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Faults and tectonic stresses of the Baikal rift zone
Abstract
The Cenozoic Baikal rift system consists of a central extensional segment which links a southwestern and a northeastern sinistral shear segment.Statistical studies on fault length, amount and sense of slip, depth of crustal penetration and spacing has led to the establishment of empirical relationships which appear to apply also for non-extensional regimes. Results are graphically displayed in Figs. 4–7.Tectonic stress regimes were determined by means of fault and joint analyses and on the basis of seismological data. Results of both approaches are compatible and explain the evolution of the Baikal rift zone in terms of tensional reactivation of pre-existing crustal discontinuities.
... Сопоставление карт кумулятивной сейсмической энергии, плотности эпицентров землетрясений [Radziminovich et al., 2018], поверхностной плотности разломов [Sherman, 1992] и теплового потока [Duchkov, 1985;Lysak, 2002;Golubev, 2007] показывает хорошую общую сходимость аномалий: области с высокой сейсмической активностью характеризуются высокой плотностью разломов и высокими значениями теплового потока (рис. 8). ...
... Оценки сейсмической добротности как по прямым волнам, так и по коде показывают сильную Рис. 8. Распределение суммарной сейсмической энергии за период 1950-2017 гг. (а), поверхностной плотности разломов (по данным [Sherman, 1992]) (б)и среднего теплового потока (по данным [Duchkov, 1985;Lysak, 2002;Golubev, 2007]) (в), в Южнобайкальской впадине и окружающих территориях. На врезке вверху -карта плотности эпицентров землетрясений за 2003-2014 гг. ...
... На карте плотности разломов цифрами показано количество разломов на единицу площади (трапеция с размерами 6570 км). [Sherman, 1992]; (в)mean heat flow [Duchkov, 1985;Lysak, 2002;Golubev, 2007]. Inset -map showing the density of earthquake epicenters in 2003-2014 (K≥6.0 events from [Radziminovich et al., 2018]). ...
Our detailed study of the crust and upper mantle of the South Baikal basin focused on seismic coda and seismic S‐waves attenuation and estimated seismic quality factor (QS and QC), frequency parameter (n), attenuation coefficient (δ), total attenuation (QT), and the ratio of two components the total attenuation: intrinsic attenuation (Qi), and attenuation due to scattering caused by the inhomogeneities of the medium (QSC). We calculated the sizes of inhomogeneities revealed in the block medium, which put their effect on the attenuation of seismic waves in different frequency ranges. The seismic wave attenuation field was analyzed in comparison with the geological and geophysical characteristics of the medium, and a direct relationship was established between attenuation, composition and active processes in the crust and upper mantle of the studied area. According to the estimated intrinsic attenuation (Qi) and scattering attenuation (QSC) contributions into the total attenuation, intrinsic attenuation is generally dominant in the studied area, while the QSC component increases in the areas of large active faults.
... 13). Также мы использовали карту поверхностной плотности разломов в Байкальском рифте [Sherman, 1992], на которой приведены как активные, так и неактивные разломы (рис. 14). ...
... Получены оценки затухания сейсмических волн в литосфере и верхней мантии трех крупных рифтовых систем мира: Байкальской рифтовой системы (Евразия), Северо-Танзанийской дивергентной зоны (Африка) и северной части Провинции Бас-Рис. 14. Карта поверхностной плотности разломов (заимствовано из работы [Sherman, 1992]). На врезке показана зависимость добротности на частоте 1 Гц и частотного параметра от поверхностной плотности разломов для отдельных тектонических структур. ...
... На врезке показана зависимость добротности на частоте 1 Гц и частотного параметра от поверхностной плотности разломов для отдельных тектонических структур. [Sherman, 1992]). The inset shows the dependence of the seismic quality factor at the frequency of 1 Hz, and the dependence of the frequency parameter from the surface density of faults in separate tectonic structures. ...
Attenuation of seismic waves in the crust and the upper mantle has been studied in three global rift systems: the Baikal rift system (Eurasia), the North Tanzanian divergence zone (Africa) and the Basin and Range Province (North America). Using the records of direct and coda waves of regional earthquakes, the single scattering theory [Aki, Chouet, 1975], the hybrid model from [Zeng, 1991] and the approach described in [Wennerberg, 1993], we estimated the seismic quality factor (QC), frequency parameter (n), attenuation coefficient (δ), and total attenuation (QT). In addition, we evaluated the contributions of two components into total attenuation: intrinsic attenuation (Qi), and scattering attenuation (Qsc). Values of QC are strongly dependent on the frequency within the range of 0.2-16 Hz, as well as on the length of the coda processing window. The observed increase of QC with larger lengths of the coda processing window can be interpreted as a decrease in attenuation with increasing depth. Having compared the depth variations in the attenuation coefficient (δ) and the frequency (n) with the velocity structures of the studied regions, we conclude that seismic wave attenuation changes at the velocity boundaries in the medium. Moreover, the comparison results show that the estimated variations in the attenuation parameters with increasing depth are considerably dependent on utilized velocity models of the medium. Lateral variations in attenuation of seismic waves correlate with the geological and geophysical characteristics of the regions, and attenuation is primarily dependent on the regional seismic activity and regional heat flow. The geological inhomogeneities of the medium and the age of crust consolidation are secondary factors. Our estimations of intrinsic attenuation (Qi) and scattering attenuation (Qsc) show that in all the three studied regions, intrinsic attenuation is the major contributor to total attenuation. Our study shows that the characteristics of seismic wave attenuation in the three different rift systems are consistent with each other, and this may suggest that the lithosphere in the zones of these different rift systems has been modified to similar levels.
... Folds and strike-slip faults revealed in seismic cross sections of lake sediments (Figs. 2 and 3) correlate with deformation in subaerial sections (see Table 2 for correlation of TLSC and SSS). The recognized reversal events are generally consistent with paleostress reconstructions on the basis of structural data (Sherman and Dneprovsky, 1989;Sherman, 1992;Delvaux et al., 1995Delvaux et al., , 1997Sankov et al., 1997;Parfeevets and Sankov, 2006). ...
... In the Quaternary, basinal evolution proceeded in a purely extensional setting, without compressive interventions, judging from the predominantly normal faulting (Zonenshain et al., 1995;Sherman, 1992;Levi and Sherman, 2005) and the layered and undeformed SSS-3 (Figs. 2, 3 and 7). ...
... The stress regime changed to transpression and shear, and then to transtension Sankov et al., 1997;Levi and Sherman, 2005;Parfeevets and Sankov, 2006) with related strikeslip faulting considered to be critical for rift formation (Balla et al., 1990). Structural evidence for strike-slip strain and faulting was extensively reported from the Baikal Basin (Sherman, 1992;Delvaux et al., 1997;Levi and Sherman, 2005;Parfeevets and Sankov, 2006;Cheremnykh, 2010). Compression events are recorded by Late Oli-goceneePliocene folds in submarine (SSS-2) and subaerial (Fig. 6) sections. ...
A new model is suggested for the history of the Baikal Rift, in deviation from the classic two-stage evolution scenario, based on a synthesis of the available data from the Baikal Basin and revised correlation between tectonic–lithological–stratigraphic complexes (TLSC) in sedimentary sections around Lake Baikal and seismic stratigraphic sequences (SSS) in the lake sediments. Unlike the previous models, the revised model places the onset of rifting during Late Cretaceous and comprises three major stages which are subdivided into several substages. The stages and the substages are separated by events of tectonic activity and stress reversal when additional compression produced folds and shear structures. The events that mark the stage boundaries show up as gaps, unconformities, and deformation features in the deposition patterns.
The earliest Late Cretaceous–Oligocene stage began long before the India–Eurasia collision in a setting of diffuse extension that acted over a large territory of Asia. The NW–SE far-field pure extension produced an NE-striking half-graben oriented along an old zone of weakness at the edge of the Siberian craton. That was already the onset of rift evolution recorded in weathered lacustrine deposits on the Baikal shore and in a wedge-shaped acoustically transparent seismic unit in the lake sediments. The second stage spanning Late Oligocene–Early Pliocene time began with a stress change when the effect from the Eocene India–Eurasia collision had reached the region and became a major control of its geodynamics. The EW and NE transpression and shear from the collisional front transformed the Late Cretaceous half-graben into a U-shaped one which accumulated a deformed layered sequence of sediments. Rifting at the latest stage was driven by extension from a local source associated with hot mantle material rising to the base of the rifted crust. The asthenospheric upwarp first induced the growth of the Baikal dome and the related change from finer to coarser molasse deposition. With time, the upwarp became a more powerful stress source than the collision, and the stress vector returned to the previous NW–SE extension that changed the rift geometry back to a half-graben. The layered Late Pliocene–Quaternary subaerial tectonic–lithological–stratigraphic and the Quaternary submarine seismic stratigraphic units filling the latest half-graben remained almost undeformed. The rifting mechanisms were thus passive during two earlier stages and active during the third stage.
The three-stage model of the rift history does not rule out the previous division into two major stages but rather extends its limits back into time as far as the Maastrichtian. Our model is consistent with geological, stratigraphic, structural, and geophysical data and provides further insights into the understanding of rifting in the Baikal region in particular and continental rifting in general.
... Based on the correlation ratio, the seismic moments were calculated for M ≥ 4 earthquakes for the period 1960-2013, providing a 50 × 50 km grid map of cumulative seismic moment (Fig. 13). We have also used the map of the surface fault density in the Baikal rift (Sherman, 1992) which combines active and non-active faults (Fig. 14). A trapezoid with sizes of 65 × 70 km was taken per square unit. ...
... As discussed in the cases of regions of the Muya and North Lake Baikal, the increased heat flow likely affects the attenuation of seismic waves in these areas of active faulting and high seismic activity. (Sherman, 1992) and spatial distribution of Q 0 (Q-factor at frequency 1 Hz) and frequency parameter n: 1. Q 0 /n; 2. Isoline and values of fault frequency; 3. Crustal age of tectonic blocks (same as Fig. 13). ...
... Based on the correlation ratio, the seismic moments were calculated for M ≥ 4 earthquakes for the period 1960-2013, providing a 50 × 50 km grid map of cumulative seismic moment (Fig. 13). We have also used the map of the surface fault density in the Baikal rift (Sherman, 1992) which combines active and non-active faults (Fig. 14). A trapezoid with sizes of 65 × 70 km was taken per square unit. ...
... As discussed in the cases of regions of the Muya and North Lake Baikal, the increased heat flow likely affects the attenuation of seismic waves in these areas of active faulting and high seismic activity. (Sherman, 1992) and spatial distribution of Q 0 (Q-factor at frequency 1 Hz) and frequency parameter n: 1. Q 0 /n; 2. Isoline and values of fault frequency; 3. Crustal age of tectonic blocks (same as Fig. 13). ...
... Similarly, discussion of the origin of the intracontinental Baikal rift has focused on active versus passive models; the active model accepts mantle forces as the major cause of the rifting, whereas the passive model considers the crustal stress as the driving force for rifting. The framing of these discussions started with the seminal paper of Molnar and Tapponnier (1975) and pro-versus contra arguments have followed (Logatchev et al., 1983;Sherman, 1992;Delvaux et al., 1997;Ivanov, 2004;Petit and Deverchere, 2006;Lunina et al., 2010;Seminsky, 2009;San'kov et al., 2011;Jolivet et al., 2013b among many others). The study of the volcanism in this area may be the key to differentiating between the passive and active models of rifting. ...
... Stress is not uniformly distributed along the Baikal rift however. Only central parts experience clear extension, whereas southwestern Baikal rift is under general compression with local extension along strike-slip faults (Sherman, 1992;Levi et al., 1997;Petit and Deverchere, 2006;Arzhannikova et al., 2011;Jolivet et al., 2013b). The crust to the south of Bolnai fault in Mongolia does not exhibit extensional structures (Fig. 2), though some faults in the Hangai region clearly indicate normal faulting in the Miocene (Cunningham, 2001;Parfeevets and Sankov, 2012). ...
In this review we focus on the volcanism, that occurred in Transbaikalia, Siberia after the closure of the Mongolia-Okhotsk Ocean. The closure happened in the Early Jurassic. After that time, lithosphere in Transbaikalia went through two phases of rifting; in the Early Cretaceous and again in the Late Cretaceous until present. The latter rifting event is known as the Baikal rifting. We consider the chronology of the volcanism and basin formation in the Baikal rift and show that there has been a complex relationship between the two. Extension initiated in the central part of the rift system; this area is now occupied by Lake Baikal. Sedimentary basins initially developed by deepening and widening of the central part of the rift system and then by bilateral propagation of basin formation outwards. Volcanism was generally offset from the axial rift. Considering along axis distribution of volcanism, it initiated in the central part of the system and propagated bilaterally to the modern rift ends. We argue that tectonic stress controlled localization of the eruptive centres. Extension and shearing probably caused melting at mantle depth, suggestive of the passive model of volcanism. However, when considering the Baikal rift and adjacent non-rifted regions of Mongolia in a wider context of tectonics and volcanism of Central and East Asia, it is not possible to rule out that the volcanism may be associated with mantle transition zone diapirs; thus the active model of volcanism may also apply. The diapirs are located by regional isostatic gravity anomalies and considered as upwelling parts of the upper mantle convective cell controlled by the Pacific subduction and slab stagnation in the mantle transition zone. We do not see any geochemical, geophysical and geochronological evidence for involvement of deeper mantle to explain volcanism in either Baikal rift, non-rifted regions of Mongolia or anywhere else within Central and East Asia.
... The lithosphere parameters determining the seis mic process in the BSZ and SSZ are the same: the thinned (to 35 km) crust [1,5,8,9], thermal flow [3,4], similar main vectors of regional tectonic stress fields [5,12], and, finally, many similarities in the gen eral characteristics of seismicity. One of them is that both seismic zones are subdivided into controlling seg ments, whose S like shape forms the zonal structure of epicentral and hypocentral earthquake fields. ...
... For the BSZ, whose fault tectonics has been studied well, the section reflects its basic structure and rheological changes with depth. It follows from the calculations that the approximate depth of brittle and quasi brittle destructions of the Earth's crust in ten sion zones can be up to 30-40 km [12]. Down through the section, there is the zone of quasi plastic flow and boudinage structures. ...
... NE of the North Baikal basin, the BRS continues through a series of NE-SW-oriented, en echelon and generally asymmetric basins, namely the Kichera, Upper Angara, Muyakan, Upper Muya, Muya, Tsipa-Baunt and Chara basins (Fig. 7). This large-scale structure has been interpreted either as a major shear zone (or transform fault) linking the BRS with the Stanovoy strike-slip zone to the east (Sherman, 1992), or as a zone of oblique extension (San'kov et al., 2000). The faults that define the basin margins are strongly controlled by inherited crustal structures (San'kov et al., 2000;Petit and Déverchère, 2006). ...
... These structures, parallel to the general direction of the strike-slip system, allow the BDHT-Baikal system to be connected with the Stanovoy system to the east. The North Baikal basins system is interpreted as a large-scale shear zone within which inherited crustal structures oriented NE-SW induce local extension (Sherman, 1992;San'kov et al., 2000;Petit and Déverchère, 2006). ...
The Transbaikal region extends over several hundreds of kilometres east
of the Baikal Rift System. It is characterized by a number of
sub-parallel Mesozoic grabens or half grabens generally filled with late
Jurassic to Early Cretaceous clastic sediments interbedded with coal
layers (1). Similar basins occur on an even larger area spanning from
the Transbaikal region down to Korea implying a large-scale extensional
process affecting most of the Amuria plate during the Mesozoic. In the
Transbaikal region, the normal faults controlling the edges of the
Mesozoic basins are generally superimposed to Palaeozoic ductile shear
zones implying a strong localisation of the extensional deformation on
inherited structures. Recent studies, associated to our own fieldwork
demonstrated that some of the faults were again activated (2), still as
extensional faults, during the Tertiary or Quaternary, and that some of
them are presently active. The closure of the Mongol-Okhotsk ocean
separating the Siberian plate from the Amurian block during the Mesozoic
corresponds to a major event in the growth process of the East Asian
continent. The oceanic suture zone is situated on the southern edge of
the Transbaikal region and its roughly SW-NE direction is parallel to
the basins (3). The timing of the closure of the Mongol-Okhotsk ocean is
still highly debated: while sedimentological and tectonic data suggest
that the oceanic closure and the following collision occurred in early
Middle Jurassic (4), paleomagnetic studies advocate for a Early
Cretaceous collision (5). Furthermore, several other questions remain on
the localization, the size and the fate of the relief that most probably
formed during the collision between the Amuria block and the Siberian
craton. In order to answer those questions we used low temperature
thermochronology data associated to tectonic, sedimentology and
palinology to investigate the evolution of the Transbaikal grabens from
Mesozoic to Present. Tectonic and thermochronology data provide
evidences of exhumation and erosion along the eastern edge of the
Siberian craton during the Middle Jurassic as well as a potential
continuum of deformation between the Mesozoic extension and the
initiation of the Baikal Rift System (6). Sedimentology and palinology
reveals that the sediments deposited in the Transbaikal basin did not
registered large-scale compressive deformation during or after their
Late Jurassic - Early Cretaceous deposition and that they do not
correspond to the dismantling of a strong compressive relief.
(1) Tsekhovsky and Leonov, (2007), Lithology and Min. Res., 42,
doi:10.1134/S0024490207040037 (2) Lunina and Gladkov, (2009),
Geotectonics, doi:10.1134/S0016852109010051 (3) Zorin, (1999),
Tectonophysics, 306, pp. 33-56 (4) Ermikov, (1994), Bull. Cent. Rech.
Explor. Prod. Elf Aquitaine, 18, pp.123-134 (5) Metelkin et al., (2010),
Gondwana Research, doi:10.1016/j.gr.2009.12.008 (6) Jolivet et al.,
(2009), Tectonics, doi:10.1029/2008TC002404
... The major tectonic events can be correlated with the reversals of tectonic modes from extension to compression as shown by structural data (Sherman, 1992;Delvaux et al., 1995Delvaux et al., , 1997Parfeevets and Sankov, 2006). In the BRZ basins, those reversals match the early-late Oligocene, middle-late Miocene, early-late Pliocene and Pliocene-Quaternary tectono-stratigraphic boundaries (Mats and Perepelova, 2011). ...
In this paper we present a review of sedimentological, geomorphological, lithological, geochronological and geophysical data from major, minor and satellite basins of the Baikal Rift Zone (BRZ) and discuss various aspects of its evolution. Previously, the most detailed sedimentological data have been obtained from the basins of the central BRZ, e.g., Baikal, Tunka and Barguzin, and have been used by many scientists worldwide. We add new information about the peripheral part and make an attempt to provide a more comprehensive view on BRZ sedimentation stages and environments and their relations to local and regional tectonic events. A huge body of sedimentological data was obtained many years ago by Soviet geologists and therefore is hardly accessible for an international reader. We pay tribute to their efforts to the extent as the format of a journal paper permits. We discuss structural and facial features of BRZ sedimentary sequences for the better understanding of their sedimentation environments. In addition, we review tectono-sedimentation stages, neotectonic features and volcanism of the region. Finally, we consider the key questions of the BRZ evolution from the sedimentological point of view, in particular, correlation of Mesozoic and Cenozoic basins, bilateral growth of the Baikal rift, Miocene sedimentation environment and events at the Miocene/Pliocene boundary, Pliocene and Pleistocene tectonic deformations and sedimentation rates. The data from deep boreholes and surface occurrences of pre-Quaternary sediments, the distribution of the Pleistocene sediments, and the data from the Baikal and Hovsgol lakes sediments showed that 1) BRZ basins do not fit the Mesozoic extensional structures and therefore hardly inherited them; 2) the Miocene stage of sedimentation was characterized by low topography and weak tectonic processes; 3) the rifting mode shifted from slow to fast at ca. 7–5 Ma; 4) the late Pleistocene high sedimentation rates reflect the fast subsidence of basin bottoms.
... In the 1960's, SHERMAN and co-workers initiated paleostress investigations in the BRZ using geological and structural methods. Compilation of the results obtained are presented by SHERMAN & DNEPROVSKY (1989) and SHERMAN (1992), They reveal that normal faulting regime with NW-trending Shmin is typical for the central part of the BRZ. Local stress fields, with principal axes of extension and compression both subhorizontal and trending respectively NW and NE, are dominant in the northeastern and southwestern segments of the BRZ. ...
BCREDP, l.'etude detaillee des structures cenozoiques dans la zone de rift du Bafkal montre la presence conjointe de structures extensives (failles normales) et compressives (failles decrochanrcs, chevauchantes et plissements), L'analyse microstructurale des structures cassantes et I'inversion des donnees de faille avec stries pour en deduire les tenseurs reduits de contrainte ont permis de reconstruire les champs de contrainte cenozolque de la zone de rift du Baika! La presence de deux types ditterents de champ de contrainte dans l'evolution spatio-temporelle de la zone de rift du Ba'ikal est etablie , systeme en extension (<< type rift ») et systems docrochant a compressif (" type Asie Centrale »). Le champ de contrainte actuel est reconstruit a partir de I'inversion des donnees de meca-nisme au foyer des tremblements de terre, La rnajorite de la zone de rift du Barkal est actuellement en extension avec un Shmm oriente NW-SE, tandis que la partie sud-ouest de la zone de rift (bassin de Tunka) est soumise a un regime transpressif. Le modele classique devolution de la zone de rift du Baikal en deux eta pes est rein-terprete en fonction de la fluctuation spatio-temporelle du champ de contrainte. l.'evolution progressive du champ de contrainte d'un regime transpressif au stade initial de rittoqenese. a un regime extensif pendant Ie Ouaternaire est mise en evidence, La direction Shmm reste stable dans une direction NW-SE durant toute cette evolution, Le segment sud-ouest de la zone de rift marque la transition depuis la province en extension de type" rift ", jusqu'a la province de Mongolie et d'Altal, en compression, Nos resultats suqqorent un mecanisme de riftogenese impliquant une superposition de forces tectoniques a l'echelle continentale, avec un S max oriente NE-SW, et des forces extensives generees par "ascension d'un diapir mantel~que sous la bordure sud-est du craton siberien. ABSTRACT Detailed investigation of Cenozoic structures in the Baikal rift zone (BRZ) revealed the existence of both extensional structures (normal faults) and compressional structures (strike-slip faults, thrusts and folding), Microstructural analysis of brittle structures and stress inversion of fault-slip data permitted the reconstruction of the Cenozoic paleostress field for the central and southwest parts of the BRZ, The presence of two different types of stress field in the spatio-temporal evolution of the BRZ is established: stress tensors of extensional "rift-type" regime and stress tensors of strike-slip to compressional "Central Asian-type" regime. The present-day stress field is reconstructed by stress inversion of earthquake focal mechanism data. Most part of the BRZ is presently in an extensional setting, with NW-trending Shmin' while the southwestern part of the rift zone (Tunka basin) is in a transpressional setting. The classical two-stage evolution model of the BRZ is interpreted in function of stress field fluctuation in time and space, Regular evolution of the stress field from a strike-slip or transpressional regime in the initial stage of rifting to pure extensional regime in the Ouaternary is revealed, The Shmin direction remained stable in a NW-SE orientation during all this evolution. The southwestern segment of the BRZ marks the transition from the "rift-type" extensional stress field, to the com-pressional stress field of Mongolia and of Altai. Our results rather suggest a mechanism of Baikal rift formation involving the superposition of continental-scale tectonic forces with NE-trending SHmax and extensional forces generated by a mantle diapir rising beneath the southeastern boundary of the Siberian Craton.
... In Cenozoic, the high-angle reverse faults were preferably rejuvenated by normal movement, controlling the development of rift major border faults (Zamaraev and Ruzhich, 1978). Kinematic investigations show that the central portion of the Baikal rift evolved mainly under tensional stress field, with NW-SE subhorizontal principal extension axis during rift evolution (Sherman, 1992; Delvaux and Levi, in press). ...
Detailed investigation of a sector of the Pribaikal fold-and-thrust belt along the northwestern coast of Lake Baikal allows a new insight in the structural evolution of Precambrian and early Paleozoic series, at the contact with the Siberian platform. The Khibelen area, 150 km south of the northern termination of Lake Baikal. is characterized by well exposed imbricated thrust structures. These are later dissected by late high-angle reverse faults, and then by strike-slip faults. The high-angle faults, together with the low-angle thrusts are likely to control the location of major rift border faults, during Cenozoic reactivation in a ten-sional stress field. The Pribaikal fold-and-thrust pattern is thought to have developped during a final stage of the Cale-donian Central-Asian event, during late Silurian-early De-vornan. The relative succession of Paleozoic brittle deformations in the Khibelen area is : (I) low-angle thrusting, (2) high-angle reverse faulting and (3) strike-slip faulting. Paleostress tensors reconstructed from the analysis of minor faults and associated slip lines show the existence of at least two major stages in the kinematic evolution of stress field during Paleo-zoic. The oldest paleostress field obtained refers to the development of high-angle reverse fault, and the second, to the subsequent development of conjugated strike-slip faults. The first stress field is pure-radial compressive with a NW-SE principal direction of compression. It then evolved by a slight anticlockwise rotation of horizontal stress axes, together with a decrease in the relative intensity of the horizontal minimum compression, leading to strike-slip regime. This evolution of the stress field is not only valid for the area surrounding the Khibelen cape, but also for the whole length of the Pribaikal area, adjacent to the northwestern coast of Lake Baikal. Resume.-L'etude detaillee d'un petit secteur de la chaine plissee de Pribaikal, Ie long de la cote nord-ouest du lac Baikal permet une nouvelle approche de l'evolution structu-rale des series precarnbriennes et paleozoiques inferieures, en contact avec la plateforme siberienne. La region du cap Khibelen, situee 150 km au sud de la terminaison septentrio-nale du lac Baikal, est caracterisee par une structure en
... The final relative travel time residuals after these corrections are shown inFigure 3. [12] To study event location dependence of the relative travel time residuals, we group the events by their back azimuth (f) and epicentral distance (Á) relative to station 13,Figure 1. Topographic map of the Baikal rift zone and adjacent areas showing the locations of major tectonic units and seismic stations used in the study (squares). Arrows show regional stress fields of the Baikal rift zone obtained from surface geological structure analysis and earthquake focal mechanism studies [Sherman, 1992], and open circles are local events that occurred during the field experiment.Figure 2. A map with azimuthal equidistant projection (which preserves distances and azimuths relative to the center of the projection) centered at station 13, showing epicenters of events (triangles) used in the study. Letters indicate names of event groups used for dividing and averaging the travel time residuals. ...
Inversion of teleseismic P wave travel time residuals collected along a
1280-km-long profile traversing the Baikal rift zone (BRZ) reveals the
existence of an upwarped lithosphere/asthenosphere interface, which
causes a travel time delay of about 1 s at the rift axis ("central
high"). An area with early arrivals relative to the stable Siberian
platform of up to 0.5 s is observed on each side of the rift, about 200
km from the rift axis ("flank lows"). While the location of the central
high is approximately fixed in the vicinity of the rift axis, those of
the flank lows vary as much as 200 km with the azimuth of the arriving
rays. We use three techniques to invert the travel time residuals for
velocity anomalies beneath the profile. Two of the techniques assume an
isotropic velocity structure, and one of them considers a transversely
isotropic velocity model with a vertical axis of symmetry. We use
independent geophysical observations such as gravity, active source
seismic exploration, and crustal thickness measurements to compare the
applicability of the models. Other types of geophysical measurements
suggest that the model involving transverse isotropy is a plausible one,
which suggests that the central high and flank lows are caused by the
combined effects of an upwarped asthenosphere with a 2.5% lateral
velocity reduction, and a velocity increase due to transverse isotropy
with a vertical axis of symmetry. We consider the anisotropy to be the
result of the vertical component of a lithosphere/asthenosphere
small-scale mantle convection system that is associated with the
rifting.
... The dots denote seismic recorders, the stars denote shotpoints, the square shows the location of the Babushkin super vibrator, and the solid line in Lake Baikal shows the location of the air gun shooting line. geometry of the three lake basins are controlled by major (>80 km in length) pre-Cenozoic master basement faults along the rigid Siberian Platform [Florensov, 1969;Zamarayev and Ruzhich, 1978;Logatchev and Zorin, 1992;Sherman, 1992;Sherman et al., 2004]. The BRZ is by Logatchev and Florensov [1978], Zorin [1987, 1992], and Zorin et al. [2003] considered to be located within a rift-related updoming structure, referred to as the Sayan-Baikal domal uplift. ...
The late Cenozoic Baikal Rift Zone (BRZ) in southern Siberia is composed of several individual topographic depressions and half grabens with the deep Lake Baikal at its center. We have modeled the seismic velocity structure of the crust and uppermost mantle along a 360 km long profile of the Baikal Explosion Seismic Transects (BEST) project across the rift zone in the southern part of Lake Baikal. The seismic velocity structure along the profile is determined by tomographic inversion of first arrival times and 2-D ray tracing of first arrivals and reflections. The velocity model shows a gently deepening Moho from the Siberian Platform (41 km depth) into the Sayan-Baikal Fold Belt (46 km depth). We can exclude the presence of any Moho uplift around the ∼10 km deep sedimentary graben structure of southern Lake Baikal. The lower crust includes a distinct 50–80 km wide high-velocity anomaly (7.4–7.6 ± 0.2 km/s), slightly offset to the northeast from the rift axis. We interpret this feature as resulting from mafic intrusions. Their presence may explain the flat Moho by compensation of lower crustal thinning by intrusion of mafic melts. The Pn wave velocities (8.15–8.2 km/s) are normal for the area and do not show any sign of decompression melting in the sub-Moho mantle, such that possible lithosphere thinning has not reached the base of the crust. On the basis of the results of the BEST project, we suggest that the BRZ is formed by passive rifting in the rheologically weak suture between the Siberian Platform and the Amurian plate.
... Surface geologic analyses and focal mechanism studies show that the regional stress in the central area of the BRZ is extensional with a direction of extension orthogonal to the rift axis. In the northern and southern 1/3 of of rift zone, the regional stress field has a left-lateral strike-slip component [Sherman, 1992]. [3] While most other continental rifts such as the Rio Grande and East African rifts were found to be underlain by a low-velocity zone in the upper mantle and even the mantle transition zone (MTZ, 410 – 660 km depth range) [e.g., Davis, 1991; Slack et al., 1996; Achauer and Masson, 2002; Wilson et al., 2005a] , contradictory seismic tomography results were obtained beneath the BRZ. ...
Like most other major continental rifts, the Baikal rift zone (BRZ) in Siberia is presumably underlain by a hot and partially molten mantle, which has a reduced seismic velocity relative to surrounding areas. Recent seismic tomography studies, however, gave conflicting results about the depth extent and even the existence of the low-velocity anomaly beneath the BRZ, suggesting that additional constraints are needed. Here we present results from stacking of about 1700 radial P-to-S receiver functions from a single long-running seismic station, TLY, located at the SW tip of Lake Baikal. A clear uplift of the 410 km discontinuity (d410) with a magnitude of about 47 km relative to the south margin of the Siberian platform is observed beneath the rift. Currently available seismic results suggest that the uplift is unlikely to be caused by addition of water to mantle transition zone (MTZ) silicates but is the result of a 550°C reduction in temperature in the vicinity of the d410. In addition, the 660 km discontinuity (d660) shows a downward trend toward the rift from the south, suggesting that the entire MTZ might have a low temperature beneath the rift. The thickening of the MTZ suggests a high-velocity anomaly of about 2% in the MTZ, and rules out the possibility that the rifting is caused by a mantle plume originated in or beneath the mantle transition zone.
... It thus is considered as an area of continental extension. The DORIS beacon of Badary (51.77øN latitude, 102.23øE longitude) is located in the southeast part of this region near the Khubsugul lake in the south flank of the rift where the motion is supposedto be eastward[Zonenshain and Sarostin, 1981;Sherman, 1992]. The relative velocity of Badary to Eurasia measured with DORIS is 1.5 cm/yr in the northeast direction (azimuth N41 øE). ...
Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) data acquired between January 1993 and December 1996 from the SPOT-2, SPOT-3, and TOPEX/Poseidon satellites have been analyzed to determine velocities for 45 sites on eight major tectonic plates. For 28 sites far from deformation zones, the velocity estimates agree with plate model predictions. Least squares computation of poles of rotation, which model the plate motions, shows that for Eurasia, Africa, Pacific, and South America plates, the agreement is better with NUVEL-1, while for Australia, Antarctica, Nazca, and North America plates the DORIS Euler vectors are closer to NUVEL-1A. In general, DORIS results do not differ significantly from other space geodetic techniques determinations but provide better estimates for plates poorly or inhomogeneously covered by Global Positioning System (GPS), Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) networks, such as Africa. The DORIS coverage of this plate allows discussion of intraplate deformations due to the motion of the eastern Africa part which constitutes the Somalia plate. Sites located in deformation zones, such as western Eurasia boundaries, central Asia, southwestern America coast, South East Asia, show motion with respect to their own plates. Comparisons with other geodetic measurements for colocated stations, or with regional geodynamical models, show the interest of DORIS in active zones where global plate models are not valid.
... At the surface, the rift opening is controlled by the Primorsky fault zone, an old inherited structure which probably merges with the suture zone at depth (Logatchev and Florensov, 1978; Delvaux et al., 1995). Since at least 4 Ma extension here is nearly orthogonal to the rift axis and generates normal faulting (Sherman, 1992; Petit et al., 1996; Delvaux et al., 1997). ...
A scaled elasto-plastic lithosphere model lying upon a liquid substratum is subjected to a uni-axial horizontal tension. In a homogeneous plate extension localises along a linear zone (rift) oriented at an angle of ~60 to the tension axis. This orientation is preserved even when the divergent displacement of the opposite plate boundaries is not plain-parallel but rotational. In the latter case the strain localization zone is rapidly propagating. When the plate length to width ratio is greater than 2.5-3, the necking develops along two branches conjugated at an angle of about 120, which is frequently observed in actual rift systems. If the model contains a local weak zone (hot spot or fault zone), the rift junction is located at this zone. In the lithospheric models comprising strong (cratonic) and weak segments, strain localization depends on the configuration of the boundary between different lithospheres. The necking starts to form in the vicinity of the cratonic promontories and propagates in opposite direc- tions again at an angle of ca. 60 to the tension axis. In the models containing both a strong lithosphere and local weak zones, the rift configuration depends on their shape and relative positions, with necking always going through the weak zones. In a set of models we have reproduced a geometry of the boundary between the Siberian craton and the thermally much younger (~100 Ma) Sayan-Baikal lithosphere in the Baikal rift area. In these models we were able to obtain the well known three-branch con- figuration of the Baikal rift system only by introducing a weak zone in the area of Lake Baikal. Such a zone simulates the Paleozoic suture existing in this area. As in nature, two outer branches (eastern and western) are oblique to the regional tension axis and the central one is narrow and orthogonal to the tension direction. In nature and in the model, rifting starts in the central branch corresponding to Lake Baikal. The modelling also predicts the formation of a fourth oblique ~NS-trending branch to the south of Baikal. Although poorly expressed in the field, this branch has some seismo-tectonic and magmatic manifestations. The orientations of all four branches with respect to each other and with respect to the regional tension direction in nature and in the model are remarkably similar.
... The knowledge of motion geometry is essential having critical implications for the rift origin. There are two basic hypotheses of the tectonic evolution of the area which interpret it either as a large shear zone (a transform fault) (Sherman, 1992) or as a zone of oblique extension that has acted at least during "fast rifting" (San'kov et al., 2000). ...
We discuss the patterns of Late Cenozoic faulting and crustal stress in the northeastern flank of the Baikal rift system. The Late Cenozoic faults are mainly of NE and ENE strikes. Faults of these trends, along with W–E faults, have been principal seismogenic structures. They have normal or left-lateral oblique geometry with different amounts of horizontal motion. The N–S, NW, and NNW faults bear signature of mostly right-lateral strike slip, and those of the WNW direction are left-lateral strike-slip faults, often with a normal component. The rift basins in this part of the rift system have had different evolution dynamics, with the most rapid faulting and sedimentation in the Muya basin. According to structural and seismological data, regional stress in the area has been stable and dominated by NW extension through the Pleistocene-Holocene and the Present. The directions and obliquity angles of principal normal stresses and percentages of stress types show local lateral variations. The evolution of the northeastern flank of the Baikal rift system can be explained by a model of oblique rifting which accounts for the architecture of rift basins, the pattern of Late Cenozoic active faults, and the stress pattern derived from structural and seismicity data. The model is consistent with centrifuge models of magma emplacement during continental oblique rifting. Oblique extension associated with underplated magma can be maintained in the area by mantle plumes beneath the Kichera, Upper Angara, and Chara rift basins. The presence of subcrustal magma chambers may explain why the three basins formed before other basins in the NE flank of the rift system.
... Deformation in the SBR is highly localized and mainly implies three major active faults: the South Baikal, Sayan, and Tunka faults. Information on their geometry comes from satellite (SPOT) images and Russian field studies [Sherman, 1992]. The South Baikal and Tunka faults strike about N80øE and dip southwards. ...
Intraplate extension, in a frame of a global compres-sional stress field, seems linked to local lithospheric perturbations (lithospheric thinning or thickening) able to modify the resulting state of stress [Zoback, 1992]. The Baikal Rift Zone (BRZ), Siberia, is located north of the India-Asia collision zone and exhibits no direct communication with any oceanic domain. It can thus.be fully considered as an area of continental extension, dominated by the "global compressional intraplate stress field" resulting from plate driving forces. In order to address the problem of its dynamics and kinematics and their links with the India-Asia collision, a comprehensive stress tensor analysis is presented, based on 319 focal mechanisms of earthquakes located along the whole Baikal rift. The stress field is varying at different scales of observation: when looking at central Asia (several thousands kilometers), the maximum horizontal stress S • m • , • directions remain rather constant (with a fan-shape geometry) when the tectonic regime goes from compressional (Himalayas) to extensional (Baikal). When observing the Baikal rift (about 1000 km long), clear variations ofthe stress regime are observed, from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends. Finally, at the scale of 100 km, systematic S • x reorientations occur close to major rift faults. We thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics. We then use computed stress tensors to predict slip vectors on major rift faults. Deformation patterns show two distinct parts of the rift: the South Baikal Rift (SBR) is characterized by a constant trending (around N100øE) slip vector, meanwhile the North Baikal Rift (NBR) exlfibits a complex block rotation behavior involving at least three crustal blocks. We propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the India-Asia collision and the geometry of the hardly deformable Siberian platform. This particular setting can explain most of the surficial deformation patterns, which suggest a large-scale cracking of the lithosphere in the Baikal region. Other possible sources of stress could also be considered, like deep mantellic upwelling, or trench suction linked to the Pacific subduction.
... At first, most researchers interpreted the Baikal Rift Zone as a purely extension structure [12,44]. After the execution of experimental works (see above) and investigation of the stress fields, this concept was complemented with the recognition of left-lateral strike-slip faulting [1, 31,32] or oblique extension at the margins [3,13,42]. ...
In order to identify the major factors of the formation of the Baikal Rift Zone, tectonophysical analysis is carried out based
on physical modeling with the application of similarity criteria. A single-layer model of elastoplastic clayey paste is superposed
on two metal plates. One of the metal plates is displaced leftward according to the simple shear mechanism and its contact
with the second metal plate has a bend morphologically similar to the Baikal segment of a marginal suture of the Siberian
Craton. This scheme of loading corresponds, to a great extent, to the passive mechanism of rifting; i.e., the deformed layer
is destructed due to strikeslip displacement of blocks and development of pull-apart structure in the model without any uplift
and thermal impact related to the influence of mantle asthenolith in nature. The series of runs reproduces the major spatiotemporal
trends in the evolution of the Baikal Rift Zone. Some experiments achieve for the first time a high degree of similarity in
the morphology and mutual alignment of its main basins. This makes it possible to conclude that the evolution of basins and
faults in the Baikal Rift Zone was governed by the following factors: (1) elastoplastic response of the substrate with a regular
localization of strain; (2) left-lateral displacement of blocks; and (3) the presence of curved (in plan view) initiating
structural heterogeneity.
The oldest structures in a rift basin define incipient rift architecture, and commonly modulate the patterns of landscape evolution, sedimentation, and associated hazards in subsequent phases of rift development. However, due to deep burial beneath younger, thick syn‐rift sequences, and limited resolution of seismic imaging, critical early‐rift processes remain poorly understood. In the Tanganyika Rift, East Africa, we augment existing 2‐dimensional (2‐D) seismic reflection data with newly acquired aeromagnetic and Full‐Tensor Gradiometry data to assess the deep basin and underlying basement structure. Aeromagnetic and gravity grids show a dominance of NW‐trending long‐wavelength (>5 km) structural fabrics corresponding to the deeper basement, and dominant NW‐trending with a secondary NNE‐trending shorter‐wavelength (<3 km) fabric representing shallower, intra‐basin structures. Seismically‐constrained 2‐D forward modeling of the aeromagnetic and gravity data reveals: (a) an anomalously high‐density (2.35–2.45 g/cc) deep‐seated, fault‐bounded wedge‐shaped sedimentary unit that directly overlies the pre‐rift basement, likely of Mesozoic (Karoo) origin; (b) ∼4 km‐wide sub‐vertical low‐density (2.71 g/cc) structures within the 3.2 g/cc basement, interpreted to be inherited basement shear zones, (c) early‐rift intra‐basin faults co‐located with the modeled shear zone margins, in some places defining a persistent structurally‐controlled intra‐basin “high,” and (d) a shallow intra‐sedimentary V‐shaped zone of comparatively dense material (∼2.2 g/cc), interpreted to be a younger axial channel complex confined between the intra‐basin “high” and border fault. These results provide new insight into the earliest basin architecture of the Tanganyika Rift, controlled by inherited basement structure, and provide evidence of their persistent influence on the subsequent basin evolution.
—This paper presents a schematic summary of comprehensive analysis of seismic, reflection profiling, and hydroacoustic data on faults which caused sediment deformation in the central segment of the Central Baikal basin. According to the tectonophysical analysis results, the fault pattern within sediment fill has been recognized as zone-block, i.e., it represents a network of high-density fracture zones limiting weakly deformed blocks. The structure of large NE-trending fault zones (Olkhon, Beregovoy, Gydratny, and Svyatoy Nos) is controlled by main fault planes (or their segments) bounded by subsidiary faults. Geomorphic expression of NW cross faults in the sedimentary cover as broad zones of smaller-scale fractures accounts for early stages of the evolution of basement faults. In a longitudinal direction, they divide the basin into large fragments. The zone–block structure of the sedimentary strata was developed in different stress regimes: strike-slip and extension at the early and late orogenic rifting stages, respectively. At the modern stage of tectogenesis, the established network of fault zones controls the gaseous (including hydrate formation) and seismic activity expression in the subsurface. Hydrate-bearing mud volcanoes and seeps are confined to major faults, while earthquake epicenters are confined to fault zones and form clusters at junctions of large NE-trending faults with NW-oriented extension zones and E–W left-lateral strike-slip faults.
Tectonic and paleo‐environmental reconstructions of rift evolution typically rely on the interpretation of sedimentary sequences, but this is rarely possible in early‐stage rifts where sediment volumes are low. To overcome this challenge, we use geomorphology to investigate landscape evolution and the role of different forcing mechanisms during basin development. Here, we focus on the humid Middle Shire River basin, located within the zone of progressive interaction and linkage between the southern Malawi Rift and Shire Rift Zone, East Africa. We used a digital elevation model to map knickpoints and knickpoint morphologies in the Middle Shire River basin and examined the relationships with pre‐rift and syn‐rift structures within the rift interaction zone. The main axial stream, Shire River, descends steeply, 372 m over a 50 km distance, across exposed metamorphic basement along the rift floor, exhibiting a strongly disequilibrated longitudinal elevation profile with both ‘mobile’ and ‘fixed’ knickpoints. In particular, we identify two clusters of mobile knickpoints, which we interpret as associated with baselevel fall events at the downstream end of the exposed basement that triggered knickpoint migration through the fluvial network since at least the Mid. Pleistocene. We infer that after the integration of the axial stream across the Middle Shire Basin, the knickpoints migrate upstream in response to fault‐related subsidence in the Shire Rift Zone. Conversely, the fixed knickpoints are interpreted to reflect local differential bedrock erodibility at lithologic contacts or basement‐hosted fault scarps along the basin floor. The results suggest that Middle Shire basin opening, associated with rift linkage, is likely a recent event (at least Mid. Pleistocene) relative to the Late Oligocene activation of Cenozoic rifting in the East African Rift’s Western Branch. These findings support the hypothesis that the Western Branch developed from the gradual propagation, linkage, and coalescence of initially nucleated distinct rift basins.
This work investigates the Cenozoic tectonic deformations on the southern edge of the
Guanabara Gráben. The study area is located in the metropolitan area of the city of Rio de
Janeiro. It is bordered on the west by Morro Dois Irmãos (southern zone of the city of Rio de Janeiro) and, on the east, by Serra da Tiririca (oceanic region of Niterói). The southern border of Guanabara graben has NE-SW direction to ENE-WSW and Cenozoic deformations controlled by the Rio Bonito Fault Zone. Regionally it is correlated with the border faults of the basins of São José de Itaboraí and Macacu. In the study area corresponds to the fault of the Paineiras, which deforms the Carioca Coastal Massif. Besides, this fault zone controls the intrusion and deformation of the alkaline massifs, the plug and dikes of Cabuçu, and the plugs of Jardim Cabuçu. These deformations are also recorded in the Tanguá alkaline massifs and Itaboraí plug. To the east of the Rio Bonito high, this fault zone has probable continental continuity up to the Cape of São Tomé , and also probable relation with the deformations in Gráben of Barra de São João and Campos basin. In the distal part of Gráben de Barra de São João, the pinching of the layers towards the continent, and the NE-SW direction of the graben suggest a correlation of this structure with the continental basins of Macacu and São José de Itaborái. The employed method involves the kinematic analysis of fracture failures and characterization of the affected lithology, including minerals newly formed by basement hydrothermal alterations. Five distinct paleotension fields were identified, with the interaction between the upper Cretaceous and the Holocene. The geochronological characterization was made based on the regional events and field criteria such as the overlap of
striations, the timing relation of fault movements and hidrotermalized rocks and their
mineralizations. The results obtained in the field were correlated to regional tectonic events of a brittle nature deforming the basins and alkaline massifs contained in the Fault Zone of Rio Bonito. The deformations recognized in the study area, the corresponding paleostress field and regional correlation will be listed below:Event 1 – σ3 paleogenic NE-SW: deforms diabase dikes and silicified breccias with percolation of oxidizing fluids. Regionally controls the intrusion of the alkaline massifs and implantation of the São José de Itaboraí basin.Event 2 – σ1 upper neogenic NW-SE formation of low-angle and high-angle dips reverse faults associated with neoformed minerals. Regionally correlated to the second inversion pulse of the Itaboraí basin and control of the formation of fluorite veins in the Tanguá alkaline massif. Event 3 – σ3 E-W forms N-S fractures in NW-SE direction diabase dike. Regionally it forms N-S faults in the Macacu basin. From the correlation of these events with literature data, it was proposed that the tectonic events responsible for the formation and reactivation of the southern border of Guanabara Gráben exerted control in the implantation and deformation of the alkaline bodies
contained therein, and inversion of the basins of Itaboraí and Macacu, characterizing a corridor of brittle deformation from the south zone of Rio de Janeiro to the Rio Bonito High.
Volcanic rocks from deep-seated sources of the crust-mantle transition (CMT) are geochemically distinguished from those of ocean island basalts (OIB). Here, we report geochemical data on tectonic pseudotachylytes from the Main Sayan Fault (MSF) and volcanic rocks from the Kamar-Stanovoy Zone of Hot Transtension (KSZHT) that represent the deep-seated CMT magmatic sources in the central part of the Baikal Rift System (BRS). The tectonic generation of the KSZHT magmas between 18.1 and 11.7 Ma is compared with present-day seismogenic deformations in the middle-upper crust of the South Baikal Basin and adjacent Tunka Valley, where strong earthquakes are distributed along the Main Sayan and Primorye sutures of the Siberian paleocontinent. From a detail seismic tomography model and geological evidence, we infer that the KSZHT crust–mantle magmatic processes were due to delamination and lamination of a thickened root part of the South Baikal Orogen existed in the Late Cretaceous and Paleogene. In addition, we identify similar deep-seated CMT sources for melts erupted in the past 17 Ma from a delaminated root part of the East Hangay Orogen and adjacent Orkhon-Selenga Saddle in the southwestern BRS. We suggest that both in the central and in the southwestern BRS, the deep-seated CMT magma sources designate cooperative pull-to-axis and convergent effects created in the Japan-Baikal Geodynamic Corridor and in the Indo-Asian interactional region, respectively.
Active continental rifts are ideal sites for understanding the break-up of continents, and long-lived rift lake environments are known as important reservoirs for endemic communities and biodiversity. The sedimentary fill of the Lake Tanganyika Rift records a long history of continental extension and variable tropical climate, that is unparalleled in its duration and fidelity. Recently acquired, state-of-the-art 2D seismic reflection data, together with reprocessed legacy data, are used to evaluate the evolution and distribution of sedimentary facies over the rift lake. Using seismic stratigraphic analysis, we reconstruct past depositional environments and the paleogeography of the lake and assess how tectonic-driven subsidence and hydroclimate variability modified the lake basin. We identify six syn-rift seismic units that overly the acoustic basement and identify depositional units beneath the syn-rift sequence that suggest episodes of pre-rift sedimentation. Based upon the seismic facies analysis, the earliest seismic stratigraphic unit is interpreted as deposited in an early-stage rift system of low-relief, that was dominated by alluvial, fluvial and shallow lacustrine conditions. Subsequent units exhibit attributes of a lacustrine environment of much greater water depth, enhanced catchment relief and accommodation, consistent with a more mature rift. In Seismic units 2–5, we observe extensive deltaic deposits and deep-water fans, and locally canyons, channels, channel-levee complexes, turbidites, slumps and other mass flow deposits. In the latter part of its history, erosional surfaces and abundant lowstand delta facies are observed, indicating the rift experienced dramatic hydroclimate cycles. We assess the relative timing of key features of the rift, including the emergence of major structures and rift segment boundaries, development of major drainages and linkages to upstream rift lakes. The evolving sedimentary facies of the rift illustrate a shallow- to-deep progression of rift valley environments and the more limited littoral habitats that influenced the evolution of its unique endemic organisms.
Statistical distribution of instrumental seismicity in the Baikal rift system was used to outline persistent earthquake clusters, which together make up a zone of lithospheric failure existing as a single active seismic and tectonic unit. Large earthquakes regularly delineate its axis, and the pattern of smaller events is controlled by its separate fragments. Earthquake sources show space and time oscillations along and across the strike of damage zones around the failure zone and its fragments. Earthquake migration and the hierarchic association of events of different magnitudes with active faults reflect the general features of faulting in the rifted lithosphere. Faulting-seismicity interaction is approached at comparable scales of lithospheric failure as rare large events mark the evolution of the whole active unit and more frequent smaller earthquakes record processes in its subunits. Time-dependent migration of earlier large events along the axis of the failure zone thus can be a guide to predict large earthquakes in the region, and smaller events can be predicted proceeding from the patterns of comparable seismicity within zone fragments. The available collection of data is extensive enough to approach new composite tectonophysical modeling of seismicity accompanying lithospheric failure in different geodynamic environments. These models will open up new avenues in medium-term earthquake prediction on the geological and geophysical basis.
In the course of the study of the problem of continental rifting, mapping of the Baikal and Teletsk oe bottom surfaces has been carried out by using a side-scanning system. Many local morphologic elements connected with active tectonic structures of different scale and genesis have been revealed. Significant fault dislocations in combination with normal faults resulted from the extention have been found. Some dislocations and their mutual correlation are refined. The kinematic interpretation of the new data is proposed.
A model of formation of tectonic extension structures with faults has been developed and tested on the Teletskoe Lake (Corny Altai) rift. The dynamic model of formation of the U-shaped structure of the Earth's crust extension was based on the rheology of a nonlinearly hardened plastoelastic body. The designed software is universal and is intended to solve plane plastoelastic problems. The author considers the problems of plane stressed state and plane deformation. The calculations show that a 4-5 km wide rift can form when the medium undergoes a tensile stress no less than 1.5 kbar. The dynamics of development of a plastic-flow zone and the position of the boundary of the elastic-deformation-plastic-deformation transition are studied. The paper also discusses possible mechanisms of formation of zones of radial-faults joint which border the margins of the main rift basin.
Analysis of the fractal dimensions of faults and seismicity has been first carried out in the Baikal Rift Zone (BRZ). It is shown that in general these dimensions are close but have slight differences on the flanks and in the central part of the BRZ. The conclusion is drawn that the development of the BRZ has become stabilized and today faulting is in balance with seismicity. The difference in fractal dimensions on the BRZ flanks is caused by different lithosphere stresses in the central and flank parts of the zone. Comparison of the fractal dimensions of faults and epicentral field may be used for a long-term forecast of seismicity.
The article presents major results which have been obtained during 30 years of researches conducted by the Laboratory of Tectonophysics at the Institute of the Earth’s Crust.General regularities in organization of fault-block structures in the brittle lithosphere are established. Relations between main parameters of faults are investigated, and their connection with the lithospheric structure and recent crustal movements is shown. A rheological model of vertical zoning of faults is proposed. Internal structures of faults are studied; stages in faulting are generally defined in terms of time; regularities of patterns of joints inside faults are described; and original methods of mapping such joints are proposed to reveal tectonic conditions of faulting.Based on seismic monitoring, new methods of quantitative assessment of relative activity rates of faults in real time are developed. Such methods are applied to delineate zones of recent destruction of the lithosphere in the Central Asian region. The state of stresses of the lithosphere is mapped, and the new map allows us to reveal regularities in the spatial mosaic of regions differing by types of stress fields.Our physical experiments conducted in compliance with similarity conditions are aimed at studying faulting mechanisms with regard to variable loads. A special set of experiments is devoted to the Baikal rift system. Cases of application of tectonophysical methods to study fault tectonics, the state of stresses and seismicity of the lithosphere are described. Prospects of tectonophysical researches conducted in the Laboratory and potentials of integration with studies of other research teams are considered.
The history of tectonophysical studies in Irkutsk began in the 1950s at the initiative of Prof. V.N. Danilovich. Tectonophysics as a new scientific field in geology was enthusiastically supported by research institutes of the actively developing Siberian Branch of the USSR Academy of Sciences, including the Institute of the Earth's Crust (IEC). In late 1950s, V.N. Danilovich, G.V. Charushin, O.V. Pavlov, P.M. Khrenov, S.I. Sherman and other scientists began to conduct large-scale studies of faults and rock fracturing with application of methods of structural analysis of fault tectonics and taking into account types of physical and mechanical destruction of the crust. In 1979, the IEC Scientific Council reviewed the initiative of Prof. S.I. Sherman, who was supported by Academician N.A. Logachev and Doctor of Geology and Mineralogy O.V. Pavlov, and approved the decision to establish the Laboratory of Tectonophysics, that has been and is the only scientific research team of the kind in the territory of Russia eastward of the Urals and, in fact, the second in the Russian Federation. Its studies are based on concepts dealing with physical regularities of crustal faulting that are described in the monograph published by S.I. Sherman [Sherman, 1977], three co-authored volumes of Faulting in the Lithosphere [Sherman et al., 1991, 1992, 1994] and other scientific papers. These publications have consolidated results of studies conducted by the team of researchers from the Laboratory, which can be called the Irkutsk school of tectonophysics. On the eve of the 21st century, the Laboratory successfully extended application of physics of destruction of materials and mathematical methods of analysis to studies of structural patterns of faults varying in ranks in the crust and the upper lithosphere.We conducted comprehensive studies of tectonophysical regularities of formation of large crustal faults, pioneered in establishing quantitative relationships between main parameters of faults, i.e. length and depth, length and amplitude of displacement, length and density, and estimated the factors determining such parameters. A model showing the fault structure was proposed with account of changes of physical properties of the crust with depth. It was shown that faulting in the crust follows the laws of deformation and destruction of Maxwell body.With accumulation of the knowledge on regularities of faulting in the lithosphere, analyses the state of stresses in the lithosphere has become prioritised, and this is one of the top challenges in geodynamics and tectonophysics. Tectonophysics from Irkutsk published the first map of the state of stresses of the Baikal rift zone and proposed new concepts for studying crustal stresses by structural geological methods. Based on such concepts, a new map of the state of stresses of the upper lithosphere was constructed.Studies of faulting included researches of areas around virtual axes of faults and variations of sizes of such areas, and a concept of an area of dynamic influence of large lithospheric faults was proposed. It is established that internal patterns of areas of dynamic influence of faults are composed of zones that can be revealed both laterally and in depth, and such zonal patterns depend on the degree of tectonical and dynamo-metamorphical transformation of the rocks.The internal structure of continental fault zones was studied, and three main disjunctive stages were revealed, each corresponding to a specific type of deformation behaviour of the medium, its state of stresses, pathogenesis of faults varying in ranks, and variations of parameters in space and time.Triple paragenesises of fractures were revealed and analysed for a number of regions, and such studies provided the basis to propose a method of specialized mapping of the crust, which provides for determination of locations of fault zones and their boundaries, conditions of their formation and major specific features of their internal structures. This method can be effectively applied within the framework of conventional geological surveys of any scale.Results of studies of tectonic divisibility of the Earth based on advanced tectonophysical concepts were referred to establish the zone-block structure (ZBS) of the lithosphere. Analyses of faults at various scales showed a strict hierarchy of ranks in the ZBS of the lithosphere in Central Asia, and actual characteristics of 11 hierarchic levels (from global to local) were revealed and described in quantitative terms. With reference to the ZBS concept, the Baikal rift was studied, and the soil radon concentration pattern of Pribaikalie was analysed and its main spatial and temporal regularities were revealed.Comprehensive geological, structural, tectonophysical and geoelectrical studies were conducted in the Cenozoic and Mesozoic basins of Pribaikalie and Transbaikalie, and results were consolidated and published. The fault-block patterns, the deep structure, the state of stresses and seismicity of the crust were studied in a number of areas in the region.Complex tectonophysical studies were initiated in the Yakutian diamond-bearing province to reveal structural factors that control the kimberlite locations, and the first results were reported. By applying tectonophysical methods, it was established that periods of formation of kimberlite bodies are related to stages of formation and activation of the fault pattern of the platform cover. A pioneering conclusion was stated that in the structural control over kimberlite magmatism of the Siberian platform, the dominant role is played by fault zones of the orthogonal network, which were activated in the regime of alternating-sign displacements at different stages of the platform's development in the Paleozoic and Mesozoic.Physical modelling experiments using an original installation were conducted, and, among its main achievements, an important result is modelling of the process of formation of the Baikal rift zone (BRZ) by an elasto-plastic model in conformity with criteria of similarity. The Shanxi rift system was also modelled, and its physical modelling study was conducted jointly with scientists from China under the Russian-Chinese project supported by the Russian Foundation for Basic Research.Besides, the article informs about commencement of original experimental studies of deformation waves in elasto-plastic mediums and describes objectives of tectonophysical studies for the nearest future.
Magmatism related to the Eastern Siberia rift system and the geodynamics. [Magmatisme essocie au rift de Siberie orientale: implications geodynamiques]. -Bul/. Centres Rech. Explor.-Prod. Elf Aquitaine, 18,2,437-452, 13 fig.; Pau, December 26, 1994. -ISSN : 0396-2687. CODEN: BCREDP. En se reterant aux variations spatiale et temporelle de composition des roches volcaniques associees au rifting de la region Trans-Baikal, I'auteur montre que les phenomenes cenozorques profonds sont herites des activites tectoniques et mag-matiques pendant la periods du Paleozolque moyen au Mesozolque. Le systerne de rift est-siberien est divise en trois groupes de bassins: I'ensemble Tunka-Eravna avec un volcanisme associe, actif du Cretace superieur au Cenozotque moyen, et les groupes essentiellement non volcaniques du Baikal-Chara et du Khubsugul-Dar-khat formes au Cenozolque superieur. Quoique Ie developpernent du rift cst-siberien soit contemporain de la collision Inde-Asie, la rotation relative des blocs separes par Ie groupe Balkal-Chara autour de sa terminaison nord-est indique que les forces d'extension etaient locales. Le modele de « hot spot» propose pour expliquer les phenomenes geodynami-ques dans Ie systerne de rift est-siberien implique I'existence d'un flux thermique puissant Ie long de la bordure meridionale du craton siberien, entre -29 et -12 Ma (Oligocene superieur a Miocene). II s'est manifeste par un «doming» un rifting et un volcanisme peu alcalin vigoureux. La migration des episodes volcaniques dans la partie occidentale du groupe Khubsugul-Darkhat pourrait indiquer un deplacement lent (0,8 a 0,9 ern/an), vers I'est, de la plaque eurasienne chevauchant un panache mantellique fixe. Sergey V Rasskazov, Institut of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences, Lermontova Street 128, 664033 Irkutsk, Russia. -May 14, 1994. ABSTRACT Based on the variation in space and time of the composition of rift-related volcanic rocks in the Trans-Baikal region, the Cenozoic deep-seated processes are shown to be inherited from the tectonic and magmatic activity during the Mid-Paleozoic to Mesozoic. The Eastern Siberia rift system is divided into three groups of rift basins: the Tunka-Eravna group with associated volcanics formed in the Late Cretaceous to Mid-Cenozoic, and the essentially non-volcanic Baikal-Chara and Khubsugul-Darkhat groups which formed during the Late Cenozoic. Although development of the Eastern Siberia rift system was contemporaneous with the Indian-Asian collision, the relative rotation of terranes separated by the Baikal-Chara group of rift basins around the pole at its northeastern termination, indicates that extensional forces were local. A hot spot model which proposed to explain the geodynamics within the Eastern Siberia rift system, implies a powerful heat impulse along the southern edge of the Siberian craton from -29 to -12 My (Late Oligocene to Miocene). This was expressed by doming, rifting and vigorous mildly alkaline basaltic volcanism. A migrating sequence of volcanism within the western area of the Khubsugul-Darkhat group might indicate a slow (0.8-0.9 crn/yr) eastward motion of the Eurasian plate overriding a fixed mantle hot plume. The role of the Indian-Asian collision in the tectonic and magmatic reactivation of Inner Asia has been discussed since the hypothesis of MOLNAR & TAPPoNIER (1975) was published (ZONENSHAIN & SAVOSTIN, 1981; ZONENSHAIN at al., 1990; KISELEV, 1982; SAMOYLOV & YARMOLYUK, 1992; LOGATCHEV at al., 1983; LOGATCHEV & ZORIN, 1992; LOGATCHEV, 1993, and others). Volcanism within the Eastern Siberia rift system (ESRS) has also been considered to be due to the colli-sion-related syntectonic decompressive melting of the litho-sphere (KISELEV, 1982; SAMOYLOV & YARMOLYUK, 1992). LOGATCHEV at al. (1983) suggested that the evolution of the Eastern Siberia rift system was driven mainly by the heating and upwelling of the asthenosphere, rather than being di-rectly caused by a collisional deformation of the lithosphere. To evaluate the role of various deep-seated processes in the reactivation of the lithosphere, this paper presents a general space-time analysis of rift-related magmatism during the Mid-Paleozoic to Mesozoic, the Late Mesozoic to Mid-Cenozoic, and the Late Cenozoic episodes in the folded terranes, south of the Siberian craton.
Institute of the Earth's Crust, Siberian Branch of the RAS, 128 ul. Lermontova, Irkutsk, 664033, Russia The morphology, structural setting, and evolution of the Baikal rift system has been controlled by its position at the junction of the Siberian craton and the Central Asian mobile belt, two major tectonic units with contrasting thermomechanical properties. Rifting initiated in South Baikal, in place of the present Selenga delta, where first Late Cretaceous-Paleocene pulses of extension produced a large basin. The basin began to draw in the regional surface runoff as the Selenga valley broke through the Khamar-Daban Ridge and captured the drainage of Western Transbaikalia and Northern Mongolia inherited from the Late Mesozoic. Rifting propagated on both sides off South Baikal as far as the youngest rift basins and faults in Mongolia in the southwest and in the Olekma region in the northeast. The Baikal basin includes two rather than three structurally equal subbasins, South and North Baikal, separated by a diagonal link of Olkhon island — submerged Akademichesky Ridge — Ushkan'i isles. The South Baikal basin is in turn bisected by the Selenga saddle, the oldest and largest deposition center filled with about 10,000 m thick sediments strongly deformed in Pliocene-Quaternary time. The neotectonics, crustal thickness, and 3D velocity structure of the region between the rift and the India/Eurasia collision front indicate that rifting in East Siberia evolved under the joint effect of local and far-field geodynamic mechanisms.
The Baikal Rift System in southern Siberia is one of the main intracontinental extensional features on Earth. The rift system represents the northwestern boundary of the Amuria plate and in that respect can be considered as an evolving plate boundary. The Baikal Rift System has been widely studied both in terms of geology and geophysics and many models have been proposed for its formation and evolution. However, the age of the initiation of deformation and the mechanism driving this deformation are still largely debated. While major extension has occurred since the Late Miocene–Pliocene, the onset of extension seems older than the India–Asia collision, implying that several driving mechanisms may have acted together or in relay through time. In this work, we review the available data and models for deformation in an area encompassing the Baikal Rift System, the Sayan ranges to the west and the Transbaikal to the east. Using a synthesis of this data and our own field and mapping observations, we show that the Baikal Rift System, along with transpressional deformation in the Sayan ranges and transtension in the Transbaikal area, can be explained through major left-lateral strike–slip systems. The deformation is strongly controlled by inherited crustal and lithospheric structures, and is distributed over a wide area within the western Amuria plate that consequently cannot be considered as a rigid block. Such distributed deformation is likely to have a strong effect on the structure of the future continental margin if extension evolves towards the formation of oceanic crust.
Synthesis of the available stratigraphic data on the Baikal basin sediments exposed around the lake and their correlation with offshore lake sediments and with onshore sections in the Baikal Foredeep allows a new perspective of the Baikal rift history. The basin sediments on the Baikal shore comprise three tectonic–lithological–stratigraphic complexes (TLSC) which correspond to three seismic stratigraphic sequences (SSS) in the lake sediments and to three complexes in the Baikal Foredeep.
The oldest unit, TLSC-1, has a particular lithology being deposited in an environment which never repeated in the later history of the area. This proves the validity of its lithostratigraphic correlation with Masstraichtian–Early Oligocene sediments of the Baikal Foredeep constrained by biostratigraphy. Further support comes from isotope dating, paleontology, and other evidence.
Unlike seismostratigraphy-derived models, SSS-1 is correlated in the new model with TLSC-1 rather than with the Tankhoi Formation (which actually represents TLSC-2), and the onset of rifting is placed at the Late Cretaceous–Paleogene rather than at the Oligocene (or Miocene). Thus, the Baikal rifting began prior to the India–Eurasia collision, and the first rifting pulse originally had other causes. This inference agrees with fission-track apatite thermochronology indicating Cretaceous ages of samples from the Barguzin rift basin in the northeastern flank of the rift system.
Rifting developed successively in three different tectonic settings and was driven by different geodynamic mechanisms at each stage. First it was a passive response to Late Cretaceous–Eocene distributed continent-wide extension in Asia (purely passive rifting). At the second stage spanning Late Oligocene–Early/Late Pliocene time, the area was subject to compressive impact from the India–Eurasia collision, which propagated from the southwest since the Eocene and reached the region about 30 Ma ago to take control over its geodynamics (conventionally passive “impactogenic” rifting). Finally, the Pliocene–Quaternary evolution has been driven by extension from a local source associated with hot mantle material rising to the base of the rifted crust (active rifting).
The major rifting stages are further subdivided into substages: two substages in the second stage with the boundary at ~ 10 Ma (Middle–Late Miocene) and three substages in the third stage, with boundaries at 1.0–0.8 and 0.15–0.12 Ma. The stages and substages of rifting are separated by events of tectonic activity and stress reversal when additional compression produced folds and shear structures. The events that mark the stage boundaries show up as gaps, unconformities, and deformation features in the deposition patterns.
Thus, the three units of synrift sediments composed of eluvium, early molasse, and late molasse, respectively, were deposited during preorogenic, early orogenic, and postorogenic stages of rifting driven by passive (first purely passive and then “impactogenic”) and active mechanisms.
The new model of the Baikal rift history agrees with data obtained by different other methods.
The first tectonophysical model of the Baikal seismic zone represents a separate complex region of the lithosphere. It has a pinnate structure with a backbone belt of current deformation, which is a concentrator of largest earthquakes, and branching, repeatedly reactivated large and small faults. In its vertical section, the seismic zone is tree-like, the stem and the branches being faults of different size ranks which can generate earthquakes when reactivated. The real-time short-period fault motions and the respective seismicity occurring at a certain time and in certain places are triggered by strain waves, which disturb the metastable state of the faulted lithosphere subject to regional stress. The modeling work includes developing general requirements for tectonophysical models of continental rifts and special methods for identifying the faults that become active within short historic time spans, as well as techniques for locating potential events in space and time in specific active faults. The methods and model testing for medium-term earthquake prediction are described by the example of the well-documented Baikal seismic zone, which is the most active part of the Baikal rift system. The tectonophysical model for the Baikal zone is statistically supported by field data, and this allows estimating the velocities and periods of strain waves for different zone segments and faults, with implications for nearest-future earthquake prediction.
Rock-magnetic measurements of sediment cores from the Academician Ridge region of Lake Baikal, Siberia show variations related to Late Quaternary climate change. Based upon the well-dated last glacial-interglacial transition, variations in magnetic concentration and mineralogy are related to glacial-interglacial cycles using a conceptual model. Interglacial intervals are characterized by low magnetic concentrations and a composition that is dominated by low coercivity minerals. Glacial intervals are characterized by high magnetic concentrations and increased amounts of high coercivity minerals. The variation in magnetic concentration is consistent with dilution by diatom opal during the more productive interglacial periods. We also infer an increased contribution of eolian sediment during the colder, windier, and more arid glacial conditions when extensive loess deposits were formed throughout Europe and Asia. Eolian transport is inferred to deliver increased amounts of high coercivity minerals as staining on eolian grains during the glacial intervals. Variations in magnetic concentration and mineralogy of Lake Baikal sediment correlate to the SPECMAP marine oxygen-isotope record. The high degree of correlation between Baikal magnetic concentration/mineralogy and the SPECMAP oxygen-isotope record indicates that Lake Baikal sediment preserves a history of climate change in central Asia for the last 250 ka. This correlation provides a method of estimating the age of sediment beyond the range of the radiocarbon method. Future work must include providing better age control and additional climate proxy data, thereby strengthening the correlation of continental and marine climate records.
The Tunka fault is a major normal-oblique transform fault within the NNE trending Baikal rift that displays geomorphic evidence of recurrent Quaternary movement. A flight of six fanhead terraces of the Kyngarga River is displaced by several parallel faults and has scarps up to 32 m high at Arshan. The main fault zone is exposed in an abandoned gravel quarry about 1 km east of the Kyngarga River. Thirteen radiocarbon dates from the quarry, a shallow trench in a graben, and natural streamcuts constrain the timing of the latest two or three Holocene paleoearthquakes. The latest earthquake is bracketed by the 1024-1315 calendar years (cal. year) B.P. age of an unfaulted terrace and by the 1947-2179 cal. year B.P. age of a soil buried by scarp colluvium in the graben trench. The penultimate earthquake is also relatively well constrained, assuming that the displacement event slightly younger than 7091-7867 cal. year B.P. in the upper quarry exposure is the same as the fissuring event slightly older than 6733-7385 cal. year B.P. in the lower quarry exposure. Evidence for earlier event(s) between 9.2-12.7 ka depends on ambiguous stratigraphic evidence in the lower quarry exposure. On the basis of only the two well-dated earthquakes, the recurrence interval at Arshan may range from 2.9 to 6.8 ka for earthquake displacements of at least 1.3 m or a slip rate of 0.19-0.44 mm/yr. Intermediate-term (100 ka) and long-term (circa 500 ka) slip rates computed from terrace age estimates and fault scarp heights are 0.08-0.16 mm/yr and 0.07-0.11 mm/yr, respectively, rates that are considerably lower than the late Holocene rate and the approximately 0.5 mm/yr that might be inferred from the tectonic geomorphology of the Tunka Range front.
New surficial data (field, Landsat TM and topography) define morpho-tectonic domains and rift flank segmentation in the Ol'khon region of the Central Baikal rift. Deformation, drainage and depositional patterns indicate a change in the locus of active extension that may relate to a recent (<1 Ma) change in the kinematics of the Siberian plate boundary. The westwards migration of the border fault location has broadened the rift with concomitant shifts in depocentres. Within the hanging wall of the new western border fault, distinct segments control the location of drainage paths and syn-rift deposits. Morphology, sediment thicknesses and fault scarp amplitude indicate that a segmented rift flank graben has propagated southwards along the rift flank and is still actively fragmenting. These surficial data are used to constrain a model for the time-dependent topographic variations during progressive subsidence along a rift flank, involving the transfer of footwall units to hanging-wall domains. Rapid changes in border fault footwall relief in this model are associated with change in the active border fault location with widespread mass-wasting. The model shows that time-dependent histories need to be integrated with flexural uplift models for active normal faults. The active, syn-rift depositional systems of the Ol'khon region provide a valuable analogue for the early evolution of continental margins and the structural controls on syn-rift hydrocarbon sources and reservoirs.
The stress fields in the Tunka Rift at the southwestern flank of the Baikal Rift Zone are reconstructed and analyzed on the
basis of a detailed study of fracturing. The variation of these fields is of a systematic character and is caused by a complex
morphological and fault-block structure of the studied territory. The rift was formed under conditions of oblique (relative
to its axis) regional NW-SE extension against the background of three ancient tectonic boundaries (Sayan, Baikal, and Tuva-Mongolian)
oriented in different directions. Such a geological history resulted in the development of several en echelon arranged local
basins and interbasinal uplifted blocks, the strike-slip component of faulting, and the mosaic distribution of various stress
fields with variable orientation of their principal vectors. The opening of basins was promoted by stress fields of a lower
hierarchical rank with a near-meridional tension axis. The stress field in the western Tunka Rift near the Mondy and Turan
basins is substantially complicated because the transform movements, which are responsible for the opening of the N-S-trending
rift basins in Mongolia, become important as Lake Hvsgl is approached. It is concluded that, for the most part, the Tunka
Rift has not undergone multistage variation of its stress state since the Oligocene, the exception being a compression phase
in the late Miocene and early Pliocene, which could be related to continental collision of the Eurasian and Indian plates.
Later on, the Tunka Rift continued its tectonic evolution in the transtensional regime.
A new structural map of the western Sea of Okhotsk, based on grids of multichannel seismic data, provides several new insights into the Tertiary deformation of Asia, especially in regard to the extension of ``extrusion'' tectonics, linked to the India-Eurasia collision, into northeastern Asia. The sedimentary basins in this offshore region are the result of two regional shear systems. In the west, the north trending Sakhalin-Hokkaido dextral fault system transects Sakhalin Island. Accessory structures closely related to its dextral shear include northeast trending normal faults as well as northwest trending en echelon folds and thrusts. The accessory normal faults are predominantly Eocene to lower Miocene and indicate transtensional shear, whereas the folds and thrusts are mostly younger and indicate late Miocene and Pliocene transpression. In the north, an east-west trending sinistral shear zone enters the sea at its northwest corner; this sinistral shear system links farther west to extensional faults of the early Tertiary Baikal rift. Extending northeastward from the terminus of this sinistral shear zone are a series of long, predominantly northwest dipping listric normal faults that form a large ``lazy-S'' shaped pull-apart basin, the Shantar-Liziansky basin (SLB). Normal faults in this 500×140 km basin accommodate 15 to 20% extension. Extension and related sinistral shear appear to be largely Eocene to Oligocene in age, with lesser later activity. To the northeast of SLB, a further extension of the sinistral shear zone appears to bend northeastward, transecting Pustorets and Penzhina basins and following the course of the older, Mesozoic Mongol-Okhotsk-Chukotsk active margin and suture; it possibly reaches as far as the Arctic. As suggested by previous workers, the Baikal rift and its associated sinistral shear zone may be interpreted as by-products of the India-Eurasia collision in Eocene time; mapping results included here show that this diffuse sinistral shear system extends much farther to the northeast. The sinistral shear zone is transected by the dextral Sakhalin-Hokkaido fault zone in the Kashevarov region of central SLB, where a fan of north to northwest striking late Tertiary transpressional fault splays marks the northern termination of the dextral fault zone. Older SLB normal faults are locally deformed, and half grabens are partly inverted, by the slightly younger dextral splays. The two regional fault systems are essentially conjugate, and the pattern of intersection closely resembles that predicted by recent modelling studies of India/Eurasia collision tectonics.
We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift-orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift-parallel again, although a small number of measurements are still rift-orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N-S trending rift. All three rifts are underlain by low-velocity upper mantle as determined from teleseismic tomography. Rift-related mantle flow provides a plausible interpretation for the rift-orthogonal fast directions. The rift-parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small-scale mantle convection with rift-parallel flow. The agreement between stress estimates and correqponding crack orientations lends some weight to the suggestion that the rift-parallel fast directions are caused by oriented magmatic cracks.
The available geological and geophysical methods fail to estimate variations of fault activity in real time intervals, such as, months, years or decades. Using geo-informational technologies, this challenge can be resolved with an algorithm and software based on a quantitative index of seismic activity. Such an approach has been applied to studies on the Baikal rift system (BRS) and its adjacent territories. It is discovered that fault activity is variable in intervals of several years, which cannot be attributed to changes in regional stress fields. An active fault map of BRS and curves of the quantitative index of seismic activity of faults are constructed for the BRS cross-section. The proposed method ensures a detailed classification of active faults by the quantitative index of seismic activity, and thus significantly extends options for finding solutions of problems related to the middle-term forecast of earthquakes. This method has been applied to study spatial and temporal variations, sources, and mechanisms of the recent fault activation. It is shown that fault activation has a relatively high frequency on the real time scale because of the slow deformation waves of disturbance that are generated by inter-plate and intra-block movements of the brittle lithosphere. Throughput velocity of deformation waves allow classification of active faults into groups that differ in geological and geophysical parameters. They also allow an estimation of the direction of the deformation wave's front and identification of an area of dominating fault activation in real time (geologically instant) intervals.
This chapter describes the Baikal rift system, which is 1800 km to 2400 km long and is situated at the boundary between the Siberian Platform to the northwest and the Caledonian Sayan-Baikal fold belt to the southeast. This rift system has been the object of intensive study by Soviet scientists for many years. The Baikal rift system is composed of fifteen individual topographic depressions which are associated with an approximately 1500-km long domal uplift. The central portion of Baikal Rift system is almost entirely located on the relatively weak and anisotropic basement of the Sayan-Baikal fold belt. The sub-vertical crustal boundary between the Siberian Platform and the fold belt forms an abrupt western boundary for the central portion of the rift system and its domal uplift, and in particular runs along the west side of the Lake Baikal depressions.
We studied the morphotectonic framework of the southwestern termination of the North Baikal basin including the Olkhon Island and its surroundings located in the central part of the Baikal Basin. The morphotectonic pattern of the region is produced by a distal series of en-echelon structures along strike and of lateral series of rift faults consisting of several parallel distal series of structures. The lateral series of faults in the Olkhon Region includes four successive distal series of structures arranged in the seaward direction as: Primorsky fault zone, Buguldeika-Chernorud graben-Maloye More rift basin-Ushkaniy fault zone, Tazheran Plateau-saddles of Olkhon Island and submerged Akademichesky ridge; Olkhon fault zone. The distal series are cut by transverse faults into a sequence of separate segments with their age increasing progressively northeastward with the youngest ones at the southwestern end. The vertical offset of the faults in the segments increases in the same direction from tens of meters to over 2000 m, and the fault zones become broader and acquire a more complex structure. The Primorsky fault zone changes northeastward from a simple linear fault scarp at its southwestern end to a system of fault-bounded blocks, then to a system of uplifted and subsided blocks (basins), and finally grading into a basin; the rift border faults are changed suddenly into within-basin structures in the same direction. This pattern of morphotectonic structures records the space and time evolution of faults, which are subject to progressive subsidence and broadening to finally develop into basins. Following this tendency, in the conditions of a lake basin, land structures eventually become submerged marine ones. The tendency of submergence of land structures and formation of within-basin structures out of tilted fault-bounded blocks may be a common feature of continental rifting associated with rotational listric faulting and controlled by general rift opening.
The Cenozoic Baikal rift zone is superimposed on the Caledonian Baikal fold belt, representing the suture between the Precambrian Siberian craton and several microcontinents. The Baikal rift zone consists of a system of disconnected fault-bounded basins and extensional and wrench faults that straddles a major arch, having a topographic relief of 2–3 km. Rifting activity commenced during the Oligocène and is still active as evident from high seismicity of the Baikal rift zone.Across the Baikal rift zone, upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8–11 km across the Baikal basins as compared to the adjacent unextended areas. The discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir. Indeed, geophysical data indicate that the Sayan-Baikal dome coincides with such a diapir, the top of which is located at the crust/mantle boundary. This intrusion is held responsible for Oligocene and Quaternary doming of the rift zone. Development of the intra-continental Baikal rift system is thought to be related to asthenospheric diapirism (active rifting); however, intra-plate stresses in conjunction with the Himalayan collision may have played some role.
Within the framework of the Baikal Drilling Project (BDP), a 192 m long sediment core (BDP-96-1) was recovered from the Academician Ridge, a submerged topographic high between the North and Central Basins of Lake Baikal. Sedimentological, clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth, corresponding to ca. 2.4–3.4 Ma. The aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of Northern Hemisphere glaciation in Late Pliocene time. A major climate change occurred in the Lake Baikal area at about 2.65 Ma. Enhanced physical weathering in the catchment, mirrored in the illite to smectite ratio, and temporarily reduced bioproduction in the lake, reflected by the diatom abundance, evidence a change towards a colder and more arid climate, probably associated with an intensification of the Siberian High. In addition, the coincident onset of distinct fluctuations in these parameters and in the Zr/Al ratio suggests the beginning of the Late Cenozoic high amplitude climate cycles at about 2.65 Ma. Fluctuations in the Zr/Al ratio are traced back to changes in the aeolian input, with high values in warmer, more humid phases due to a weaker Siberian High. Assuming that the sand content in the sediment reflects tectonic pulses, the Lake Baikal area was tectonically active during the entire investigated period, but in particular around 2.65 Ma. Tectonic movements have likely led to a gradual catchment change since about 3.15 Ma from the western towards the eastern lake surroundings, as indicated in the geochemistry and clay mineralogy of the sediments. The strong coincidence between tectonic and climatic changes in the Baikal area hints at the Himalayan uplift being one of the triggers for the Northern Hemisphere Glaciation.
To better understand how active deformation localizes within a continental plate in response to extensional and transtensional tectonics, a combined analysis of high-quality gravity (Bouguer anomaly) and seismicity data is presented consisting of about 35000 earthquakes recorded in the Baikal Rift Zone. This approach allows imaging of deformation patterns from the surface down to the Moho. A comparison is made with heat flow variations in order to assess the importance of lithospheric rheology in the style of extensional deformation. Three different rift sectors can be identified. The southwestern rift sector is characterized by strong gravity and topography contrasts marked by two major crustal faults and diffuse seismicity. Heat flow shows locally elevated values, correlated with recent volcanism and negative seismic P-velocity anomalies. Based on earthquake fault plane solutions and on previous stress field inversions, it is proposed that strain decoupling may occur in this area in response to wrench-compressional stress regime imposed by the India–Asia collision. The central sector is characterized by two major seismic belts; the southernmost one corresponds to a single, steeply dipping fault accommodating oblique extension; in the centre of lake Baikal, a second seismic belt is associated with several dip-slip faults and subcrustal thinning at the rift axis in response to orthogonal extension. The northern rift sector is characterized by a wide, low Bouguer anomaly which corresponds to a broad, high topographic dome and seismic belts and swarms. This topography can be explained by lithospheric buoyancy forces possibly linked to anomalous upper mantle. At a more detailed scale, no clear correlation appears between the surficial fault pattern and the gravity signal. As in other continental rifts, it appears that the lithospheric rheology influences extensional basins morphology. However, in the Baikal rift, the inherited structural fabric combined with stress field variations results in oblique rifting tectonics which seem to control the geometry of southern and northeastern rift basins.
We used the Digital Faults geoinformation system that we developed to propose an algorithm for quantitative estimation of
seismic activity on faults. The resulting technique was used to study the spatiotemporal patterns in the present-day activity
of faults in Central Asia. Fault activity was found to vary at frequencies of a few years and cannot be explained by changes
in the regional stress fields. We studied the tendency of seismic events to be localized to areas of dynamic influence due
to faults. The active faults were grouped by the criteria of seismicity organization in the influence areas of these faults.
It was shown that fault activity and its comparatively high frequency on real time scales are caused by strain waves, which
may be generated by interplate and interblock movements in the brittle lithosphere. Judging by the speed of strain waves,
the active faults are classified into groups that differ in their geological and geophysical parameters. They can be used
to estimate the directions of strain wave fronts and to identify areas of dominant fault activation over intervals of real
(geologically speaking) time. We give a map showing active faults in Central Asia, plots of a quantitative index of their
seismic activity, and the directivity vectors of strain waves that excite fault activity. The methods we developed for classifying
active faults by the quantitative index of seismic activity and for determining the vectors of strain waves that excite fault
activity are all tools that significantly expand our possibilities when developing tectonophysical models of the seismic process
in earthquake-generating zones of the lithosphere and open new methods for attacking problems in intermediate-term earthquake
prediction.
Displacement gradients on single fault surfaces are a function of the maximum displacement on a fault and the dimensions of the fault surface. Data on the maximum lateral dimensions (widths) and maximum displacements on normal faults and thrusts, with maximum displacements from 4 mm to 40 km, are used to derive an expression relating width, displacement and material properties. The basis of this expression is a fault growth model in which width is proportional to the square root of displacement. Width/displacement ratios vary systematically with the size of a fault from values of ca 30,000, which are characteristic of a single slip event, to about 10 in the case of thrusts with displacements of 40 km. Rocks from which the fault data are derived have a likely range of shear moduli from ca 0.1 to ca 30 GPa, which is sufficient to account for the range of data.Data on widths and maximum displacements of 308 fault traces recorded on British coalmine plans are shown to be consistent with variation of shear modulus of about half an order of magnitude. Data on 58 further fault traces are shown to be consistent with the fault growth model. Synsedimentary faults may have growth curves characteristically different from those of other faults.It is suggested that the increase in dimensions of a fault is a postseismic process of subcritical crack propagation for which the significant material property is fracture toughness.
The variation of displacement along fifteen traces of minor normal faults was measured in the multilayered Quaternary sediments of Kyushu, Japan. In the diagrams of distance along a fault trace (L) vs displacement (D) two distinct types of faults, a cone-shaped L-D pattern (C-type) and mesa-shaped one (M-type), were detected. Because the L-D pattern is subject to slip-parallel strain in the wall rocks, a D-constant pattern is ascribed to the competent (rigid) material and a D-variable pattern is found in the incompetent material. Therefore, C-type faults are characteristic of homogeneous incompetent materials, whereas M-types are representative of faults that cut through a rigid unit. However, the steep slopes in the flanking sections of M-type patterns indicate that the faulting of a rigid unit should terminate in a strain absorber of incompetent materials. The concept of lithologic control in the L-D pattern is important for the better understanding of faulting processes as well as the localization of faults.
Strike-slip faults in continental crust are shown to exhibit non-linear positive correlation between length and offset. This correlation can be empirically explained as the combined effect of the changes in the growth rates of offset and length during the time of tectonic activity of a fault.
Baikal rift-zone faults range in magnitude from major through regional to local. The major, transcrustal faults of pre-Cenozoic initiation frame the structural pattern of the rift zone. Rifting causes a rejuvenation of all important faults regardless of their original type, many becoming oblique-slip faults. The displacement directions correlate well with the strike of the faults in terms of a single strain field for the region. Amplitudes of vertical and horizontal displacements are discussed. The general directions of the main crustal stresses are shown on a schematic diagram which illustrates the origin of different morphogenic groups of faults, and the main stages of their evolution.
Polya Napryageniya Zemnoi Kory i Geologo-Structurnie Melody ikh Izucheniy (Crustal Stress Fields and their Structural Study)
- Sherman
Sherman, S.I. and Dneprovsky, Yu.I., 1989. Polya Naprya-geniya Zemnoi Kory i Geologo-Structurnie Metody ikh Izucheniy (Crustal Stress Fields and their Structural Study).
Fizicheskie Zakonomernosti Razvitiya Razlomov Zemnoi Kory (Physical Regularities of Evolution of Crustal Faults)
- Sherman
Sherman, S.I., 1977. Fizicheskie Zakonomernosti Razvitiya Razlomov Zemnoi Kory (Physical Regularities of Evolu-tion of Crustal Faults). Nauka, Novosibirsk, 101 pp. (in Russian).
Fault system of the Lake Tanganyika rift at the Kigoma area, Western Tanzania
- F Yairi
- Mizutani
- Sh
Yairi, F. and Mizutani, Sh., 1969. Fault system of the Lake Tanganyika rift at the Kigoma area, Western Tanzania. J. Earth Sci. Nagava IJniv. 17: 71-95.
Fault system of the Lake Tanganyika rift at the Kigoma area
- Yairi