Active faults, seismicity and recent fracturing in the lithosphere of the Baikal rift system

A map of major active faults has been constructed for the Baikal rift system (BRS). Recent active faults are identified using seismological data. The BRS seismicity of the past 40 years is statistically analyzed. Areas of a “stable” concentration of epicenters are revealed. On this basis, a zone of recent fracturing of the lithosphere is identified and its relation to active and developing faults of the BRS is analyzed. The zone of the lithosphere fracturing is a major tectonic structure, which controls both the recent seismic process and the reactivation of ancient faults. It is demonstrated that the available seismological data can provide a basis for a detailed classification of faults by degree of their tectonic activity. Regularities in the distribution of strong earthquakes along the zone of the recent fracturing of the lithosphere are established, as well as regularities in the distribution of strong and weak seismic events relative to transform and other faults. The degree of the fault reactivation is determined by their spatial closeness to the axial zone of the recent rupturing of the lithosphere.     

The Baikal system of rift valleys

The Baikal system of rift valleys has no evident structural connections with the World Rift System. The peculiar features of its structure, morphology and volcanicity reflect this isolation. The spatial position and major structural features of the system are determined where the central segment (the South Baikal depression) is confined to the junction of two major lithospheric plates, the Precambrian Siberian platform and the heterogenous folded framework of Sayan—Baikal. The contrasting structures and thermodynamic conditions of these two plates, and the deep nature of the suture zone developed between them, have been responsible for the location of crustal extension and proto-rift formation within the Baikal depression proper, first initiated not later than Eocene and then propagating to zones both west and northeastwards.     

New data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system

The investigation data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system obtained with a seismic coda envelope and sliding window are considered. Eleven local districts were described by one-dimensional attenuation models characterized by alternation of high and low attenuation layers, which are consistent with the results obtained previously by Yu.F. Kopnichev for the southwestern flank of the Baikal rift system [9]. The subcrust of the lithosphere contains a thin layer with high attenuation of seismic waves likely related to higher heterogeneity (fragmentation) and occurrence of fluids. The lithosphere basement depth varies from 100–120 km in the west within the Baikal folded area to 120–140 km in the east within the Siberian Platform. It is concluded that there are two asthenosphere layers. Based on specific features of the lithosphere and upper mantle structure, it can be assumed that they were subject to gradual modification involving fluidization processes and partial melting in the Late Cenozoic extension under the influence of distant tectogenesis sources.     

Attractor structures of riftogenesis as an attribute of the cenozoic stage in evolution of the lithosphere within the Baikal rift system

Doklady Earth Sciences -     

山西断陷带南部三维P波速度结构及地震分布特征

本研究使用山西测震台网记录的2010年1月～2019年12月地震观测数据,使用TomoDD方法,反演得到了山西断陷带南部（110°～114°E,34.5°～38.5°N）分辨率为0.2°的三维P波速度结构以及该区域地震重定位结果。反演结果显示：研究区的地壳速度结构与该区域的地表地质构造和沉积作用有关,5～10 km太原盆地、临汾盆地显示明显V_p低速分布,灵石隆起是以沉积作用为主导的地质活动,存在较大范围的沉积物,在5～10 km同样显示低速分布;峨眉山地台、吕梁山脉、太行山山区显示高速分布。而吕梁山脉在10 km以上为低速分布,可能与大同火山的上地幔岩浆构造活动有关;太原盆地自15 km深度不再延续5～10 km的低速分布而显示高速分布,说明太原盆地不受大同火山区上地幔构造活动影响,受青藏高原的推挤作用形成的可能性更大。重定位结果显示：地震丛集在断陷带内分布,震源深度集中在0～30 km。太原盆地内地震丛集事件发生在太原盆地北部,深度集中在20～25 km之间,速度剖面显示位于低速向高速转换区域内;交城断裂的应力集中以及介质结构的高低速变换是太原北部地震从集的主要原因。运城盆地内地震分布除盐湖序列外没有明显的丛集性。2016年3月12日发生的M_L4.8盐湖序列,主震发生在低速向高速过度区域内,其余震震源深度较主震浅,且基本发生在下方存在高速分布的低速区域内。盐湖序列M_L4.8主震的震源机制解与附近中条山北麓断裂的高角度正断层性质一致,说明主震受中条山北麓断裂活动影响。余震震源类型复杂,其中,逆断和逆断兼走滑机制与该地区区域背景应力场不符。综合机制解和速度结构的结果认为盐湖序列的发生机理较复杂,可能受该区域介质结构、隐伏断裂分布等综合作用,还需进一步研究。     

The Baikal rift zone: the effect of mantle plumes on older structure

The main chain of SW–NE-striking Cenozoic half-grabens of the Baikal rift zone (BRZ) follows the frontal parts of Early Paleozoic thrusts, which have northwestern and northern vergency. Most of the large rift half-grabens are bounded by normal faults at the northwestern and northern sides. We suggest that the rift basins were formed as a result of transformation of ancient thrusts into normal listric faults during Cenozoic extension.Seismic velocities in the uppermost mantle beneath the whole rift zone are less than those in the mantle beneath the platform. This suggests thinning of the lithosphere under the rift zone by asthenosphere upwarp. The geometry of this upwarp and the southeastward spread of its material control the crustal extension in the rift zone. This NW–SE extension cannot be blocked by SW–NE compression generated by pressure from the Indian lithospheric block against Central Asia.The geochemical and isotopic data from Late Cenozoic volcanics suggest that the hot material in the asthenospheric upwarp is probably provided by mantle plumes. To distinguish and locate these plumes, we use regional isostatic gravity anomalies, calculated under the assumption that topography is only partially compensated by Moho depth variations. Variations of the lithosphere–asthenosphere discontinuity depth play a significant role in isostatic compensation. We construct three-dimensional gravity models of the plume tails. The results of this analysis of the gravity field are in agreement with the seismic data: the group velocities of long-period Rayleigh waves are reduced in the areas where most of the recognized plumes are located, and azimuthal seismic anisotropy shows that these plumes influence the flow directions in the mantle above their tails.The Baikal rift formation, like the Kenya, Rio Grande, and Rhine continental rifts [Achauer, U., Granet, M., 1997. Complexity of continental rifts as revealed by seismic tomography and gravity modeling. In: Jacob, A.W.B., Delvaux, D., Khan, M.A. (Eds.), Lithosphere Structure, Evolution and Sedimentation in Continental Rifts. Proceedings of the IGCP 400 Meeting, Dublin, March 20–22, 1997. Institute of Advanced Studies, Dublin, pp. 161–171], is controlled by the three following factors: (i) mantle plumes, (ii) older (prerift) linear lithosphere structures favorably positioned relative to the plumes, and (iii) favorable orientation of the far-field forces.     

Seismicity of the Baikal rift system from regional network observations

In the paper we report the state-of-the-art of seismicity study in the Baikal rift system and the general results obtained. At present, the regional earthquake catalog for fifty years of the permanent instrumental observations consists of over 185,000 events. The spatial distribution of the epicenters, which either gather along well-delineated belts or in discrete swarms is considered in detail for different areas of the rift system. At the same time, the hypocenters are poorly constrained making it difficult to identify the fault geometry. Clustered events like aftershock sequences or earthquake swarms are typical patterns in the region; moreover, aftershocks of M ⩾ 4.7 earthquakes make up a quarter of the whole catalog. The maximum magnitude of earthquakes recorded instrumentally is M_LH7.6 for a strike-slip event in the NE part of the Baikal rift system and M_LH6.8 for a normal fault earthquake in the central part of the rift system (Lake Baikal basin). Predominant movement type is normal faulting on NE striking faults with a left lateral strike-slip component on W–E planes. In conclusion, some shortcomings of the seismic network and data processing are pointed out.     

Seismotectonics of the Baikal rift zone

High seismicity in the Baikal rift zone is controlled by the development of conjugate rising and subsiding block structures. Many types of seismological phenomena resulting from large earthquakes are manifested in the rift zone and include seismotectonic (regional, zonal and local), gravity-seismotectonic and seismogravitational deformations. Impulsive as distinct from gradual seismogenetic crustal movements play a dominant role in the recent development of the Baikal geomorphology.     

Faults of the Baikal rift zone

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.     

Cenozoic lithogenesis in the Baikal rift zone

New data on the geological history and Cenozoic lithogenesis in depressions of the Baikal rift zone are considered with areas adjacent to Lake Baikal as example. In this region, rifting developed during the plain (Late Oligocene?Early Pliocene) and orogenic (Late Pliocene?Holocene) stages and was accompanied by the accumulation of plain coaliferous fan and orogenic molasses formations, respectively. The examination of Quaternary sequences in the Baikal region reveals that deposition and postsedimentary transformations of riftogenic sediments were intensely influenced by deep-seated water sources of the so far undivided stratal?infiltration, elision, and exfiltration types, according to the classification in (Kislyakov and Shchetochkin, 2000). Deep processes in this region determined the elevated heat flow, volcanism, and extensive discharge of hydrothermal solutions and gas fluids. In our opinion, gaseous?hydrothermal activity stimulated the formation of hydrothermal?sedimentary rocks (dolomitic and calcitic travertines, geyserites, aluminosulfates), the accumulation of diatomaceous and carbonaceous oozes in Baikal, and the formation of a large methane gas hydrate deposit.     

GPS-measurements of recent crustal deformation in the junction zone of the rift segments in the central Baikal rift system

Based on multiyear measurements of present-day motions in the central area of the Baikal rift system, new data on the kinematics of horizontal motions, relative horizontal deformation rates, and rotation velocities in the area of junction of the South Baikal, North Baikal, and Barguzin rift basins have been obtained. This area is an intricate structure with two transfer zones: Ol’khon–Svyatoi Nos and Ust’-Barguzin.It is shown that crustal blocks are moving southeastward, normally to the structures of transfer zones and at an acute angle to the Baikal Rift strike, which corresponds to the right-lateral strike-slip extensional faulting along the major structure. The average horizontal velocities increase from 3.0 mm yr–1 in the northern South Baikal basin to 6.5 mm yr–1 in the Barguzin basin. The elongation axes prevailing in the study region are mainly of NW–SE direction. The areas of intense deformations are confined to structures with high seismic activity in the South Baikal and, partly, Barguzin basins. This confirms the existence of a present-day zone of the Earth’s crust destruction in the Baikal rift system, which is the most likely source of strong earthquakes in the future. Two zones with rotations in opposite directions are recognized in the rotation velocity field. Clockwise rotation is typical of structures of N–NE strike (Maloe More basin, southern North Baikal basin, Barguzin Ridge rise). Counterclockwise rotation is determined for NE-striking structures (northern South Baikal basin, southern Barguzin basin). In general, the obtained data show an intricate pattern of present-day horizontal dislocations and deformations in the area of junction of NE- and N–NE-striking rift structures. This suggests left- and right-lateral strike-slip faults, respectively, within them.     

Hierarchy of earthquakes in the Baikal rift system: implications for lithospheric stress

We investigated space-time lithospheric stress patterns of the Baikal rift system according to the hierarchy of earthquakes using mechanisms of 265 K_P ≥ 10 events recorded from 1950 to 1998 and seismic moments of 802 K_P ≥ 11 events from 1968 to 1994. The lithosphere of the region was confirmed to undergo rifting with mostly normal-slip events, while local areas of frequent strike-slip and reverse motions may record stress heterogeneity. The dominance of rifting, although being evident in the stress dynamics, is unstable, which is indicated by increase in strike-slip and reverse motions to as many as normal slip events in the latest 1980s–earliest 1990s. The lithospheric stress patterns inferred from seismic-moment data are generally consistent with those derived from the classical focal mechanism method. The suggested approach of seismic zoning according to earthquake slip geometry may provide a more reliable background for successful mitigation of seismic hazard in the region.     

Oligocene deposits of the Baikal rift valley

New data are reported on the stratigraphy of the Oligocene deposits of Lake Baikal (the Tankhoi field, the outcrops near the mouths of the Osinovka, Polovinka, and Klyuevka Rivers). Detailed paleontological analysis of the key sections on the continental part of the Baikal eastern coast revealed four floristic horizons that could be used as indicator horizons and showed that the Tankhoi Formation formed throughout the Oligocene and at the early Early Miocene. Lithologically, blue vivianite clays and coal beds could be used as the most important indicator horizons. Formation of the deposits began after a long stratigraphic break from the Early Cretaceous to the Early Oligocene. The deposits were formed by erosion and denudation of weathering crust and accumulation of redeposited weathering residues in the Tankhoi paleobasin with a lacustrine-marsh landscape. Coarse-clastic foehn deposits of the Osinovka Formation, containing rich Miocene palynological assemblages, were eroded and overlie concordantly the Upper Tankhoi Subformation and redeposited weathering residues of the weathering crust, and underlie the Anosovka Formation. The study suggests that the Baikal rift valley began to form at 38 Ma.     

东亚西太平洋巨型裂谷体系岩石圈与软流圈结构及动力学

笔者根据地震面波层析成像结果，对欧亚大陆及西太平洋岩石圈和软流圈速度结构进行了研究，发现东亚至西太平洋间存在一巨型低速异常带，结合构造地质学、地幔岩石学、地球化学及其他地球物理特性的研究，确认该区存在巨型裂谷体系。该巨型裂谷体系的岩石圈和软流圈三维Vs速度结构与太平洋洋中脊、大西洋洋中脊和印度洋洋中脊及其邻区的岩石圈和软流圈地震Vs速度结构十分相似，而与东太平洋边缘现代板块俯冲带的岩石圈与软流圈Vs速度结构有显著差异。在进一步论述该区动力学特征后认为，该巨型裂谷体系是中生代中晚期以来岩石圈整体主动伸展变形，大型裂陷盆地形成，岩石圈强烈拆沉减薄，以及软流圈物质上涌加热引起的。边缘海是在大陆裂谷系形成基础上发展起来的，主导扩张期为中渐新世至中中新世（32－13Ma），这些边缘海在17－15Ma后停止扩张，因而未能将所有边缘海和洋中脊联通。据此划分出4期构造变形动力学演化阶段，现今东亚至西太平洋间大陆裂谷、边缘海与沟弧体系是新生代中晚期以来，邻区各板块构造相互作用叠加的结果。     

Mechanism of rifting and some features of the deep-seated structure of the Baikal rift zone

The mechanism of rifting in the Baikal rift zone is a complex process, with stages of crustal fracturing alternating with stages of plastic extension. Data on the form and size of the anomalous mantle region lying below the rift zone is given in the present work. Divergent flow in the upper part of the anomalous mantle is considered the cause of extension of the crust in this region.