining high-resolution multi-proxy records that reflect environmental changes spanning most of the Cenozoic. Previous geophysical and sedimentary studies indicate that the basin hosts 6000–8000 m thick fluvial–lacustrine sedimentary successions spanning the Eocene to Holocene. This sedimentary record provides an excellent and unique archive to decipher long-term tectonic–climate interactions related to the uplift of the Tibetan Plateau, the onset/evolution of the Asian monsoon, and the development of the biogeography of East Asia. Owing to its location at the interface of the opposing westerly and Asian monsoon circulation systems, the Weihe Basin also holds enormous promise for providing a record of changes in these circulation systems in response to very different boundary conditions since the Eocene. To develop an international scientific drilling programme in the Weihe Basin, the Institute of Earth Environment, Chinese Academy of Sciences, organized a dedicated workshop with 55 participants from eight countries. The workshop was held in Xi'an, China, from 15 to 18 October 2019. Workshop participants conceived the key scientific objectives of the envisaged Weihe Basin Drilling Project (WBDP) and discussed technical and logistical aspects as well as the scope of the scientific collaboration in preparation for a full drilling proposal for submission to the International Continental Scientific Drilling Program (ICDP). Workshop participants mutually agreed to design a two-phase scientific drilling programme that will in a first phase target the upper 3000 m and in a second phase the entire up to 7500 m thick sedimentary infill of the basin. For the purpose of the 7500 m deep borehole, the world's only drill rig for ultra-deep scientific drilling on land, Crust 1, which previously recovered the entire continental Cretaceous sediments in the Songliao Basin, will be deployed in the WBDP.