Tectonic evolution of the Dabieshan orogen: In the view from polyphase deformation of the Beihuaiyang metamorphic zone

The Dabieshan and its geological counterpart in the Sulu area represent the eastern part of the Qinling-Dabie orogenic belt in Eastern China. This Orogen corresponds to the collision zone between the North and South China blocks (denoted as NCB and SCB, respectively) during the Early Mesozoic. Since the discovery of ultra-high-pressure (UHP) metamor- phism[1?3], research of the Dabieshan has made great progress from petrological work (e.g. Cong and Wang, 1999 and enclosed references)[…     

Genesis of the Hongzhen metamorphic core complex and its tectonic implications

The Hongzhen metamorphic core complex is situated in the Yangtze plate to the east of the Dabie oro- genic belt. Its ductile detachment zone in the foot wall overprints on the metamorphic complex of the Proterozoic Dongling Group. The present profile of the ductile shear zone with consistent SW-dipping mineral elongation lineation shows antiform and reversed S-shape from northeast to southwest respectively. Exposure structures, microstructures and quartz C-axis fabric all indicate top-to-SW movement for the ductile shear zone. Recrystallisation types of quartz and feldspar in the mylonites demonstrate that the shear zone was developed under the amphibolite facies condition and at mid-crust levels. The metamorphic core complex formed in the Early Cretaceous with a muscovite plateau age of 124.8±1.2 Ma. Regional NE-SW extension along a SW-dipping, gentle detachment zone was responsible for formation of the core complex. Intrusion of the Hongzhen granite with a biotite plateau age of 124.8±1.2 Ma rendered the ductile shear zone curved, uplifted and final localization of the core complex. The Hongzhen metamorphic core complex suggests that the Early Cretaceous magma- tism in this region took place under the condition of regional extension and the eastern Yangtze plate also experienced lithospheric thinning.     

Geochemistry and chronology of the Jiehekou Group metamorphic basic volcanic rocks in the Lüliang Mountain area,Shanxi, China

Samples were systematically collected from metamorphic basic volcanic rocks in the Jiehekou and Xiyupi areas on both sides of the Lüliang Mountains, Shanxi Province and analyzed for their major elements, trace elements and rare earth elements (REE). The geochemical characteristics of their major, trace and rare-earth elements indicated that the metamorphic basic volcanic rocks in this area were emplaced in the tectonic environment like a modern continental rift. Sm-Nd and Rb-Sr isotope chronological studies demonstrated that the Jiehekou Group metamorphic basic volcanic rocks were formed during the 2600-Ma crust/mantle differentiation event, and were transformed by granulite facies metamorphism during the late Neo-Archaean period (2500 Ma ±), making the Sm-Nd systematics of the rocks reset. During the late Paleoproterozoic period (1800 Ma ±) the Rb-Sr systematics of the rocks were disturbed again in response to the Lüliang movement. Since the extent of disturbance was so weak that the Sm-Nd systematics was not affected, the age of 1600 Ma ± obtained from this area seems to be related to local magmatic activities within the craton. Research results lend no support to the idea that the Lüliang Group was formed during the Archaean. Instead, it should be formed during the Proterozoic.     

Geochemistry and chronology of the Jiehekou Group metamorphic basic volcanic rocks in the LǖIiang Mountain area,Shanxi, China

Relative to the Wutai Mountain and Heng Mountain areas, the Lliang Mountain area has been poorly documented geologically. In Geology of Shanxi Province[1], the detailed stratigraphic division was conducted in the L黮iang Mountain area. It was considered t…     

Zircon Hf isotope composition of metamorphic eclogite from Xindian,Dabie Terrain

Hf isotope measurement has been carried out for UHP metamorphic eclogite from Xindian by using LA-MC-ICP-MS technique. The result indicates that metamorphic growth zircon has high 176Hf/177Hf (0.282544―0.282612) and low 176Lu/177Hf (0.000004―0.000211) ratio,inherited and recrystallized proto-lith zircon has low 176Hf/177Hf (0.282266―0.282466) and high 176Lu/177Hf (0.000090―0.002144) composi-tions. The low 176Lu/177Hf of growth zircon comes from its decreasing of Lu and increasing of Hf during UHP process. The high 176Hf/177Hf deduced from high radiogenic 176Hf,which was produced from long-term evolution of high Lu/Hf ratio minerals. Partial recrystallization of protolith zircon would not cause reworking of Lu/Hf isotope in zircon. Compared to U-Pb,zircon Lu-Hf system has better stability. The initial Hf isotope composition of metamorphic growth zircon may represent the Hf isotope compo-sition of whole rock system at the same time. The initial εHf of 3.0 for metamorphic precursor formation of Xindian eclogite indicates that the source material mainly derived from weak depleted mantle or mixing of depleted mantle with old crust.     

Evidence of microstructures and fluid inclusions for the origin of polycrystalline quartz ribbons in high-grade metamorphic rocks in Daqingshan region

Polycrystalline quartz ribbons in high-grade metamorphic rocks from the Daqingshan region, are typi- cal microfabrics of, and provide information for, deep crust deformation and metamorphism. The quartz ribbons have straight boundaries and extend stably along gneissosity. They truncate other mineral grains in the rocks and may contain inclusions of such minerals that are lens-shaped and oriented. They frequently end into branching termination. Analysis fluid inclusions in polycrystalline quartz rib- bons reveal that the complex types of fluid inclusions are inhomogeneously distributed. They are ob- viously different from inclusions captured at granulite facies, in both fluid compositions and T-P esti- mations. Based on microfabric and fluid inclusion analysis, the polycrystalline quartz ribbons are suggested to be formed by SO2-rich fluids filling micro-fractures that are parallel to early gneissosity. The SO2 composition is derived from the deformed host rocks. The fluid phase has significant effects on the rheological characteristics, fracturing of rocks, and formation of quartz ribbons.     

The role of aerial algae in the formation of the landscape of the Yunnan Stone Forest,Yunnan Province, China

~~The role of aerial algae in the formation of the landscape of the Yunnan Stone Forest,Yunnan Province, China~~     

Study of the earthquake of the January 23, 1880, in San Cristóbal,Cuba and the Guane fault

All available data on the January 23, 1880, earthquake near San Cristobal, Western Cuba, are compiled and presented here. The earthquake reached a maximum intensity of eight degrees (MSK) and caused three fatalities. It was accompanied by 65 aftershocks and was felt as far away as the Florida Keys. Twentieth century specialists has associated this event, in its day the strongest recorded (Ms = 6.2) in the region, with the Pinar fault. The Pinar fault is well expressed topographically as the boundary between the Guaniguanico Range in the north and an alluvial plain to the south. Most of the major damage caused by the earthquake was located on the alluvial plain, which in consequence has been considered the epicenter area. In the study presented here, the data compiled from the first reports of Father Benito Vines Martorell, S.J., and Pedro Salteraín y Legarra, indicate that the seismic structure was located in the alluvial plain, and that it was the Guane fault, and not the Pinar fault, that was responsible for the earthquake. The Guane fault, found below the alluvial sediments, extends NE-SW for over 110 km. Its eastern extreme, near San José de las Lajas (La Habana), is linked to another active fault which represents a seismoactive knot responsible for the earthquake of March 9, 1995 (I = 5 degrees, MSK). Seismic events of the Western Cuban region are related to the transpressive interaction of the North American and Caribbean Plates, damped by oceanic structures.     

Double-difference Relocation of the Aftershocks of the Tecom��n, Colima, Mexico Earthquake of 22 January 2003

On 22 January 2003, the M _w?=?7.6 Tecomán earthquake struck offshore of the state of Colima, Mexico, near the diffuse triple junction between the Cocos, Rivera, and North American plates. Three-hundred and fifty aftershocks of the Tecomán earthquake with magnitudes between 2.6 and 5.8, each recorded by at least 7 stations, are relocated using the double difference method. Initial locations are determined using P and S readings from the Red Sismológica Telemétrica del Estado de Colima (RESCO) and a 1-D velocity model. Because only eight RESCO stations were operating immediately following the Tecomán earthquake, uncertainties in the initial locations and depths are fairly large, with average uncertainties of 8.0?km in depth and 1.4?km in the north?Csouth and east?Cwest directions. Events occurring between 24 January and 31 January were located using not only RESCO phase readings but also additional P and S readings from 11 temporary stations. Average uncertainties decrease to 0.8?km in depth, 0.3?km in the east?Cwest direction, and 0.7?km in the north?Csouth direction for events occurring while the temporary stations were deployed. While some preliminary studies of the early aftershocks suggested that they were dominated by shallow events above the plate interface, our results place the majority of aftershocks along the plate interface, for a slab dipping between approximately 20° and 30°. This is consistent with the slab positions inferred from geodetic studies. We do see some upper plate aftershocks that may correspond to forearc fault zones, and faults inland in the upper plate, particularly among events occurring more than 3?months after the mainshock.     

A comparative study on the illite crystallinity and the clay mineral reflectance spectral index for subdividing the very low-grade metamorphic belt along the Lizhou-Hekou geological section in the Youjiang sedimentary basin, Guangxi, China

To examine the application potential of hyperspectral remote sensing techniques in classifying very low-grade metamorphic belts, the composition of clay minerals and the cyrstallinity of illite from mudstones were measured using XRD and VIS-SWIR (400-2500 nm) reflectance spectroscopy. Based on the illite cyrstallinity, Kubler Index (KI), the Early Triassic LuoLou Group and the Middle Triassic lower Baifeng Formation were classified as the lower Epizone with KI△2θ° ranging from 0.22 to 0.25, the upper Baifeng Formation as upper anchizone with KI△2θ°ranging from 0.26 to 0.33, and the Hekou Formation as lower anchizone with KI△2θ° ranging from 0.38 to 0.40. According to a KI△2θ° value of 0.43, it is possible that there may exist a local diagenetic zone in the upper strata. The illite cyrstallinity Kubler index and the metamorphic grade increase from the bottom to the top of the stratigraphic sequence. The metamorphic grade boundaries nearly match the stratigraphic boundaries, indicating a burial metamorphism nature for the stratigraphic sequence. From the bottom to the top of the sequence, the spectral absorption band center of clay minerals from fresh rocks is around 2200 nm. The absorption band centers change towards shorter wavelengths: the Luolou Group being at 2220 nm, the Baifeng Formation at 2217-2213 nm, the lower member of the Hekou Formation at 2214-2206 nm, and the upper member of the Hekou Formation at 2205-2197 nm. The spectral absorption band center of illite shows the same change pattern. These results indicate that very low-grade metamorphic belts can be subdivided using spectral indices of clay minerals, which are measured by using field portable spectroradiometers. However, it may not work well with satellite and airborne sensors.     

Variation of stress during the rupture process of the 1995 M_L=4.1 Shacheng, Hebei, China, earthquake sequence

Introduction An earthquake could be caused by the failure of focal material or fast slip on the pre-existed faults under the tectonic stress based on the understanding of the occurrence process of earth-quakes in which the stress change could play a key role. Therefore to examine the stress change is beneficial to understanding the physic process actually occurring in the source region deeply. The apparent stress is defined as the product of seismic efficiency and the average stress on the foc…     

Is the invasion of the common reed,Phragmites australis,into tidal marshes of the eastern US an ecological disaster?

Studies of effects of the invasive brackish marsh plant Phragmites australis (common reed) on estuarine biota are reviewed. With few exceptions, most field studies indicate that these P. australis-dominated marshes have diverse and abundant benthic biota, and are utilized by nekton, comparable to Spartina alterniflora marshes. However, larval mummichogs, Fundulus heteroclitus, appear to be reduced in P. australis marshes compared with S. alterniflora marshes. Small epifauna living on plant stems also appear to be denser on S. alterniflora than P. australis stems. Other studies indicate that the detritus produced by decaying P. australis litter provides food value comparable to that of S. alterniflora and that its production enters estuarine food webs. Therefore, the general assumption that these marshes are ecologically "useless" is untrue. This information should be considered by marsh managers when making decisions about restoration projects.     

A GIS-based multi-proxy analysis of the evolution of subglacial dynamics of the Quebec–Labrador ice dome,northeastern Quebec,Canada

The glacial records of the inner-core regions of the Laurentide Ice Sheet (LIS) document complex yet coherent patterns reflecting ice-sheet change (e.g. ice-divide migration), providing unique insights into past glacial conditions. This study develops a conceptual model of subglacial dynamics evolution within a major ice-dispersal centre of the LIS in northeastern Quebec, Canada using a GIS-based analysis of the surficial geologic record. Multiple proxies of subglacial conditions (subglacial streamlined landforms, lake density and lake area over thin drift/bedrock) were analysed through grid-overlay techniques and then classified based on different proxy variables ranging from highly mobile warm-based to immobile cold-based conditions. An additional proxy (till blanket) was used to identify areas of thick till deposition, but with few proxies (few lake or landform metrics). Based on local ice-flow reconstructions, the most ‘relict’ glacial terrain zone (GTZ1) has warm-based conditions over 66% of its area and is remarkably well preserved, suggesting laterally extensive warm-based conditions during the oldest identified ice-flow phase. This relict glacial terrain is partially overprinted by two subsequent ice-flow phases in spatially restricted zones in the northeast (73% warm-based), east-central (41% warm-based), and northwest (33% warm-based) of the study area. A zone of more sluggish conditions (only 3% warm-based) was identified in the highlands at the centre of the study area, characterized by thin till cover, few landforms, yet with large patches of relatively abundant small lakes, indicative of areal scouring. No clear evidence of sustained cold-based conditions (i.e. high chemical index of alteration values or high ¹⁰Be abundances) was found in the study area. These results suggest that warm-based conditions (active erosion and/or deposition) were uniformly widespread during the earliest ice-flow phase, later becoming more spatially restricted with broader sluggish ice conditions. These spatially restricted regions of warm-based subglacial regimes were likely controlled by surrounding and down-flow ice streaming. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

The calculation of TV, VT, VV, VV′ - rate coefficients for the collisions of the main atmospheric components

The first-order perturbation approximation is applied to calculate the rate coefficients of vibrational energy transfer in collisions involving vibrationally excited molecules in the absence of non-adiabatic transitions. The factors of molecular attraction, oscillator frequency change, anharmonicity, 3-dimensionality and quasiclassical motion have been taken into account in the approximation. The analytical expressions presented have been normalized on experimental data of VT-relaxation times in N₂ and O₂ to obtain the steric factors and the extent of repulsive exchange potentials in collisions N₂–N₂ and O₂–O₂. The approach was applied to calculate the rate coefficients of vibrational-vibrational energy transfer in the collisions N₂–N₂, O₂–O₂ and N₂–O₂. It is shown that there is good agreement between our calculations and experimental data for all cases of energy transfer considered.     

On the Dynamics of the Seasonal Components of Induced Seismicity in the Koyna–Warna Region,Western India

The spatial and time dynamics are analyzed for the seasonal components of induced seismicity in the Koyna–Warna region of Western India. The peculiarities of the variations in these components are compared to the changes in the local tectonic regime inferred from the focal mechanism data of the earthquakes. Based on this, the hypotheses about the probable nature of the dynamics in the seasonal components of seismicity are suggested. It is noted that the variations in the seasonal seismic activity after the impoundment of the Koyna reservoir in the north are caused by the spatial migration of the induced seismicity and activation of the normal faults in the south. It is hypothesized that the process of fracture migration from the north to the south at this stage advanced the diffusion of the fluid from the Koyna reservoir, and as the water front reached the southern zone of normal faulting, this caused reactivation of the seasonal seismicity. An explanation is suggested for the stronger response of the seasonal activity in the region of Warna reservoir compared to the Koyna area: in contrast to Koyna, filling the Warna reservoir was geographically close to the area of activated seismicity. It is shown that the localization and sizes of the areas of the instantaneous and delayed components in the seasonal activity of the induced seismicity are determined by the localization and sizes of the areas of high stresses created by the increase in the pore pressure in highly permeable fault zones.     

Megacrysts in the Cenozoic basalt of the Tuoyun Basin,Southwest Tianshan

Abundant megacrysts of clinopyroxene, amphibole, anorthoclase, and phlogopite are found together with deep-seated xenoliths in the Cenozoic basalt of the Tuoyun Basin, Southwest Tianshan. The megacrysts are mainly in the cone sheet formed at the early stage of the volcanic activity. Clinopyrox-ene megacrysts are located in the lower part of the profile, with amphibole and phlogopite megacrysts in the middle part and anorthoclase megacrysts in the upper part. The crystal integrity, absence of de-formation fabric and their relation to the host basalt suggest that they were crystallized from the host magma and quickly transported to the surface. The mineralogical studies imply that the clinopyroxene megacrysts are of Al-augite with higher Al2O3 (>9%). Amphibole megacrysts are kaersutite rich in TiO2 (>4.5%). Sulfide inclusions such as pyrrhotite occur in some clinopyroxene and amphibole megacrysts. Thermodynamic calculations reveal that pyroxene megacrysts formed under the temperature of 1185.85―1199.85℃ and the pressure between 1.53 and 1.64 GPa comparable to the crust-mantle boundary and amphibole megacrysts crystallized under the pressure of around 0.85 GPa, temperature about 1000℃ comparable to the depth of 30 km. Anorthoclase megacrysts crystallized under the pressure between 0.8―1 GPa,temperature about 900℃.The absence of Ti-rich inclusions such as rutile can be considered as an evidence of quick magma ascending. The P-T conditions estimated via py-roxene megacrysts and phenocrysts compose a P-T path with a steep slope. It can be considered as another evidence of quick magma ascending. However, the estimated temperatures for amphibole megacrysts are markedly lower than those for pyroxene megacrysts given the same pressure. It probably shows that the amphiboles have crystallized at the vanguard of magma and under the vola-tile-rich condition. Thus, we can conclude that the Cenozoic basalts are produced in an extensional tectonic setting and the processes governing crystallization and ascending of the megacrysts are very complex.     

Geochemistry of the volcano-hydrothermal system of El Chichón Volcano, Chiapas, Mexico

 The 1982 eruption of El Chichón volcano ejected more than 1 km³ of anhydrite-bearing trachyandesite pyroclastic material to form a new 1-km-wide and 300-m-deep crater and uncovered the upper 500 m of an active volcano-hydrothermal system. Instead of the weak boiling-point temperature fumaroles of the former lava dome, a vigorously boiling crater spring now discharges  / 20 kg/s of Cl-rich (∼15 000 mg/kg) and sulphur-poor ( / 200 mg/kg of SO₄), almost neutral (pH up to 6.7) water with an isotopic composition close to that of subduction-type magmatic water (δD=–15‰, δ¹⁸O=+6.5‰). This spring, as well as numerous Cl-free boiling springs discharging a mixture of meteoric water with fumarolic condensates, feed the crater lake, which, compared with values in 1983, is now much more diluted (∼3000 mg/kg of Cl vs 24 030 mg/kg), less acidic (pH=2.6 vs 0.56) and contains much lower amounts of S ( / 200 mg/kg of SO₄, vs 3550 mg/kg) with δ³⁴S=0.5–4.2‰ (+17‰ in 1983). Agua Caliente thermal waters, on the southeast slope of the volcano, have an outflow rate of approximately 100 kg/s of 71  °C Na–Ca–Cl water and are five times more concentrated than before the eruption (B. R. Molina, unpublished data). Relative N₂, Ar and He gas concentrations suggest extensional tectonics for the El Chichón volcanic centre. The ³He/⁴He and ⁴He/²⁰Ne ratios in gases from the crater fumaroles (7.3R_a, 2560) and Agua Caliente hot springs (5.3R_a, 44) indicate a strong magmatic contribution. However, relative concentrations of reactive species are typical of equilibrium in a two-phase boiling aquifer. Sulphur and C isotopic data indicate highly reducing conditions within the system, probably associated with the presence of buried vegetation resulting from the 1982 eruption. All Cl-rich waters at El Chichón have a common source. This water has the appearence of a "partially matured" magmatic fluid: condensed magmatic vapour neutralized by interaction with fresh volcaniclastic deposits and depleted in S due to anhydrite precipitation. Shallow ground waters emerging around the volcano from the thick cover of fresh pumice deposits (Red waters) are Ca–SO₄–rich and have a negative oxygen isotopic shift, probably due to ongoing formation of clay at low temperatures. Received: 21 July 1997 / Accepted: 4 December 1997     

Oxysulfates of copper, sodium, and potassium in the lava flows of the 2012–2013 Tolbachik Fissure Eruption

Samples from the surface of lava flows discharged by the 2012–2013 Tolbachik Fissure Eruption were found to contain oxysulfates of copper, sodium, and potassium: K₂Cu₃O(SO₄)₂ (fedotovite), NaKCu₂O(SO₄)₂, and Na₃K₅Cu₈O₄(SO₄)₈. The last two phases have no naturally occurring or synthetic analogues that we are aware of. They form flattened crystals of prismatic to long-prismatic habits. The crystals of Na₃K₅Cu₈O₄(SO₄)₈ have a chemical composition corresponding to the empirical formula Na_2.22K_5.47Cu_8.02S_8.05O₃₆. An X-ray analysis of this compound showed that it has a monoclinic symmetry, P2/c, a = 13.909(4), b = 4.977(1), c = 23.525(6) Å, β = 90.021(5)°, V = 1628.3(7) ų. The crystal structure was determined by direct techniques and refined to yield R ₁ for 3955 reflexes//web// with F ² > 4σF. The compound NaKCu₂O(SO₄)₂ also belongs to the monoclinic system, P2/c, a = 14.111(4), b = 4.946(1), c = 23.673(6) Å, β = 92.052(6)°, V = 1651.1(8) ų. The structure was determined by direct techniques to yield a tentative structural model that has been refined up to R ₁ = 0.135 for 4088 reflexes with F ² > 4σF. The crystal structure of Na₃K₅Cu₈O₄(SO₄)₈ is based on chains of [O₂Cu₄]⁴⁺ consisting of rib-coupled oxy-centered tetrahedrons of (OCu₄)⁶⁺. The chains are surrounded by sulfate radicals, resulting in columns of {[O₂Cu₄](SO₄)₄}^4? aligned along the b axis. The interchain space contains completely ordered positions of Na⁺ and K⁺ cations. The principle underlying the connection of NaKCu₂O(SO₄)₂ columns in the crystal structure of {[O₂Cu₄](SO₄)₄}^4? is different, in view of the relation Na:K = 1 as contrasted with 3:5 for the compound Na₃K₅Cu₈O₄(SO₄)₈. The presence of oxy-centered tetrahedrons in the structure of these new compounds furnishes an indirect hint at the importance of polynuclear copper-oxygen radicals with centering oxygen atoms as forms of transport of copper by volcanic gases.     

Paleoseismic study of the Kamishiro Fault on the northern segment of the Itoigawa–Shizuoka Tectonic Line,Japan

The M_w 6.2 (Mj 6.8) Nagano (Japan) earthquake of 22 November 2014 produced a 9.3-km long surface rupture zone with a thrust-dominated displacement of up to 1.5 m, which duplicated the pre-existing Kamishiro Fault along the Itoigawa–Shizuoka Tectonic Line (ISTL), the plate-boundary between the Eurasian and North American plates, northern Nagano Prefecture, central Japan. To characterize the activity of the seismogenic fault zone, we conducted a paleoseismic study of the Kamishiro Fault. Field investigations and trench excavations revealed that seven morphogenic paleohistorical earthquakes (E2–E8) prior to the 2014 M_w 6.2 Nagano earthquake (E1) have occurred on the Kamishiro Fault during the last ca. 6000 years. Three of these events (E2–E4) are well constrained and correspond to historical earthquakes occurring in the last ca. 1200 years. This suggests an average recurrence interval of ca. 300–400 years on the seismogenic fault of the 2014 Kamishiro earthquake in the past 1200 years. The most recent event prior to the 2014 earthquakes (E1) is E2 and the penultimate and antepenultimate faulting events are E3 and E4, respectively. The penultimate faulting event (E3) occurred during the period of AD 1800–1400 and is associated with the 1791 M_w 6.8 earthquake. The antepenultimate faulting event (E4) is inferred to have occurred during the period of ca. AD 1000–700, likely corresponding to the AD 841 M_w 6.5 earthquake. The oldest faulting event (E8) in the study area is thought to have occurred during the period of ca. 5600–6000 years. The throw rate during the early Holocene is estimated to be 1.2–3.3 mm/a (average, 2.2 mm/a) with an average amount of characteristic offset of 0.7–1.1 m produced by individual event. When compared with active intraplate faults on Honshu Island, Japan, these slip rates and recurrence interval estimated for morphogenic earthquakes on the Kamishiro Fault along the ISTL appear high and short, respectively. This indicates that present activity on this fault is closely related to seismic faulting along the plate boundary between the Eurasian and North American plates.