The thermal-rheological structure of the continental lithosphere is an important indicator of geodynamic processes. Tectonic compression and extension may change thermal-rheological structure of ...lithosphere. However, the thermal-rheological evolution over geological time may be difficult to determine because of overprints of subsequent geological events. Mantle xenolith temperature and pressure conditions can differentiate the lithospheric thermal structure over geological time and indicate the lithospheric rheological strength. Chronological and temperature-pressure condition data from 11 mantle xenoliths in Cenozoic basalts from the Wuyishan terrane (WYT) and coastal terrane (CT) on both sides of the Zhenghe-Dapu Fault (ZDF) in the Cathaysia Block indicate that these terranes have experienced distinct thermal-rheological evolution since the Eocene. Moreover, the thermal-rheological evolution of the CT and WTY were differently influenced by the West Pacific tectonic domain, respectively, because the ZDF has been relatively active since 23 Ma and thus has partly accommodated and thus reduced the tectonic impact of Eurasia-Pacific convergence. Therefore, we classified the thermal-rheological structural evolution of the Cathaysia Block into three stages (35.5-20 Ma, 20-6.5 Ma, and 6.5-0 Ma) that sequentially recorded the opening and closing dynamics of the northeastern margin of the South China Sea and uplift of Taiwan Island on the eastern margin of Eurasia.
The present-day stress state at depth in Southern Cathaysia block remains poorly understood. We retrieved twelve high-quality granite core samples in a 3 km deep borehole of the Huangshadong ...geothermal field in South China and conducted core-based stress measurements using the anelastic strain recovery (ASR) method to estimate the full stress tensors. The measured stress data of hydraulic fracturing in an adjacent, 600 m-deep geothermal borehole was also collected. The results show that the maximum principal stress σ1 is sub-horizontal from 1566 m to 3008 m in depth. The maximum and minimum horizontal principal stresses, SH and Sh, and the vertical principal stress, Sv, generally follow SH > Sv ≈ Sh in their magnitudes, thus indicating a thrust/strike-slip faulting transitional stress regime in the study area. Our stress orientations by ASR method based on paleomagnetic analysis and the impression test of hydraulic fracturing further suggest that SH is approximately oriented N72° ± 5°W, and this is consistent with both the regional stress field and the observed motion pattern of the Tonghu strike-slip fault in the study area. Interestingly, we found that the difference between the magnitudes of SH and Sh is small, indicating a low level of shear stress in the shallow crust. Also, because of the presence of the lithological changes and fracture zones, there are some snapshots of localized heterogeneous stress states in the granite geothermal reservoir. The present stress state has an impact on the occurrence and exposure of hot springs in the Huangshadong geothermal field. We indicated that the fracture zones with weak stress accumulation are potential transport channels of hot water. The stress field of the Southern Cathaysia block is mainly constrained by the southeast push of the Tibetan Plateau and the northwest subduction of the Philippine Sea plate.
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•Stress state is measured by anelastic strain recovery method in a 3 km deep geothermal borehole in Southern Cathaysia block.•The stress state is a thrust/strike-slip faulting transitional regime with ∼N72°W-oriented SH.•The presence of lithological changes and fracture zones lead to a heterogeneous stress profile in geothermal reservoir.•The fracture zones with weak stress accumulation are prone to hot-water conduction.•The stress field of the Southern Cathaysia block may be in a relatively weak compressive state.
•The Jinzhou detachment fault root in the middle crust.•Microstructural and fabric analysis supports a middle-shallow crust progressive shearing.•The north China cratonic lithosphere is weak during ...the Early Cretaceous extension.•The Liaonan metamorphic core complex is not of a typical Cordilleran type.
Although exhumation of metamorphic core complexes (MCCs) have been traditionally attributed to collapse of orogenically overthickened crust, recent studies reveal that they can also result from extension of lithosphere with normal crustal thickness. For example, there is an ongoing debate about the mechanisms responsible for exhumation of the Liaonan MCC, which occurred during late Mesozoic lithospheric thinning of the North China Craton. This paper attempts to present rheological constraints on middle to upper crustal detachment faulting during the exhumation of the Liaonan MCC and, therefore, on the genesis of the MCC based on a comprehensive study of the microstructural and fabric characteristics of tectonites from the Jinzhou master detachment fault of the Liaonan MCC.
The Jinzhou detachment fault zone comprises a thick sequence of fault rocks of middle to shallow crustal depths. Three types of mylonitic rocks characterize middle crustal deformation along the detachment fault during early Cretaceous lithospheric extension. Microstructural and fabric studies reveal that these fault rocks were formed via different mechanisms of crystal plastic deformation and dynamic recrystallization at temperatures from ca. 300 to 650°C. The flow stresses at different crustal depths were calculated using classical paleopiezometers. This study suggests that a pre-heated crust was responsible for the low flow stresses during the Jinzhou detachment faulting, which is somehow different from, e.g., the Whipple Mountain detachment faulting in the North American Cordillera. Based on inferences about the properties of middle to upper crustal flow associated with the Jinzhou detachment faulting, it is suggested that the Liaonan metamorphic core complex was formed by tectonic extension of cratonic lithosphere with a normal crustal thickness, instead of being a typical Cordilleran-type core complex that occurred in a setting with overthickened crust.
Hot dry rock (HDR) is an important geothermal resource and clean energy source that may play an increasingly important role in future energy management. High‐temperature HDR resources were recently ...detected in deep regions of the Gonghe Basin on the northeastern edge of the Tibetan Plateau, which led to a significant breakthrough in HDR resource exploration in China. This research analyzes the deep temperature distribution, radiogenic heat production, heat flow, and crustal thermal structure in the Qiaboqia Valley, Guide Plain, and Zhacanggou area of the Gonghe Basin based on geothermal exploration borehole logging data, rock thermophysical properties, and regional geophysical exploration data. The results are applied to discuss the heat accumulation mechanism of the HDR resources in the Gonghe Basin. The findings suggest that a low‐velocity layer in the thickened crust of the Tibetan Plateau provides the most important source of constant intracrustal heat for the formation of HDR resources in the Gonghe Basin, whereas crustal thickening redistributes the concentrated layer of radioactive elements, which compensates for the relatively low heat production of the basal granite and serves as an important supplement to the heat of the HDR resources. The negative effect is that the downward curvature of the lithospheric upper mantle caused by crustal thickening leads to a small mantle heat flow component. As a result, the heat flows in the Qiaboqia Valley and Guide Plain of the Gonghe Basin are 106.2 and 77.6 mW/m2, respectively, in which the crust‐mantle heat flow ratio of the former is 3.12:1, indicating a notably anomalous intracrustal thermal structure. In contrast, the crust‐mantle heat flow ratio in the Guide Plain is 1.84:1, which reflects a typical hot crust‐cold mantle thermal structure. The Guide Plain and Zhacanggou area show the same increasing temperature trend with depth, which reflects that their geothermal backgrounds and deep high‐temperature environments are similar. These results provide important insight on the heat source mechanism of HDR resource formation in the Tibetan Plateau and useful guidance for future HDR resource exploration projects and target sites selection in similar areas.
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•Quartz geothermometers are suitable for Yangyi and Yangbajing.•Multicomponent geothermometry yields more reasonable temperatures for diluted waters.•Na/K and K/Mg results could ...represent the deep reservoir and mixing temperatures for Gulu.•Intermediate and high temperature reservoirs of any one geothermal field are within the same hydrothermal system.•Enthalpy vs. chloride plot provide insights on the cooling processes of deep parent fluids.
This study defines reasonable reservoir temperatures and cooling processes for geothermal fluids in three representative high temperature geothermal fields – Yangyi, Yangbajing, and Gulu – distributed along the active geothermal belt of Nimu–Nagchu in south-central Tibet. It uses a combined analysis of hydrochemical compositions, chemical geothermometers, and multicomponent chemical equilibrium analyses of geothermal fluid with an enthalpy vs. chloride plot. There are two geothermal reservoirs in Yangyi and Yangbajing, and both the intermediate and high temperature reservoirs of any one geothermal field are essentially within the same hydrothermal system. The subsurface geothermal fluids from Yangyi cooled mainly by mixing with abundant cold water in the intermediate (163–172°C) and high temperature (192–200°C) reservoir. The subsurface geothermal fluids from Yangbajing experienced adiabatic cooling and mixing with colder water, which formed the high temperature reservoir ZK4001 (255°C) and intermediate temperature reservoirs (164–177°C), respectively, and then emerged on the surface with adiabatic cooling during the ascent. The subsurface geothermal fluids from Gulu ascended to high temperature geothermal reservoirs (211–234°C) mainly cooled by adiabatic boiling or mixing with cooler water. Most of the high temperature fluids mixed with colder water (mixing temperatures range from 149°C to 176°C) during the ascent, and then emerged on the surface as hot springs mainly cooled by conduction. The deep parent fluid of Yangbajing is calculated to have a Cl− concentration of 767mgL−1 and enthalpy of 1350Jg−1 (water temperature of 321°C), which agrees well with the maximum temperature measured in well ZK4002 (329°C). The deep parent fluid of Gulu is calculated to have a Cl− concentration of 845mgL−1 and enthalpy of 1290Jg−1 (water temperature of 307°C).
As a significant part of geothermal resources, the hot dry rock (HDR) resources have drawn more and more attentions because it potentially can provide clean, stable, and huge potential of ...high-temperature geothermal energy. China started research on HDR resources since 1990s, relatively later than advanced countries. Until now, researches on the genetic mechanisms and the occurrence of HDR resources are still inadequate, which hinders the exploration and precise potential estimation of HDR resources in China. Based on the geological indicators of the occurrence of HDR resources, by combining the genetic analysis of HDR resources in the world and crustal structure in China, we classified the HDR resources into four types, i.e., the high radioactive heat production type, the sedimentary basin type, the modern volcano type, and the inner-plate active tectonic zone type. This classification could provide geological guidance for HDR resource targeting in certain areas and facilitate further developments.
•CO2 degassing took place during deep geothermal fluid ascent to the surface in Qialagai only.•Equilibrium temperatures and water-rock interactions in the geothermal reservoirs are similar below the ...Bujimu and Qialagai valleys.•Mixing ratio of cold groundwater in the up-flow zone of Bujiemu is 58–66% calculated by Cl− concentrations.•Deep geothermal fluid reacts with shallow wallrock and does not reach water-rock re-equilibrium in the up-flow zone of Bujiemu.•Genetic mechanisms of siliceous and calcareous sinters are clarified.
Yangyi is a typical high temperature geothermal field in the Tibetan Plateau. Massive siliceous and calcareous sinters have developed in the northern Qialagai Valley and the southern Bujiemu Valley, respectively. This suggests that the deep geothermal fluids in the northern and southern areas may experience different hydrogeochemical processes during ascent to the surface. By combining the analysis of hydrochemical composition, classical chemical geothermometers and multicomponent geothermometry with water-rock interaction models, this study analyzes the genetic mechanisms of the different types of sinters developed in the southern and northern parts of the same geothermal field. The equilibrium temperatures and degrees of water-rock interaction in the deep reservoirs are similar in the Bujimu and Qialagai valleys. CO2 degassing took place during deep geothermal fluid ascent to the surface in Qialagai only. This process leads to calcite precipitation, thus, Ca and HCO3 are consumed, leading to a deficiency in the formation of calcareous sinters. Deep geothermal fluids supersaturated with respect to SiO2 have cooled and precipitated rapidly as siliceous sinters in low-temperature and low-pressure environments. The deep geothermal fluids in Bujiemu mix with cold groundwater during ascent, and the mixing ratio of the cold groundwater is 58–66%, as calculated using Cl− concentrations. In the case of Bujiemu hot springs, the water-rock interaction models built with the PHREEQC program shows that calcite and fluorite dissolve along the flow path, which leads to an increase in the concentrations of Ca, HCO3, and F. The SiO2 concentration of the deep geothermal fluid is diluted by mixing with cold water, which leads to a deficiency of material for the formation of siliceous sinters. However, Ca and HCO3 concentrations are not significantly influenced, and the decrease in water temperature and dissolution of calcite and fluorite provides an advantage for the formation of calcareous sinters in the Bujiemu Valley.
Metamorphic core complexes are developed in crustal activity belts at the continental margins or within continents, and their main tectonic feature is that the ductile middle crust is exhumed at the ...surface. The deformation properties are closely related to the geodynamic process affecting the continental crust. However, the evolution of the metamorphic core complexes after their formation is still unclear. The Cretaceous Liaonan metamorphic core complex developed in the eastern North China craton provides an ideal environment to study its evolution. In this study, we estimate the paleo-temperature and paleo-stress at the time of formation of the metamorphic core complex dynamical recrystallization of quartz and calculate the thermo-rheological structure of the present Liaonan metamorphic core complex by one-dimensional steady-state heat conduction equation and power-creep law. The results show that compared with the Cretaceous period, the geothermal heat flow value of the present Liaonan metamorphic core complex decreases from 70–80 mW/m2 to 49.4 mW/m2, the thermal lithosphere thickness increases from 59–75 km to 173 km, and the brittle transition depth increases from 10–13 km to about 70 km, showing coupling of the crust–mantle rheological structure. We speculate that the evolution of the thermo-rheological structure of the Liaonan metamorphic core complex is possibly caused by rapid heat loss or lithospheric mantle flow in the Bohai Bay Basin.
As an important geothermal resource, hot dry rock （HDR） reserves have been studied in many countries. HDR resources in China have huge capacity and have become one of the most important resources for ...the potential replacement of fossil fuels. However, HDR resources are difficult to develop and utilise. Technologies for use with HDR, such as high-temperature drilling, reservoir characterisation, reservoir fracturing, microseismic monitoring and high-temperature power stations, originate from the field of oil and drilling. Addressing how to take advantage of these developed technologies is a key factor in the development of HDR reserves. Based on the thermal crustal structure in China, HDR resources can be divided into four types： high radioactive heat production, sedimentary basin, modern volcano and the inner-plate active tectonic belt. The prospective regions of HDR resources are located in South Tibet, West Yunnan, the southeast coast of China, Bohai Rim, Songliao Basin and Guanzhong Basin. The related essential technologies are relatively mature, and the prospect of HDR power generation is promising. Therefore, analysing the formation mechanisms of HDR resources and promoting the transformation of technological achievements, large-scale development and the utilisation of HDR resources can be achieved in China.
