Crustal structure of mainland China from deep seismic sounding data

Since 1958, about ninety seismic refraction/wide angle reflection profiles, with a cumulative length of more than sixty thousand kilometers, have been completed in mainland China. We summarize the results in the form of (1) a new contour map of crustal thickness, (2) fourteen representative crustal seismic velocity–depth columns for various tectonic units, and, (3) a Pn velocity map. We found a north–south-trending belt with a strong lateral gradient in crustal thickness in central China. This belt divides China into an eastern region, with a crustal thickness of 30–45 km, and a western region, with a thickness of 45–75 km. The crust in these two regions has experienced different evolutionary processes, and currently lies within distinct tectonic stress fields. Our compilation finds that there is a high-velocity (7.1–7.4 km/s) layer in the lower crust of the stable Tarim basin and Ordos plateau. However, in young orogenic belts, including parts of eastern China, the Tianshan and the Tibetan plateau, this layer is often absent. One exception is southern Tibet, where the presence of a high-velocity layer is related to the northward injection of the cold Indian plate. This high-velocity layer is absent in northern Tibet. In orogenic belts, there usually is a low-velocity layer (LVL) in the crust, but in stable regions this layer seldom exists. The Pn velocities in eastern China generally range from 7.9 to 8.1 km/s and tend to be isotropic. Pn velocities in western China are more variable, ranging from 7.7 to 8.2 km/s, and may display azimuthal anisotropy.     

Crustal structure of Japan as derived from explosion seismic data

Crustal structures of Japan were investigated under the Upper Mantle Project in three profiles, Kurayosi-Hanabusa, western Japan; Atumi-Noto, central Japan; Kesennuma-Oga, northeastern Japan. These investigations have revealed that the crust of Japan is of continental type. The variation of the crustal structure reflects the topography, especially the water depth; so the thinning of the crust occurs near the shore where the water depth increases rapidly. The velocity below the Mohorovi?i? discontinuity is smaller than 8.0 km/sec, but it is possible that a deeper layer with a velocity of over 8.0 km/sec may exist. The basaltic layer in central Japan, if existing at all, must be thin.     

Crustal structure of the western Arafura Sea from ocean bottom seismograph data

Refraction data taken from ocean bottom seismograph recordings in the western Arafura Sea indicate a continental‐type structure for the region. This structure is characterised by a thin column (2 km) of sediments, with velocities ranging from about to 2 to 4 km s^‐1, overlying an essentially two layer crust. The compressional wave velocities in the upper and lower crust are 5.97 and 6.52 km s^‐1, respectively, with the boundary between the layers at a depth of 11 km. Very weak mantle‐refracted arrivals with a velocity of about 8.0 km s^‐1 were recorded. Large‐amplitude, later arrivals, beginning at distances near 100 and 150 km, have been interpreted to be part of the retrograde branches from the 8.0 and 7.33 km s^‐1 layers, respectively. Model studies indicate that a small positive velocity gradient is required between 17 and 30 km, and that the Moho is at a depth of 34 km. A third set of large amplitude, later arrivals starting at a distance near 250 km has been interpreted as most probably multiple refraction‐reflection arrivals from the 5.97 and 6.52 km s^‐1 layers. Correlation of this structure with the stratigraphic logs from exploratory oil wells in the Arafura Sea using layer velocities indicates that rocks younger than Jurassic appear to thin towards the east.     

Crust structure of the northeastern Barents Sea basin area

Processing of data from regional geophysical surveys completed in the northern Barents Sea has provided updates to gravity and magnetic databases, structural maps of seismic interfaces, and positions of anomaly sources, which made a basis for 3D density and magnetic models of the crust. The new geological and geophysical results placed constraints on the boundaries between basement blocks formed in different settings and on the contours of deposition zones of different ages in the northeastern Barents Sea. The estimated thicknesses of sedimentary sequences that formed within certain time spans record the deposition history of the region. There is a 20-50 km wide deep suture between two basins of Mesozoic and Paleozoic ages in the eastern part of the region, where pre-Late Triassic reflectors have no clear correlation. The suture slopes eastward at a low angle and corresponds to a paleothrust according to seismic and modeling data. In the basement model, the suture is approximated by a zone of low magnetization and density, which is common to active fault systems. The discovery of the suture has important geological and exploration implications.     

Crustal studies of the Koyna region using explosion data from deep seismic soundings

This study is based on the seismic data collected as a result of explosions carried out during the 1976 and 1978 Deep Seismic Sounding (DSS) field operations in the Koyna region. These shots were exploded from twelve shot points by the National Geophysical Research Institute along the Guhagar-Chorochi and Kelsi-Loni profiles.Refraction studies of the records reveal a two-layered crust. The top layer consists 17 km of granite and the second layer 19 km of basalt, giving the average depth of the Moho as 36 km in the region. The velocities of the phases P_g, P* and P_n have been computed as 5.82 ± 0.01, 6.61 ± 0.05 and 8.23 ± 0.05 km/sec respectively and those of S_g, S* and S_n as 3.41 ± 0.00, 4.09 ± 0.07 and 4.60 ± 0.08 km/sec respectively. The shear wave velocity in the basement rock has been found to be lower in comparison with other regions of the peninsular India.In some cases reflections were recorded both from the Moho as well as from the intermediate layer. These reveal a crustal thickness of 39 km with 19 km of granitic and 20 km of basaltic layers.Coda signal durations from DSS explosions recorded by microearthquake seismographs indicate a lateral heterogeneity in the crust on either side of Karad in an east-west direction.     

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.     

Crustal structure of the northern part of the Proterozoic Cuddapah basin of India from deep seismic soundings and gravity data

A crustal depth section was obtained from Deep Seismic Soundings (DSS) along the Alampur-Koniki-Ganapeshwaram profile, cutting across the northern part of the Proterozoic Cuddapah basin, India, running just south of latitude 16° N and between longitude 78° E and 81°E. The existence of a low-angle thrust fault at the eastern margin of the Cuddapah basin (Kaila et al., 1979) was confirmed along a second profile. Another low-angle thrust, along which charnockites with the granitic basement are upthrust against the Dharwars was delineated further east. The contact of the khondalites (lower Precambrian) with quaternary sediments near the east coast of India seems to be a fault boundary, which may be responsible for the thick sedimentary accumulation in the adjoining offshore region.The basement in the western part of the Cuddapah basin is very shallow and is gently downdipping eastward, to a depth of 1.7 km about 20 km west of Atmakur. It attains a depth of about 4.5 km in the deepest part of the Kurnool sub-basin, around Atmakur. Under the Nallamalai ranges its depth varies between 3.5 and 6.5 km, with an easterly dip. In the region north of the Iswarkuppam dome, the basement is at a depth of about 5.0 km, to about 6.8 km in the eastern part of the Cuddapah basin. Outside the eastern margin of the basin, the depth of the basement is about 1.8 km and further eastwards it is exposed. A fault at the contact of the khondalites with quaternary sediments near the east coast brings the basement down to a depth of approximately 1.3 km.In the Kurnool sub-basin the depth to the Moho discontinuity varies from 35 km under Atmakur to 39 km under the Nallamalai hills. In the region of the Iswarkuppam dome it is at a depth of about 36 km, deepening to about 39 km before rising to 37 km towards the east. Two-dimensional velocity modelling using ray-tracing techniques tends to confirm these results.Gravity modelling of the crustal structure, utilizing a four-layer crustal model in most parts along this profile, conforms to the observed gravity values. A weak zone in the eastern part of the profile where high-density material (density 3.05 g/cm³) has been found seems to be responsible for the gravity high in that part.     

Crustal structure across southern Mexico inferred from gravity data

We present a gravity model of the crustal structure in southern Mexico based on interpretation of a detailed marine gravity profile perpendicularly across the Middle America Trench offshore from Acapulco, and a regional gravity transect extending into continental Mexico across the Sierra Madre del Sur, the central sector of the Trans-Mexican Volcanic Belt, the Sierra Madre Oriental, the Coastal Plain, and into the Gulf of Mexico. The elastic thickness of the Cocos lithospheric plate was found to be 30 km. In agreement with a previous seismic refraction study, no major differences in crustal structure were observed on both sides of the O’Gorman Fracture Zone. The gravity high seaward of the trench is interpreted as due to the incipient flexure and crustal thinning. The gravity low at the axis of the trench is explained by the increase in water depth and the existence of low-density accreted or continental-derived sediments (2.25 and 2.40 g/cm³). A gravity high of 50 mGal extending about 100 km landward is interpreted as caused by local shoaling of the Moho. The crust attains a thickness of 42 km under the Trans-Mexican Volcanic Belt but thins beneath the Coastal Plain and the continental slope of the Gulf of Mexico. Gravity highs around the Sierra de Tamaulipas are interpreted in terms of relief of the lower–upper crustal interface, implying a shallow basement.     

Crustal structure of the Eastern Alps along the TRANSALP profile from wide-angle seismic tomography

The objective of the TRANSALP project is an investigation of the Eastern Alps with regard to their deep structure and dynamic evolution. The core of the project is a 340-km-long seismic profile at 12°E between Munich and Venice. This paper deals with the P-wave velocity distribution as derived from active source travel time tomography. Our database consists of Vibroseis and explosion seismic travel times recorded at up to 100 seismological stations distributed in a 30-km-wide corridor along the profile. In order to derive a velocity and reflector model, we simultaneously inverted refractions and reflections using a derivative of a damped least squares approach for local earthquake tomography. 8000 travel time picks from dense Vibroseis recordings provide the basis for high resolution in the upper crust. Explosion seismic wide-angle reflection travel times constrain both deeper crustal velocities and structure of the crust–mantle boundary with low resolution. In the resulting model, the Adriatic crust shows significantly higher P-wave velocities than the European crust. The European Moho is dipping south at an angle of 7°. The Adriatic Moho dips north with a gentle inclination at shallower depths. This geometry suggests S-directed subduction. Azimuthal variations of the first-break velocities as well as observations of shear wave splitting reveal strong anisotropy in the Tauern Window. We explain this finding by foliations and laminations generated by lateral extrusion. Based on the P-wave model we also localized almost 100 local earthquakes recorded during the 2-month acquisition campaign in 1999. Seismicity patterns in the North seem related to the Inn valley shear zone, and to thrusting of Austroalpine units over European basement. The alignment of deep seismicity in the Trento-Vicenza region with the top of the Adriatic lower crust corroborates the suggestion of a deep thrust fault in the Southern Alps.     

Crustal structure beneath the east side of Pearl River Estuary from onshore-offshore seismic experiment

ABSTRACT

The land-sea transition zone in the northern South China Sea (SCS) records important information from the continental rifting to the seafloor spreading. The crustal structure is the key to explore the deep tectonic environment and the evolution of the SCS. In 2015, the onshore-offshore 3D deep seismic experiment was carried out on the Pearl River Estuary (PRE). Explosions and air guns were used as sources on land and at sea respectively in this experiment.Onshore seismic stations and Ocean Bottom Seismographs (OBSs) synchronously recorded the seismic signals. We focus on an onshore-offshore seismic profile (L2, SE-trending) along the eastern side of the PRE. By modelling the seismic travel times, we constructed a P-wave velocity model along the profile. The model shows that the sediment on land is thin and has seismic velocities of 4.5–5.5 km/s. In contrast, thickness of the offshore sediment gradually increases to more than 4.0 km, and the velocities vary between 2.0 km/s and 4.5 km/s. The onshore and offshore crustal velocities are 5.8–6.8 km/s and 5.5–6.8 km/s, respectively. At depth between 15 km and 20 km, a low-velocity layer (LVL; only 5.9 km/s) is detected, pinching out under the Littoral Fault Zone (LFZ). The LVL has probably accommodated the crustal extension beneath the land area, resulting in low extent of the crustal thinning. A slightly uplifted Moho exists beneath the Dongguan fault depression zone, representing a place where hot mantle materials ascend. Localized thickening of the sediments and rapid thinning of the crust characterize the LFZ, and it can be regarded as a tectonic boundary between the South China (SC) with normal continental crust and the northern SCS margin with extended continental crust.     

Crustal structure from two-ship multichannel seismic profiles on the continental shelf off Northwest Scotland

We report a series of two-ship multichannel seismic experiments carried out on the continental shelf off northwest Scotland in February 1981. Both expanding spread and constant offset profiles were shot using explosives and a 16.4-1 airgun. The expanding spread shot-receiver configurations yield detailed information on the variation of seismic velocity with depth, whilst constant offset profiles allow us to monitor lateral variations in the upper crustal velocity structure. We show an example of marked variations in the efficiency of compressional to shear wave conversion along a constant offset profile over shallow Lewisian basement west of the Outer Hebrides which we interpret as due to changes in the uppermost basement velocity structure. From profiles north of Cape Wrath we show that where the Precambrian Lewisian basement is buried beneath Mesozoic sediments of variable thickness we can use the constant offset profile of the basement refractor to strip off the effect of the sediments and so correct deeper crustal returns on the coincident expanding spread profiles.     

Crustal structure of the Gulf of Aden and the Red Sea

Seismic refraction profiles now number 9 in the Gulf of Aden and 15 in the Red Sea with a further intensive study by the Cambridge University group between latitudes 22 and 23°N. The results of these surveys indicate that the main trough of the Gulf of Aden is underlain by oceanic crust while only the deep axial zone and a questionable amount of the main trough of the Red Sea are underlain by oceanic crust.

Seismic reflexion profiles reveal the nature of Layer 1 and the upper surface of Layer 2. A strong subbottom reflector is found beneath the main trough of the Red Sea at 0.5 km but is found to be absent in the axial zone. This survey together with the refraction work and geological evidence suggests a complex history for the main trough of the Red Sea. Reflexion profiles and dredging in the Gulf of Aden indicate that the thickness of sediments increases away from the central rough zone and that the sediment is underlain by volcanic material.     

Granite-free area in Black Sea trough,from seismic data

Absence of granite in the crust under the central part of the Black Sea depression is confirined by the two methods. The sediments are underlain by basalt; granite is detectable only on the depression's periphery. Boundaries of the granite-free zone are clearly defined in the north and east of the sea; so far, no deep seismic sounding has been conducted in the west (fig. 5) where the "wedging-Out" of granite, toward the center of the depression, outside the competence of unaided seismological analysis, has been indicated on geomorphological premises (fig. 1).--V.P.Sokoloff.     

A possible Caledonide arm through the Barents Sea imaged by OBS data

The assembly of the crystalline basement of the western Barents Sea is related to the Caledonian orogeny during the Silurian. However, the development southeast of Svalbard is not well understood, as conventional seismic reflection data does not provide reliable mapping below the Permian sequence. A wide-angle seismic survey from 1998, conducted with ocean bottom seismometers in the northwestern Barents Sea, provides data that enables the identification and mapping of the depths to crystalline basement and Moho by ray tracing and inversion. The four profiles modeled show pre-Permian basins and highs with a configuration distinct from later Mesozoic structural elements. Several strong reflections from within the crystalline crust indicate an inhomogeneous basement terrain. Refractions from the top of the basement together with reflections from the Moho constrain the basement velocity to increase from 6.3 km s⁻¹ at the top to 6.6 km s⁻¹ at the base of the crust. On two profiles, the Moho deepens locally into root structures, which are associated with high top mantle velocities of 8.5 km s⁻¹. Combined P- and S-wave data indicate a mixed sand/clay/carbonate lithology for the sedimentary section, and a predominantly felsic to intermediate crystalline crust. In general, the top basement and Moho surfaces exhibit poor correlation with the observed gravity field, and the gravity models required high-density bodies in the basement and upper mantle to account for the positive gravity anomalies in the area. Comparisons with the Ural suture zone suggest that the Barents Sea data may be interpreted in terms of a proto-Caledonian subduction zone dipping to the southeast, with a crustal root representing remnant of the continental collision, and high mantle velocities and densities representing eclogitized oceanic crust. High-density bodies within the crystalline crust may be accreted island arc or oceanic terrain. The mapped trend of the suture resembles a previously published model of the Caledonian orogeny. This model postulates a separate branch extending into central parts of the Barents Sea coupled with the northerly trending Svalbard Caledonides, and a microcontinent consisting of Svalbard and northern parts of the Barents Sea independent of Laurentia and Baltica at the time. Later, compressional faulting within the suture zone apparently formed the Sentralbanken High.     

Crustal properties from S-wave and gravity data along a seismic refraction profile in Romania

VRANCEA'99 is a seismic refraction line that was carried out in 1999 to investigate the deep structure and physical properties of the upper lithosphere of the southeastern Carpathians and its foreland. It runs from the city of Bacau to the Danube River, traversing the Vrancea epicentral area of strong intermediate-depth seismicity and the city of Bucharest.

Interpretation of P-wave arrivals led to a velocity model that displays a multi-layered crust with velocities increasing with depth. The range of P wave velocities in the sedimentary cover increases from N to S and a structuring of the autochthonous basement of the Moesian Platform is observed. The crystalline crust displays thickness variations, but at the same time the lateral velocity structure along the seismic line remains almost constant. An intra-crustal boundary separates an upper crust from the lower crust. Within the upper mantle a low velocity zone is detected at a depth of about 55-km.

The interpretation of observable S-waves resulted in a velocity model that shows the same multi-layered crust, with S-velocities increasing similarly with depth as the P-waves. The corresponding Poisson's ratio is highly variable throughout the crust and ranges from 0.20–0.35 for the sedimentary cover to 0.22–0.25 for the crystalline crust. The interpretation of the V_p, V_s and Poisson's ratio in petrological terms suggests a large variety of rocks from sand and clay to sandstone, limestone and dolomite within the sedimentary cover. Within the crystalline crust the most probably rock types are granite, granodiorite, granite–gneiss and/or felsic amphibolite–gneiss in the upper part and gneiss and /or amphibolite in the lower part.

Based on the 2-D seismic velocity model, a density model is developed. Density values are assigned to each layer in agreement with the P-wave velocity model and with values accepted for the geological units in the area. After several iterations a good fit between the computed and observed Bouguer anomalies was obtained along the seismic line.     

Crustal structure of the southern Dabie ultrahigh-pressure orogen and Yangtze foreland from deep seismic reflection profiling

A new 140‐km‐long seismic reflection profile provides a high‐resolution crustal‐scale image of the southern Dabieshan high‐pressure (HP) metamorphic belt and the Yangtze foreland fold‐and‐thrust belt. The seismic image of the stacked section shows that the southern Dabieshan metamorphic terrane and Yangtze foreland belt are separated by a large north‐dipping fault. In the foreland the upper crust is dominated by a series of folds and thrusts formed during the collisional stage in the mid‐Triassic; it was reworked by crustal extension resulting in the formation of a late Jurassic and Cretaceous red‐bed basin. The southern Dabieshan profile shows stacked crustal slabs developed along the margin of the collisional orogenic belt. The Moho reflectors at 10–11 s (～30–33 km) are seismically prominent and segmented by a number of south‐verging thrusts that were probably developed by foreland‐directed thrusting of the deeply subducted continental crust during exhumation. The seismic reflection profile suggests that structures related to the Triassic–Jurassic subduction and exhumation of the Yangtze plate are preserved despite the severe crustal extension superimposed during the late Mesozoic and Cenozoic.     

Crustal structure and sedimentation history over the Alleppey platform,southwest continental margin of India:Constraints from multichannel seismic and gravity data

The Alleppey Platform is an important morphological feature located in the Kerala-Konkan basin off the southwest coast of India. In the present study, seismic reflection data available in the basin were used to understand the sedimentation history and also to carry out integrated gravity interpretation. Detailed seismic reflection data in the basin reveals that:(1) the Alleppey Platform is associated with a basement high in the west of its present-day geometry(as observed in the time-structure map of the Trap Top(K/T boundary)),(2) the platform subsequently started developing during the Eocene period and attained the present geometry by the Miocene and,(3) both the Alleppey platform and the Vishnu fracture zone have had significant impact on the sedimentation patterns(as shown by the time-structure and the isochron maps of the major sedimentary horizons in the region). The 3-D sediment gravity effect computed from the sedimentary layer geometry was used to construct the crustal Bouguer anomaly map of the region.The 3-D gravity inversion of crustal Bouguer anomaly exhibits a Moho depression below the western border of the platform and a minor rise towards the east which then deepens again below the Indian shield. The 2-D gravity modelling across the Alleppey platform reveals the geometry of crustal extension,in which there are patches of thin and thick crust. The Vishnu Fracture Zone appears as a crustal-scale feature at the western boundary of the Alleppey platform. Based on the gravity model and the seismic reflection data, we suggest that the basement high to the west of the present day Alleppey platform remained as a piece of continental block very close to the mainland with the intervening depression filling up with sediments during the rifting. In order to place the Alleppey platform in the overall perspective of tectonic evolution of the Kerala-Konkan basin, we propose its candidature as a continental fragment.     

Crustal structure under the Sydney Basin and Lachlan Fold Belt,determined from explosion seismic studies

The Lachlan Fold Belt has the velocity‐depth structure of continental crust, with a thickness exceeding 50 km under the region of highest topography in Australia, and in the range 41–44 km under the central Fold Belt and Sydney Basin. There is no evidence of high upper crustal velocities normally associated with marginal or back‐arc basin crustal rocks. The velocities in the lower crust are consistent with an overall increase in metamorphic grade and/or mafic mineral content with depth. Continuing tectonic development throughout the region and the negligible seismicity at depths greater than 30 km indicate that the lower crust is undergoing ductile deformation.

The upper crustal velocities below the Sydney Basin are in the range 5.75–5.9 km/s to about 8 km, increasing to 6.35–6.5 km/s at about 15–17 km depth, where there is a high‐velocity (7.0 km/s) zone for about 9 km evident in results from one direction. The lower crust is characterised by a velocity gradient from about 6.7 km/s at 25 km, to 7.7 km/s at 40–42 km, and a transition to an upper mantle velocity of 8.03–8.12 km/s at 41.5–43.5 km depth.

Across the central Lachlan Fold Belt, velocities generally increase from 5.6 km/s at the surface to 6.0 km/s at 14.5 km depth, with a higher‐velocity zone (5.95 km/s) in the depth range 2.5–7.0 km. In the lower crust, velocities increase from 6.3 km/s at 16 km depth to 7.2 km/s at 40 km depth, then increase to 7.95 km/s at 43 km. A steeper gradient is evident at 26.5–28 km depth, where the velocity is about 6.6—6.8 km/s. Under part of the area an upper mantle low‐velocity zone in the depth range 50–64 km is interpreted from strong events recorded at distances greater than 320 km.

There is no substantial difference in the Moho depth across the boundary between the Sydney Basin and the Lachlan Fold Belt, consistent with the Basin overlying part of the Fold Belt. Pre‐Ordovician rocks within the crust suggest fragmented continental‐type crust existed E of the Precambrian craton and that these contribute to the thick crustal section in SE Australia.     

Methane cycle in the Barents Sea

Biogeochemical cycle of methane in the Barents Sea was studied using isotope geochemistry to determine the rates of microbial methane oxidation. It was established that microbiological processes (glucose consumption, ¹⁴CO₂ assimilation, sulfate reduction, and slow methane oxidation) in oxidized surface and weakly reduced sediments are marked by only insignificant change in SO ₄ ^2? concentration and absence of notable growth of total alkalinity and N/NH₄ downward the sediment core. Microbial methane productivity was 0.111 × 10⁶ mol day^?1. Taking into account the volume of water column, microbial methane consumption therein can be as much as 1.8 × 10⁶ mol day^?1.