Lahars risk at the Tacaná Volcano Complex (México–Guatemala) from numerical simulations and physical vulnerability analysis

Natural Hazards - The Nile River provides Egypt with most of its water resources. Medium- and long-rage forecasts of Nile flows at Aswan have been recognized as of significant importance to allow...     

The geochemistry of a dying continental arc: the Incapillo Caldera and Dome Complex of the southernmost Central Andean Volcanic Zone (~28°S)

The Pleistocene Incapillo Caldera and Dome Complex (5,570 m) marks the southernmost siliceous center of the Andean Central Volcanic Zone (~28°S), where the steeply dipping (~30°) segment of the subducting Nazca plate transitions into the Chilean “flatslab” to the south. The eruption of the Incapillo Caldera and Dome Complex began with a 3–1 Ma effusive phase characterized by ~40 rhyodacitic dome eruptions. This effusive phase was terminated by an explosive “caldera-forming” event at 0.51 Ma that produced the 14 km³ Incapillo ignimbrite. Distinctive and virtually identical chemical signatures of the domes and ignimbrites (SiO₂ = 67–72 wt%; La/Yb = 37–56; Ba/La = 16–28; La/Ta = 30–50; ⁸⁷Sr/⁸⁶Sr = 0.70638–0.70669; ε _Nd = −4.2 to −4.6) indicate that all erupted lavas originated from the same magma chamber and that differentiation effects between units were minor. The strong HREE depletion (Sm/Yb = 6–8) that distinguishes Incapillo magmas from most of the large ignimbrites of the Altiplano–Puna plateau can be explained by the extent and degree of partial melting at lower crustal depths (>40 km) in the presence of garnet. At upper crustal depths, this high-pressure residual geochemical signature, also common to adjacent late Miocene/Pliocene Pircas Negras andesites, was partially overprinted by shallow-level assimilation and fractional crystallization processes. Energy-constrained AFC modeling suggests that incorporation of anatectic upper crustal melts into a fractionated “adakite-like” dacitic host best explains the petrogenesis of Incapillo magmas. The diminution of the sub-arc asthenospheric wedge during Nazca plate shallowing left the Incapillo magma chamber unreplenished by both mafic mantle-derived and lower crustal melts and thus stranded at shallow depths within the Andean crust. Based on its small size and distinctive high-pressure chemical signature, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone.     

Quaternary bimodal volcanism in the Niğde Volcanic Complex (Cappadocia,central Anatolia,Turkey): age,petrogenesis and geodynamic implications

The late Neogene to Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia is one of the most impressive volcanic fields of Turkey because of its extent and spectacular erosionally sculptured landscape. The late Neogene evolution of the CVP started with the eruption of extensive andesitic-dacitic lavas and ignimbrites with minor basaltic lavas. This stage was followed by Quaternary bimodal volcanism. Here, we present geochemical, isotopic (Sr–Nd–Pb and δ¹⁸O isotopes) and geochronological (U–Pb zircon and Ar–Ar amphibole and whole-rock ages) data for bimodal volcanic rocks of the Ni?de Volcanic Complex (NVC) in the western part of the CVP to determine mantle melting dynamics and magmatic processes within the overlying continental crust during the Quaternary. Geochronological data suggest that the bimodal volcanic activity in the study area occurred between ca. 1.1 and ca. 0.2 Ma (Pleistocene) and comprises (1) mafic lavas consisting of basalts, trachybasalts, basaltic andesites and scoria lapilli fallout deposits with mainly basaltic composition, (2) felsic lavas consisting of mostly rhyolites and pumice lapilli fall-out and surge deposits with dacitic to rhyolitic composition. The most mafic sample is basalt from a monogenetic cone, which is characterized by ⁸⁷Sr/⁸⁶Sr = 0.7038, ¹⁴³Nd/¹⁴⁴Nd = 0.5128, ²⁰⁶Pb/²⁰⁴Pb = 18.80, ²⁰⁷Pb/²⁰⁴Pb = 15.60 and ²⁰⁸Pb/²⁰⁴Pb = 38.68, suggesting a moderately depleted signature of the mantle source. Felsic volcanic rocks define a narrow range of ¹⁴³Nd/¹⁴⁴Nd isotope ratios (0.5126–0.5128) and are homogeneous in Pb isotope composition (²⁰⁶Pb/²⁰⁴Pb = 18.84–18.87, ²⁰⁷Pb/²⁰⁴Pb = 15.64–15.67 and ²⁰⁸Pb/²⁰⁴Pb = 38.93–38.99). ⁸⁷Sr/⁸⁶Sr isotopic compositions of mafic (0.7038–0.7053) and felsic (0.7040–0.7052) samples are similar, reflecting a common mantle source. The felsic rocks have relatively low zircon δ¹⁸O values (5.6 ± 0.6 ‰) overlapping mantle values (5.3 ± 0.3 %), consistent with an origin by fractional crystallization from a mafic melt with very minor continental crustal contamination. The geochronological and geochemical data suggest that mafic and felsic volcanic rocks of the NVC are genetically closely related to each other. Mafic rocks show a positive trend between ⁸⁷Sr/⁸⁶Sr and Th, suggesting simultaneous assimilation and fractional crystallization, whereas the felsic rocks are characterized by a flat or slightly negative variation. High ⁸⁷Sr/⁸⁶Sr gneisses are a potential crustal contaminant of the mafic magmas, but the comparatively low and invariant ⁸⁷Sr/⁸⁶Sr in the felsic volcanics suggests that these evolved dominantly by fractional crystallization. Mantle-derived basaltic melts, which experienced low degree of crustal assimilation, are proposed to be the parent melt of the felsic volcanics. Geochronological and geochemical results combined with regional geological and geophysical data suggest that bimodal volcanism of the NVC and the CVP, in general, developed in a post-collisional extensional tectonic regime that is caused by ascending asthenosphere, which played a key role during magma genesis.     

Continental rift and oceanic protoliths of mafic–ultramafic rocks from the Kechros Complex,NE Rhodope (Greece): implications from petrography,major and trace-element systematics,and MELTS modeling

The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe–Ti (LFT) eclogites have low-TiO₂ content (<0.67 wt%), negative high field strength element anomalies, and variable enrichments in large ion lithophile elements (LILE). The rare earth element (REE) patterns are characterized by strong light-REE (LREE) enrichment and heavy-REE (HREE) depletion. The high-Fe–Ti (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE–HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO₂ loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite.     

Biogeochemical Behavior of Arsenic Species at Paranaguá Estuarine Complex, Southern Brazil

The behavior and dynamics of arsenic at Ilha do Mel, Guaraque?aba, and Paranaguá, located in the Paranaguá Estuarine Complex, Brazil, were investigated in this work. Samples were collected in March, September, and December of 2005. With respect to arsenic behavior, the total concentrations were higher for Paranaguá (22.5?±?2.5?μg?L^?1) on average, followed by the samples of Guaraque?aba (14.4?±?3.2?μg?L^?1) and Ilha do Mel (8.7?±?1.1?μg?L^?1). The concentrations found in this work were consistently greater when compared to other estuaries. The results can be attributed to geological factors. Nevertheless, it was also possible to realize that higher concentrations were found in Paranaguá, the more impacted environment. These results represent a strong indication that human activities in the area contribute to the enrichment of the estuary with this metalloid. Arsenic was mainly present in the dissolved phase, which corresponded to more than 70% of the total concentration. This behavior can represent a greater residence time of this metalloid in the water column. The marine phytoplankton presence was decisive in dictating the distribution of As in the waters of Paranaguá estuary. Organic species, which are considered the less toxic, were found in higher concentrations in the eutrophic areas such as Guaraque?aba and Paranaguá. This was a typical behavior during the summer probably due to higher biological activity.     

Ceboruco hazard map: part II—modeling volcanic phenomena and construction of the general hazard map

Natural Hazards - Ceboruco volcano in the western Trans-Mexican Volcanic Belt is one of the eleven most active stratovolcanoes in Mexico. Due to its recent eruptive history including a large...     

Coupled surface and subsurface flow modeling of natural hillslopes in the Aburrá Valley (Medellín,Colombia)

Numerical results are presented of surface-subsurface water modeling of a natural hillslope located in the Aburrá Valley, in the city of Medellín (Antioquia, Colombia). The integrated finite-element hydrogeological simulator HydroGeoSphere is used to conduct transient variably saturated simulations. The objective is to analyze pore-water pressure and saturation variation at shallow depths, as well as volumes of water infiltrated in the porous medium. These aspects are important in the region of study, which is highly affected by soil movements, especially during the high-rain seasons that occur twice a year. The modeling exercise considers rainfall events that occurred between October and December 2014 and a hillslope that is currently monitored because of soil instability problems. Simulation results show that rainfall temporal variability, mesh resolution, coupling length, and the conceptual model chosen to represent the heterogeneous soil, have a noticeable influence on results, particularly for high rainfall intensities. Results also indicate that surface-subsurface coupled modeling is required to avoid unrealistic increase in hydraulic heads when high rainfall intensities cause top-down saturation of soil. This work is a first effort towards fostering hydrogeological modeling expertise that may support the development of monitoring systems and early landslide warning in a country where the rainy season is often the cause of hydrogeological tragedies associated with landslides, mud flow or debris flow.     

Numerical modeling of porosity waves in the Nankai accretionary wedge décollement,Japan: implications for aseismic slip

Seismic and hydrologic observations of the Nankai accretionary wedge décollement, Japan, show that overpressures at depths greater than ～2 km beneath the seafloor could have increased to near lithostatic values due to sediment compaction and diagenesis, clay dehydration, and shearing. The resultant high overpressures are hypothesized then to have migrated in rapid surges or pulses called ‘porosity waves’ up the dip of the décollement. Such high velocities—much higher than expected Darcy fluxes—are possible for porosity waves if the porous media through which the waves travel are deformable enough for porosity and permeability to increase strongly with increasing fluid pressure. The present study aimed to test the hypothesis that porosity waves can travel at rates (kilometers per day) fast enough to cause aseismic slip in the Nankai décollement. The hypothesis was tested using a one-dimensional numerical solution to the fluid mass conservation equation for elastic porous media. Results show that porosity waves generated at depths of ～2 km from overpressures in excess of lithostatic pressure can propagate at rates sufficient to account for aseismic slip along the décollement over a wide range of hydrogeological conditions. Sensitivity analysis showed porosity wave velocity to be strongly dependent on specific storage, fluid viscosity, and the permeability–depth gradient. Overpressure slightly less than lithostatic pressure could also produce porosity waves capable of traveling at velocities sufficient to cause aseismic slip, provided that hydrogeologic properties of the décollement are near the limits of their geologically reasonable ranges.     

Leaiid conchostracans from the uppermost Permian strata of the Paraná Basin,Brazil: Chronostratigraphic and paleobiogeographic implications

Conchostracan fossils are abundant and relatively diversified in the Rio do Rasto Formation (Passa Dois Group, Paraná Basin, southern Brazil), but leaiids (‘Leaia pruvosti’ [Reed, F.R.C., 1929. Novos Phyllopodos Fósseis do Brasil. Boletim do Serviço Geológico e Mineralógico do Brasil 34, 2–16]) were previously found at only one locality of the formation in the northern Santa Catarina State. New specimens of the Family Leaiidae, collected from two outcrops in central Paraná State near the top of the formation, stimulated a revision of related taxa. Both the new and the previously known leaiids are herein assigned to Hemicycloleaia mitchelli [Etheridge Jr., R., 1892. On Leaia mitchelli Etheridge. Proceedings of the Linnean Society of New South Wales 7, 307–310] based on the presence of three carinae and subovate shape. This species was originally recorded in the upper Tatarian (Wuchiapingian, Late Permian) of Sydney Basin, eastern Australia and therefore corroborates the interpretation that the leaiid bearing strata of the Rio do Rasto Formation cannot be younger than Permian. H. mitchelli possibly was one of the most widespread, eurytopic and conservative Late Paleozoic conchostracans of Gondwana (although records from Africa, India and Antarctica must still be confirmed) and it was also found in the Tatarian of Russia. The sudden disappearance of leaiids after their apparent success is consistent with the hypothesis about the biotic crisis around the Permo-Triassic boundary.     

Karst landslides hazard during 1940–2002 in the mountainous region of Guizhou Province, Southwest China

Mountain landslide has been an environmental geological problem and occurs frequently in China, especially in the karst region of Guizhou Province, Southwest China. The data of karst mountain landslide are collected and analysed, which occurred in the period of 1940–2002. The collection includes 321 events in the karst region of Guizhou Province. The characteristics of mountain landslides may be classified as two types, namely natural mountain landslides (287 events) and the other induced by human activities (34 events). The results indicate that natural mountain landslide causes especially high damage and is still the main type of natural hazard in the study area owing to the extremely fragile karst geological environment.     

Volcanic threat in developing countries of the Asia�CPacific region: probabilistic hazard assessment, population risks, and information gaps

The importance of disaster reduction has gained increased awareness within the international development community and thereby highlighted a need for a preliminary assessment of natural hazard risk in developing countries of the Asia?CPacific, including that for volcanic eruption. In this paper, we present a key component of such an assessment, which involved qualifying the frequency and potential consequences of large??Volcanic Explosivity Index of four or more??volcanic eruptions. The frequencies of large eruptions from volcanoes grouped by region were determined from frequency?Cmagnitude plots using data provided by the Smithsonian Institution??s Global Volcanism Program. However, calculated frequencies represent only minimum values due to an incomplete eruption record. Unfortunately, limited data precluded the calculation of eruption frequencies for the Solomon Islands, Fiji and Samoa. A first-order analysis of the populations potentially impacted by large volcanic eruptions suggest that (1) volcanic disasters affecting populations of >100,000 can be expected at least every decade in Indonesia and once every few decades in the Philippines and (2) a volcanic disaster impacting >1% of the population can be expected twice a century in Vanuatu, twice a millennium for Indonesia and the Philippines, and around every millennium in Papua New Guinea and Tonga.     

Geochemical and Sr–O isotopic constraints on magmatic differentiation at Gede Volcanic Complex, West Java, Indonesia

The Gede Volcanic Complex (GVC) of the Sunda island arc (West Java, Indonesia) consists of multiple volcanic centres and eruptive groups with complex magmatic histories. We present new petrological, mineralogical, whole-rock major and trace element and Sr–O isotopic data to provide constraints on the relative importance of fractional crystallisation and magma mixing in petrogenesis, as well as on the role and nature of the arc crust. Banded juvenile scoria from Young and Old Gede provide unequivocal evidence for the (late-stage) interaction of distinct magmas at Gede volcano. However, the relatively small-degree compositional zoning observed in plagioclase phenocrysts of all eruptive groups (up to ~20 mol% An) may be attributed to physical changes in magma properties (e.g. P, T, and PH₂O) rather than changes in melt composition. Major element and trace element variations within each eruptive series are inconsistent with magmatic evolution through simple mixing processes. Instead, mixing of variably fractionated magma batches is suggested to account for the significant scatter in some element variation diagrams. No correlation is observed between textural complexity and/or mineral disequilibrium and whole-rock geochemistry. REE data and geochemical modelling indicate that fractional crystallisation involving amphibole in the mid- to lower crust, and fractionation of plagioclase, clinopyroxene, Fe–Ti oxide ± olivine ± orthopyroxene provide strong control on the geochemical evolution of GVC rocks. Two-pyroxene geothermobarometry provides pre-eruption crystallisation temperatures of 891–1,046°C and pressures of 3.4–6.5 kbar, equivalent to ~13–24 km depth beneath the volcanoes (mid- to lower crust). Low, mantle-like clinopyroxene δ¹⁸O values of GVC lavas and poor correlation of Sr isotope ratios with indices of differentiation precludes significant assimilation of isotopically distinct crust during magmatic differentiation. Therefore, we suggest that the geochemical character of the moderately thick West Javan arc crust is relatively immature compared to typical continental crust. Trace element ratios and strontium isotopes show that the magmatic source composition of the older geographical units, Gegerbentang and Older Quaternary, is distinct from the other GVC groups.     

Petrology and P–T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex,China

The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M₁ assemblage is composed of plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M₂ stage is represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P–T conditions for this assemblage have been estimated using the program Tweequ at 10–12 kbar and 600–650°C. The M₃ assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yields P–T conditions of 6.0–7.0 kbar and 600–650°C. M₄ is represented by chlorite and epidote rimming garnet, chlorite rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400–500°C and relatively lower pressures. Taken together, the qualitative P–T estimates from M₁ and M₄ and the quantitative P–T estimates from M₂ and M₃ define a clockwise P–T path for the Wutai amphibolites.The estimated P–T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M₁+M₂), subsequent isothermal exhumation (M₃), and finally cooling and retrogression (M₄). This tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton.     

U-Pb chronology of the Northampton Complex, Western Australia – evidence for Grenvillian sedimentation, metamorphism and deformation and geodynamic implications

Conventional and SHRIMP U-Pb analyses of zircon, monazite, titanite and apatite from the high grade rocks of the Northampton Complex in Western Australia provide constraints on the timing of metamorphic processes and deformation events in the northern Darling Mobile Belt (western margin of the Archean Yilgarn Craton). Paragneisses and mafic volcanics and/or intrusions have undergone granulite facies metamorphism in a probable extensional tectonic setting prior to formation of W- to NW-verging folds and thrusts cut by normal shears (interpreted as late collapse structures) during the main deformation event (D₁). These structures are folded by open to tight folds with NW-striking axial surfaces developed in a second, NE-SW contractional event (D₂). Zircons from a mafic granulite provide an age of 1079 ± 3 Ma attributed to new zircon growth prior to, or at the peak of regional granulite facies metamorphism. Metamorphic monazites extracted from a paragneiss yield an identical age of 1083 ± 3 Ma. The similarity of ages between zircons from the mafic granulite (1079 ± 3 Ma) and monazites from the paragneiss (1083 ± 3 Ma) is interpreted to reflect fast cooling and/or rapid uplift, which is consistent with thrusting of the gneissic units during the first deformation event (D₁) associated with the onset of retrograde metamorphism. Granitic activity at 1068 ± 13 Ma was followed by intrusion of post-D₂ pegmatite (989 ± 2 Ma), which constrains the end of metamorphism and associated deformation. Cooling of the complex to about 500 °C is timed by the apatite age of 921 ± 23 Ma. SHRIMP U-Pb ages of detrital zircons from a paragneiss sample yield a maximum age of 2043 Ma, with no evidence of an Archean Yilgarn signature. A majority of ages between 1.6 and 1.9 Ga are consistent with derivation from the Capricorn Orogen on the northern margin of the Yilgarn Craton. Younger detrital zircons with 1150–1450 Ma ages, however, indicate an additional source that had undergone early Grenvillian igneous or metamorphic event(s) and also places a maximum age constraint upon deposition. The source of this clastic material may have been from within the southern Darling Mobile Belt or from Greater India (adjacent to the Northampton Complex in Rodinia reconstructions). This study documents an extended Grenvillian history, with basin formation, sedimentation, granulite facies metamorphism, contractional tectonics (two periods with orthogonal directions of shortening) and late pegmatite emplacement taking place between 1150–989 Ma on the western margin of the Yilgarn Craton. Ages recorded in this study indicate that the proposed global distribution of Grenvillian belts during assembly of the Rodinia supercontinent should be reassessed to include the Darling Mobile Belt. Received: 7 January 1998 / Accepted: 10 March 1999     

Paleoclimatic implications (Late Cretaceous–Paleogene) from micromorphology of calcretes,palustrine limestones and silcretes,southern Paraná Basin,Uruguay

Sedimentologic and petrographic analyses of outcroping and subsurface calcretes, palustrine carbonates, and silcretes were carried out in the southern Paraná Basin (Uruguay). The aim of this work is to describe the microfabric and interpret the genesis of these rocks through detailed analyses, since they contain significant paleoenvironmental and paleoclimatic evolution information.The main calcrete and silcrete host rock (Mercedes Formation) is represented by a fluvial thinning upward succession of conglomerate and sandstone deposits, with isolated pelitic intervals and paleosoils. Most of the studied calcretes are macroscopically massive with micromorphological features of alpha fabric, originated by displacive growth of calcite in the host clastic material due to evaporation, evapotranspiration and degassing. Micromorphologically, calcretes indicate an origin in the vadose and phreatic diagenetic environments. Micrite is the principal component, and speaks of rapid precipitation in the vadose zone from supersaturated solutions. The abundance of microsparite and secondary sparite is regarded as the result of dissolution and reprecipitation processes.Although present, brecciated calcretes are less common. They are frequent in vadose diagenetic environments, where the alternation between cementation and non-tectonic fracturing conditions take place. These processes generated episodes of fragmentation, brecciation and cementation. Fissures are filled with clear primary sparitic calcite, formed by precipitation of extremely supersaturated solutions in a phreatic diagenetic environment. The micromorphological characteristics indicate that calcretes resulted from carbonate precipitation in the upper part of the groundwater table and the vadose zone, continuously nourished by lateral migration of groundwater.The scarcity of biogenic structures suggests that they were either formed in zones of little biological activity or that the overimposed processes related to water table fluctuations produced intense recrystallization completely obliterating the biogenic fabric.Limestone beds containing terrestrial gastropods are geographically restricted. Situated at the top of the calcrete successions, they exhibit brecciated and peloidal-intraclastic textures but lack lamination, edaphic structures, aggregates and vertical rhizoliths. This indicates they correspond to low-energy palustrine deposits, generated in shallow, local and ephemeral ponds developed in topographic depressions. When water table levels dropped, the palustrine deposits were exposed. This favours the presence of terrestrial gastropods, seeds and insect nests. The combination of calcretes and palustrine carbonates indicates periods and areas with a reduced clastic input and a predominantly semiarid climate, with well-defined humid and dry seasons.Characteristics of the later developed massive and nodular horizons of silcretes, such as, preservation of the internal structure of the host rock, the small areal extent, the formation of massive lenses, the complex pore infillings and the lack of a columnar upper section, indicate that they were generated from groundwaters. Every silcretized horizon shows different positions of the groundwater table and relates to the dissection of landscape.The age of calcretization and silcretization is bracketed between the Late Cretaceous (Campanian–Maastrichtian) and the Early Eocene. Paleoclimate indicates changing conditions from warm and humid at the end of the Cretaceous (Mercedes Formation) to semiarid and seasonal during Paleocene (groundwater calcretes and palustrine deposits) and subtropical and seasonal in the early Eocene (Asencio Formation).     

Terrane character of the north-east margin of the Moldanubian Zone: the Kutná Hora Crystalline Complex,Bohemian Massif

Three major allochthonous units have been distinguished on the north-eastern border of the Moldanubian Zone, which differ each from other in lithology and structural and metamorphic evolution. Their present day position displays significant metamorphic and structural inversion resulting from progressive nappe stacking during the Variscan orogeny. The uppermost-Gföhl Unit consists of anatectic rocks containing high temperature/high pressure relics, i.e. granulites, eclogites and garnet peridotites. The rocks of the Gföhl Unit were strongly mylonized during uplift and later also extensively migmatized in the kyanite stability field. The Kouiim Nappe is built up by a sequence of fine-grained leucocratic migmatites with preserved relics of a pre-Variscan deformation event. This event was terminated by the intrusion of coarse-grained porphyritic granites, converted into augen orthogneisses by the Variscan orogeny. The lowermost Micaschist Zone was formed from a sequence of metapelites intercalated with diopsidic amphibolites.During uplift from deep crustal zones the Gföhl Unit cut off a thick slice of the basement crustal material represented by the Kourim Nappe. The quartzo-feldspathic material of the Kourim Nappe acted as a major shear interface because of its extreme ductility under the conditions found in the middle crust. This process occurred under amphibolite facies metamorphism. The continuous uplift of the nappe pile induced another crustal segment in the nappe stack, represented by the Micaschist Zone. The whole nappe sequence was then thrust over the Moldanubian Zone. A westward sense of shear is suggested for the whole uplift history. The kinematic pattern was complicated by later strike-slip ductile faults which finished the recent geological configuration.Correspondence to: J. Synek     

U‒Pb Age of Sphene and the Petrochemical,Mineralogical, and Geochemical Features of Alkaline Rocks of the Bogdo Complex (Arctic Siberia)

Doklady Earth Sciences - In the northern part of the Siberian Platform, east of the Anabar Shield, several massifs of alkaline rocks with carbonatites identified (Tomtor, Bogdo, Promezhutochnyi)...     

Seismic hazard at a triple plate junction: the state of Chiapas (México)

The state of Chiapas (SE México) conforms a territory of complex tectonics and high seismic activity. The interaction among the Cocos, North American and Caribbean tectonic plates, as well as the active crustal deformation inside Chiapas, determines a variety of seismogenic sources of distinct characteristics and particular strong ground motion attenuation. This situation makes the assessment of seismic hazard in the region a challenging task. In this work, we follow the methodology of probabilistic seismic hazard analysis, starting from the compilation of an earthquake catalogue, and the definition of seismogenic source-zones based on the particular seismotectonics of the region: plate-subduction-related sources (interface and intraslab zones), active crustal deformation zones and the shear zone between the North American and Caribbean plates formed by the Motagua, Polochic and Ixcán faults. The latter source is modelled in two different configurations: one single source-zone and three distinct ones. We select three ground motion prediction equations (GMPEs) recommended for South and Central America, plus two Mexican ones. We combine the GMPEs with the source-zone models in a logic tree scheme and produce hazard maps in terms of peak ground acceleration and spectral acceleration for the 500-, 1000- and 2500-year return periods, as well as uniform hazard spectra for the towns of Tuxtla Gutiérrez, Tapachula and San Cristóbal. We obtain higher values in comparison with previous seismic hazard studies and particularly much higher than the output of the Prodisis v.2.3 software for seismic design in México. Our results are consistent with those of neighbouring Guatemala obtained in a recent study for Central America.