Late Cenozoic faulting and the stress state in the south-eastern segment of the Siberian platform (original) (raw)
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Geodynamics & Tectonophysics
This article gives a chronological review of the main published research results concerning the Cenozoic crustal stress-strain state in Mongolia and adjacent territories. The studies commenced in the southern Baikal rift zone in the 1970s and were extended further southwards to cover mobile regions neighbouring the Siberian platform. Geological, structural and morphostructural data were collected and analysed to define the crustal stress types and spatial characteristics. The authors have consolidated their reconstructions of the crustal stress-strain state of Mongolia, which were based on tectonic fracturing data and displacements along fractures in fault zones active in the Cenozoic. We consolidated a database of reconstructed stress tensors, which now contains more than 750+ solutions. The Late Cenozoic stress field was mapped. The map shows domains differing in types of the paleostress state of the crust. The reconstructions were compared to our calculations of the present-day crustal stress state, which were based on earthquake focal mechanisms, and to calculations by other authors. At the Late Cenozoic and current stages, the maximum horizontal compression axis (S Hmax) has varying orientations, from submeridional (Western Mongolia) to NE and ENE (Eastern Mongolia). The role of compression increases from the northern domains, where the reconstructions show shear and transtension, to the southern domains with prevailing transpression and compression. Regular changes occur in the stress state and rupture parageneses along the largest latitudinal faults, North Khangai and Dolinoozersky; such changes are related to left-lateral strike-slip faulting. We analysed the sequence of the occurrence of stress fields differing in types and spatial characteristics, and revealed the main regularities in the evolution of the crustal stress-strain state in time. In the Cenozoic history of crust deformation in Mongolia, we can distinguish several episodes that differ in the dominant impacts of various tectonic force sources or combinations of such impacts. At the beginning of the Cenozoic, tectonic structures developed mainly under the influence of the interaction of East Asia and the Pacific Plate, which was manifested in the southeastern domains of the study area. The long-term SE-trending asthenospheric flow caused crustal stretching, which initiated the formation of tectonic structures comprising the Baikal rift system. Starting from the Pliocene, crustal stretching took place in combination with NNE compression caused by the India-Eurasia convergence. As a result, shearing occurred along the large faults. At this background, the Khangai and Khentei uplifts (including crust extension zones at their crests) are large structures that developed due to the dynamic effect of local mantle anomalies.
Geodynamics & Tectonophysics, 2013
The zoneblock structure of the lithosphere is represented by a hierarchically organized pattern of stable blocks and mobile zones which border such blocks and contain highly dislocated geological medium (Fig. 1). Today, different specialists adhere to different concepts of blocks and zones, which are two main elements of the lithosphere structure. Differences are most significant in determinations of 'interblock zones' that are named as deformation / destructive / contact / mobile / frac ture zones etc. due to their diversity in different conditions of deformation. One of the most effective approaches to studying the zoneblock structure of the lithosphere is a combination of geological and geophysical studies of interblock zones tectonic features on various scales, which can make it possible to reveal the most common patterns of the interblock zones, general regularities of their development and relationships between the interblock zones. The main objectives of our study were (1) to identify the zoneblock structure of the crust in the southern regions of East Siberia from tectonophysical analysis of geological and geophysical surveys conducted on four different scales along the 500 km long ShertoyKrasny Chikoy transect crossing the marginal segment of the Siberian block, the Baikal rift and the Trans baikalian block (Fig. 2); (2) to clarify structural features of the central part of the Baikal rift (representing the tectonic type of interblock extension zone) by applying new research methods, such as radon emanation survey, to the ShertoyKrasny Chi koy transect and using the previously applied methods, such as magnetotelluric sounding, on a smaller scale; and (3) to study manifestation of interblock zones of various ranks in different geological and geophysical fields, to reveal common specific features of their structural patterns for the upper crust, and to establish regularities of hierarchic and spatial relationships be tween the interblock zones. On the global scale, the object of our study at the ShertoyKrasny Chikoy transect was the Baikal interblock zone (Fig. 2, 15, and 16). On the transregional scale, large fault zones were studied (Fig. 6, 11, and 14). On the regional and local scales, the objects of our study were systems of faults and fractures of various ranks which were active at the late Cenozoic stage of tectogenesis (Fig. 4, and 5). The set of geological and geophysical surveys included application of methods for identification of faults and fractures using different criteria, with account of the fact that clusters of such structures are indicative of the interblock zones of the crust. We used structural geological methods for studying faults and fractures, morphostructural analysis (including interpretation of satellite images), selfpotential (SP) and resistivity profiling, magnetotelluric (MT) sounding, radon emanation survey, and hydrogeological studies of water occurrences. The region of Lake Baikal is one of the most studied geodynamically active regions of Russia; therefore, published data from previous studies of the Baikal region were used to interpret the data obtained by the authors. By interpreting the obtained data from the unified tectonophysical positions, the three objectives were met, and the fol lowing research results were stated: 1. The principal specific features of the geological structure of the crust along the ShertoyKrasny Chikoy transect are specified. It is established that the divisibility pattern complies with tectonophysical definitions of the hierarchically organ ized zoneblock structure of the lithosphere (Fig. 2, 6, 11, 14, and 16). It is clearly evidenced, within the depth interval from the nearsurface to about 30 km, that the crust is split into slightly broken blocks that are in contact with each other via wide zones that are marked by higher fracturing and fluid saturation. To a first approximation, such blocks are shaped as subhori zontal plates in the stable southern regions of East Siberia (e.g., the southern part of the Siberian platform) and subvertical plates in the areas being active in the Cenozoic (e.g., the Baikal rift). Within the framework of the given model of the zone block structure of the southern regions of East Siberia, strict hierarchical subordination is established that manifests in spatial relationships of interblock zones (the closed network of the zones, imbedded blocks); its quantitative characteristics are stated at the global, transregional and three regional levels (Table 1, Fig. 2, Fig. 22). Average sizes of the zones, that were crossed by the transect, are estimated from the depth of their penetration into the crust; it is shown that Scale Invariant 2.2 (previously set for estimates of square areas) is valid also for the analysis of volumes of interblock structures. Detailed observations show GEODYNAMICS & TECTONOPHYSICS
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A gap in sedimentation in the Silurian section of the Subpolar Urals at the Ludlow-Pridoli boundary
A gap in sedimentation in the Silurian section of the Subpolar Urals at the Ludlow-Pridoli boundary, 2020
Research subject. The article discusses the results of a new detailed study of a reference section of the Upper Silurian in the Subpolar Urals. This study was undertaken to clarify the existing contradictions concerning the age of the Ludlow-Pridoli boundary deposits and the definition of the Ludlow-Pridoli boundary, which is based on the study of different fauna groups. Materials and methods. The newly collected collections contained more than 100 samples of sedimentary rocks with fossil macro fauna, 22 tests on microfauna, 198 tests on chemical analysis for determining the content of Ba, Sr and δ13C and δ18 O isotopes in carbonates. The results of experiments were confirmed by the authors’ bio-sedimentological, paleo-ecological and chemostratigraphic data. Results. The conducted research confirmed the existence of a gap in sedimentation at the end of Ludlow; clarified the thickness of the Sizim stage in the reference section; elucidated its sedimentological and chemostratigraphic characteristics; allowed changes in biodiversity due to a change in the sedimentation regime, paleoecological impact on biota in the late Ludlow and restoration of biota in the early Pridoli to be traced. The study also demonstrated that the time boundaries of the transgressive and regressive stages in the development of the Northern Ural sea basin and the event-stratigraphic boundary of the Ludlow-Pridoli were directly related to the main global events in the Late Silurian (Lau Event, Lower Pridolian Event), the traces of which are preserved in the studied section. Conclusions.The intensification of regressive tendencies across the largest part of the Northern Ural paleobasin in the Late Ludlow, widespread development of microbial biota, cessation of the Silurian reef formation, as well as the extinction of Pentamerida brachiopods – exclusively, indicate a significant ecosystematic restructuring in the late Ludlow. It can be assumed that the absence of a significant positive deviation of the δ13 C global Lau Event in this section is associated with the identified gap, the amplitude of which correlates with the Ozarkodina snajdri and Ozarkodina crispa zones located above the Polygnathoides siluricus zone in the conodont sequence of the Upper Ludlow.
LITOSFERA, 2018
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