Abstract Radon-222 and carbon dioxide (CO 2 ) emissions were studied around four remote Nepalese thermal springs near the Main Central Thrust: Timure and Chilime in the upper Trisuli Valley, central Nepal; and Sulighad and Tarakot in Lower Dolpo, western Nepal. A total of 279 radon fluxes and 670 CO 2 fluxes were measured on the ground, complemented by radon concentration measurements in soil and water, and assisted by thermal infrared imaging. In Lower Dolpo, mean radon fluxes ranging from 270×10 −3 to 450×10 −3 Bq m −2 s −1 , radon concentration in water greater than 100 Bq l −1 , low mean CO 2 fluxes (18–32 g m −2 day −1 ), and integrated radon and CO 2 discharges of 70–180 Bq s −1 and (2.3–3.8)×10 −3 mol s −1 , respectively, suggest shallow-water-dominated transport with simultaneous radon and CO 2 degassing from the hydrothermal water. In the upper Trisuli Valley, mean radon fluxes ranging from 140×10 −3 to 570×10 −3 Bq m −2 s −1 , larger mean CO 2 fluxes that range from 430 to 2930 g m −2 day −1 , radon concentration in water of less than 6 Bq l −1 , and integrated radon and CO 2 discharges of 290–840 Bq s −1 and (390–830)×10 −3 mol s −1 , respectively, indicate fast gas-dominated transport of deep metamorphic-origin CO 2 charged in radon along a fault network. Radon can thus give precious information on the gas transport properties of the shallow continental crust. Supplementary material: Additional radon and carbon dioxide flux measurement profiles are available at https://doi.org/10.6084/m9.figshare.c.3582128

Various types of evidence, including the size and volume of the Siberian Traps, the timing and duration of eruptions, paleotectonic and paleogeographic reconstructions, lithospheric structure, heatflow, and the trace-element and radiogenic isotope compositions of lava, are reviewed in this chapter. The major evidence may be summarized as follows. The Siberian Traps erupted in a number of brief volcanic events from the Late Permian until the end of the Middle Triassic. They occupied a vast region (∼7 × 10 6 km 2 ) in a back-arc tectonic setting. The overall volume of erupted rocks was as much as ∼4 × 10 6 km 3 , with most of the volume erupted within the Tunguska syncline. This syncline experienced long-term subsidence before initiation of the volcanism, and the region is now underlain by a relatively thin lithosphere, which is ∼180 km thick. Two types of trace-element patterns are observed in the Siberian Traps: subordinate high-Ti ocean island basalt–like patterns and dominant low-Ti island arc basalt–like patterns. In radiogenic isotope and trace-element coordinates, mixing trends between these two types of magma are absent, or at least not evident. Some volcanic rocks contain primary magmatic mica. These are considered in light of different models. Each model can explain, or was thought to explain, particular observations. However, some evidence can be fatal for some models. For example, the enormous size and volume of the Siberian Traps cannot be explained in the framework of impact and edge-driven convection models and are problematic for lithospheric delamination models. Plume models face problems in explaining the uplift and subsidence pattern and the absence of mixing curves between expected high-Ti primary plume melts and contaminated low-Ti melts. Therefore, a model that relates Siberian Trap magmatism and subduction is suggested. In this model, subducting slabs brought significant amounts of water into the mantle transition zone. Consequent release of water from the transition zone lowered the solidus of the upper mantle, leading to voluminous melting. Major supporting observations for this model include (1) the tectonic position of the Siberian Traps in a back-arc setting of Permian subduction systems, (2) island arc basalt–like trace-element patterns for the majority of the erupted basalts, (3) primary mica found in volcanic rocks, and (4) experimental data on the high water capacity of the mantle transition zone and its recharging via the subduction process.