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Large deformation has always been a focus and difficult issue in the construction of deep-buried tunnels in squeezing rock. Previous studies mainly focused on the large deformation of medium and small span railway/highway tunnels in soft ground. However, there are limited researches on the large deformation control methods for large-span (three-lane) highway tunnels constructed in unfavorable geological environment. Based on the Lianchengshan Tunnel of the Baoji-Hanzhong expressway in Shaanxi Province, China, this paper studied the deformation behaviors and mechanical mechanisms of a large-span tunnel excavated in chlorite schist formation with single primary lining method and double primary lining method by in-situ test and numerical simulation. The achieved results indicate that the double primary lining method is much more effective than that of the single primary lining method in restraining the deformation of surrounding rock, and the maximum vertical displacement and horizontal convergence are reduced by 67% and 66%, respectively. The support method of double HK200b-type steel sets combined with large-diameter foot reinforcement bolt (FRB) and deep invert could effectively control the large deformation of the case tunnel, which effectively avoided the supporting structure failure, repeated clearance invasion and multiple reshaping work caused by the single primary lining method and conformed to the energy-saving construction concept of “no clearance interfering, no support reshaping” of tunnels in squeezing ground. Simulation analysis of surrounding rock deformation, supporting structure stress and plastic zone distribution was performed to evaluate the support effect of the two deformation-controlled methods. Finally, the deformation and stress characteristic curves of rock-support of the two deformation-controlled methods were established, which revealed the supporting mechanism of double primary linings for large-span tunnels in chlorite schist. The research results can provide a theoretical basis and practical reference for the large-deformation control of similar large-span tunnels in squeezing rock.

Subduction zones are critical sites for recycling of Li and B into the mantle. The way of redistribution of Li and B and their isotopes in subduction settings is debated, and there is a lack of detailed studies on Li and B partitioning between minerals of different types of eclogites and the host rocks of the eclogites. We present Li and B concentration data of minerals and Li and B whole-rock isotope data for low- T and high- T eclogites and their phengite schist host rocks from the Changning–Menglian suture zone, SW China. Omphacite controls the Li budget in both the low- T and high- T eclogites. Low- T eclogites have Li and δ 7 Li values (8.4–27.0 ppm, – 5.5 to + 3.2 ‰) similar to the phengite schists (8.7–27.0 ppm, – 3.8 to + 3.0 ‰), suggesting that Li was added to low- T eclogites from the phengite schists. In contrast, high- T eclogites have much lower δ 7 Li values (– 13.2 to – 5.8 ‰) than the phengite schists, reflecting prograde loss of Li or exchange with wall rocks characterized by low δ 7 Li values. Phengite and retrograde amphibole/muscovite are the major B hosts for low- T and high- T eclogites, respectively. The budgets and isotopic compositions of B in eclogites are affected by the infiltration of fluids derived from phengite schists, as indicated by eclogite δ 11 B values (– 15.1 to – 8.1 ‰) overlapping with the values of the phengite schists (– 22.8 to – 9.5 ‰). Lithium and B in eclogites are hosted in different mineral phases that may have formed at different stages of metamorphism, implying that the contents and isotopic compositions of Li and B may become decoupled during subduction-related fluid-mediated redistribution. We suggest a mineralogical control on the redistribution of Li and B in eclogites during subduction and the exchange of Li and B with the immediate wall rocks. The observed contrasting Li and B isotopic signatures in eclogites are likely caused by a fluid-mediated exchange with different types of wall rocks during both prograde metamorphism and exhumation.

The Krishna region of south India comprises Eastern Ghats Mobile Belt (EGMB), Nellore Schist Belt (NSB) of Eastern Dharwar Craton (EDC) margin and Nallamalai Fold Belt (NFB) including Cuddapah Basin (CB) from east to west. The gravity surveys are carried out across it, so as to delineate the different litho-tectonic belts and salient structural features. The gravity data is processed to generate regional, residual and derivative maps along with three 2D gravity models. Two major gravity highs over the EGMB and NSB and a wide gravity low across the NFB, along with a linear gravity low representing as Transitional Zone (TZ) between these two highs are delineated. Two curvilinear steep gravity gradients between the NFB-NSB and NSB-EGMB are differentiated as Cuddapah Eastern Margin Thrust and Eastern Ghats Boundary Thrust along with a low angle Malakondasatram Thrust in the central part. The NSB comprises Eastern (EA) and Western (WA) arms of coeval different environmental facies of foreland and back-arc setups. The EA with intense gravity high due to a high-density layer at a depth of ~10 km is evidenced from 2D gravity model. The thickness of high-density layer (EA) gradually decreases towards westerly and wedges out below the WA suggesting the entire NSB as a single Late Archaean segment. The major linear gravity high of covered eastern part in Kavali–Nellore–Gudur region indicates the southern continuation of EGMB. The occurrence of thin unconformable high-grade schists in two doubly plunging structures and as tectonic lenses, including a major E–W folded erosional remnants in the low-medium grade late Archaean NSB domain are found as eastern continuation of Mesoproterozoic upper Cuddapah extensions at the EGMB front. These erosional remnants are reflected as isolated residual gravity lows in the west and as residual highs in overall EA of NSB in the east. The isolated relative highs and lows within the major low zone of NFB are linked to differential basement configuration due to superposed effects with the N–S non-cylindrical fold.

Acoustic emission (AE) location technique and moment tensor analysis were used to evaluate the temporal–spatial evolution and damage of micro-cracks of schist during true triaxial compression and strain burst tests. The results show that the AE locations coincide with the macroscopic cracks for true triaxial compression while they are scattered during unloading strain burst tests. A shearing concentration occurs at the bottom of ejection position, but a tensile zone is located in the fracture plane of the ejection block. The ratios of shear and mixed-mode micro-cracks to total micro-cracks for true triaxial compression are both larger than those for strain burst. However, the strain burst has more tensile micro-cracks. Additionally, the damage caused by tensile micro-cracks for a strain burst is larger than that for a true triaxial compression. Moreover, for strain burst, the difference of damage between shear and tensile micro-cracks is in direct proportion to the loading rates after unloading.

Cuddapah Basin (CB) is an intracontinental, Proterozoic basin flanked by Eastern Dharwar Craton (EDC) in the west, Nellore Schist Belt (NSB) and Eastern Ghat Mobile Belt (EGMB) in the east, represents second largest Proterozoic basin of India. Gravity and magnetic surveys were carried out at the interface of Cuddapah Basin (CB) and Nellore Schist Belt (NSB) covering ~2880 km 2 area. Gravity map has brought out some distinct zones. The thrusted contact of NSB and Cuddapah sediments has been well delineated from the gravity map by NE–SW trending steep gradient of contours. Relatively high gravity values are observed over NSB in the southeastern part, moderately high values are observed over Cumbum Formation, but distinct low is observed over Baironkonda Formation. These gravity highs and lows are mainly the manifestation of basement characteristics and intrusives. The magnetic map shows two distinct domains, viz., moderate to low zone in the southern part, and moderate to high zone in the northern part. Regional gravity map suggests a change in basement characteristics from felsic to mafic from NW to SE. Presence of mafic basement may be representing EGMB group of rocks underneath the Cuddapah sediments at the eastern part of the study area. The joint gravity and magnetic modelling reveal varied nature of sedimentary units in terms of density and susceptibility and change in basement characteristic.

The constitutive behavior of faults intervenes in virtually every aspect of the seismic phenomenon but is poorly understood, particularly regarding how effective normal stress affects the boundaries of the seismogenic zone. Here, we explore the mechanical properties of Pelona schist, Westerly granite, phyllosilicate‐rich gouge, gabbro, hornblende, lawsonite blueschist, montmorillonite, and smectite in hydrothermal conditions at various confining pressures and explain the laboratory observations with a physical model of fault friction. The thermobaric activation of healing and deformation mechanisms explains the boundaries of unstable slip as a function of slip‐rate, temperature, and effective normal stress for a given lithology. The constitutive law affords extrapolation of laboratory data in the conditions relevant to seismic cycles throughout the crust, explaining the focus of large earthquakes in collision, subduction, and continental and oceanic transform settings. Plain Language Summary An important goal of earthquake physics involves predicting the failure of rocks under the various physical conditions encountered during the seismic cycle. Here, we analyze mechanical data for Pelona schist, Westerly granite, phyllosilicate‐rich gouge, gabbro, hornblende, lawsonite blueschist, montmorillonite, and smectite that reveal how normal stress, temperature, and slip‐rate affect the frictional properties of rocks. We capture these effects consistently at constant coefficients with a physics‐based constitutive friction law. The boundaries of the seismogenic zone follow a thermobaric activation, whereby the transition temperature is a function of pressure. Increasing confining pressure may induce or inhibit velocity‐weakening behavior, depending on the constitutive properties controlling the healing and deformation mechanisms. The constitutive model provides an increasingly realistic representation of fault behavior during seismic cycles applicable to a wide range of tectonic contexts. Key Points The temperature boundaries of the seismogenic zone depend on confining pressure, implying a thermobaric activation of fault friction The model explains schist, granite, gabbro, hornblende, clays, and natural gouge friction evolution with velocity, temperature, and pressure The constitutive model provides a realistic representation of fault behavior during seismic cycles applicable to all tectonic contexts

The Ife-Ilesha schist belt of the Nigerian Basement Complex is renowned for artisanal gold mining activities owing to the occurrence of gold-bearing eluvial deposits within and around the belt. However, gold mining operations in Ile-Ijesha axis are carried out haphazardly without consideration of probable health risks that might be posed to the miners and local community by heavy metals and naturally occurring radionuclides materials (NORMs) contents in the soil. This study was therefore conducted to determine the mineralogy, heavy metals contents and radionuclide activity of soil samples collected from a gold mining site in Ile-Ife with a view to assessing the consequential health and radiological risks. Thirty-five (35) soil samples were collected from abandoned and active excavating pits over the entire area using stainless-steel spade. The heavy metals and radionuclides concentrations and mineralogical compositions of the samples were obtained by Atomic Absorption Spectroscopic (AAS), gamma ray spectrometry and X-ray diffraction (XRD) methods, respectively. The contamination indices consisting of Enrichment Factor (EF) and Index of geoaccumulation (Igeo) were calculated from heavy metals concentrations. The sequence of the average concentrations of heavy metals in the soil samples decreases as follows: Pb (72.93 ± 4.60) > Cd (58.26 ± 3.25) > Cr (42.81 ± 3.00) > Zn (33.66 ± 2.71) > Mn (24.60 ± 1.80) > Ni (23.93 ± 2.51). The mean EF values of the measured heavy metals occurred in the order of Cr > Pb > Zn > Cd > Ni. The Igeo values of the measured heavy metals (except Pb) were less than one (< 1) revealing practically uncontaminated condition of the studied soil. The radiation dose and radiological hazard risks were estimated by employing absorbed dose, effective dose, radium equivalent, gamma index, external hazard index, excess life cancer risk and representative level. The hazard index values of all the measured heavy metals were < 1 indicating no significant non-carcinogenic effects. The average activity concentrations of 40 K, 238U and 232Th are 259.23 ± 84.20 Bqkg−1, 185.48 ± 73.31 Bqkg−1, 11.93 ± 4.68 Bqkg−1, respectively. All the radiological indices were below the world average values stipulated by the radiation monitoring bodies. The result of the XRD indicates the preponderance of quartz, and biotite, as well as subordinate amounts of plagioclase, kaolinite, ilmenite, andradite, actinolite and microcline in the soil samples, pointing to a schistose source rock, which is probably gold-bearing. The current study indicated practically low contamination of the Ile-Ife gold mining site soils by heavy metals, low radiological risk from NORMs and prospect of mineral exploration in the area if well-established procedure is followed.

The Juomasuo (Co-Au) and Hangaslampi (Au-Co) deposits are located in the Kuusamo Schist Belt, part of the 1.9–1.8 Ga Svecokarelian Orogenic Belt, central Finland. The deposits are hosted by metasedimentary and mafic rocks, and structurally controlled by the F2 Kayla-Konttiaho Antiform. Hydrothermal alteration is spatially zoned, from early albite through biotite and then chlorite and late muscovite alteration. Pipe-like chlorite-quartz-pyrrhotite bodies plunge parallel to the F2 fold axis and are cut by tabular quartz-muscovite-pyrite gold lodes in the S2 axial plane orientation. The chlorite alteration zone contains 10–40 vol.% pyrrhotite containing approximately 0.25 wt% Co but whole-rock Co grades are enhanced by the late-stage and heterogeneous enrichment of minor pyrite (up to 3% Co) and introduction of cobaltian pyrite and cobalt pentlandite. Cobaltite formed where muscovite replaces chlorite within metres of the gold lodes. Cobaltite and Co-enriched pyrite formed because Co and S were mobilized from pyrrhotite in relatively S-poor gold lodes by an As-rich gold ore fluid, during late-D2. Potassium, cobalt, sulfur and arsenic were transported into the mainly chlorite-rich wallrocks, via veinlets and fractures, by the now depleted gold ore fluid. Approximately 5 vol.% pyrite in the gold lodes displays extremely variable cobalt contents, which is interpreted to reflect mobility of Co during formation of the gold lodes. The origin of the gold ore fluid is enigmatic but is most plausibly an orogenic gold fluid, despite the paucity of carbonate minerals in the gold lodes.

The spatio-temporal evolution of the Paleo-Tethys Ocean has been a hot and controversial issue in the world. Here we carry out petrographic, chronological, and geochemical study on garnet–phengite–quartz schist and mafic rocks in the Rongma area from the northern margin of the Southern Qiangtang block to determine the early Mesozoic tectonic evolution of the Shuanghu Paleo-Tethys Ocean in northern Tibet. The zircons from a phengite–quartz schist sample yielded concordant ages of 1936–393 Ma, indicating that its protolith deposited after ~ 393 Ma. Overgrowth zoning garnet with three stages of metamorphic evolution from garnet core to rim (i.e., Peak metamorphic, early retrograde metamorphic, and late retrograde metamorphic stages) in the schist was recognized, indicating that two subduction in a short time might be involved for its genesis. Two groups of phengite in the schist yielded 40Ar/39Ar plateau ages of 229 ± 1.4 Ma and 225 ± 1.3 Ma, respectively; thus, the late retrograde metamorphism of the schist might occur at ~ 229–225 Ma. Zircon U–Pb dating of a diabase in the area yielded crystallization age of 241 ± 1.1 Ma implying its formation in the early Triassic. The Hf-in-zircon and whole-rock Nd isotopes of the diabase show εHf(t) of − 0.6- + 19.1 and εNd(t) of − 0.8 to  + 0.9, respectively. Combined with the whole-rock geochemical features of the early Triassic diabase (241 Ma) and gabbro (237 Ma), they indicate that these mafic rocks are formed in a back-arc extensional setting related to the subduction of the oceanic plate between the Northern and Southern Qiangtang blocks beneath the latter. Combined with regional data, our study favors that view bi-directional subduction of the Shuanghu Paleo-Tethys ocean in the Early Triassic and it was finally closed at ~ 237 to ~ 229 Ma. Our model will help us better understand the tectonic evolution of the Paleo-Tethys Ocean.