Quantitative analysis of the shallow crustal tectonic stress field in China mainland based on in situ stress data

The latest hydraulic fracturing and stress relief measurement data in the Chinese mainland were collected. The total of 3856 data entries are measured at 1474 locations. The measured area covers 75–130°E and 18–47°N, and the depth range varies from surface to 4000 meters depth, which generally includes each active tectonic block of China and each segment of North–South seismic belt. We investigated the tectonic stress field by removing the effect of gravity. For this, we assume lateral constraints and Heim’s rule. The gravity contribution is removed by using the assumption of lateral constraint and Heim’s rule. Our results show: (1) the maximum and the minimum horizontal principal stress σ_H, σ_h and the vertical stress σ_V in the shallow crust of China all increase linearly with depth: σ_H = 0.0229D + 4.738, σ_h = 0.0171D + 1.829, σ_V = 0.0272D. Maximum and minimum horizontal tectonic stress varies as a function of depth D linearly 4.738 < σ_T < 0.0139D + 4.738 and 1.829 < σ_t < 0.0162D + 1.829. The horizontal tectonic differential stress is σ_T − σ_t = 0.0058D + 2.912. (2) The intermediate value of σ_T1 (regression value of tectonic stress inferred from the assumption of lateral constraint at 2000 m depth) changes in different areas, the maximum value of which is 45.6 MPa, while the minimum value of which is 26.8 MPa. Horizontal tectonic differential stress σ_T − σ_t increases linearly with depth and the maximum and minimum of σ_T − σ_t is 25.3 MPa and 13.0 MPa, respectively. In general, the stress magnitude is much higher in western than in eastern China. This indicates that the strong Indo-Eurasian collision dominates the present tectonic stress field in Chinese mainland. (3) Compared with other study regions, the northward crustal compression to the Qinghai-Tibet block is relatively lower in magnitude in the shallow subsurface and higher at deeper depth. (4) The orientations of σ_T in China mainland generally form a radial scattering pattern centered in Tibetan Plateau. From western to eastern China, they rotate gradually clockwise from NS to NNE, NE, NEE, and SE, which is consistent with the result of focal mechanism solutions.     

Plio-Quaternary stress states in NE Iran: Kopeh Dagh and Allah Dagh-Binalud mountain ranges

NE Iran, including the Kopeh Dagh and Allah Dagh-Binalud deformation domains, comprises the northeastern boundary of the Arabia–Eurasia collision zone. This study focuses on the evolution of the Plio-Quaternary tectonic regimes of northeast Iran. We present evidence for drastic temporal changes in the stress state by inversion of both geologically and seismically determined fault slip vectors. The inversions of fault kinematics data reveal distinct temporal changes in states of stress during the Plio-Quaternary (since ～ 5 Ma). The paleostress state is characterized by a regional transpressional tectonic regime with a mean N140 ± 10°E trending horizontal maximum stress axis (σ₁). The youngest (modern) state of stress shows two distinct strike-slip and compressional tectonic regimes with a regional mean of N030 ± 15°E trending horizontal σ₁. The change from the paleostress to modern stress states has occurred through an intermediate stress field characterized by a mean regional N trending σ₁. The inversion analysis of earthquake focal mechanisms reveals a homogeneous, transpressional tectonic regime with a regional N023 ± 5°E trending σ₁. The modern stress state, deduced from the youngest fault kinematics data, is in close agreement with the present-day stress state given by the inversions of earthquake focal mechanisms. According to our data and the deduced results, in northeast Iran, the Arabia–Eurasia convergence is taken up by strike-slip faulting along NE trending left-lateral and NNW trending right-lateral faults, as well as reverse to oblique-slip reverse faulting along NW trending faults. Such a structural assemblage is involved in a mechanically compatible and homogeneous modern stress field. This implies that no strain and/or stress partitioning or systematic block rotations have occurred in the Kopeh Dagh and Allah Dagh-Binalud deformation domains. The Plio-Quaternary stress changes documented in this paper call into question the extrapolation of the present-day seismic and GPS-derived deformation rates over geological time intervals encompassing tens of millions of years.     

Static stress drop inferred from near-fault parameters for the 1999 Chi–Chi earthquake in Taiwan

In 2001, a special issue of the Bulletin of the Seismological Society of America (BSSA) featured seismological research for the 1999 Chi–Chi Taiwan earthquake. This study uses source parameters suggested by the first author in this special issue to estimate static stress drop associated with the Chi–Chi earthquake. The waveform simulation method was used to carefully examine these source parameters. The simulation results indicate that source parameters, inferred from near-fault observations, are well determined. According to the rupture area and slip, the static stress drops (Δσ_s) obtained were distributed between a small value of 47 bars near the epicentral region and a much larger value (>200 bars) to the north. Similar trends in dynamic stress drop (Δσ_d) were also recognized by the first author in his paper published in 2001 BSSA special issue. Comparing the Δσ_s with Δσ_d, satisfies the relation Δσ_s/Δσ_d ≈ 1. This relation suggests that fault motion is mostly spent releasing seismic wave energy during the rupture process of the Chi–Chi earthquake. The consistency between static and dynamic stress drops thus provides a measure of energy-moment (E_s/M₀) ratios, which range from 9.0 × 10⁻⁵ to 6.5 × 10⁻⁴. The average E_s/M₀ ratio estimated for the northern portions of the fault is 3.4 × 10⁻⁴, which is about 3 times that of the south. Such a high E_s/M₀ ratio can be interpreted as having low strength in the rupture for the northern portions of the fault, where the fault would release less energy per unit rupture surface to create the new rupture.     

Is earthquake activity along the French Atlantic margin favoured by local rheological contrasts?

The seismological study of recent seismic crises near Oleron Island confirms the coexistence of an extensional deformation and a transtensive regime in the Atlantic margin of France, which is different from the general western European stress field corresponding to a strike-slip regime. We argue that the switch of the principal stress axes σ₁/σ₂ in a NW–SE vertical plane is linked with the existence of crustal heterogeneities. Events of magnitude larger than 5 sometimes occur along the Atlantic margin of France, such as the 7 September 1972 (M_L = 5.2) earthquake near Oleron island and the 30 September 2002 (M_L = 5.7) Hennebont event in Brittany. To test the mechanism of local strain localization, we model the deformation of the hypocentral area of the Hennebont earthquake using a 3D thermo-mechanical finite element code. We conclude that the occurrence of moderate earthquakes located in limited parts of the Hercynian shear zones (as the often reactivated swarms near Oleron) could be due to local reactivation of pre-existing faults. These sporadic seismic ruptures are favoured by stress concentration due to rheological heterogeneities.     

Interpretation of strains in torsion shear tests

The normal and shear strains obtained in torsion shear tests may be interpreted in two different ways to gain insight into (1) the type of plastic potential to be employed in hardening plasticity stress–strain models, and (2) the coincidence in physical space of the plastic strain increment direction with the stress direction during principal stress rotation. Thirty-four drained torsion shear tests were performed on hollow cylinder specimens of Santa Monica Beach sand deposited by dry pluviation. Twenty-six tests were performed on tall specimens with height of 40 cm, and eight tests were performed on short specimens with height of 25 cm to investigate the effect of the specimen height on the soil behavior in hollow cylinder specimens. Each test was conducted with the same, constant inside and outside confining pressure, σ_r, thus tying the value of b = (σ₂–σ₃)/(σ₁– σ₃) to the inclination, β, of the major principal stress. The directions of strain increment vectors at failure are compared with the directions corresponding to associated and non-associated flow. The relation between the directions of major principal strain increment and major principal stress during rotation of principal stress axes in physical space are investigated.     

Present-day crustal deformation along the Philippine Fault in Luzon,Philippines

The Philippine Fault results from the oblique convergence between the Philippine Sea Plate and the Sunda Block/Eurasian Plate. The fault exhibits left-lateral slip and transects the Philippine archipelago from the northwest corner of Luzon to the southeast end of Mindanao for about 1200 km. To better understand fault slip behavior along the Philippine Fault, eight GPS surveys were conducted from 1996 to 2008 in the Luzon region. We combine the 12-yr survey-mode GPS data in the Luzon region and continuous GPS data in Taiwan, along with additional 15 International GNSS Service sites in the Asia-Pacific region, and use the GAMIT/GLOBK software to calculate site coordinates. We then estimate the site velocity from position time series by linear regression. Our results show that the horizontal velocities with respect to the Sunda Block gradually decrease from north to south along the western Luzon at rates of 85–49 mm/yr in the west–northwest direction. This feature also implies a southward decrease of convergence rate along the Manila Trench. Significant internal deformation is observed near the Philippine Fault. Using a two dimensional elastic dislocation model and GPS velocities, we invert for fault geometries and back-slip rates of the Philippine Fault. The results indicate that the back-slip rates on the Philippine Fault increase from north to south, with the rates of 22, 37 and 40 mm/yr, respectively, on the northern, central, and southern segments. The inferred long-term fault slip rates of 24–40 mm/yr are very close to back-slip rates on locked fault segments, suggesting the Philippine Fault is fully locked. The stress tensor inversions from earthquake focal mechanisms indicate a transpressional regime in the Luzon area. Directions of σ₁ axes and maximum horizontal compressive axes are between 90° and 110°, consistent with major tectonic features in the Philippines. The high angle between σ₁ axes and the Philippine Fault in central Luzon suggests a weak fault zone possibly associated with fluid pressure.     

Influence of the 2008 Wenchuan earthquake (Mw 7.9) on the occurrence probability of future earthquakes on neighboring faults

The Longquan–Shan fault and the Huya fault are two major neighboring faults of the Longmen–Shan fault zone where the 12 May 2008 Wenchuan earthquake (M_w 7.9) occurred. To study the influence of the Wenchuan event on these two active faults, we calculate changes of Coulomb stress on the Longquan–Shan fault and the Huya fault caused by the Wenchuan mainshock. Our results indicate that the Coulomb stress in the northern section (Zone A) of the Longquan–Shan fault is increased by 0.07–0.10 bars, that in the middle section (Zone B) by 0.04–0.11 bars, and that in the southern section (Zone C) shows almost no change. For the Huya fault, the Coulomb stress is decreased by 0.01–0.03 bars in the northern section (Zone A), 0.10–0.35 bars in the middle section (Zone B), and nearly 0.5 bars in the southern section (Zone C). The epicenter distribution of small earthquakes (M_L ⩾ 1.5) on the Longquan–Shan fault and the Huya fault after the Wenchuan earthquake is consistent with the distribution of the Coulomb stress change. This implies that the Wenchuan earthquake may have triggered small events on the Longquan–Shan fault, but inhibited those on the Huya fault. We then use the rate/state friction law to calculate the occurrence probability of future earthquakes in the study region for the next decade. They include the distribution of b-values, magnitude of completeness (M_c), the background seismicity rate, a value of Aσ_n and the duration for the transient effect (t_a) in the study region. We also estimate the earthquake occurrence probabilities on the neighboring faults after the Wenchuan earthquake. Our results show that, the occurrence probability of future earthquakes in the Longquan–Shan has a slight increase, being 7% for M ⩾ 5.0 shocks during the next decade, but the earthquake probability in the Huya region is reduced obviously, being 5–20%, 7–26% and 3–9% for M ⩾ 5.0 shocks during the next decade in sections A, B and C of the Huya fault, respectively.     

Hierarchy of earthquakes in the Baikal rift system: implications for lithospheric stress

We investigated space-time lithospheric stress patterns of the Baikal rift system according to the hierarchy of earthquakes using mechanisms of 265 K_P ≥ 10 events recorded from 1950 to 1998 and seismic moments of 802 K_P ≥ 11 events from 1968 to 1994. The lithosphere of the region was confirmed to undergo rifting with mostly normal-slip events, while local areas of frequent strike-slip and reverse motions may record stress heterogeneity. The dominance of rifting, although being evident in the stress dynamics, is unstable, which is indicated by increase in strike-slip and reverse motions to as many as normal slip events in the latest 1980s–earliest 1990s. The lithospheric stress patterns inferred from seismic-moment data are generally consistent with those derived from the classical focal mechanism method. The suggested approach of seismic zoning according to earthquake slip geometry may provide a more reliable background for successful mitigation of seismic hazard in the region.     

The 10 June 2012 Fethiye Mw 6.0 aftershock sequence and its relation to the 24–25 April 1957 Ms 6.9–7.1 earthquakes in SW Anatolia,Turkey

The 10 June 2012 M_w 6.0 aftershock sequence in southwestern Anatolia is examined. Centroid moment tensors for 23 earthquakes with moment magnitudes (M_w) between 3.7 and 6.0 are determined by applying a waveform inversion method. The mainshock is a shallow focus strike-slip with reverse component event at a depth of 30 km. The seismic moment (M_o) of the mainshock is estimated as 1.28 × 10¹⁸ Nm and rupture duration of the Fethiye mainshock is 38 s. The focal mechanisms of the aftershocks are mainly strike-slip faulting with a reverse component. The geometry of the focal mechanisms reveals a strike-slip faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to obtain a more accurate picture of the Fethiye earthquake stress field. The stress tensor inversion results indicate a predominant strike-slip stress regime with a NW–SE oriented maximum horizontal compressive stress (S_H). According to variance of the stress tensor inversion, to first order, the Fethiye earthquake area is characterized by a homogeneous interplate stress field. The Coulomb stress change associated with the mainshock and the largest aftershock are also investigated to evaluate any significant enhancement of stresses along the Gulf of Fethiye and surrounding region. Positive lobes with stress more than 0.4 bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards NNW–SSE and E–W directions.     

Differential timing of vertical-axis block rotations in the northern Ryukyu Arc: Paleomagnetic evidence from the Koshikijima Islands,Japan

Over 300 samples for paleomagnetic analysis and K–Ar dating were collected from 27 sites at NW–SE and NE–SW trending dike swarms (herein, NW dikes and NE dikes, respectively) in the Koshikijima Islands, northern Ryukyu Arc. The NW dikes are Middle Miocene in age and have directions (D = ? 37.7^°, I = 51.8^°, α₉₅ = 9.6^°, and κ = 40.8) that are deflected westward relative to the stable eastern Asian continent. Conversely, the NE dikes, of Late Miocene age, have directions (D = 16.1^°, I = 57.7^°, α₉₅ = 7.1^°, and κ = 41.9) that show no such deflection. These differences are interpreted as indicating that the Koshikijima Islands underwent approximately 40^° of counter-clockwise rotation during the Middle to Late Miocene. A synthesis of the paleomagnetic and structural data suggests a three-stage history of extensional deformation: (1) displacement upon normal faults (F₁ faults) without vertical-axis block rotation, (2) strike-slip reactivation of F₁ faults and oblique-normal displacement on NE–SW-trending faults (F₂ faults) with vertical-axis block rotation, and (3) oblique-normal displacement on F₂ faults without vertical-axis block rotation. Regional differences in the timing and amount of counter-clockwise vertical-axis block rotations indicate that the northern Ryukyu Arc rotated as several distinct rigid blocks.     

Sulfur cycling at the Mid-Atlantic Ridge: A multiple sulfur isotope approach

The role of sulfur in two hydrothermal vent systems, the Logatchev hydrothermal field at 14°45′N/44°58′W and several different vent sites along the southern Mid-Atlantic Ridge (SMAR) between 4°48′S and 9°33′S and between 12°22′W and 13°12′W, is examined by utilizing multiple sulfur isotope and sulfur concentration data. Isotope compositions for sulfide minerals and vent H₂S from different SMAR sites range from + 1.5 to + 8.9‰ in δ³⁴S and from + 0.001 to + 0.051‰ in Δ³³S. These data indicate mixing of mantle sulfur with sulfur from seawater sulfate. Combined δ³⁴S and Δ³³S systematics reveal that vent sulfide from SMAR is characterized by a sulfur contribution from seawater sulfate between 25 and 33%. This higher contribution, compared with EPR sulfide, indicates increased seawater sulfate reduction at MAR, because of a deeper seated magma chamber and longer fluid upflow path length, and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid-ocean ridges.Additionally, isotope data uncover non-equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 °C. δ³⁴S values between + 0.2 to + 8.8‰ for dissolved and precipitated sulfide from Logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at SMAR. δ³⁴S values between ? 24.5 and + 6.5‰ and Δ³³S values between + 0.001 and + 0.125‰ for sulfide-bearing sediments and mafic/ultramafic host rocks from drill cores taken in the region of Logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction. Basalts and basaltic glass from SMAR sites with Δ³³S = ? 0.008‰ reveal lower Δ³³S lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials.We conclude that the combined use of both δ³⁴S and Δ³³S provides a more detailed picture of the sulfur cycling in hydrothermal systems at the Mid-Atlantic Ridge and uncovers systematic differences to hydrothermal sites at different mid-ocean ridge sites. Multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow. Furthermore, based on Δ³³S we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when δ³⁴S were identical.     

New Lu–Hf and Sm–Nd geochronology constrains the subduction of oceanic crust during the Carboniferous–Permian in the Dabie orogen

Eclogites from the Huwan shear zone in the western Dabie were investigated in terms of their P–T evolution, geochemistry, and combined Lu–Hf and Sm–Nd geochronology. Trace element and isotope data suggest a normal mid-ocean ridge rather than an intraplate or ocean island setting for the protoliths of the eclogites. Electron microprobe analyses of representative garnets show typical prograde zoning profiles. Estimated peak metamorphic temperatures of 540–590 °C most likely did not exceed the closure temperature of the Lu–Hf and Sm–Nd systems. The consistent Lu–Hf and Sm–Nd ages, therefore, most likely reflect garnet growth and are interpreted to reflect high-pressure eclogite-facies metamorphism due to the occurrence of omphacite inclusions from core to rim in garnets and the spherical geometry effect despite the well-preserved prograde zoning in the garnets. The high-pressure mineral assemblage of the eclogite yielded a statistically robust Lu–Hf age of 260.0 ± 1.0 Ma (2σ, 10 points, MSWD = 1.0) and a Sm–Nd age of 260.4 ± 2.0 Ma (2σ, 9 points, MSWD = 1.4), which are younger than the Carboniferous zircon U–Pb ages of ca. 310 Ma. The new Lu–Hf and Sm–Nd data, in combination with published geochronological data, define two distinct Carboniferous and Permian population ages for the oceanic-type eclogites from the Huwan shear zone, which may require that these rocks experienced two episodes of high-pressure metamorphism within less than 50 Myr.     

Migration of brines in the basement rocks of the Athabasca Basin through microfracture networks (P-Patch U deposit,Canada)

The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted near the unconformity between a middle Proterozoic intracratonic sedimentary basin and an Archean to Paleo-Proterozoic metamorphic and plutonic basement. These deposits, which are considered to be the richest U deposits in the world, are the result of massive basinal fluid migrations in the basement rocks.This study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths (hundred metres below the unconformity) and their chemical modification (salinity increase) by interaction with basement lithologies. These processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity-type mineralization.Microfracture networks, which opened during the basinal brine stage (ca. 1600–1400 Ma) are interpreted as sets of mode I cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing. The stress field at that stage (σ₁ = N130–150 °E, subvertical) partly reopened the earlier microcrack networks (σ₁ = N80–110 °E and N130–150 °E, subvertical) issued from the Trans-Hudson Orogeny late retrograde metamorphic stage (ca. 1795–1720 Ma). The circulation of the two types of fluids (carbonic and brines) occurs thus at two distinct events (Trans-Hudson Orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines) but the same main microfissure geometry was used by the fluids. This demonstrates the existence of a similar stress field direction acting before and after the basin formation. Moreover, the brine circulations in the basement acted in a wider volume than the clay-rich alteration halo surrounding the U-ores, generally considered as the main envelope of fluid percolation outside the fault systems. The data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity-related uranium mineralization in the Athabasca Basin.     

Geochronology and geochemistry of Mesozoic mafic–ultramafic complexes in the southern Liaoning and southern Jilin provinces,NE China: Constraints on the spatial extent of destruction of the North China Craton

LA–ICP–MS zircon U–Pb ages, geochemical and Sr–Nd–Pb isotope data are presented for mafic–ultramafic complexes from the southern Liaoning–southern Jilin area with the aim of determining the nature of the Mesozoic lithospheric mantle and to further constrain the spatial extent of destruction of the North China Craton (NCC). The complexes consist of olivine-websterite, gabbro, dolerite, and gabbro-diorite. Zircons from the complexes show typical zoning absorption, are euhedral–subhedral in shape, and yield high Th/U ratios (1.23–2.87), indicating a magmatic origin. Zircon U–Pb age data indicate that they formed in the Early Cretaceous (129–137 Ma). Geochemically, they have SiO₂ = 44.3–49.8%, MgO = 6.8–26.5%, Cr = 102–3578 ppm, and Ni = 31–1308 ppm, and are characterized by enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs), as well as a wide range of Sr–Nd–Pb isotopic compositions [(⁸⁷Sr/⁸⁶Sr)i = 0.70557–0.71119; ε_Nd (t) = ?5.4 to ?20.1; (²⁰⁶Pb/²⁰⁴Pb)i = 15.13–17.85; Δ7/4 = ?11.49 to 16.00; Δ8/4 = 102.64–203.48]. Compared with the southern Liaoning mafic–ultramafic rocks, the southern Jilin mafic–ultramafic rocks have high TiO₂ and Al₂O₃ contents, high ε_Nd (t) values, low (La/Yb)_N values, low initial ⁸⁷Sr/⁸⁶Sr ratios, and low radiogenic Pb isotopic compositions. These findings indicate that the primary magmas of the southern Jilin complexes were derived from lithospheric mantle that was previously metasomatized by a melt derived from the delaminated ancient lower crust, whereas the primary magmas of the southern Liaoning complexes originated from partial melting of a lithospheric mantle source that was previously modified by melt derived from the broken-off Yangtze slab. Therefore, the lateral extent of the NCC destruction should include the southern Liaoning–southern Jilin area.     

Re–Os and Sr–Nd–Pb isotope constraints on source of fluids in the Zhifang Mo deposit,Qinling Orogen,China

The Zhifang Mo deposit is located in the northeastern Qinling Orogen along the southern margin of the North China Craton. The deposit represents a quartz-vein system hosted in the Mesoproterozoic Xiong'er Group volcanic rocks. We identify three hydrothermal stages (early, middle and late), characterized by veinlets of quartz–pyrite, quartz–molybdenite–pyrite–chalcopyrite–galena–sphalerite, and quartz–carbonate assemblages, respectively. Five molybdenite samples from the Zhifang deposit yield Re–Os ages ranging from 241.2 ± 1.6 Ma to 247.4 ± 2.5 Ma, with an isochron age of 246.0 ± 5.2 Ma (2σ, MSWD = 7.4), and a weighted mean age of 243.8 ± 2.8 Ma (2σ, MSWD = 5.5). The Re–Os age shows that the Mo mineralization occurred during the Indosinian Orogeny, and suggests that the mineralization is unrelated to the Yanshanian magmatism or the Paleo-Mesoproterozoic volcanic–hydrothermal event.This study also reports a new Sr–Nd–Pb isotope dataset from ore sulfides in an attempt to constrain the source of the ore-forming fluids. Ten sulfide samples from middle stage of the Zhifang Mo deposit yield I_Sr(t) ratios of 0.710286–0.711943, with an average of 0.711004; ε_Nd(t) values between − 19.5 and − 14.8, with an average of − 16.7; and (²⁰⁶Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i and (²⁰⁸Pb/²⁰⁴Pb)_i ratios of 17.126–17.535, 15.374–15.466 and 37.485–37.848, with averages of 17.380, 15.410 and 37.631, respectively. One pyrite from the early stage yield I_Sr(t) of 0.722711–0.722855, with an average of 0.722783, which is higher than those of the middle stage sulfides and suggests equilibration with wallrocks. The ε_Nd(t) values are in the range of − 17.3 to − 16.6 with a mean at − 17.0; and (²⁰⁶Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i and (²⁰⁸Pb/²⁰⁴Pb)_i ratios are 17.386, 15.405 and 37.622, respectively. The ore sulfides show higher Pb-isotope ratios, higher ε_Nd(t) and lower I_Sr(t) values than the host rocks. The results suggest that the ore-forming fluids had lower I_Sr(t), and higher ε_Nd(t) values than the ore sulfides, and were possibly sourced from the Dengfeng Complex. The southward subduction of the North China Craton beneath the Huaxiong Block during the Triassic was possibly responsible for the formation of the Waifangshan orogenic Mo system.     

Geochemistry and U–Pb zircon geochronology of the Alvand plutonic complex in Sanandaj–Sirjan Zone (Iran): New evidence for Jurassic magmatism

This paper presents geochemical, Sr–Nd isotopic, and U–Pb zircon geochronological data on the Alvand plutonic complex in Sanandaj–Sirjan zone (SSZ), Western Iran. The gabbroic rocks show a trend of a calc-alkaline magma suite and are characterized by low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7023–0.7037) and positive εNd(t) values (2.9–3.3), which suggest derivation from a moderately depleted mantle source. Geochemical features of the granites illustrate a high-K calc-alkaline magma series, whereas the leucocratic granitoids form part of a low-K series. Granites have intermediate ⁸⁷Sr/⁸⁶Sr ratios (0.707–0.719) and negative εNd_(t) values (−1.0 to −3.4), while leucocratic granitoids have higher initial ⁸⁷Sr/⁸⁶Sr ratio (0.713–0.714) and more negative εNd_(t) values (−3.5 to −4.5). Potential basement source lithologies for the granites are Proterozoic granites and orthogneisses, and those for the leucocratic granites are plagioclase-rich sources such as meta-arkoses or tonalites. The U–Pb dating results demonstrate that all granitoids were exclusively emplaced during the Jurassic instead of being Cretaceous or younger in age as suggested previously. The pluton was assembled incrementally over c. 10 Ma. Gabbros formed at 166.5 ± 1.8 Ma, granites between 163.9 ± 0.9 Ma and 161.7 ± 0.6 Ma, and leucocratic granitoids between 154.4 ± 1.3 and 153.3 ± 2.7 Ma. Granites and leucocratic granitoids show some A-type affinity. It is concluded that the Alvand plutonic complex was generated in a continental-arc-related extensional regime during subduction of Neo-Tethyan oceanic crust beneath the SSZ. The U/Pb zircon age data, recently corroborated by similar results in the central and southern SSZ, indicate that Jurassic granitoids are more areally extensive in this belt than previously thought.     

High-pressure generation in the Kawai-type multianvil apparatus equipped with tungsten-carbide anvils and sintered-diamond anvils,and X-ray observation on CaSnO3 and (Mg,Fe)SiO3

We extended the attainable pressure of the Kawai-type multianvil apparatus to 71.3 GPa and 120.3 GPa at room temperature by equipping it with tungsten carbide (WC) and sintered diamond (SD) cubic anvils, respectively. In the experiments with WC anvils, pressure decreased largely, ΔP ∼−20 GPa, on heating from room temperature to 1500 K. In the experiments with SD anvils, pressure also dropped to 105 GPa from 120 GPa at 1673 K. In order to generate higher pressure and temperatures, therefore, innovation of SD material in both quality and size are essential, together with improvements of cell assembly. Besides pressure generation, we conducted in situ energy-dispersive X-ray diffraction observations on CaSnO₃ and (Mg,Fe)SiO₃ in the experiments with WC and SD anvils, respectively. We observed the growth of new peaks, which can be assigned to the post-perovskite phase, transformed from a starting material of CaSnO₃ perovskite at 48.4 GPa and 1500 K, although they are not clearly identified. In contrast, we could not observe the post-perovskite phase of (Mg,Fe)SiO₃ in the present P–T conditions generated by experiments with SD anvils.     

The structure of mantle flows and stress fields in a two-dimensional convection model with non-Newtonian viscosity

The structure of mantle convection and spatial fields of superlitho static pressure and vertical and horizontal stresses in the Earth’s mantle are studied in a 2D numerical model with non-Newtonian viscosity and heat sources. The model demonstrates a jump-like motion of subduction zones and reveals abrupt changes in the stress fields depending on the stage of slab detachment. The stresses decrease dramatically in the areas without slabs. The horizontal stresses o_xx, superlitho static pressure, and vertical stresses o_zz in the part of the mantle lacking intense near-vertical flows are approximately equal, varying within ± 6, ± 8, and ± 10 MPa, respectively. However, these fields are stronger in the areas of descending slabs, where the values of the above parameters are about an order of magnitude higher (± 50 MPa).This result agrees with the current views of the oceanic slabs as the most important gent of mantle convection. We have found significant differences among the o_xx, o_zz, and pressure fields. The pressure field reveals both the vertical and horizontal features of slabs and plumes, clearly showing their long thermal conduits with broader heads. The distributions of o_xx are sensitive to the near-horizontal parts of the flows, whereas the o_zz fields reveal mainly their vertical substructures. The model shows the presence of relatively cold remnants of slabs in the lower mantle above the thermal boundary layer. Numerous hot plumes penetrating through these high-viscosity remnants, as well as the new descending slabs, induce intense stress fields in the lower mantle, which are strongly inhomogeneous in space and time.     

Analytical solution of undrained cylindrical cavity expansion in saturated soil under anisotropic initial stress

This paper presents an analytical solution for undrained elasto-plastic cylindrical cavity expansion in saturated soil under anisotropic initial stress. The problem is formulated by assuming small-strain deformation in the elastic zone and large-strain deformation in the plastic zone. Plastic yielding is determined by the Tresca failure criterion and an associated flow rule. Two stress functions are used to describe the stress state in the two zones around the cavity. The elasto-plastic boundary can subsequently be determined by solving the two stress functions under the stress boundary conditions. Additionally, the cavity pressure-expansion relationship, the total stress and the excess pore pressure around the cavity wall under anisotropic initial stress can be easily obtained by application of a numerical integration. The results show that the cavity pressure and excess pore pressure under the isotropic initial total stress (K = 1) are larger than those under the anisotropic initial stress (K > 1 and K < 1), which is true at all states of the expansion. The higher value of K develops lower stress and pore pressure around the cavity wall at the ultimate states. However, the stress and excess pore pressure are not sensitive to the value of K. The present solution may be used for analyzing the uplift capacity of plate anchors in soils and Horizontal Directional Drilling (HDD) problems such as the tunneling, and pipeline installation.     

Geology,geochemistry, and geochronology of the Wangjiazhuang porphyry–breccia Cu(–Mo) deposit in the Zouping volcanic basin,eastern North China Block

The Wangjiazhuang porphyry–breccia Cu(–Mo) deposit is located in the Zouping volcanic basin, western Shandong Province. Seven molybdenite samples yield a Re–Os weighted mean age of 127.8 ± 0.7 Ma (2σ), which is identical within error to the zircon weighted mean ²⁰⁶Pb/²³⁸U age of 128.3 ± 1.3 Ma (2σ) determined for quartz monzonite samples. The host rock is characterized by high concentrations of K₂O (4.26–4.53 wt.%), Na₂O (4.97–5.76 wt.%), LILEs and LREEs, and high Mg# (> 40), and low concentrations of HFSEs and HREEs, with K₂O/Na₂O ratios of 0.76–0.88. The quartz monzonite also has high Sr/Y (69.9–112.5) and (La/Yb)_N (22.0–30.0) ratios, similar to adakitic rocks worldwide. Relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70549–0.70556), high ε_Nd(t) values (2.58–3.06), high radiogenic Pb [(²⁰⁶Pb/²⁰⁴Pb)i = 18.3424–18.4606, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.5692–15.5985, (²⁰⁸Pb/²⁰⁴Pb)_i = 38.1714–38.2734] and high zircon ε_Hf(t) values (− 2.1 to + 4.3) indicate that the magma was likely derived from the partial melting of subducted oceanic crust which then reacted with the peridotitic mantle wedge. Both the breccia and porphyry ores have a narrow range of δ³⁴S (− 4.8 to + 2.1‰) and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.295–18.402, ²⁰⁷Pb/²⁰⁴Pb = 15.551–15.573, and ²⁰⁸Pb/²⁰⁴Pb = 38.215–38.331), suggesting that the ore metals were extracted primarily from the quartz monzonite or similar source. Subduction of the Paleo-Pacific slab during the Early Cretaceous resulted in the formation of the Wangjiazhuang quartz monzonite and associated Cu(–Mo) deposit in western Shandong Province.