Subaqueous pyroclastic flows and ignimbrites: an assessment

An assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary, hot gas-rich pyroclastic flows, mass-flows which resulted from the transformation of gassupported flows into water-supported ones, and secondary mass-flows carrying redeposited pyroclastic debris. Based on subaerial pyroclastic flows, the term pyroclastic flow should be restricted to demonstrably hot, gas-rich mass-flows of pyroclastic debris. Using this definition, very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described. In the majority of these cases, the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate. The only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the Ordovician nearshore, shallow marine ignimbrites of Ireland and Wales, and Miocene ignimbrites of southwest Japan, resulting from the passage of subaerially erupted pyroclastic flows into shallow water. Other possible examples are near-vent dense clast deposits in the Donzurobo Formation of Japan, possible submarine intra-caldera ponded ignimbrite successions in California and Wales, and near-vent pumiceous deposits of Ramsay Island, Wales. All other purported cases are either clearly the result of water-supported mass-flow transportation and deposition (debris avalanches, debris flows, turbidity currents), or lack adequate supporting evidence regarding the heat state or the palaeoenvironment. Only the shallow marine ignimbrites of Ireland and Wales show adequate evidence of welding, but even these could have been nearly wholly exposed above sea-level when welding occurred. We conclude that when pyroclastic flows enter water they are generally disrupted explosively and/or ingest water and transform into water-supported mass-flows, and we suggest the various scenarios in which this occurs. There is no evidence to suggest that welding in wholly subaqueous environments is common.     

The East European Platform in the late Ediacaran: new paleomagnetic and geochronological data

The paleogeography of the Earth, including the East European Platform, is very inaccurately defined for the interval 500–700 Ma. The quantity and quality of Late Precambrian–Cambrian paleomagnetic data on this platform are absolutely insufficient for reliable paleogeographical or paleotectonic reconstructions. Since there are almost no unstudied objects in the platform that could be used for paleomagnetic studies, it seems reasonable to consider the deformed platform margins. Of particular interest is the Bashkir anticlinorium (South Urals) with numerous Ediacaran sedimentary sections, some of which contain tuff beds suitable for isotope dating. We present paleomagnetic and geochronological data on the Upper Ediacaran Zigan Formation, sampled in the western part of the western limb of the Bashkir anticlinorium. The East European Platform must have been at near-equatorial latitudes at ～550 Ma.     

Tectono-geophysical model of the Southern Caspian in the context of the presence of oil and gas

In the paper, the subduction of the South Caspian microplate beneath the continental lithosphere of Central Caspian is substantiated based on the GPS measurements, crustal density model, seismological data, and seismic section along the Elburs–Apsheron–Balkhan Sill regional profile. In this context, the geodynamical model is suggested for the formation and spatial distribution of the oil and gas fields in the South Caspian Basin (SCB). It is shown that the heterogeneous oil-and-gas saturation of the sedimentary section within the South Caspian Depression (SCD), which was revealed by the long-term prospecting and exploration works, is, according to the suggested model, controlled by the subduction zone. The developed geodynamical approach to the problem of the formation of hydrocarbon deposits in the South Caspian Basin calls for rethinking the strategy of further prospecting for hydrocarbons in this region.     

断裂引起的应变量计算方法

本文介绍了断裂引起的应变量计算方法。断裂作用可导致连续应变和非连续应变。连续应变与断裂位移,断裂长度比值及断裂面上有效应力成正相关关系。影响非连续应变的因素有：断裂几何形态、断裂的旋转性、断裂规模。已经提出三种断裂旋转机制：刚性旋转,垂直剪切和斜向剪切。对于这三种机制,我们分别建立了断裂非连续应变的计算公式。这些公式与断裂的旋转角度和位移大小相关。刚性旋转时,断块内部没有任何塑性变形,因此地层的长度没有变化。它引起的非连续应变最小。垂直剪切作用使断块内地层变形,但水平方向的地层长度不变。推算的公式表明,对于相同的原始数据,它引起的非连续应变比刚性旋转机制引起的非连续应变大。斜向剪切也使断块内地层变形,但水平方向的长度也不变。在同等条件下,它引起的非连续应变比垂直剪切机制引起的非连续应变大。     

Paleomagnetism of sedimentary strata and the origin of the structures in the western slope of South Urals

Paleomagnetic data may contribute to studying the formation history of orogens; in particular, these data can promote identifying the pattern and scales of deformations at the final stages of orogeny. We have conducted paleomagnetic studies of the Paleozoic and Neoproterozoic sediments in the western part of the Western Ural Megazone in South Urals. The detailed thermal demagnetization revealed the intermediate temperature magnetization component in most samples. This magnetization has a reversed polarity and has been acquired before folding or at the early stages of the deformations. The directions of this component are narrowly grouped in rocks of a different age in all the segments of the studied part of South Urals, and the regional average direction closely agrees with the reference paleomagnetic direction of 270 Ma for the East European Platform. The results of our study suggest the following conclusions: (1) the main magnetization component in the studied sedimentary rocks has a secondary origin; (2) this component has an age of ~270 Ma and has been formed during the Kungur deformations (279–272 Ma ago) of the western part of South Urals; (3) neither a general rotation of the studied part of the Urals relative to the East European Platform nor local rotations of the individual tectonic blocks relative to each other are revealed; (4) the changes in the strike of the structures from NE within the Karatau uplift to the submeridional in the remaining part of the Urals is not an oroclinal bend.     

Isotope (δ13C and δ18O) compositions of dolomites from the Permian evaporitic sequences of the Eastern Russian Plate: Evidence from the Syukeevo gypsum deposit

Carbon and oxygen isotope compositions has been studied in dolomites of the Syukeevo gypsum deposit located in the Eastern Russian Plate. Values of δ¹³C in the dolomites vary from 0.3 to 6.6 ‰; δ¹⁸O, from 28.0 to 36.6‰. It is shown that the dolomites were formed in the epicontinental evaporitic basin in different paleoecological settings. This led to the formation of diverse lithological types of dolomites with peculiar isotope-geochemical features.     

X-Ray Computed Microtomography of Sulfide Minerals: Studies of Microinclusions and Implications for the Sm–Nd Dating of Ore Genesis

Doklady Earth Sciences - A microtomographic study of the internal structure of sulfide minerals from the ore-bearing rock varieties of two economically significant deposits in the Arctic zone of...