High‐pressure metamorphic evolution of eclogite and associated metapelite from the Chuacús complex (Guatemala Suture Zone): Constraints from phase equilibria modelling coupled with Lu‐Hf and U‐Pb geochronology

As is common in suture zones, widespread high‐pressure rocks in the Caribbean region reached eclogite facies conditions close to ultrahigh‐pressure metamorphism. Besides eclogite lenses, abundant metapelitic rocks in the Chuacús complex (Guatemala Suture Zone) also preserve evidence for high‐pressure metamorphism. A comprehensive petrological and geochronological study was undertaken to constrain the tectonometamorphic evolution of eclogite and associated metapelite from this area in central Guatemala. The integration of field and petrological data allows the reconstruction of a previously unknown segment of the prograde P–T path and shows that these contrasting rock types share a common high‐pressure evolution. An early stage of high‐pressure/low‐temperature metamorphism at 18–20 kbar and 530–580°C is indicated by garnet core compositions as well as the nature and composition of mineral inclusions in garnet, including kyanite–jadeite–paragonite in an eclogite, and chloritoid–paragonite–rutile in a pelitic schist. Peak high‐pressure conditions are constrained at 23–25 kbar and 620–690°C by combining mineral assemblages, isopleth thermobarometry and Zr‐in‐rutile thermometry. A garnet/whole‐rock Lu‐Hf date of 101.8 ± 3.1 Ma in the kyanite‐bearing eclogite indicates the timing of final garnet growth at eclogite facies conditions, while a Lu‐Hf date of 95.5 ± 2.1 Ma in the pelitic schist reflects the average age of garnet growth spanning from an early eclogite facies evolution to a final amphibolite facies stage. Concordant U‐Pb LA‐ICP‐MS zircon data from the pelitic schist, in contrast, yield a mean age of 74.0 ± 0.5 Ma, which is equivalent to a U‐Pb monazite lower‐intercept age of 73.6 ± 2.0 Ma in the same sample, and comparable within errors with a less precise U‐Pb lower‐intercept age of 80 ± 13 Ma obtained in post‐eclogitic titanite from the kyanite‐bearing eclogite. These U‐Pb metamorphic ages are interpreted as dating an amphibolite facies overprint. Protolith U‐Pb zircon ages of 167.1 ± 4.2 Ma and 424.6 ± 5.0 Ma from two eclogite samples reveal that mafic precursors in the Chuacús complex originated in multiple tectonotemporal settings from the Silurian to Jurassic. The integration of petrological and geochronological data suggests that subduction of the continental margin of the North American plate (Chuacús complex) beneath the Greater Antilles arc occurred during an Albian‐Cenomanian pre‐collisional stage, and that a subsequent Campanian collisional stage is probably responsible of the amphibolite facies overprint and late syncollisional exhumation.     

The 1877–1878 El Niño episode: associated impacts in South America

At times when attention on climate issues is strongly focused on the assessment of potential impacts of future climate change due to the intensification of the planetary greenhouse effect, it is perhaps pertinent to look back and explore the consequences of past climate variability. In this article we examine a large disruption in global climate that occurred during 1877–1878, when human influence was negligible. The mechanisms explaining this global disturbance are not well established, but there is considerable evidence that the major El Niño episode that started by the end of 1876 and peaked during the 1877–1878 boreal winter contributed significantly to it. The associated regional climate anomalies were extremely destructive, particularly in the Northern Hemisphere, where starvation due to intense droughts in Asia, South-East Asia and Africa took the lives of more than 20 million people. In South America regional precipitation anomalies were typical of El Niño events, with rainfall deficit and droughts in the northern portion of the continent as well as in northeast Brazil and the highlands of the central Andes (Altiplano). In contrast, anomalously intense rainfall and flooding episodes were reported for the coastal areas of southern Ecuador and Northern Perú, as well as along the extratropical West coast of the continent (central Chile, 30° S–40° S), and in the Paraná basin in the southeast region. By far the most devastating impacts in terms of suffering and loss of life occurred in the semiarid region of northeast Brazil where several hundreds of thousands of people died from starvation and diseases during the drought that started in 1877.     

A comparison of approximate techniques for non-linear seismic soil response

Some equivalent linearization techniques, applicable to problems dealing with non-linear seismic amplification of layered soils, are systematically treated and critically compared in this paper. The discussion concerns, in particular, the theoretical consistency, the operative modalities, the computational complexity and the numerical reliability of these methods. A comparison is made between the results furnished by some techniques among those examined and the results obtained through a step-by-step integration of the non-linear equations of motion, assuming as a test-problem the one-dimensional amplification of a horizontally stratified soil deposit characterized by the Ramberg-Osgood constitutive model.     

Reply to Molina-Garza et al. (2019) “Discussion of: Ortega-Flores et al. (2018) provenance analysis of Oligocene sandstone from the Cerro Pelón area,southern Gulf of Mexico”

ABSTRACT

The origin of the Oligocene turbidites from the Cerro Pelón area in south Gulf Mexico proposed by Ortega-Flores et al. (2018) is in disagree with the interpretations made by Molina-Garza et al. (2019), which main criticism is based on U-Pb ages of detrital zircons from the matrix of a conglomerate unit, which they refer to as ‘Nanchital Conglomerate’, as well as on the presence of limestone, gabbros, and mafic protolith-derived clasts. Molina-Garza et al. (2019) basically interpret the Nanchital Conglomerate as Miocene in age, which was sourced mainly from metamorphic complexes including their sedimentary covers located to the west and south of the Cerro Pelón area. For some reason, Molina-Garza et al. (2019) suppose that the Nanchital Conglomerate should have the same provenance sources that the Oligocene turbidites from Cerro Pelón area, reported by Ortega-Flores et al. (2018). Based on the foregoing, we strongly disagree with Molina-Garza et al. (2019) considering that, from the beginning, they intend to compare two units of different age. Additionally, the scarce data reported from both the matrix and the clasts of the Nanchital Conglomerate are not determinant for interpreting the provenance of this conglomeratic unit and subsequently, to consider the same rock sources from the Oligocene through Miocene time.     

Automatic time recording in determination of astronomical latitude and longitude using an astrolabe and an HP41CX

The paper presents the automatic time recording of transits of the image of a star across the hairs in the visual field of a telescope and the automatic reception of radio time signals for the determination of astronomical latitude and longitude of a station using an astrolabe. The automatization was achieved by an electronic calculator Hewlett Packard HP41CX and an especially constructed relay. The method proved very practical in the course of making the network of geoidal points more dense for the needs of geodesy.     

Seismic response of multi‐supported structures by proper orthogonal decomposition

The seismic analysis of structures is usually carried out considering the ground motion as fully‐correlated in space and determining the structural response by pseudo‐deterministic methods such as the response spectrum technique. Actually, the partial correlation of the seismic acceleration may influence heavily the behaviour of spatially extended structures, such as bridges, viaducts or pipelines. In order to take its partial correlation into account, the seismic ground motion is schematized as a stochastic process dependent on time and on space; the hypotheses of stationarity and homogeneity are used to obtain simple and general results. The influence of the partial correlation of the seismic ground motion on the structural response is investigated by introducing suitable Equivalent Spectra. The acceleration of the support‐points of the structure is represented by the Proper Orthogonal Decomposition (POD), defining the modes of the earthquake. The method is formulated for any kind of multi‐degree‐of‐freedom system and is applied, as a case study, to an ideal single‐storey multi‐supported frame with an axially rigid beam. In the case of two supports, the POD decouples the pseudo‐static and the dynamic contributions to the structural response. This property is preserved for structural systems with many supports, where only the lower modes of the earthquake, usually the first two POD modes, are responsible for the structural response. Copyright © 2003 John Wiley & Sons, Ltd.     

990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico: roots of an orogen

Inliers of 1.0–1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012±12 Ma anorthosite–mangerite–charnockite–granite (AMCG) suite; (2) El Catrı́n unit: ≥1350 Ma orthogneiss migmatized at 1106±6 Ma; and (3) El Marquez unit: ≥1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U–Pb isotopic analyses of zircon: (a) the 1106±6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004–978±3 Ma Zapotecan event produced at least two sets of structures: (Z1) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal WNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 °C at 977±12 Ma is documented by ⁴⁰Ar/³⁹Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential–longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of 25–30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modern analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events took place throughout Oaxaquia, and in various parts of the Genvillian orogen in Laurentia and Amazonia.     

Chemical classification of the water in a lowland river basin (Salado River,Buenos Aires,Argentina) affected by hydraulic modifications

The main ions were measured seasonally during two years at 13 sampling stations in the Salado River and its main tributaries. The importance of each ion was assessed by standard methods used to examine ionic composition and by multivariate methods. The K-means clustering and Principal Component Analysis were applied to the percentages of the major ions. The concentration of the major cations are in the order Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ and the major anions, Cl⁻ > SO₄²⁻ > HCO₃⁻ > CO₃²⁻, and the salinity was high (mean TDS 2,691 mg l⁻¹) due to sodium chloride. Using the proportions of the ions was possible to identify seven types of water within the basin related to discharges of different river sub-catchments and from endorheic catchments (in a sand dune region) actually connected with the basin by canals. The chemical composition of the basin is consequence of surface waters receiving salts from groundwater, evaporation and weathering of Post-Pampeano materials, and of anthropogenic impact by diversion between subcatchments for flood control. These results allowed us to test the marked effects on the ionic balance of basin at the base of a diversion management from endorheic catchments characterized by high salinity waters.     

The Proterozoic of NW Mexico revisited: U–Pb geochronology and Hf isotopes of Sonoran rocks and their tectonic implications

International Journal of Earth Sciences - Several Proterozoic basement units crop out in the Sonora State of NW Mexico, and the same can be correlated with crustal provinces of southern Laurentia...