The basic parameters of acceleration spectra for M > 5 earthquakes in the Baikal Rift Zone

We consider the acceleration spectra of S waves from M = 5-6.5 earthquakes that occurred in three regions of the Baikal Rift Zone. The main characteristics of the shape and level of the spectra are given, and the difference between the spectra in the relative positions of the focus and seismic station is shown. Average acceleration spectra have been obtained for all three regions with regard to shape and the magnitude of the earthquakes. The spectra have been transformed so that they correspond to the accelerations of M = 6.5 and M = 7.5 earthquakes. The conclusion is made that the spectra not necessarily retain their shape even with the same movements in the focus.     

New data on the late holocene seismicity of the southwestern edge of the Baikal Rift Zone

Three strong earthquakes that occurred since the end of 8th century have been identified by archeoseismic data in the southwestern part of the Tere Khol’ Depression. The dates for these events are the 9th, 12th, and first two-thirds of 19th century, and the average recurrence interval is 500 years. The relative seismic passivity of the Tere Khol’ Depression at present may be related to the relatively recent discharge of stress in seismogenerating sources.     

Eclogite trace in evolution of Late Cenozoic alkaline basalt volcanism on the southwestern flank of the Baikal Rift Zone: Geochemical features and geodynamic consequences

Eclogitized material from the oceanic lithosphere are the most likely source of alkaline basalt magmas in the formation of Late Cenozoic volcanic areas on the southwestern flank of the Baikal Rift Zone. Basaltic trachyandesites of the early stage of volcanism (Pg₃² ~ 28–23 Ma) are rich in high field strength elements (HFSE), P₂O₅, F, Zn, Ga, Sr, Sn, and light rare earth elements (LREE); they are characterized by high values of the following ratios: Fe/Mn = 72–77, Sm/Yb = 7.7–8.5, Sr/Y = 57–63, and Ga/Sc = 2.1–2.3. At this stage, magmas are formed under conditions with a 2–8% degree of partial melting of the mantle substrate enriched with the material of the eclogite source (50–70%) (Cpx/Grt = 1.5–1.7). Basaltoid magmas of the final stage of volcanism (N₁³–N₂¹ ~ 6–4 Ma) are formed from melting (1.5–4%) of a less fertilized mantle (Cpx/Grt = 2.1–3.1, Fe/Mn = 62–71, Sm/Yb = 3.5–4.6, Sr/Y = 29–44, Ga/Sc = 1.0–1.4). The directed variations of the compositions of the successive basaltoid magmas, which were formed in the Late Cenozoic, create an “eclogite trace” in this area.     

Heat flow and gas hydrates of the Baikal Rift Zone

Multi-channel seismic studies (MCS), performed during a Russian expedition in 1989 and a joint Russian-American expedition in 1992, have for the first time revealed a “bottom simulating reflector” (BSR) in Lake Baikal. These data have shown that gas hydrates occur in the southern and central basins of Lake Baikal in those places where the water depth exceeds 500–700 m. Four types of tectonic influence on the distribution of the gas hydrate were revealed: (a) Modern faults displace the BSR as they do with normal seismic boundaries. (b) Older faults displace normal reflectors, whereas the BSR is not displaced. (c) Modern faults form zones, where the BSR has been totally destroyed. (4) Processes that occur within older fault zones situated close to the base of the hydrated sediment layer lead to undulations of the BSR. The thickness of the hydrate stability field (inferred from seismic data) ranges between 35 and 450 m. Heat-flow values determined from BSR data range from 48 to 119 mW/m². A comparison between heat-flow values from BSR data and values measured directly on the lake bottom shows an overall coincidence. Changes in water level and bottom-water temperature that occurred in the past have had no noticeable influence on the present BSR depths or heat-flow values. Determination of deep heat flow from BSR data is in this case more reliable than by direct measurements. Received: 10 December 1998 / Accepted: 15 November 1999     

Variations of stress fields in the Tunka Rift of the southwestern Baikal region

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 Hövsgöl 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.     

Geothermal field and distribution for earthquake source depths in the Baikal Rift Zone

A comparison between values of thermal flux and the deep temperatures calculated by them and the depths of earthquake sources in three areas of the Baikal Rift Zone is made. It has been shown that during transit from the Baikal depression to the adjacent mountain massif, the thermal flux decreases almost 2–3 times. The corresponding deep temperatures decrease to a similar degree. The available data for these areas on earthquake depths show that their lower boundary both beneath the depression and the massif is located at almost the same depth, which is about 20 km. In this paper, the conclusion is made that the cause of the absence of an interrelation between thermal and seismic fields lies in the discrepancy between the measured values of thermal flux and its deep values. This discrepancy arises because conductive heat transfer in the upper part of the Earth’s crust, up to 5–10 km depth, is highly distorted by heat-and-mass transfer of ground waters. In the middle part of the crust, the difference in temperatures beneath depressions and ridges is leveled horizontally, which is reflected in almost the same depth for the basement of the seismogenerating layer beneath these main rift structures.     

Investigation of Migrating Seismicity in the Lithosphere of the Baikal Rift Zone

Doklady Earth Sciences - Based on “Catalog of earthquakes in the Baikal Region” (52700 earthquakes of representative energy classes KP > 8, 1964–2013) at the angular sector...     

A geoelectrical model of the Barguzin basin in the Baikal Rift Zone

We processed data from geophysical survey archives of the 1950s acquired in intermontane basins in the Baikal rift, including a large collection of vertical electric soundings (VES) from the Barguzin basin, which remained only partly interpreted. The processing and reinterpretation became possible with the advanced computing facilities and software for forward modeling and inversion, and GIS tools. We estimated the electrical parameters and modeled the complex block structure of the uppermost basement and sediments beneath the Barguzin basin.     

Dynamic causes for the opening of the Baikal Rift Zone: a numerical modelling approach

Previous dynamic models of the Baikal Rift Zone (BRZ) are mostly two-dimensional on vertical plane. In this study, a numerical model of neotectonics in the region on map view was constructed using the adapted PLATES program. The present work is an attempt to test different mechanisms for opening Baikal Rift by comparing the modelled and observed stress and strain rate fields. The following rifting scenarios were tested: (1) pure northwest–southeast extension, (2) pure northeast–southwest compression, (3) oblique rift opening and (4) combined northwest–southeast extension and northeast–southwest compression. The models are calibrated using geologically and GPS-derived strain rates and stress-tensor determinations from fault-slip data and earthquake focal mechanisms. The most successful model requires a combination of NE–SW compression and orthogonal extension. The model results indicate that the present extensional regime in BRZ can be explained by combining the India plate indentation northward into Eurasia, east–west convergence between the North America and Eurasia plates and southeastward extrusion of the Amur plate in northeastern Asia. Predicted fault-slip rates for the best-fit model are consistent with the observed Holocene fault-slip rates in the Lake Baikal region. The generally accepted rotation of the Amur and Mongolia microplates are used as independent constraints for the choice of the best-fit model. These data correlate well with the predicted direction of rotation in our best model.     

Seismic process in the lithosphere of the Baikal Rift Zone: Synchronization episodes

Doklady Earth Sciences -     

Hydrogeochemical peculiarities of the composition of nitric thermal waters in the Baikal Rift Zone

The chemical, gas, and isotopic compositions of nitric thermal waters in the Baikal Rift Zone are considered. It is shown that the behavior of sulfate and carbonate ions in hydrothermal systems is different, which indicates that they are of two different origins. The studied thermal waters are of five chemical types formed in different geologic conditions. Special attention is given to the genesis of hydrotherms, the geologic and geomorphologic conditions of their recharge, and their equilibrium with rocks. It has been established that most of chemical elements of the waters migrated from rocks, but a significant portion of them is bound by secondary minerals, which results in their deep differentiation, accumulation, or precipitation. Thus, the so-called redundant elements appear, which were earlier considered to be of mantle origin.     

Prehistoric earthquakes and fault scarps in the Barguzin fault zone (Baikal Rift system)

We studied fault scarps produced by prehistoric earthquakes in the Barguzin fault zone and estimated the ages and the magnitudes of the scarp-forming events in each scarp segment from their structure, morphology, and scarp parameters. Morphological and structural data reveal two to four surface-rupturing events with displacement up to 5–9.5 m, and two events showed ¹⁴C ages of 4.5 and 9 kyr. The area of the Barguzin Fault may have experienced six 7.5 ≤ M < 8.0 earthquakes and two M ≥ 8.0 (8.0–8.2) events for the past 10–12 kyr.     

Seismotectonics of the transitional region from the Baikal Rift Zone to orogenic rise of the Stanovoi range

This paper is based on the data obtained during the field study of active faults carried out in 2005–2006 in the Chita and Amur oblast and South Yakutia in connection with detailed seismic demarcation of the projected East Siberia-Pacific Ocean pipeline route. The comprehensive geomorphic and geophysical fieldwork was focused on paleoseismogeology and accompanied by trenching in the zones of reactivated faults. These works allowed us to specify the available information on the present-day structure, seismotectonic regime, and potential seismic hazard of the conjugation of the Baikal Rift Zone and the arched-block rise of the Stanovoi Ridge.     

Major factors of the evolution of basins and faults in the Baikal Rift Zone: Tectonophysical analysis

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.     

贝加尔裂谷带中部的新构造特征及其成因分析

位于贝加尔裂谷带中部的滨奥里洪地区主要包括3个大地构造单元:西部滨海山脉、滨奥里洪高原和小海地堑.笔者着重对该地区中新世以来的新构造活动特征,特别是对第四纪夷平面的变化、断层的发育、河流的变迁、河流阶地的发育等进行了研究,表明该地区在中新世以来具有强烈的拉张运动与升降运动特征,是贝加尔裂谷带新构造运动表现最强烈的地区.新生代贝加尔裂谷带的发育受印度板块-欧亚板块碰撞、西太平洋俯冲引起弧后扩张作用的影响;中部滨奥里洪地区受到NW-SE向拉张作用,发育了强烈的新构造.     

Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia

Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone between the Central and South basins of the lake. Incorporating one of the world's largest lacustrine deltas, this depositional system is a classic example of the influence of rift basin structural segmentation on a major continental drainage. More than 3700 km of deep basin-scale multi-channel seismic reflection (MCS) data were acquired during the 1989 Russian and the 1992 Russian–American field programs. The seismic data image most of the sedimentary section, including pre-rift basement in several localities. The MCS data reveal that the broad bathymetric saddle between these two major half-graben basins is underlain by a complex of severely deformed basement blocks, and is not simply a consequence of long-term deltaic deposition. Maximum sediment thickness is estimated to be more than 9 km in some areas around the Selenga Delta. Detailed stratigraphic analyses of the Selenga area MCS data suggest that modes of deposition have shifted markedly during the history of the delta. The present mode of gravity- and mass-flow sedimentation that dominates the northern and southern parts of the modern delta, as well as the pronounced bathymetric relief in the area, are relatively recent developments in the history of the Lake Baikal Rift. Several episodes of major delta progradation, each extending far across the modern rift, can be documented in the MCS data. The stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the Baikal Rift. An age model has been established for this stratigraphy, by tying the delta sequences to the site of the Baikal Drilling Project 1993 Drill Hole. Although the drill hole is only 100 m deep, and the base of the cores is only ∼670 ka in age, ages were extrapolated to deeper stratigraphic intervals using the Reflection-Seismic-Radiocarbon method of Cohen et al. (1993). The deep prograding delta sequences now observed in the MCS data probably formed in response to major fluctuations in sediment supply, rather than in response to shifts in lake level. This stratigraphic framework and age model suggest that the deep delta packages developed at intervals of approximately 400 ka and may have formed as a consequence of climate changes affiliated with the northern hemisphere glaciations. The stratigraphic analysis also suggests that the Selenga Basin and Syncline developed as a distinct depocentre only during the past ∼2–3 Ma. Received: 1 December 1999 / Accepted: 26 January 2000