Paleomagnetic Study of Devonian and Carboniferous Rocks of the Southern Urals: An Independent Test of Collision between the Magnitogorsk Island Arc and the Passive Margin of the Continent of Laurussia

New paleomagnetic data on Devonian and Lower Carboniferous rocks of the Magnitogorsk Zone (Southern Urals) are presented. The paleomagnetic pole calculated for the Lower–Middle Devonian is quite close to the coeval pole of the Baltica Paleocontinent (Laurussia), which indicates that the Magnitogorsk island arc was probably located close to the continent, but not being accreted with it. Paleomagnetic data obtained for Lower Carboniferous rocks may indicate that the Magnitogorsk island arc was turned in the Early Carboniferous and collided with the continent of Laurussia forming a single continent.     

Formation of Libyan Desert Glass: Numerical simulations of melting of silica due to radiation from near‐surface airbursts

Libyan Desert Glass contains meteoritic material and, therefore, its origin is most likely associated with an impact event. However, the impact crater has not been found. We performed numerical simulations of impacts of stony and cometary bodies in order to confirm the version that this glass was formed from silica heated by radiation from aerial bursts near the ground. Asteroids were treated as strengthless bodies from dunite with a density of 3.3 g cm^?3, and comets as icy bodies with a density of 1 g cm^?3. The simulations based on hydrodynamic equations included the equations of radiation transfer. Melting and vaporization of a silica target under action of radiation incident on a planar surface were modeled using a one‐dimensional hydrodynamic equation of energy and equations of radiation transfer in two‐flux approximation. We selected those variants of simulations in which a crater is not formed, a fireball touches the earth surface, and the area of a molten target corresponds to the area of the Libyan Desert Glass strewn field. Appropriate options include the impact of an asteroid with a diameter of 300 m, an entry speed of 35 km s^?1, and an entry angle of 8°, and cometary bodies with diameters from 150 to 300 m, speeds of 50–70 km s^?1, and entry angles from 15° to 45°. Impact options with crater formation are also discussed. The maximum depth of molten silica at ground zero reaches 10 cm with the cometary impacts and 3–4 cm with the asteroidal impact. Melting occurs during a period of time from 50 to 400 s.     

Crust and mantle of the Tien Shan from data of the receiver function tomography

A 3-D velocity model of the Tien Shan crust and upper mantle is constructed through the inversion of the receiver functions of P and S waves together with teleseismic traveltime anomalies at nearly 40 local seismic stations. It is found that in the vast central region, where no strong earthquakes have been known over the past century, the S wave velocity at depths of 10–35 km is lower than in adjacent regions by up to 10%. These data are evidence for mechanical weakness of the crust preventing the accumulation of elastic energy. Apparently, the lower velocity and the weakness of the crust are due to the presence of water. The weakness of the crust is one of the possible reasons for the strain localization responsible for the formation of the present Tien Shan but can also be due in part to the young orogenesis. The crustal thickness is largest (about 60 km) in the Tarim-Tien Shan junction zone. The crust-mantle boundary in this region descends by a jump as a result of an increase in the lower crust thickness. This is probably due to the underthrusting of the Tien Shan by the Tarim lithosphere. This causes the mechanically weak lower crust of the Tarim to delaminate and accumulate in nearly the same way as an accretionary prism during the subduction of oceanic lithosphere. In the upper mantle, the analysis has revealed a low velocity anomaly, apparently related to basaltic outflows of the Upper Cretaceous-Early Paleogene. The Cenozoic Bachu uplift in the northern Tarim depression is also associated with the low velocity anomaly. The Naryn depression is characterized by a high velocity in the upper mantle and can be interpreted as a fragment of an ancient platform.     

Receiver functions at the stations of the German Regional Seismic Network (GRSN)

Several years of broad-band teleseismic data from the GRSN stations have been analysed for crustal structure using P -to- S converted waves at the crustal discontinuities. An inversion technique was developed which applies the Thomson-Haskell formalism for plane waves without slowness integration. The main phases observed are Moho conversions, their multiples in the crust, and conversions at the base of the sediments. The crustal thickness derived from these data is in good agreement with results from other studies. For the Gräfenberg stations, we have made a more detailed comparison of our model with a previously published model obtained from refraction seismic experiments. The refraction seismic model contains boundaries with strong velocity contrasts and a significant low-velocity zone, resulting in teleseismic waveforms that are too complicated as compared to the observed simple waveforms. The comparison suggests that a significant low-velocity zone is not required and that internal crustal boundaries are rather smooth.     

Climatic intrannual variability of the geostrophic circulation of the main Black Sea current

A quasi-periodic (wave) spatial-temporal structure of the intrannual variability of the climatic fields of the seawater geostrophic motion within the constant halocline in the Black Sea (75–300m) has been revealed on the basis of generalized archival data on hydrological observations in the main Black Sea current (MBSC) near the Crimea and Caucasus. The variability with annual and semi-annual periods has the character of standing quasi-geostrophic waves at 2- to 4-month periods which become progressive as the direction of their propagation changes during the year. The energy characteristics for the forms of variability revealed are discussed and interpreted from the viewpoint of gradient-eddy waves and the hydrodynamic instability of the MBSC.Translated by Mikhail M. Trufanov.     

New iron resources of the South Urals

An area with brick-red loose and viscous sandy-clayey rocks and brown ores with an average Fe content of 19.84% and possible resources of 1 billion tons of metal was determined. Mn and Ti are the main alloying components; Ni, Co, Cr, V, and Zr are additional; and goethite (FeOOH) is an ore mineral.     

Joint inversion of P- and S-receiver functions and dispersion curves of Rayleigh waves: The results for the Central Anatolian Plateau

The P- and S-wave receiver functions and dispersion curves of the fundamental Rayleigh wave are used to study the lithosphere within the Central Anatolian Plateau. The results for eight broadband seismic stations are presented. It is established that within the plateau, the crust with a thickness of about 35 km is underlain by the mantle lid with its bottom at a depth of about 60 km. The velocities of longitudinal (Vp) and shear (Vs) waves in this layer are at most 7.6 and 4.5 km/s, respectively, and the Vp/Vs ratio is close to 1.7 (i.e., by 6% lower than in the standard IASP91 and PREM models). Such a low velocity ratio is characteristic of rocks having high orthopyroxene content. Beneath the high-velocity mantle lid, the S-wave velocity decreases to 4.0–4.2 km/s and the Vp/Vs ratio is close to its standard value (1.8). At most stations, the P-wave receiver functions do not contain seismic phase P410s, which is formed at the global seismic boundary at a depth of 410 km. The seismic boundary at a depth of 410 km is related to the olivine-spinel phase transformation, and its absence can indicate the anomalously low olivine content and high basalt content. This anomaly is probably associated with the subduction of a large amount of oceanic crust during the closure of the Tethys. The results of the study overall indicate the high informativity of the used method.