Linking deeply-sourced volatile emissions to plateau growth dynamics in southeastern Tibetan Plateau

Does Large-Scale Crustal Flow Shape the Eastern Margin of the Tibetan Plateau? Insights From Episodic Magmatism of Gongga-Zheduo Granitic Massif

The mechanisms driving crustal deformation and uplift of orogenic plateaus are fundamental to continental tectonics. Large-scale crustal flow has been hypothesized to occur in eastern Tibet, but it remains controversial due to a lack of geologic evidence. Geochemical and isotopic data from Cenozoic igneous rocks in the eastern Tibet-Gongga-Zheduo intrusive massif, provide a way to test this model. Modeling results suggest that Cenozoic magmas originated at depths of ∼30-40 km, the depth that crustal flow has been postulated to occur at. Detailed isotopic analyses indicate that the igneous rocks are derived from partial melting of the local Songpan-Ganzi crust, arguing against a long-distance crustal flow. Episodic magmatism during the Cenozoic showing a repeated shifting of magmatic sources can be correlated with crustal uplift. The continued indentation of the Indian Block and upwelling of the asthenosphere contribute to the crustal deformation, magmatism, and uplift.

Limited underthrusting of India below Tibet: 3He/4He analysis of thermal springs locates the mantle suture in continental collision

Our regional-scale geochemical dataset (³He/⁴He) resolves the geometry of the continental collision between India and Asia. Geophysical images have led to contradictory interpretations that India directly underthrusts Tibet as a horizontal plate or India subducts steeply into the mantle. Helium transits from mantle depths to the surface within a few millennia, such that the ratio of mantle-derived ³He to dominantly crust-derived ⁴He provides a snapshot of the subsurface. ³He/⁴He data from 225 geothermal springs across a >1,000-km-wide region of southern Tibet define a sharp boundary subparallel to the surface suture between India and Asia, just north of the Himalaya, delineating the northern limit of India at ∼80-km depth. The India–Asia collision resembles oceanic subduction with an asthenospheric mantle wedge.

During continent–continent collision, does the downgoing continental plate underplate far inboard of the collisional boundary or does it subduct steeply into the mantle, and how is this geometry manifested in the mantle flow field? We test conflicting models for these questions for Earth’s archetypal continental collision forming the Himalaya and Tibetan Plateau. Air-corrected helium isotope data (³He/⁴He) from 225 geothermal springs (196 from our group, 29 from the literature) delineate a boundary separating a Himalayan domain of only crustal helium from a Tibetan domain with significant mantle helium. This 1,000-km-long boundary is located close to the Yarlung-Zangbo Suture (YZS) in southern Tibet from 80 to 92°E and is interpreted to overlie the “mantle suture” where cold underplated Indian lithosphere is juxtaposed at >80 km depth against a sub-Tibetan incipiently molten asthenospheric mantle wedge. In southeastern Tibet, the mantle suture lies 100 km south of the YZS, implying delamination of the mantle lithosphere from the Indian crust. This helium-isotopic boundary helps resolve multiple, mutually conflicting seismological interpretations. Our synthesis of the combined data locates the northern limit of Indian underplating beneath Tibet, where the Indian plate bends to steeper dips or breaks off beneath a (likely thin) asthenospheric wedge below Tibetan crust, thereby defining limited underthrusting for the Tibetan continental collision.

Long-Lasting Boiling-Wells: Geochemical Windows into the Tectonic Activity of the Maodong Fault (China)

The Maodong Fault (China) was mainly active during the Late Pleistocene. However, in the past century, numerous destructive earthquakes have occurred along the fault zone, indicating its continuing activity. Therefore, refined monitoring of the tectonic activity along the fault is required. Boiling-Wells located in the Maodong Fault Zone were selected for this purpose. The parameters, including the rare earth elements (REE) and gas components, such as CO2, Rn, and Total Volatile Organic Compounds (TVOC), in the wells were analyzed. By combining field observations with the analytical data, we constrained the relationships between the anomalies of the hydrochemical composition and the gas composition in the Boiling-Wells and the Maodong Fault: (1) CO2 and TVOC in the Boiling-Wells originated from Cenozoic magmatism and associated intrusive rocks. High concentrations of Rn are closely linked to tectonic activities of the Maodong Fault. CO2, TVOC, and Rn are all transported to the Boiling-Wells along the Maodong Fault, with CO2 acting as a carrier gas for Rn. (2) REE in the Boiling-Wells was mainly sourced from CO2 fluids that originated from deep-seated Cenozoic magmas and intrusive rocks. The concentrations of the REE and their distribution patterns were controlled by the input of CO2 fluids and by epigenetic processes. (3) The abnormally high contents of Ca2+, HCO3−, Pb2+, and Al3+ in the Boiling-Wells are attributed to the migration of externally-derived (deep) CO2 fluids through the Maodong Fault. (4) The anomalies of the gaseous (Rn, CO2, and TVOC) and hydrochemical components (Ca2+, HCO3−, Pb2+, Al3+, ∑REE, and REE patterns) in the Boiling-Wells are closely related to the tectonic activity of the Maodong Fault. Therefore, the long-lasting Boiling-Wells provide an excellent geochemical window into the evolution of the Maodong Fault. Our study documents that the contents and variations of specific hydrochemical and gaseous components of Boiling-Wells are well-suited geochemical tracers to identify and characterize the tectonic activity of the Maodong Fault. This method is also applicable for the monitoring of tectonic activities of major faults zones with comparable preconditions worldwide.

Hydrochemical Characteristics of Earthquake-Related Thermal Springs along the Weixi–Qiaohou Fault, Southeast Tibet Plateau

The Weixi–Qiaohou Fault (WQF) is considered an important zone of the western boundary of the Sichuan–Yunnan block, and its seismicity has attracted much attention after a series of moderate–strong earthquakes, especially the Yangbi Ms6.4 earthquake that occurred on 21 May 2021. In the present research, we investigate major and trace elements, as well as hydrogen and oxygen isotopes, of 10 hot springs sites located along the WQF, which are recharged by infiltrated precipitation from 1.9 to 3.1 km. The hydrochemical types of most analyzed geothermal waters are HCO3SO4-Na, SO4Cl-NaCa, and SO4-Ca, proving that they are composed of immature water and thus are characterized by weak water–rock reactions. The heat storage temperature range was from 44.1 °C to 101.1 °C; the circulation depth was estimated to range between 1.4 and 4.3 km. The results of annual data analysis showed that Na+, Cl−, and SO42− in hot springs decreased by 11.20% to 23.80% north of the Yangbi Ms5.1 earthquake, which occurred on 27 March 2017, but increased by 5.0% to 28.45% to the south; this might be correlated with the difference in seismicity within the fault zone. The results of continuous measurements of NJ (H1) and EYXX (H2) showed irregular variation anomalies 20 days before the Yangbi Ms6.4 earthquake. In addition, Cl− concentration is more sensitive to near-field seismicity with respect to Na+ and SO42−. We finally obtained a conceptual model on the origin of groundwater and the hydrogeochemical cycling process in the WQF. The results suggest that anomalies in the water chemistry of hot spring water can be used as a valid indicator of earthquake precursors.

Mantle-Derived Helium Distribution and Tectonic Implications in the Sichuan-Yunnan Block, China

The geochemical characteristics of mantle degassing observed on the surface of the earth can indicate the origin and migration path of mantle fluids. Compared with the plate boundary tectonic environment, the intraplate tectonic environment does not have a large number of active volcanoes and active faults, and the observation of mantle volatiles in hot spring gas is relatively limited. We selected the Sichuan-Yunnan block to discuss mantle degassing based on the carbon and noble gas isotopes of the spring gases and previous studies on the fault slip rate and geophysical research. A total of five hot spring gas samples (including two free gases and three dissolved gases) were collected from the Sichuan-Yunnan block. Chemical and isotopic compositions were analyzed in N₂-dominant hot spring gases. The ³He/⁴He ratio (0.068-0.541 R _a) indicates the occurrence of mantle-derived helium throughout the Sichuan-Yunnan block, which has been diluted by a crustal radiogenic ⁴He component. The occurrence of mantle-derived helium in the study areas ranges from 0.74 to 5.67%. The lower proportion of mantle-derived helium in YNWQ and HGWQ than that in other spring gases near the Jinghe-Qinghe fault may be caused by the smaller scale of fault around YNWQ and HGWQ than the Jinghe-Qinghe fault. The correlation between ⁴He, ²⁰Ne, and N₂ concentrations implies a common trapping mechanism for ⁴He, ²⁰Ne, and N₂ in hot spring gases. The ⁴⁰Ar/³⁶Ar ratios and N₂/Ar ratios indicate that N₂ and Ar are mostly meteoric, and YNWQ and HGWQ have more crustal-derived Ar contribution (40.56 and 51.49%, respectively). The δ¹³C(CO₂)_o values calculated by Rayleigh fractionation and CO₂ concentration suggest that CO₂ has inorganic and organic origins. The plot of R _c/R _a versus δ¹³C(CO₂) indicates that the spring gas CO₂ origin in the Sichuan-Yunnan block is mainly derived from mixing of limestone and organic sediments with minor mantle CO₂. The δ¹³C(CH₄) versus CH₄/³He values indicate that the origin of methane is thermogenic and microbial oxidation. The low mantle-derived helium distribution pattern is most likely controlled by the weak fault activity rate, the small fault scale, and not obvious magmatic activity in the Sichuan-Yunnan block.

Hydrogeochemical Characteristics of Hot Springs and Their Short-Term Seismic Precursor Anomalies along the Xiaojiang Fault Zone, Southeast Tibet Plateau

Significant hydrogeochemical changes may occur prior- and post-earthquakes. The Xiaojiang fault zone (XJF), situated in a highly deformed area of the southeastern margin of the Tibetan Plateau, is one of the active seismic areas. In this study, major and trace elements, and hydrogen and oxygen isotopes of 28 sites in hot springs along the XJF were investigated from June 2015 to April 2019. The meteoric water acts as the primary water source of the hot spring in the XJF and recharged elevations ranged from 1.8 to 4.5 km. Most of the hot spring water in the study area was immature water and the water–rock reaction degree was weak. The temperature range was inferred from an equation based on the SiO2 concentration and chemical geothermal modeling: 24.3~96.0 °C. The circulation depth for the springs was estimated from 0.45 to 4.04 km. We speculated the meteoric water firstly infiltrated underground and became heated by heat sources, and later circulated to the earth’s surface along the fault and fracture and finally constituted hot spring recharge. Additionally, a continuous monitoring was conducted every three days in the Xundian hot spring since April 2019, and in Panxi and Qujiang hot springs since June 2019. There were short-term (4–35 d) seismic precursor anomalies of the hydrochemical compositions prior to the Xundian ML4.2, Dongchuan ML4.2, and Shuangbai ML5.1 earthquakes. The epicentral distance of anomalous sites ranged from 19.1 to 192.8 km. The anomalous amplitudes were all over 2 times the anomaly threshold. The concentrations of Na+, Cl−, and SO42− are sensitive to the increase of stress in the XJF. Modeling on hydrology cycles of hot springs can provide a plausible physicochemical basis to explain geochemical anomalies in water and the hydrogeochemical anomaly may be useful in future earthquake prediction research of the study area.