Intraslab seismicity and thermal stress in the subducted Cocos plate beneath central Mexico

We present a model of the subducting Cocos slab beneath Central Mexico, that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment. Based on the recently developed thermal models for the Central Mexico subduction zone, the thermal stresses due to non-uniform temperature contrast in the subducting slab are calculated using a finite element approach. The slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep. The calculation results show a  20 km slab core characterized by a tensional state of stress with stresses up to 70 MPa. On the other hand, the top of the slab experiences high compressive thermal stresses up to 110 MPa, depending on the elastic constants used and location along the flat part of the subducting plate. These compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes, and two different sources of stress are proposed.

The trenchward migration of the Mexican volcanic arc for the last 7 Ma indicates an increase of the slab dip through time. This observation suggests that the gravity torque might exceed the suction torque. Considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0.5 × 10¹⁶–1.5 × 10¹⁶ N m, the upper half would exhibit tensional stresses of 40–110 MPa that can actually balance the compressive thermally induced stresses.

An alternative stress source might come from the slab pull force caused by the slab positive density anomaly. Based on our density anomaly estimations (75 ± 20 kg/m³), a 350 km slab length, dipping at 20° into the asthenosphere, induces a slab pull force of 1.7 × 10¹²–4.6 × 10¹² N m. This force produces a tensional stress of 41–114 MPa, sufficient to balance the compressive thermal stresses at the top of the flat slab.

The linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60–180 MPa inside the flat slab core. Also, our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40–80 MPa.

This study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath Central Mexico, and also it shows that thermal stresses due to non-uniform reheating of subducting slabs play a considerable role in the total stress field.     

Optical Spectroscopy of Quasars Discovered by SRG/eROSITA with a 2.5-m Telescope at the Caucasus Mountain Observatory of SAI MSU

Astronomy Letters - Based on observations with the transient double-beam spectrograph (TDS) at the 2.5-m telescope of the Caucasus Mountain Observatory of SAI MSU, we have determined the redshifts...     

Light Curves of the Type II-P Supernova SN 2017eaw: The First 200 Days

We present the results of our UBVRI photometry for the type II-P supernova SN 2017eaw in NGC6946 obtained fromMay 14 to December 7, 2017, at several telescopes, including the 2.5-m telescope at the CaucasusHigh-Altitude Observatory of the SAIMSU. The dates andmagnitudes atmaximumlight and the light-curve parameters have been determined. The color evolution, extinction, and peak luminosity of SN 2017eaw are discussed. The results of our preliminary radiation–gasdynamic simulations of its light curves with the STELLA code describe satisfactorily the UBVRI observational data.     

Provenance of Oligocene lithic and quartz arenites of the East Carpathians: Understanding sediment routing systems on compressional basin margins

We present new sedimentological, petrographical, palaeontological and detrital zircon U–Pb data on late Oligocene–early Miocene sedimentary rocks of the thin-skinned thrust belt of East Carpathians. These data were acquired to reconstruct the sedimentary routing system for two compositionally different turbidite fans made of the regionally extensive Kliwa and Fusaru formations. On the eastern margin of the Moldavides foreland basin, large low-gradient river systems draining the East European Platform provided well-sorted quartz-rich sand forming deltas on wide shallow shelves and thick Kliwa submarine fans. Due to the westward subduction of a thinned continental plate, the western basin margin was characterized by short, steep-gradient routing systems where sediment transport to deep water was mainly through hyperpycnal flows. The Getic and Bucovinian nappes of the East Carpathians and the exhumed Cretaceous–Early Palaeogene orogenic wedge fed Fusaru fans with poorly sorted lithic sand. The Fusaru fans trend northwards in the foredeep basin having an elongate depocentre, interfingering and then overlapping on the distal part of the Kliwa depositional system due to the eastward advance of the Carpathian fold-and-thrust belt. A smaller sediment input is supplied by southern continental areas (i.e. Moesian Platform, North Dobrogea and potentially the Balkans). In general, the sandstone interfingering between distinct basin floor fan systems is less well documented because the facies would be similar and there are not many systems that have a distinct sediment provenance like Kliwa and Fusaru systems. This case study improves the understanding of regional palaeogeography and sedimentary routing systems and provides observations relevant here or elsewhere on the interfingering turbidite fan systems.     

Perspective targeting CuAuMo porphyries of Romanian Carpathians: blind porphyry mineralization and its variable distal expression as vein and skarns

Romania has a genuine accumulation of “blind” porphyry CuAuMo systems associated with Cu skarns or AuAg (PbZn) epithermal vein sets. Many such hidden structures were discovered inadvertently during state exploration based on local tradition that Romanian Carpathians are rich in vein and skarn ore types. Notable dichotomy between porphyries associated with peripheral skarns versus epithermal ores is expressed by age (Mesozoic versus Tertiary), precious metal content, alteration types and zoning, shape/size and extension of ore bodies. Preliminary conclusions on metallic/alteration assemblages around porphyries in addition to geophysical information about inferred deep-seated Alpine plutonism promote potential exploration vectors for what will be an increasingly important deposit type in the future.     

The Stability of the Light Curve of DL Cassiopeiae Between 1959–1997

A variability survey on the shape of the light curve of the classical Cepheid DL Cas has been performed on the basis of Johnson V photometry data covering about 38 years. The input parameters (mean magnitude, pulsation frequency and period, light curve amplitude, harmonics amplitudes, Fourier type structural parameters) for our study were determined through the Fourier decomposition technique applied to each data set. The analysis of the outcoming time series, using different and complementary methods, seems to indicate the constancy of these parameters within the precision limits of the available data, although the hypothesis of the presence of a low level variability cannot be excluded. The only exception is the pulsation period, which displays a possible increasing trend with a rate of 0.109 ± 0.037 s yr⁻¹. This trend may be, at least partly, an effect of the unequal precision of pulsation period estimated values, corresponding to each considered data set. This revised version was published online in July 2006 with corrections to the Cover Date.     

Meteorological conditions at the Caucasus Observatory of the SAI MSU from the results of the 2007–2015 campaign

Based on the measurements performed from 2007 to 2015 at the summit of Mount Shatdzhatmaz adjacent to the 2.5-m telescope at the Caucasus Observatory of the SAI MSU, we have determined the statistical characteristics of basic meteorological parameters: the ambient air temperature, the ground wind speed, and the relative humidity. The stability of these parameters over the entire period of our measurements and their variations within an annual cycle have been studied. The median temperature on clear nights is +3.2°C, although there are nights with a temperature below ?15°C. The typical ground wind speed is 3 m s^?1; the probability of a wind stronger than 10 m s^?1 does not exceed 2%. The losses of observing time due to high humidity are maximal in the summer period but, on the whole, are small over a year, less than 10%. We have estimated the absolute water vapor content in the atmosphere, which is especially important for infrared observations. Minimum precipitablewater vapor is observed in December–February; the median value over these months is 5 mm. We additionally provide the wind speeds at various altitudes above the ground (from 1 to 16 km) that we obtained when measuring the optical turbulence. We present the results and technique of our measurements of the annual amount of clear night astronomical time, which is, on average, 1320 h, i.e., 45% of the possible one at the latitude of the observatory. The period from mid-September to mid-March accounts for about 70% of the clear time. A maximum of clear skies is observed in November, when its fraction reaches 60% of the possible astronomical night time.     

Diamond formation during metasomatism of mantle eclogite by chloride-carbonate melt

A xenolith of bimineralic eclogite from the Udachnaya kimberlite pipe provides a snapshot of interaction between mantle rocks and diamond-forming fluids/melts. The major-element composition of the eclogite is similar to that of N-MORB and/or oceanic gabbros, but its trace-element pattern shows the effects of mantle metasomatism, which resulted in diamond formation. The diamonds are clustered in alteration veins that crosscut primary garnet and clinopyroxene. The diamonds contain microinclusions of a fluid/melt dominated by carbonate and KCl. Compared to the worldwide dataset, the microinclusions in these diamonds fall in middle of the range between saline fluids and low-Mg carbonatitic melts. The fluid/melt acted as a metasomatic agent that percolated through ancient eclogitic rocks stored in the mantle. This interaction is consistent with calculated partition coefficients between the rock-forming minerals and diamond-forming fluid/melt, which are similar to experimentally-determined values. Some differences between the calculated and experimental values may be due to the low contents of water and silicates in the chloride-carbonate melt observed in this study, and in particular its high contents of K and LILE. The lack of nitrogen aggregation in the diamonds implies that the diamond-forming metasomatism took place shortly before the eruption of the kimberlite, and that the microinclusions thus represent saline carbonate-rich fluids circulating in the basement of lithospheric mantle (150–170 km depth).