Heterogeneities of the shear wave attenuation field in the lithosphere of East Tien Shan and their relationship with seismicity

The shear wave attenuation field in the lithosphere of Eastern Tien Shan has been mapped. The method based on analysis of the ratio between amplitudes of Sn and Pn waves was used. On aggregate, about 120 seismograms made at the Makanchi station (MKAR), mainly in the period of 2003–2009, at epicentral distances of about 350–1200 km were analyzed. It was found that shear wave attenuation in the lithosphere of Eastern Tien Shan is weaker than that in the region of Central Tien Shan. This agrees with the fact that the rate of deformation of the Earth’s crust in Eastern Tien Shan is lower (based on GPS data), as is the seismicity level, in comparison to Central Tien Shan. The zones of high attenuation, where strong earthquakes with M > 7.0 have not occurred for the last 200 years, have been identified: first of all, these are the area west of Urumqi and that of the Lop Nur test site. It is suggested that in the first zone, where an annular seismicity structure has formed over the last 30 years, a strong earthquake may be being prepared. The second zone is most probably related to the uplift of mantle fluids resulting from a long-term intensive technogenic effect, analogous to what has occurred in areas of other nuclear test sites (Nevada and Semipalatinsk).     

Geodynamics and seismicity of the eastern part of Central Asia

Study of the geodynamics of the eastern part of Central Asia shows that the present-day tectonic activity of this territory is connected with its division into blocks limited by active faults and with the interaction among these blocks and with the neighboring lithospheric plates. The North China and South China platforms occupy most of this territory. The western boundary of the South China platform with Tibetan blocks is the most active. The energy volume increases up to 10¹⁰–10¹² J, and the earthquake magnitudes go up to 8–9 within this boundary. The interaction of Tibetan blocks with the Southeast China Block causes detachment and a clockwise turn of the upper layers of the Earth’s crust under the influence of the Hindustan indenter pressure.

     

中亚地区中生代以来的地貌巨变与岩石圈动力学

中亚地区的地貌自中生代以来发生过两次巨变：一次是青藏高原的隆起,另一次是中生代中国东部高原及其西侧共存的中亚准平原的兴衰。青藏高原的隆起引起了全球气候和中亚环境的巨变。对此,自80年代以来开展的国际合作已经在地质学和地球物理学等研究领域取得了丰硕的成果。不过,在解释高原隆升—气候变化—剥蚀作用的相互关系方面仍存歧见。相比之下,中亚地区中生代的地貌巨变尚属新的研究课题。人们认识到,中亚地区在中侏罗世至新近纪曾存在一个准平原,而在中国东部则存在一个中生代高原。这一中生代地貌巨变引发出许多新的思考,如：为什么这一中生代准平原能保存长达150 Ma？中国东部高原是怎样形成的,又是怎样消失的？这两次地貌巨变及其相关的岩石圈动力学将是“TOPO?CENTRAL?ASIA”这一国际岩石圈计划项目的研究主题。     

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.     

Slow deformation waves in the lithosphere: Registration,parameters, and geodynamic analysis (Central Asia)

New methods are proposed for registering deformation waves and assessing their phase (vector) velocities based on earthquake monitoring in dynamic influence areas of active faults. Active faults in Central Asia are classified on the basis of the vector velocities of deformation waves. The complex parameters of the presently active faults and the vector velocities of the deformation waves are used for the geodynamic zoning of Central Asia. Such an approach offers new opportunities for the more extensive investigation of the geodynamic characteristics of spacious intracontinental structures that were tectonically active in the Cenozoic.     

Shear wave attenuation characteristics over the Central India Tectonic Zone and its surroundings

The study area lies between latitude 18–26°N and longitude 73–83°E, and mainly covers the Central India Tectonic Zone (CITZ). The frequency-dependent shear wave quality factor (Q_s) has been estimated over the CITZ and its surroundings using Double Spectral Ratio (DSR) method. We have considered 25 local earthquakes with magnitude (M_L) varies from 3.0 to 4.7 recorded at 11 stations running under national seismic network. The Fast Fourier Transformed (FFT) spectra were computed from the recorded waveform having time-window from onset of S-phase to 1.0 s and for a frequency-band of 0.1–10 Hz. Three different shear wave velocities (i.e., 3.87, 3.39 and 3.96 km/s) were obtained over the study area based on a pair of earthquakes recorded at a pair of stations. The low Q_s values of 51–96 at 1 Hz (i.e., Q_s = 51f^0.49; Q_s = 90f^0.488 and Q_s = 96f^0.53) were found in the area covering the Son–Narmada–Tapti (SONATA) lineament, CITZ, eastern part of the Satpura fold belt, Vindhyan and Gondwana basins, Godavari and Mahanadi grabens, and southern part of Gangetic plain. Intermediate Q_s values of the order of 204–277 (i.e., Q_s = 204f^0.56 and Q_s = 277f^0.55) were noted in the cartonic areas, namely, Bundelkhand, Dharwar-Bhandara and Bastar. While the higher Q_s values of 391–628 at 1 Hz (i.e., Q_s = 391f^0.49, Q_s = 409f^0.48, Q_s = 417f^0.48, Q_s = 500f^0.66, Q_s = 585f^0.65 and Q_s = 628f^0.69) were found in the eastern part of the SONATA, CITZ, and the northeastern part of the Satpura fold belt. The low Q_s values might be attributing to the more heterogeneous SONATA rift system. Low Q_s values further may presumably be associated with lower-level of seismicity and apparently account for higher tectonic stress accumulation over long duration. The long-term accumulated stress is generally released through occasional triggering of moderate magnitude earthquakes in the SONATA zone. Surrounding the SONATA region, the higher Q_s values possibly accounts for a more homogeneous subsurface structure along the SONATA zone.     

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.     

Is there a correlation between lithosphere structure and the statistical properties of seismicity?

The elastic parameters and the thickness of the lithospheric part of the mantle, the lid, are correlated with the seismicity of Italy and surrounding areas through the estimation of several statistical parameters. In particular, the intensity of the flow of seismic events, deprived of aftershocks, reveals concentrations associated with large gradients in the lithospheric thickness. The average depth of crustal shocks tends to be very shallow in areas of thin and soft (low shear-wave velocities) lid. The positive influence, probably reflecting the thermal state in the lithosphere, is very large in connection with the thin and soft lid and where strong gradients in lid thickness are observed, and is small in connection with a thick and hard lid (high shear-wave velocities).     

Younger and older zircons from rocks of the oceanic lithosphere in the Central Atlantic and their geotectonic implications

Local U-Pb dating of zircons separated from various rocks in the crest zone of the Mid-Atlantic Ridge (MAR) and Carter Seamount (Sierra Leone Rise) is performed. Younger zircons formed in situ in combination with older xenogenic zircons are revealed in enriched basalts, alkaline volcanic rocks, gabbroic rocks, and plagiogranites. Only older zircons are found in depleted basalts and peridotites. Older zircons are ubiquitous in the young oceanic lithosphere of the Central Atlantic. The age of the younger zircons from the crest zone of the MAR ranges from 0.38 to 11.26 Ma and progressively increases receding from the axial zone of the ridge. This fact provides additional evidence for spreading of the oceanic floor. The rate of half-spreading calculated from the age of the studied zircons is close to the rate of half-spreading estimated from magnetic anomalies. The age of the younger zircons from Carter Seamount (58 Ma) corresponds to the age of the volcanic edifice. Older zircons make up an age series from 53 to 3200 Ma. Clusters of zircons differing in age reveal quasiperiodicity of about 200 Ma, which approximately corresponds to the global tectonic epochs in the geological evolution of the Earth. Several age groups of older zircons combine grains close in morphology and geochemistry: (1) Neoproterozoic and Phanerozoic (53–700 Ma) prismatic grains with slightly resorbed faces, well-preserved or translucent oscillatory zoning, and geochemical features inherent to magmatic zircons; (2) prismatic grains dated at 1811 Ma with resorbed faces and edges, fragmentary or translucent zoning, and geochemical features inherent to magmatic zircons; (3) ovoid and highly resorbed prismatic grains with chaotic internal structure and metamorphic geochemical parameters; the peak of their ages is 1880 Ma. The performed study indicates that older xenogenic zircons from young rocks in the crest zone of the MAR were captured by melt or incorporated into refractory restite probably in the sublithospheric mantle at the level of magma generation in the asthenosphere. It is suggested that zircons could have crystallized from the melts repeatedly migrating through the asthenosphere during geological history or were entrapped by the asthenosphere together with blocks of disintegrated and delaminated continental lithosphere in the process of breakup of the continents older than Gondwana. The variability in the age of older zircons even within individual samples may be regarded as evidence for active stirring of matter as a result of periodically arising and destroyed within-asthenospheric convective flows varying in orientation and scale.     

Quasi-adiabatic shear zones in the lithosphere: numerical and experimental approaches

Continental breakup, or compressive lithosphere scale faulting, requires a physical mechanism for wholesale faulting of the lithosphere. We compared numerical and experimental models for the nucleation of quasi-adiabatic shear bands in polyvinylchloride (PVC) with those in an idealized viscoelastoplastic mantle with olivine rheology. In both materials fault nucleation is caused by elastic stress concentration on pre-existing imperfections, with localized yielding confined to its vicinity. Faulting occurs rapidly after the initial elastic energy in the system is charged sufficiently to cause wholesale yielding. Propagation of the fault, monitored by looking at the dissipation of plastic energy, reveals migration of a sharp, thermal-mechanical “crack”- like instability, which appears in the temperature field as a slightly diffused signal. The initial temperature rise in the crack is subtle but increases suddenly when the plate is severed. This autocatalytic behavior has also been described in ductile polymers, which can be used as mechanical analogues. We suggest that elastoplastic coupling in quasi-adiabatic shear banding is a key for fast (< 1 Ma) nucleation of shear zones. These nonlinear phenomena will be illustrated for both experimental and numerical results by nine movies     

Mantle plumes of Central Asia (Northeast Asia) and their role in forming endogenous deposits

The Phanerozoic within-plate magmatism and the related deposits of Siberia are reviewed. The formation of post-perovskite at about 2.5 Ga in the Earth’s interior and the isotope characteristics of within-plate igneous rocks have shown that plate tectonics and deep geodynamics started to operate at about 2–2.5 Ga. The assembly and breakup of supercontinents under the effect of the superplumes formed in layer D″ is considered. Thus, the supercontinent–superplume cycles spanning about 700 Ma are recognized in the Earth’s history.The manifestations of the within-plate magmatic activity are found throughout the whole Phanerozoic. It was demonstrated earlier that between 570 and 160 Ma, the Siberian continent drifted within the African hot mantle field or large low shear velocity province (LLSVP). At least four plumes, excluding the superplume leading to the breakup of Rodinia at 750 Ma, interacted with the Siberian continent. The superplume leading to the breakup of Rodinia was also responsible for the origin of ultramafic intrusions with carbonatites hosting rare-metal (Nb, Ta, REE) mineralization as well as ultramafic–mafic intrusions with Cu–Ni–Pt mineralization localized along the rift zones.The plumes originated in other Phanerozoic cycles formed most likely at the lower-upper mantle boundary, where most of the stagnant slabs is accumulated. Those plumes were responsible for the origin of within-plate igneous rocks. The granitic batholiths formed in the centers of zonal area surrounded by rift zones containing abundant rare-metal intrusions with rare-metal mineralization. Gold, tin, base metal, and porphyry copper deposits are also related to these zonal area.The studies have shown that the formation of folded zones and related deposits which surround these zones as well as the structures of cratons and their metallogenic specialization should be considered in terms of both plate tectonics and plume tectonics.     

Microcrystalline diamonds in the oceanic lithosphere and their nature

The carbon isotope composition of microdiamonds found in products of the Tolbachik Volcano eruption, Kamchatka (porous lavas and ash), was studied. The isotope composition of microdiamonds (with an average value of δ¹³C =–25.05‰) is close to that of microsized carbon particles in lavas (from–28.9 to–25.3‰). The general peculiarities of the diamond-forming environment include (1) no evidence for high pressure in the medium; (2) a reduced environment; and (3) mineralogical evidence for the presence of a fluid. The geochemical data characterizing the type of diamonds studied allow us to suggest that they were formed in accordance with the mechanism of diamond synthesis during cavitation in a rapidly migrating fluid, which was suggested by E.M. Galimov.     

Spatial variations in the effective elastic thickness of the lithosphere in Southeast Asia

As a proxy for long-term lithospheric strength, detailed information on lateral effective elastic thickness (T_e) variations can aid in understanding the distribution pattern of surface deformation and its response to long-term forces. Here we present high-resolution maps of spatial variations of T_e for the complex SE Asian region by analyzing the coherence of topography and Bouguer gravity anomaly data. We find that after considering the gravity deficit of less dense sediment, the recovered T_e maps are more representative of the geology, particularly in elongated rift basins. The results show that the T_e variation pattern in SE Asia, in general, agrees well with its tectonic provinces and major tectonic boundaries. The oceanic basins, the Indosinian suture zones between the Indochina and Sibumasu blocks, and the Makassar Strait are characterized by low T_e, while moderate and high T_e values are recovered in the Khorat plateau, West Burma, the Singapore Ridge, the Con Song Swell, Borneo, the northern Australian margin and the Molucca Sea. The T_e pattern in the south Indonesian margin is complicated by the approach and collision of oceanic plateaus and seamounts with the fore-arc region. The heterogeneous strength features are consistent with the complex assemblage of different tectonic units, and significant deformation during Cenozoic tectonic events. In the Indochina Peninsula, the extruded displacement during the India-Eurasia collision might have been partitioned and absorbed by the combined mechanism of the extrusion and viscous tectonic models. As a result, the offshore displacements of the major strike-slip faults in the South China Sea are much smaller than originally assumed, thus having less effect on the development of the South China Sea than other mechanisms such as the slab pull of the proto-South China Sea. Since the displacement driven by the boundary tectonic forces has been greatly absorbed and decreased by subduction and deformation in the active margins and adjacent weak regions, the motion velocity of the interior regions is greatly lower than the boundary active margins, and they are largely free of seismicity and volcanism. Our results suggest that East Borneo might share a similar crustal basement, and represent a broad tectonic zone of the destroyed Meso-Tethys Ocean extending from West-Middle Java, through East Borneo to northern Borneo of the Sarawak and Sabah. The Indosinian zones between the Indochina and Sibumasu blocks might extend further southeastward across Billiton Island to offshore of southern Borneo, and the Singapore platform and SW Borneo might belong to the same block. The results also show that the internal load fraction F is high in the coastal area of South China, the northern margin of the South China Sea, and the coastal area of Indochina, which, in general, agrees with the distribution of a high-velocity lower crustal layer and Late Cenozoic basaltic rocks.     

Isotopic-geochemical characteristics of free gases of the South Sakhalin mud volcano and their relationship to regional seismicity

This paper discusses the results of long-term monitoring observations on the chemical and isotopic composition of free gases in gryphons of the South Sakhalin mud volcano on Sakhalin Island. It was shown that during seismic activation in the region, the chemical composition of free gases changes, whereas the isotopic composition of carbon in methane and carbon dioxide remains constant. A possible interpretation of the obtained data was suggested. The depth of the gas reservoir that supplies the South Sakhalin mud volcano was estimated as well.     

中亚盆地钾盐矿床盐类矿物Rb-Sr同位素体系研究及其意义

中亚盆地钾盐矿床的形成时代目前被限定为晚侏罗世至早白垩世,较为宽泛.盐类矿物沉积之后若未被改造,可测定其形成时代;若被改造则可利用同位素测年研究其沉积后作用.为了得到中亚盆地钾盐矿床的形成时代和/或了解盐类矿物受到的后期改造过程,利用Rb-Sr同位素定年对矿床中的盐类矿物进行了测定.结果表明,无法形成可靠的Rb-Sr等...     

Lateral variations in shear velocity and attenuation in the upper mantle

We have used digitally recorded long period seismic data from the HGLP and SRO networks to obtain estimates of multiple ScS travel times and spectra and have used these to place constraints on the lateral variations in upper-mantle shear velocity and attenuation structure. The travel time results are consistent with our previous findings: to a first approximation, upper-mantle heterogeneity as expressed in vertical travel time differences can be correlated with surface tectonic features. A spectral stacking procedure for retrieving the attenuation operator for SH polarized multiple ScS waves has been applied to seismograms representing a variety of tectonic provinces. The spectral analysis has yielded stable estimates of the phase and amplitude response of the attenuation operator in the frequency interval 6–60 mHz. These estimates indicate that the apparent Q can also be correlated with surface tectonic regimes.