Crustal and upper mantle S-wave velocity structures across the Taiwan Strait from ambient seismic noise and teleseismic Rayleigh wave analyses

Although orogeny tapers off in western Taiwan large and small earthquakes do occur in the Taiwan Strait, a region largely untouched in previous studies owing mostly to logistical reasons. But the overall crustal structure of this region is of particular interest as it may provide a hint of the proto-Taiwan before the orogeny.By combining time domain empirical Green’s function (TDEGF) from ambient seismic noise using station-pairs and traditional surface wave two-station method (TS) we are able to construct Rayleigh wave phase velocity dispersion curves between 5 and 120 s. Using Broadband Array in Taiwan for Seismology (BATS) stations in Taiwan and in and across the Strait we are able to derive average 1-D Vs structures in different parts of this region. The results show significant shear velocity differences in the upper 15 km crust as expected. In general, the highest Vs in the upper crust observed in the coastal area of Mainland China and the lowest Vs appears along the southwest offshore of the Taiwan Island; they differ by about 0.6–1.1 km/s. For different parts of the Strait, the upper crust Vs structures are lower in the middle by about 0.1–0.2 km/s relative to those in the northern and southern parts. The upper mantle Vs structure (Moho – 150 km) beneath the Taiwan Strait is about 0.1–0.3 km/s lower than the AK135 model. The overall crustal thickness is approximately 30 km, much thinner and less variable than under the Taiwan Island. The inversion of seismic velocity structures using shorter period band dispersion data in the sea areas with water depth deeper than 1000 m should take water layer into consideration except for the continental shelves.     

Crustal and upper mantle S-wave velocity structure beneath the Bransfield Strait (West Antarctica) from regional surface wave tomography

Regional surface wave tomography in the sub-Antarctic Scotia Sea is helpful in revealing the nature of the crust and the S-wave seismic velocity profile beneath the Bransfield Strait. The joint use of our regional network, global seismographic network stations and local temporary arrays provide better lateral resolution than that obtained in our previous studies concerning the Scotia Sea region.Tomographic analysis of data obtained using 10 broad band seismic stations and more than 300 regional events, shows that the Bransfield Basin is characterised by a strong group velocity reduction of 8% with respect to the surrounding areas, in the period range from 15 s to 50 s.The crustal and upper mantle models of the eastern, central and western Bransfield Basin are obtained by joint inversion of Rayleigh and Love local dispersion curves from 15 s to 50 s. In addition our data set is expanded to a broader period interval (1–80 s), in central Bransfield Strait in order to better constrain the upper mantle and shallow crust.The main results can be summarized as follows: (a) the crust thins distinctly from W toward E; the variation is consistent with the type of volcanism, earthquake distribution and bathymetric observations, (b) low upper mantle velocities (soft lid) extend down to depths exceeding 70 km as a consequence of elevated temperatures, (c) the crust beneath the central Bransfield Basin displays continental characteristics with a gradually increasing S-wave velocity distribution versus depth analogous to the East African Rift structure of Kenya, (d) negative velocity gradients are present in the lower crust beneath the eastern Bransfield Basin; these could be interpreted as magmatic bodies originating from decompression melting of the mantle.     

S-wave velocity structure of the North China from inversion of Rayleigh wave phase velocity

We constructed the S-wave velocity structure of the crust and uppermost mantle (10–100 km) beneath the North China based on the teleseismic data recorded by 187 portable broadband stations deployed in this region. The traditional two-step inversion scheme was adopted. Firstly, we measured the interstation fundamental Rayleigh wave phase velocity of 10–60 s and imaged the phase velocity distributions using the Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.25° × 0.25° and constructed the 3-D S-wave velocity structure of the North China. The 3-D S-wave velocity model provides valuable information about the destruction mechanism and geodynamics of the North China Craton (NCC). The S-wave velocity structures in the northwestern and southwestern sides of the North–South Gravity Lineament (NSGL) are obviously different. The southeastern side is high velocity (high-V) while the northeastern side is low velocity (low-V) at the depth of 60–80 km. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 100 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. The Zhangjiakou-Penglai fault zone (ZPFZ) may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30–50 km) beneath the Beijing–Tianjin–Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ.     

Upper crustal seismic structure of the Mazury complex and Mazowsze massif within East European Craton in NE Poland

The large-scale seismic experiment POLONAISE '97 (POlish Lithospheric ONsets—An International Seismic Experiment) was carried out in May 1997 in Poland, Lithuania, and Germany. Its main purpose was to investigate the structure of the crust and the uppermost mantle in the region of the Trans European Suture Zone (TESZ) that lies between the East European Craton (EEC) and the Palaeozoic Platform. This paper covers the interpretation of seismic data along the NW–SE-trending, 180-km-long profile P5 located on the EEC. The recordings were of a high quality with seismic energy clearly visible along the whole profile. We have not found waves refracted below the upper crust in first arrivals. In the NW part of the profile, we have delineated a high-velocity body with the P-wave velocity in the range of 6.5–6.75 km/s in the upper crust. It corresponds to the K trzyn anorthosite massif within the Mazury complex. The Mazowsze massif is rather uniformly characterized by P-wave velocities 5.9–6.05 and 6.2–6.35 km/s in two layers, respectively. Sufficient S-wave data were available to estimate the V_p/V_s ratio (as well as the Poisson ratio), being 1.80 (0.277) in the high-velocity body and 1.67 (0.220) in the upper crust.Apart from the 2-D model along the profile, results of 3-D modelling in the area of the P5 profile are presented. Using off-line recordings, we got P-wave velocity field up to 8 km/s below the P5 profile at the depth of about 40 km as well as horizontal extent of the high-velocity body.     

Structure of the lithosphere below the southern margin of the East European Craton (Ukraine and Russia) from gravity and seismic data

The present study was undertaken with the objective of deriving constraints from available geological and geophysical data for understanding the tectonic setting and processes controlling the evolution of the southern margin of the East European Craton (EEC). The study area includes the inverted southernmost part of the intracratonic Dnieper-Donets Basin (DDB)–Donbas Foldbelt (DF), its southeastern prolongation along the margin of the EEC–the sedimentary succession of the Karpinsky Swell (KS), the southwestern part of the Peri-Caspian Basin (PCB), and the Scythian Plate (SP). These structures are adjacent to a zone, along which the crust was reworked and/or accreted to the EEC since the late Palaeozoic. In the Bouguer gravity field, the southern margin of the EEC is marked by an arc of gravity highs, correlating with uplifted Palaeozoic rocks covered by thin Mesozoic and younger sediments. A three-dimensional (3D) gravity analysis has been carried out to investigate further the crustal structure of this area. The sedimentary succession has been modelled as two heterogeneous layers—Mesozoic–Cenozoic and Palaeozoic—in the analysis. The base of the sedimentary succession (top of the crystalline Precambrian basement) lies at a depth up to 22 km in the PCB and DF–KS areas. The residual gravity field, obtained by subtracting the gravitational effect of the sedimentary succession from the observed gravity field, reveals a distinct elongate zone of positive anomalies along the axis of the DF–KS with amplitudes of 100–140 mGal and an anomaly of 180 mGal in the PCB. These anomalies are interpreted to reflect a heterogeneous lithosphere structure below the supracrustal, sedimentary layers: i.e., Moho topography and/or the existence of high-density material in the crystalline crust and uppermost mantle. Previously published data support the existence of a high-density body in the crystalline crust along the DDB axis, including the DF, caused by an intrusion of mafic and ultramafic rocks during Late Palaeozoic rifting. A reinterpretation of existing Deep Seismic Sounding (DSS) data on a profile crossing the central KS suggests that the nature of a high-velocity/density layer in the lower crust (crust–mantle transition zone) is not the same as that of below the DF. Rather than being a prolongation of the DDB–DF intracratonic rift zone, the present analysis suggests that the KS comprises, at least in part, an accretionary zone between the EEC and the SP formed after the Palaeozoic.     

Geodynamics of the Riphean stage in the evolution of the northern passive margin of the East European Craton

The nearly parallel northwest-trending Onega-Kandalaksha, Kerets-Leshukonsky, and Barents paleorift zones located in the northeastern part of the East European Platform are interpreted as a common structural assemblage that was formed in the Middle-Late Riphean as a result of horizontal extension of the continental margin. Therefore, it is reasonable to combine these paleorift structural features into the common White Sea Rift System instead of subdividing them into two or more systems as done previously. The White Sea Rift System originated owing to the breakup of the ancient Paleopangea supercontinent 1300–1240 Ma ago. The latter event occurred as a result of the divergence of the Baltia and Laurentia continental plates that most probably was caused by mantle spreading within the hot equatorial belt of the Earth. The diffuse rifting of that time occurred in the form of near-parallel rifts developing progressively from the inner part of the continental plate toward its margin. A pericratonic sedimentary basin eventually formed at the passive margin of Baltia as a system of roughly parallel rift zones. The geologic and geophysical data show that the passive margin of the East European Platform formed in the Riphean, a phenomenon that corresponds with a model of large-scale extension of the lithosphere after the stage of early ocean-floor spreading. In the course of this process, the brittle upper crust was detached from the ductile lower crust. The geodynamic regime of the Riphean passive margin of the East European Platform probably was similar to the regime of the present-day Atlantic-type passive margins. The White Sea Rift System differs from the transverse Mid-Russian Paleorift System both in origin and age. The Mid-Russian Paleorift System is considered to have formed in the Late Riphean as a result of transtension along a mobile zone in the ancient basement. The lithosphere of northeastern Fennoscandia has experienced horizontal extension since the Middle Riphean, a phenomenon that is closely related to the evolution of the White Sea Rift System, i.e., to the formation of the passive margin of the Baltia continent.     

The southern margin of the East European Craton: new results from seismic sounding and potential fields between the North Sea and Poland

The extension of eastern Avalonia from Britain through the NE German Basin into Poland is, in some sense, a virtual structure. It is covered almost everywhere by late Paleozoic and younger sediments. Evidence for this terrane is only gathered from geophysical data and age information derived from magmatic rocks. During the last two decades, much geophysical and geological information has been gathered since the European Geotraverse (EGT), which was followed by the BABEL, LT-7, MONA LISA, DEKORP-Basin'96, and POLONAISE'97 deep seismic experiments. Based on seismic lines, a remarkable feature has been observed between the North Sea and Poland: north of the Elbe Line (EL), the lower crust is characterised by high velocities (6.8–7.0 km/s), a feature which seems to be characteristic for at least a major part of eastern Avalonia (far eastern Avalonia). In addition, the seismic lines indicate that a wedge of the East European Craton (EEC) (or Baltica) continues to the south below the southern Permian Basin (SPB)—a structure which resembles a passive continental margin. The observed pattern may either indicate an extension of the Baltic crust much farther south than earlier expected or oceanic crust of the Tornquist Sea trapped during the Caledonian collision. In either case, the data require a reinterpretation of the docking mechanism of eastern Avalonia, and the Elbe–Odra Line (EOL), as well as the Elbe Fault system, together with the Intra-Sudedic Faults, appear to be related to major changes in the deeper crustal structures separating the East European crust from the Paleozoic agglomeration of Middle European terranes.     

Lower-crust S-wave velocity beneath western Yunnan Province from waveform inversion of dense seismic observations

We use teleseismic body waveforms to explore S-wave layered velocity structures beneath 30 portable digital seismic stations deployed around western Yunnan Province. Results show that the Moho depth in this region is ∼40 km and decreases in general from north to south, consistent with previous geophysical studies. Associated with this lateral variation of the Moho depth, the lower crust above the Moho discontinuity has a 15–25 km thick zone with an S-wave velocity lower than that of the upper crust. This lower velocity zone might be interpreted as a lower crust weak channel, which may mechanically partially decouple the upper-crust deformation from the underlying mantle. Thus, the inverted S-wave velocity structure could provide new evidence for the lateral flow of lower crust in the build-up of the south-eastern Tibetan plateau.     

Seismic velocity model of the crust and uppermost mantle around the Mirnyi kimberlite field in Siberia

We present the first detailed seismic velocity models of the crust and uppermost mantle around the Mirnyi kimberlite field in Yakutia, Siberia. We have digitized vintage seismograms that were acquired in 1981 and 1983 by use of Taiga analogue seismographs along two perpendicular seismic profiles. The 370-km long, northwest striking profile I across the kimberlite pipe was covered by 41 seismographs, which recorded seismic signals from 21 chemical shots along the line, including one off-end shot. The perpendicular, 340-km long profile II across profile I ca. 30 km to the south of the Mirnyi kimberlite field was covered by 45 seismographs, which recorded seismic signals from 22 chemical shots, including four off-end shots. Each shot involved detonation of between 1.5 and 6.0 tons of TNT, distributed in individual charges of 100–200 kg in shallow water (< 2 m deep). The data is of high quality with high signal/noise ratio to the farthest offsets. We present the results from two-dimensional ray tracing, forward modelling.Both velocity models show normal cratonic structure of the ca. 45-km-thick crust with only slight undulation of the Moho. However, relatively small seismic velocity is detected to 25-km depth in a ca. 60-km wide zone around the kimberlite pipe, surrounded by elevated velocity (> 6.3 km/s) in the upper crust. The lower crust has a relatively constant velocity of 6.8–6.9 km/s. It appears relatively unaffected by the presence of the kimberlite field. Extremely large P-wave velocity (> 8.7 km/s) of the sub-Moho mantle is interpreted along profile I, except for a 70-km wide zone with a “normal” Pn velocity of 8.1 km/s below the kimberlite. Profile II mainly shows Pn velocities of 8.0–8.2 km/s, with unusually large velocity (> 8.5 km/s) in two, ca. 100-km wide zones, at its southwestern end, one zone being close to the kimberlite field. The nature of these exceptionally large, sub-Moho mantle velocities is not yet understood. The difference in velocity in the two profile directions indicates anisotropy, but the effect of unusual rock composition, e.g. from a high concentration of garnet, cannot be excluded.     

Banded iron formations of the calciphyre-metabasite-gneiss association in the East European Craton

The paper presents characteristics of the least studied iron formations of the East European Craton (Archean banded iron formations of the calciphyre-metabasite-gneiss association), a typical member of granulite complexes of the Ukrainian Shield, Belarussian-Baltic region, and Voronezh crystalline massif. They are mainly composed of diverse metasedimentary rocks: aluminous gneisses; silicate-magnetite, magnetite, and barren quartzites; eulysites; calciphyres; and marbles associated with metavolcanic rocks. Data on chemical compositions of the metasedimentary rocks are summarized for the first time and their possible primary mineral composition has been reconstructed using the MINLITH software. It is shown that they could be formed from a lithogenetic series of sediments linked by gradual transitions and geochemical commonness of sediments: from fine-grained terrigenous insufficiently mature sediments to chemogenic sediments depleted in terrigenous material (ferruginous-siliceous, ferruginous-siliceous-carbonate, siliceous-carbonate, and carbonate sediments). The inferred primary mineral assemblage indicates sedimentation in the central parts of large paleobasins in a reducing environment characterized by deficit of oxygen and excess of carbon dioxide. Lithological specifics of the banded iron formations in different regions presumably reflect different distances of sedimentation zones from submarine hydrothermal discharge sites and sources of terrigenous material. The banded iron formations at the present-day erosion section of basement represent metamorphosed fragments of the lateral-facies zoning of rocks of the Archean sedimentary basins (or a single basin) of the East European Craton. Unlike other Early Precambrian banded iron formations of the East European Craton, rocks of the calciphyre-metabasite-gneiss association are marked by a high Mn content.     

Position of the southern marginal suture of the East European Craton

Old and modern data are given and discussed. They allow us to decide where the real position of the south marginal suture of the East European Craton is. According to the geophysical and aerogeophysical studies, Paleozoic deposits of the Karpinskii Ridge are bedded upon deeply sunken continuation of Archean-Proterozoic complexes comprising the Voronezh massif and crystalline rocks belonging to the Astrakhan Dome. The latter have a pre-Riphean age. It is possible that the crust in the southeastern part of ridge in a narrow zone with >20 km depth of the basement surface (BS) and, partially, in the Sarpinskii trough has oceanic origin.     

Crustal velocity structure beneath the western Andes of Colombian using receiver-function inversion

Analysis of teleseismic records obtained in two broadband seismic stations of three components located on the Andean region of Colombia is presented in this work. The two stations are located at the Western Cordillera (WC), station BOL, and at the Central Cordillera (CC), station PBLA. The analysis of seismograms was performed by inversion of the receiver functions (RF) in order to obtain the crustal velocity structure beneath the receivers. The receiver function is a spectral ratio obtained from teleseismic earthquakes recorded by broadband seismic stations, which allows the calculation of the velocity structure beneath the receiver by removing source effects in the horizontal components of the seismic traces. Data stacking was performed in order to improve signal to noise ratio and then the data was inverted by using two optimization algorithms: a genetic algorithm (GA), and a simulated annealing algorithm (SA). The present work calculates the receiver functions using teleseismic earthquakes at epicentral distances (Δ) ranging between 30° and 90° and recorded at the two stations within the years 2007 and 2009.Delay times between P and PS waves converted at the Moho boundary were used to constrain the velocity structure. The receiver functions at the stations were generated from seismic events within a broad range of back azimuth. Data from gravity and magnetism were also used during the geophysical survey. The depth of the Moho boundary was found to be at 40 km in the WC beneath station BOL and at 43 km in the CC beneath station PBLA. The upper crust, with a thickness of 5 km, is characterized by a shear wave velocity of about 3.0 km s⁻¹; the shallower layers, at approximately 1.0 km, have shear wave velocities between 2.2 and 2.6 km s⁻¹, which corresponds to sediments overlying the upper crust. These observations support the hypothesis of a thickness of the crust at the root of the mountain range to be between 32 and 50 km. The calculated receiver functions were compared with artificial ones generated from the inversion of 48000 models of horizontal layers for each station using a GA and an SA that allowed a satisfactory coverage of all the sample space in order to avoid non-unique solutions. Beneath station BOL a moderate low-velocity zone (LVZ) was found, which was caused by accretionary processes of the ophiolite complex in the WC.     

Dissociation of CAS phase in the uppermost lower mantle

The high-pressure stability limit of calcium aluminosilicate (CAS) phase has been examined in its end-member CaAl₄Si₂O₁₁ composition at 18–39 GPa and 1,670–2,300 K in a laser-heated diamond-anvil cell (LHDAC). The in-situ synchrotron X-ray diffraction measurements revealed that the CAS phase decomposes into three-phase assemblage of cubic Al-bearing CaSiO₃ perovskite, Al₂O₃ corundum, and SiO₂ stishovite above 30 GPa and 2,000 K with a positive pressure–temperature slope. Present results have important implications for the subsolidus mineral assemblage of subducted sediment and the melting phase relation of basalt in the lower mantle.     

Lithochemical typification of Early Precambrian ferruginous-siliceous formations in the East European Craton

The first comparative paleolithochemical characteristics of Early Precambrian ferruginous-siliceous formations of the East European Craton confined to four stratigraphic levels—Lower Archean, Upper Archean (Lopian), Lower Karelian, and Upper Karelian—are presented. Using the MINLITH method and software package for lithochemical calculations, the possible primary composition of metasedimentary rocks is reconstructed and paleogeographic settings of sedimentation are suggested. It is shown that different age formations represented initially lithogenetic groups with different compositions and quantitative relationships between the major types of sedimentary rocks with gradual transitions and genetic affinity. They accumulated in paleotectonic and facies settings that were specific for each stage of iron ore sedimentation, resulting in the development of four genetic (Bug, Algoma, Okolovo, and Lake Superior) types of ferruginous-siliceous formations.     

Significance of inhomogeneous compression of mineral substance for explaining crystalline crust and uppermost mantle velocity profiles

Consideration of the Voronezh Crystalline Massif shows that three-layered seismic models and velocity profiles with a characteristic distribution of layer velocities and their gradients in each layer, reflect a crystalline crust and uppermost mantle structure formed through regional metamorphism and magmatism processes.Velocity and seismic parameter vertical gradients from the lithosphere profile are several times higher than those for homogeneous compression of minerals and rocks.For the identified layers the contribution of different factors for each seismic parameter gradient value is estimated. Anomalously high rates of increase with depth for V_p, V_s and ø are shown to be caused by the combined effects of structure-phase transformations and substance composition variations in lithospheric complex mineral parageneses.