Vladimir San'kov

Present‐day stress field changes along the Baikal rift and tectonic implications

Intraplate extension, in a frame of a global compressional 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 SHmax 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 of the 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 SHmax 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 N100°E) slip vector, meanwhile the North Baikal Rift (NBR) exhibits 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.

Seismicity, active faults and stress field of the North Muya Region, Baikal Rift: New insights on the rheology of extended continental lithosphere

The eastern Baikal rift is characterized by a succession of young en echelon half-grabens distributed in a 300-km-wide zone of high seismic activity. In this study we present the overall seismotectonic setting of the north Muya region, which is an along-strike transfer zone between two en echelon dip-slip fault systems. Using both regional and local seismological networks, we refine the hypocenters and the single-event focal mechanisms of 704 earthquakes recorded during 3 years, including the main period of activity of a very dense cluster, the Angarakan swarm. Our final hypocenter locations, obtained after determining a region-specific velocity model and applying a master event technique, allow us to observe a widespread and unusual seismic deformation throughout the upper 30 km of the continental crust. Deep earthquakes (15–30 km) define a planar surface related to a large north-dipping basement fault (Upper Muya), and also occur in a small cluster at 23 km depth at the intersection of two major structural trends. Along more than 150 km of strike length from west to east, maximum focal depths continuously increase from 20 to 30 km, whereas fault dip concomitantly increases from 30° to 55°. On the other hand, most activity of the Angarakan swarm is confined in the “classical” depth range of 0–15 km and depicts a narrow and steeply south-dipping seismic band related to a very recent fault scarp at the surface (eastern Kovokta). This fault geometry, which is observed at depth and in the field, and the opposite tilting of regularly 30-km-spaced horsts and grabens, reveal an asymmetrical rift system. The Agreement of fault lengths estimated from field studies with rupture lengths estimated from large historical earthquakes strengthens the inference that large earthquakes may occur near the base of the seismogenic layer of the crust, i.e., 30 km. Most of the 39 focal mechanisms determined in the study region show a dominant normal dip-slip displacement on nodal planes striking in the main inherited structural direction WSW-ENE, whereas some limited pure strike-slip faulting mainly occurred in the highly fractured Angarakan zone. From inversion of 37 focal mechanisms, we deduce a tensional stress tensor with a σ3 axis striking N160° and a σ2 axis slightly compressional (shape factor R of 0.4). Comparing these results with stress tensors computed along the eastern Baikal rift and with the focal mechanisms of three strong earthquakes that occurred in the Muya region during this century shows that small-magnitude earthquake data give consistent and accurate constraints on the regional stress regime. Whether this present stress tensor remained stable since the beginning of opening of the Baikal rift and during Quaternary is unknown. Our results confirm that continental lithosphere submitted to rapid extension in an early stage of rifting may retain significant rigidity.

Crustal deformation in the Baikal Rift from GPS measurements

Three years and four campaigns of Global Positioning System (GPS) measurements (1994–1997) in the Baikal rift zone, largest active continental rift system in Eurasia, show crustal extension at a rate of 4.5±1.2 mm/yr in a WNW-ESE direction. A comparison with moment release of large historical earthquakes suggests that elastic strain is currently accumulating in the Baikal rift zone along active faults that currently have the potential for a M=7.5 earthquake. The GPS-derived extension rate in the Baikal rift zone is at least two times greater than the prediction of most deformation models of Asia. This result could reflect the dynamic contribution of the Pacific-Eurasia subduction to intracontinental deformation in Asia, in addition to the effect of the India-Eurasia collision.

How old is the Baikal Rift Zone? Insight from apatite fission track thermochronology

Apatite fission track analysis (AFTA) data are used to bring new light on the long-term and recent history of the Baikal rift region (Siberia). We describe the evolution of the topography along a NW-SE profile from the Siberian platform to the Barguzin range across the Baikal-southern Patom range and the northern termination of Lake Baikal. Our results show that the Baikal-Patom range started to form in the Early Carboniferous and was reactivated in Middle Jurassic-Lower Cretaceous times during the orogenic collapse of the Mongol-Okhotsk belt. Samples located in the Siberian platform recorded a continuous sedimentation up to the early Carboniferous but remain unaffected by later tectonic episodes. The Barguzin basin probably started to form as early as Late Cretaceous, suggesting a continuum of deformation between the postorogenic collapse and the opening of the Baikal Rift System (BRS). The initial driving mechanism for the opening of the BRS is thus independent from the India-Asia collision. AFTA show a late Miocene-early Pliocene increase in tectonic extension in the BRS that confirms previous thoughts and might reflect the first significant effect of the stress field generated by the India-Asia collision.

Geometry and rate of faulting in the North Baikal Rift, Siberia

We present a detailed morphotectonic analysis of late Quaternary faulting in the North Baikal Rift (NBR), a region characterized by ranges and basins distributed over more than 800 km along strike in eastern Siberia. Remote sensing techniques (SPOT, METEOR scenes, and aerial photographs) are used to map the active fault network which displays a general en echelon distribution from the northern Lake Baikal to the easternmost basin, with ∼30-km-spaced overstepping segments of 10–80 km in length. Most faults have a dominant dip-slip component over their Cenozoic history. The inherited crustal fabric strongly influences the overall geometry of the rifted basins. We use 54 14C ages of postglacial terraces near the foot scarps of the Muya basin to date offsets measured inside alluvial fans. The last main postglacial event in this area appears to be the early Holocene optimum dated at ∼10 ± 2 ka, following the onset of deglaciation at ∼13 ka. Using these time constraints, a detailed leveling across two terraces offset by the Taksimo fault (West Muya basin) shows consistent minimum vertical slip rates of 1.6±0.6 mm yr−1. Using 30 other active scarps analyzed in the field, we find a lower bound for horizontal velocity of 3.2±0.5 mm yr−1 across the NBR, a rate close to the one found in the southern rift from Global Positioning System measurements. We then compare directions of slip vectors from Holocene field data and slip directions from earthquake fault plane solutions: although local discrepancies appear, the mean directions of lesser horizontal stress (σ3) inverted from theses values are ∼N130°E and ∼N155°E, respectively, which are comparable within uncertainties and favor a rifting obliquity of ∼30°–40°. Extrapolating our Holocene rates, we estimate basin ages younger than those generally believed (less than 7 Ma) and propose a spatial and temporal evolution of rifted basins consistent with experimental models of oblique rifting. Total amounts of extension and vertical throw (∼7 and ∼12 km, respectively) across major faults appear rather constant from the central to the northern rift. These results favor a progressive development of asymmetric grabens in a rift zone that widens with times and they indicate a strong rheological control on deformation which seems enhanced by other contributions than the far-field effects of the Indo-Eurasian collision.

Velocity Structure of the Lithosphere on the 2003 Mongolian-Baikal Transect from SV Waves

The S wave velocity distribution in the Earth’s crust and the first two hundred kilometers of the upper mantle is inferred from data of a seismological linear network including 18 broadband stations installed in the framework of the international teleseismic experiment carried out in 2003 in the south of Siberia and in Mongolia. Models were constructed by using P-to-S received function inversion beneath each station. Vertical cross sections of S wave velocities from the surface to depths of 65 and 270 km covering the entire 1000-km profile are constructed by the linear spline interpolation of individual velocity models. The vertical sections are also represented as anomalies relative to the standard velocity model. The most intense low velocity anomalies (from −3 to −6%) in the crust and upper mantle are identified beneath the Sayan, Khamar-Daban, and Khangai highlands and the Djida fold zone and agree both with other geophysical data and with the distribution of Late Cenozoic volcanic fields. The results of this work suggest that the activation of Mongolian-Siberian highlands is largely connected with uplift of the asthenosphere to the base of the crust.

Interaction compression–extension à la limite Mongolie–Sibérie : analyse préliminaire des déformations récentes et actuelles dans le bassin de Tunka

Abstract The Tunka basin was initiated during Oligocene, under transtensional regime (normal-sinistral) as shown by large-scale structures and geomorphology. Nevertheless, a preliminary analysis of the most recent deformations allows us to evidence transpression on several sites within the basin. These tectonic features together with focal mechanisms and preliminary GPS data, suggest that the kinematics of the Tunka basin has undergone a very recent change, which could be due to the northward propagation of the India–Eurasia collisional strain field.

Contemporary horizontal movements and seismicity of the south Baikal Basin (Baikal rift system)

The contemporary horizontal movements and deformations in the central and southern parts of the Baikal depression are analyzed, and their relationship with contemporary seismicity is studied. Based on the long-term measurements by the Baikal geodynamical GPS monitoring network, the refined estimate is obtained for the velocity of the divergence of the Siberian and Transbaikalian blocks, which is found to occur in the southeastward direction (130°) at 3.4 ± 0.7 mm per annum. This agrees with the parameters of the long-term extension component estimated from the geological data and with the direction of extension determined from the seismic data. The distribution of the displacement velocity across the strike of the rift, which gradually increases from one block to another, suggests a nonrigid behavior of the continental lithospheric plates at the divergent boundary. About 30% (1.0–1.5 mm per annum) of the total increase in the velocity is accommodated by the Baikal Basin. The strain rate within the trough reaches 3.1 × 10−8 yr−1 and decreases on either side across the structure. The character of distribution of the horizontal displacement velocities on the Baikal divergent boundary between the Eurasian and Amurian plates favors the model of passive rifting. The zones of highly contrasting topography and increased seismicity are localized within the area of contemporary deformations, and the seismic moment release rate directly depends on the strain rate. Here, the rate of the seismic moment release rate makes up a few percent of the geodetic moment accumulation rate calculated by the approach suggested by Anderson (1979). Based on the coherence between the graphs of the rates of geodetic moment accumulation and seismic moment release rate by the earthquakes with M ≥ 5.0 during the historical and instrumental observation periods, the contemporary seismic hazard for the South Baikal Basin could be assessed at a level of seismic event with M = 7.5–7.6.

Coupling of late cenozoic faulting of the Siberian Platform margin and Baikal Rifting

The relationship between the Late Cenozoic tectonic deformations at the Siberian Platform (SP) margin and the adjacent Baikal Rift System is considered. This study was aimed at estimation of the stress state and conditions of neotectonic reactivation of faults in the sedimentary cover of the eastern Irkutsk Amphitheater (the Angara‐Lena Uplift and Fore-Baikal Trough coupling zone). On the basis of geological, structural, and geomorphic methods, the Late Cenozoic fault assemblages were investigated and the related stress field was reconstructed to provide evidence for interrelated deformation of the Baikal Rift and the SP margin. The dynamic interaction of the SP block with the surrounding fold systems at the neotectonic stage and the evolution of the Late Cenozoic faulting remain poorly studied. A.G. Zolotarev developed the concept of fore-rift troughs [5 among others]. The brittle fracturing of the upper Pleistocene sediments was documented in the Fore-Sayan Foredeep [1, 8]. The existence of recent seismic activity of the platform has been proven in [2]. Seminsky et al. [8 and others] attempted to demonstrate the interrelation between the seismic activity and deformations in the Baikal Rift. However, these issues remain insufficiently studied because of short-term instrumental measurements and the absence of seismic station network on the platform. The northeastern part of the Irkutsk Amphitheater of the SP is characterized by medium-intensity neotectonic uplifting with an amplitude of 800 m relative to the initial surface [4]. Such values have been estimated for the Angara‐Lena Uplift. The Fore-Baikal Trough, which extends on the eastern side along the SP margin, ascended less intensely (amplitude of vertical movements was not greater than 200 m). The difference in the present-day hypsometric position of these structures suggests a rather high velocity gradient of neotectonic vertical movements in their coupling zone, providing prerequisites for differential motions along fault zones. The schematic map of faults in the Fore-Baikal Trough and Angara‐Lena Uplift coupling zone (Fig. 1) is based on the interpretation of satellite images and aerial photographs, analysis of a digital 3D model of topography, results of field geostructural and geomorphic observations, and compilation of the State Geological Mapping data. The faults in the study territory make up a rather dense network in comparison with inner sectors of the SP, probably owing to the closeness with the Sayan‐ Baikal mobile region. Paleozoic folds and faults significantly controlled the development of local neotectonic structures. The northern part of the territory is dominated by submeridional faults that reflect the general strike of older linear structures in the study region (Kirenga fold zone [3]). The NE-trending faults inherit structures of the Zhigalovo‐Tulukmur fold zone. The neotectonic reactivation stage is characterized by nonuniform manifestation over the study area (Fig. 1). Judging from the density of neotectonic faults, the northern Fore-Baikal Trough and the zone of its coupling with the Angara‐Lena Uplift were most active. The maximums of fault density are confined to the Khanda (southern end), Baldakhin’ya, and Novoselovo basins. The platformal sedimentary cover in the northern Angara‐Lena Uplift is marked by minimum faulting.

Active Faulting in the Earth’s Crust of the Baikal Rift System Based on the Earthquake Focal Mechanisms

The destruction of the lithosphere with the formation of fault zones is one of the leading geological processes determining the structure of the continents, both in the past and at the present stage. Seismicity providing information on the structure and dynamics of formation of large fault zones in real time reflects the modern fault formation in the crust. For its study, both the epicentral field of earthquakes (see, for example, Sherman 2009) and the data on the position of their hypocenters are actively used (see, for example, Kaven and Polland 2013). To determine the orientation of modern faults of various orders, one can also use data on the earthquake focal mechanism solutions preliminarily distinguishing the true fault planes in the source. In the case of strong earthquakes, the geological data (the outcrop of the fault on the surface, the existing of faults with similar geometry, etc.), the data on the orientation of the aftershock field, the shape of the first isoseits, and other data are indirect features that help to choose one plane or another as the true fault plane. These approaches are inapplicable in study of weak earthquakes (magnitude M ≤ 4.0) and the only information available on them is concerned with their waveforms.