Average Crustal Velocity Research Articles

Enigmatic crustal and upper mantle structure in the NE Sino-Korean Craton based on nuclear explosion seismic data

We interpret the crustal and upper mantle structure along ∼2500 km long seismic profiles in the northeastern part of the Sino-Korean Craton (SKC). The seismic data with high signal-to-noise ratio were acquired with a nuclear explosion in North Korea as source. Seismic sections show several phases including Moho reflections (PmP) and their surface multiple (PmPPmP), upper mantle refractions (P), primary reflections (PxP, PL, P410), exceptionally strong multiple reflections from the Moho (PmPPxP), and upper mantle scattering phases, which we model by ray-tracing and synthetic seismograms for a 1-D fine-scale velocity model. The observations require a thin crust (30 km) with a very low average crustal velocity (ca. 6.15 km/s) and exceptionally strong velocity contrast at the Moho discontinuity, which can be explained by a thin Moho transition zone (< 5 km thick) with strong horizontal anisotropy. We speculate that this anisotropy was induced by lower crustal flow during delamination dripping. An intra-lithospheric discontinuity (ILD) at ∼75 km depth with positive velocity contrast is probably caused by the phase transformation from spinel to garnet. Delayed first arrivals followed by a long wave train of scattered phases of up to 4 s duration are observed in the 800–1300 km offset range, which are modelled by continuous stochastic velocity fluctuations in a low-velocity zone (LVZ) below the Mid-Lithospheric Discontinuity (MLD) between 120 and 190 km depth. The average velocity of this LVZ is about 8.05 km/s, which is much lower than the IASP91 standard model. This LVZ is most likely caused by rocks which are either partially molten or close to the solidus, which explains both low velocity and the heterogeneous structure.

Constraining crustal structure in the presence of sediment: a multiple converted wave approach

SUMMARYReceiver functions are sensitive to sharp seismic velocity variations with depth and are commonly used to constrain crustal thickness. The H–κ stacking method of Zhu &amp; Kanamori is often used to constrain both the crustal thickness (H) and ${V_P}$/${V_S}$ ratio ($\kappa $) beneath a seismic station using P-to-s converted waves (Ps). However, traditional H–κ stacks require an assumption of average crustal velocity (usually ${V_P}$). Additionally, large amplitude reverberations from low velocity shallow layers, such as sedimentary basins, can overprint sought-after crustal signals, rendering traditional H–$\ \kappa $ stacking uninterpretable. We overcome these difficulties in two ways. When S-wave reverberations from sediment are present, they are removed by applying a resonance removal filter allowing crustal signals to be clarified and interpreted. We also combine complementary Ps receiver functions, Sp receiver functions, and the post-critical P-wave reflection from the Moho (SPmp) to remove the dependence on an assumed average crustal ${V_P}$. By correcting for sediment and combining multiple data sets, the crustal thickness, average crustal P-wave velocity and crustal ${V_P}$/${V_S}$ ratio is constrained in geological regions where traditional H–$\ \kappa $ stacking fails, without making an initial P-wave velocity assumption or suffering from contamination by sedimentary reverberations.

Crustal P-wave velocity structure in the northeastern margin of the Qinghai-Tibetan Plateau and insights into crustal deformation

The transitional area between the northeastern margin of the Qinghai-Tibetan Plateau, Ordos Block and Alxa Block, also being the northern segment of the North-South Seismic Belt, is characterized by considerably high seismicity level and high risk of strong earthquakes. In view of the special tectonic environment and deep tectonic setting in this area, this study used two seismic wide-angle reflection/refraction cross profiles for double constraining, so as to more reliably obtain the fine-scale velocity structure characteristics in both the shallow and deep crust of individual blocks and their boundaries in the study area, and further discuss the seismogenic environment in seismic zones with strong historical earthquakes. In this paper, the P-wave data from the two profiles are processed and interpreted, and two-dimensional crustal velocity structure models along the two profiles are constructed by travel time forward modeling. The results show that there are great differences in velocity structure, shape of intra-crustal interfaces and crustal thickness among different blocks sampled by the two seismic profiles. The crustal thickness along the Lanzhou-Huianbu-Yulin seismic sounding profile (L1) increases from ~43 km in the western margin of Ordos Block to ~56 km in the Qilian Block to the west. In the Ordos Block, the velocity contours vary gently, and the average velocity of the crust is about 6.30 km s−1; On the other hand, the velocity structures in the crust of the Qilian Block and the arc-like tectonic zone vary dramatically, and the average crustal velocities in these areas are about 0.10 km s−1 lower than that of the Ordos Block. In addition, discontinuous low-velocity bodies (LVZ1 and LVZ2) are identified in the crust of the Qilian Block and the arc-like tectonic zone, the velocity of which is 0.10–0.20 km s−1 lower than that of the surroundings. The average crustal thickness of the Ordos Block is consistently estimated to be around 43 km along both Profile L2 (Tongchuan-Huianbu-Alashan left banner seismic sounding profile) and Profile L1. In contrast to the gently varying intra-crustal interfaces and velocity contours in the Ordos Block along Profile L1, which is a typical structure characteristic of stable cratons, the crustal structure in the Ordos Block along Profile L2 exhibits rather complex variations. This indicates the presence of significant structural differences in the crust within the Ordos Block. The crustal structure of the Helan Mountain Qilian Block and the Yinchuan Basin is featured by “uplift and depression” undulations, showing the characteristics of localized compressional deformation. Moreover, there are low-velocity zones with alternative high and low velocities in the middle and lower crust beneath the Helan Mountain, where the velocity is about 0.15–0.25 km s−1 lower than that of the surrounding areas. The crustal thickness of the Alxa Block is about 49 km, and the velocity contours in the upper and middle-lower crust of the block vary significantly. The complex crustal velocity structure images along the two seismic sounding profiles L1 and L2 reveal considerable structural differences among different tectonic blocks, their coupling relationships and velocity structural features in the seismic zones where strong historical earthquakes occurred. The imaging result of this study provides fine-scale crustal structure information for further understanding the seismogenic environment and mechanism in the study area.

Crust and uppermost-mantle structure of Greenland and the Northwest Atlantic from Rayleigh wave group velocity tomography

The Greenland landmass preserves ∼4 billion years of tectonic history, but much of the continent is inaccessible to geological study due to the extensive inland ice cap. We map out, for the first time, the 3-D crustal structure of Greenland and the NW Atlantic ocean, using Rayleigh wave anisotropic group velocity tomography, in the period range 10–80 s, from regional earthquakes and the ongoing GLATIS/GLISN seismograph networks. 1-D inversion gives a pseudo-3-D model of shear wave velocity structure to depths of ∼100 km with a horizontal resolution of ∼200 km. Crustal thickness across mainland Greenland ranges from ∼25 km to over 50 km, and the velocity structure shows considerable heterogeneity. The large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper ∼5–15 km. Within the upper continental basement, velocities are systematically lower in northern Greenland than in the south, and exhibit a broadly NW–SE trend. The thinning of the crust at the continental margins is also clearly imaged. Upper-mantle velocities show a clear distinction between typical fast cratonic lithosphere (Vs ≥4.6 km s−1) beneath Greenland and its NE margin and anomalously slow oceanic mantle (Vs ∼4.3–4.4 km s−1) beneath the NW Atlantic. We do not observe any sign of pervasive lithospheric modification across Greenland in the regions associated with the presumed Iceland hotspot track, though the average crustal velocity in this region is higher than that of areas to the north and south. Crustal anisotropy beneath Greenland is strong and complex, likely reflecting numerous episodes of tectonic deformation. Beneath the North Atlantic and Baffin Bay, the dominant anisotropy directions are perpendicular to the active and extinct spreading centres. Anisotropy in the subcontinental lithosphere is weaker than that of the crust, but still significant, consistent with cratonic lithosphere worldwide.

Crustal velocity structure of central Gansu Province from regional seismic waveform inversion using firework algorithm

The firework algorithm (FWA) is a novel swarm intelligence-based method recently proposed for the optimization of multi-parameter, nonlinear functions. Numerical waveform inversion experiments using a synthetic model show that the FWA performs well in both solution quality and efficiency. We apply the FWA in this study to crustal velocity structure inversion using regional seismic waveform data of central Gansu on the northeastern margin of the Qinghai-Tibet plateau. Seismograms recorded from the moment magnitude (MW) 5.4 Minxian earthquake enable obtaining an average crustal velocity model for this region. We initially carried out a series of FWA robustness tests in regional waveform inversion at the same earthquake and station positions across the study region, inverting two velocity structure models, with and without a low-velocity crustal layer; the accuracy of our average inversion results and their standard deviations reveal the advantages of the FWA for the inversion of regional seismic waveforms. We applied the FWA across our study area using three component waveform data recorded by nine broadband permanent seismic stations with epicentral distances ranging between 146 and 437 km. These inversion results show that the average thickness of the crust in this region is 46.75 km, while thicknesses of the sedimentary layer, and the upper, middle, and lower crust are 3.15, 15.69, 13.08, and 14.83 km, respectively. Results also show that the P-wave velocities of these layers and the upper mantle are 4.47, 6.07, 6.12, 6.87, and 8.18 km/s, respectively.

Joint Inversion of Body‐Wave Receiver Function and Rayleigh‐Wave Ellipticity

Abstract In recent years, surface‐wave dispersion has been used to image lithospheric structure jointly with receiver function or Rayleigh‐wave ellipticity. Because surface‐wave dispersion is the total propagation effect of the travel path, the joint inversion relies on dense seismic arrays or high seismicity to obtain local velocity structure. However, both receiver function and Rayleigh‐wave ellipticity are single‐station measurements with localized sensitivities and could be combined for joint inversion naturally. In this article, we explored the feasibility of the joint inversion of Rayleigh‐wave ellipticity and receiver function. We performed sensitivity tests with forward modeling and found that the receiver function is sensitive to sharp velocity interfaces but shows weak sensitivity to long‐wavelength structure, almost complementary to Rayleigh‐wave ellipticity. Therefore, joint inversion with two single‐station measurements provides tighter constraints on the velocity structure beneath the seismic station. A joint inversion algorithm based on the fast simulated‐annealing method is developed to invert Rayleigh‐wave ellipticity and receiver function for the lithospheric structure. Application of the algorithm to the Indian craton and the Williston basin in the United States demonstrates its effectiveness in reducing the nonuniqueness of the inversion. However, the joint inversion may fail to resolve the average crustal velocity, suggesting the need to combine surface‐wave dispersion (or other type of observations), receiver function, and Rayleigh‐wave ellipticity to more accurately resolve the velocity structure.

Deep seismic refraction experiment in northeast Brazil: New constraints for Borborema province evolution

The Borborema Province of northeastern Brazil is a major Proterozoic crustal province that, until now, has never been explored using deep crustal seismic methods. Here are reported the first results obtained from a high-quality seismic refraction/wide-angle reflection profile that has defined the internal seismic velocity structure and thickness of the crust in this region. Almost 400 recording stations were deployed in the Deep Seismic Refraction (DSR) experiment through an NW–SE ca. 900 km linear array and 19 shots were exploded at every 50 km along the line. Data from the 10 southeastern most shots of the seismic profile were processed in this work. The main features and geological structures crossed by the studied portion of the profile belong to the so-called Central Sub-province of the Borborema tectonic province. The crustal model obtained is compatible with a typical structure of extended crust. The model was essentially divided into three layers: upper crust, lower crust, and a half-space represented by the shallower portion of the mantle. The Moho is an irregular interface with depth ranging between 31.7 and 34.5 km, and beneath the Central Sub-province it varies from 31.5 to 33 km depth, where its limits are related to major crustal discontinuities. The distribution of velocities within the crust is heterogeneous, varying vertically from 5.7 to 6.3 km/s in the upper crust and from 6.45 to 6.9 km/s in the lower crust. From the average crustal velocity distribution it is evident that the Central Sub-province has seismic characteristics different from neighboring domains. The crust is relatively thin and crustal thickness variations in the profile are subtle due to stretching that occurred in the Cretaceous, during the fragmentation of Pangaea, opening of the South Atlantic Ocean and separation of South America from Africa.

Accretionary nature of the crust of Central and East Java (Indonesia) revealed by local earthquake travel-time tomography

Reassessment of travel time data from an exceptionally dense, amphibious, temporary seismic network on- and offshore Central and Eastern Java (MERAMEX) confirms the accretionary nature of the crust in this segment of the Sunda subduction zone (109.5–111.5E). Traveltime data of P- and S-waves of 244 local earthquakes were tomographically inverted, following a staggered inversion approach. The resolution of the inversion was inspected by utilizing synthetic recovery tests and analyzing the model resolution matrix. The resulting images show a highly asymmetrical crustal structure. The images can be interpreted to show a continental fragment of presumably Gondwana origin in the coastal area (east of 110E), which has been accreted to the Sundaland margin. An interlaced anomaly of high seismic velocities indicating mafic material can be interpreted to be the mantle part of the continental fragment, or part of obducted oceanic lithosphere. Lower than average crustal velocities of the Java crust are likely to reflect ophiolitic and metamorphic rocks of a subduction melange.

LITHO1.0: An updated crust and lithospheric model of the Earth

AbstractWe present the LITHO1.0 model, which is a 1° tessellated model of the crust and uppermost mantle of the Earth, extending into the upper mantle to include the lithospheric lid and underlying asthenosphere. The model is parameterized laterally by tessellated nodes and vertically as a series of geophysically identified layers, such as water, ice, sediments, crystalline crust, lithospheric lid, and asthenosphere. LITHO1.0 is created by constructing an appropriate starting model and perturbing it to fit high‐resolution surface wave dispersion maps (Love and Rayleigh, group and phase) over a wide frequency band (5–40 mHz). We examine and discuss the model with respect to key lithospheric parameters, such as average crustal velocity, crustal thickness, upper mantle velocity, and lithospheric thickness. We then compare the constructed model to those from a number of select studies at regional and global scales and find general consistency. It appears that LITHO1.0 represents a reasonable starting model of the Earth's shallow structure (crust and uppermost mantle) for the purposes in which these models are used, such as traveltime tomography or in efforts to create a 3‐D reference Earth model. The model matches surface wave dispersion over a frequency band wider than the band used in the inversion. There are several avenues for improving the model in the future by including attenuation and anisotropy, as well as making use of surface waves at higher frequency.

Lateral variation of crustal structure in the Ordos block and surrounding regions, North China, and its tectonic implications

Crustal thicknesses and Poissonʼs ratios in the Ordos block and surrounding regions were estimated by the use of the H–k stacking method on teleseismic receiver functions. The data came from 353 temporary and permanent seismic stations in 2006–2011. Results show that the crustal thickness and Poissonʼs ratio gently vary within the Ordos block, with an average of 41.3 km and 0.265, respectively, consistent with a felsic composition of the crust. Crustal thicknesses predicted on the basis of Airy isostasy are consistent with the estimated thicknesses, implying that the topography is approximately compensated. The reactivated portion of the North China Craton that has undergone Mesozoic–Cenzoic lithospheric thinning is also characterized by the thinning crust, while the Ordos block maintains normal crustal thickness and average crustal velocity. Inferred higher densities in the lower crust and the anti-correlation between Poissonʼs ratio and crustal thickness in the Ordos block may be the result from underplating of mafic magmas in the Precambrian. Around the Ordos block, the Paleoproterozoic khondalite zone in the northern edge has higher Poissonʼs ratio and thickened crust, which is consistent with the lower crust being of more mafic composition. The Weihe–Shanxi graben in the southeastern edge has mid-high Poissonʼs ratio, high heat flow and thinning crust, which is consistent with the known transtensional tectonic setting. In the Liupanshan thrust belt, along the southwestern edge of the Ordos plateau, significant variations in crustal thicknesses and Poissonʼs ratios occur on two sides. Besides a thickened crust, a concaved Moho implies horizontal shortening of this edge of the Ordos block due to its collision with the northeastern Tibetan Plateau. The structural differences between the eastern and western edges of the Ordos block reflect that the Ordos block is in tectonic stress environment of the western compression and the eastern extension.

A controlled‐source seismic and gravity study of the High Lava Plains (HLP) of Eastern Oregon

This study employs data collected during the High Lava Plains (HLP) controlled‐source seismic experiment conducted in September 2008. In this experiment, 2612 short‐period seismic recorders and 120 3‐component recorders were deployed across eastern Oregon and adjacent parts of Nevada and Idaho to record 15 seismic sources. Seismic and gravity data were integrated to create 2‐D crustal scale P‐wave velocity and density models for the ∼400 km long NW‐SE and N‐S profiles to provide a better understanding of the crustal and upper mantle structure and ultimately the magmatic and tectonic evolution of the region. These models are the first high‐resolution images beneath the path of migratory, bi‐modal volcanism that dotted the High Lava Plains beginning at 16 Ma. Our models show that the crustal structure across the HLP region is similar to that of the northern Basin and Range which has experienced extension since ∼35 Ma but with moderate magmatic modification of the crust. A thick layer (5–7 km) of sediments and volcanics extends over most of the area and is thickest in the Harney Basin area. We interpret denser/faster material in the lower to middle crust under the southern Harney Basin area to be mafic intraplating. We have also identified a region of denser/faster material in the upper crust in the vicinity of Oregon‐Idaho border. The crust thickens from 34 to 37 km, and the lower crust increases in density (2.8–2.85 g/cm3), from west to east across eastern Oregon in close proximity to the interpreted position of the 0.706 Sr isotope line, suggesting a decrease in extension. In the lowermost crust below the southeastern HLP, there is relatively high velocity (7.2–7.4 km/s) and density (2.95 g/cm3) that suggests the presence of underplating. The HLP region has undergone moderate extension, and the average crustal velocity is somewhat higher than in the adjacent Basin and Range, suggesting some magmatic modification in the lower crust, but not as much as might be expected given the voluminous surface volcanism.

A rootless rockies—Support and lithospheric structure of the Colorado Rocky Mountains inferred from CREST and TA seismic data

Support for the Colorado high topography is resolved using seismic data from the Colorado Rocky Mountain (CRM) Experiment and Seismic Transects. The average crustal thickness, derived from P wave receiver function imaging, is 48 km. However, a negative correlation between Moho depth and elevation is observed, which negates Airy‐Heiskanen isostasy. Shallow Moho (&lt;45 km depth) is found beneath some of the highest elevations, and therefore, the CRM are rootless. Deep Moho (45–51 km) regions indicate structure inherited from the Proterozoic assembly of the continent. Shear wave velocities from surface wave tomography are mapped to density employing empirical velocity‐to‐density relations in the crust and mantle temperature modeling. Predicted elastic plate flexure and gravity fields derived from the density model agree with observed long‐wavelength topography and Bouguer gravity. Therefore, low‐density crust and mantle are sufficient to support much of the CRM topography. Centers of Oligocene volcanism, e.g., the San Juan Mountains, display reduced crustal thicknesses and lowest average crustal velocities, suggesting that magmatic modification strongly influenced modern lithospheric structure and topography. Mantle velocities span 4.02–4.64 km/s at 73–123 km depth, and peak lateral temperature variations of 600°C are inferred. S wave receiver function imaging suggests that the lithosphere‐asthenosphere boundary is 100–150 km deep beneath the Colorado Plateau, and 150–200 km deep beneath the High Plains. A region of negative arrivals beneath the CRM above 100 km depth correlates with low mantle velocities and is interpreted as thermally modified/metasomatized lithosphere resulting from Cenozoic volcanism.

Tomography of crustalPandStravel times across the western United States

[1] Data from the USArray combined with local, regional, and national networks provide an unprecedented opportunity for imaging the crust of the western United States. We employ a simple tomography scheme to invert Pg and Sg travel times for velocity structure. The approach is analogous to Pn tomography: we assume a great circle arc between source and receiver and solve for station and event terms along with 2-D velocity structure. Pg velocities are high in the Snake River Plain, western Washington and the Columbia Plateau, and in central California. The Colorado Plateau falls midrange while the Basin and Range is slow, as are the Geysers region north of the San Francisco Bay and the Salton Trough. For Sg, the Snake River plain is fast, as is the entire Cordillera from the Southern Sierra Nevada up through the Cascades. Both the Colorado Plateau and most of the Rocky Mountains are also fast. At the highest point of the Rocky Mountains, the Aspen Anomaly is a low-velocity region. The Basin and Range is a composite of slower to the west abutting the Sierra Nevada and faster in eastern Nevada. Coastal California is slow, with the exception of the San Francisco Bay region and far northern California. The continental crust of the North American craton appears slightly fast for both Pg and Sg. Our observations and their trends compare well with middle to upper crustal velocity estimates from previous studies. They also compare reasonably well with average crustal velocities from the same studies.

A Crust and Upper-Mantle Model of Eurasia and North Africa for Pn Travel-Time Calculation

Abstract We develop a regional seismic travel-time (RSTT) model and method for use in routine seismic analysis. The model parameterization is a global tessellation of nodes with a velocity profile at each node. Interpolation of the velocity profiles generates a 3D crust and laterally variable upper-mantle velocity. The upper-mantle velocity profile at each node is represented as a linear velocity gradient, which enables travel-time computation in approximately 1 millisecond. This computational speed allows the model to be used in routine analyses in operational monitoring systems. We refine the model using a tomographic formulation that adjusts the average crustal velocity, mantle velocity at the Moho, and the mantle velocity gradient at each node. While the RSTT model is inherently global, our first RSTT tomographic effort covers Eurasia and North Africa, where we have compiled a data set of approximately 600,000 Pn arrivals. Ten percent of the data set is randomly selected and set aside for testing purposes. Travel-time residual variance for the validation data is reduced by 32%. Based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better, epicenter error using 16 Pn arrivals is reduced by 46% from 17.3 km (ak135 model) to 9.3 km (RSTT model) after tomography. The median uncertainty ellipse area is reduced by 68% from 3070 km 2 (ak135) to 994 km 2 (RSTT), and the number of ellipses with area less than 1000 km 2 , which is the area allowed for onsite inspection under the Comprehensive Nuclear Test Ban Treaty, is increased from 0% (ak135) to 51% (RSTT).

Southern African continental margin: Dynamic processes of a transform margin

Dynamic processes at sheared margins associated with the formation of sedimentary basins and marginal ridges are poorly understood. The southern African margin provides an excellent opportunity to investigate the deep crustal structure of a transform margin and to characterize processes acting at these margins by studying the Agulhas‐Falkland Fracture Zone, the Outeniqua Basin, and the Diaz Marginal Ridge. To do this, we present the results of the combined seismic land‐sea experiments of the Agulhas‐Karoo Geoscience Transect. Detailed velocity‐depth models show crustal thicknesses varying from ∼42 km beneath the Cape Fold Belt to ∼28 km beneath the shelf. The Agulhas‐Falkland Fracture Zone is embedded in a 50 km wide transitional zone between continental and oceanic crust. The oceanic crust farther south exhibits relatively low average crustal velocities (∼6.0 km/s), which can possibly be attributed to transform‐ridge intersection processes and the thermal effects of the adjacent continental crust during its formation. Crustal stretching factors derived from the velocity‐depth models imply that extension in the Outeniqua Basin acted on regional as well as more local scales. We highlight evidence for two episodes of crustal stretching. The first, with a stretching factor β of 1.6, is interpreted to have influenced the entire Outeniqua Basin. The stresses possibly originated from the beginning breakup between Africa and Antarctica (∼169–155 Ma). The second episode can be associated with a transtensional component of the shear motion along the Agulhas‐Falkland Transform from ∼136 Ma. This episode caused additional crustal stretching with β = 1.3 and is established to only have affected the southern parts of the basin. Crustal velocities directly beneath the Outeniqua Basin are consistent with the interpretation of Cape Supergroup rocks underlying most parts of the basin and the Diaz Marginal Ridge. We propose that the formation of this ridge can be either attributed to a transpressional episode along the Agulhas‐Falkland Transform or, more likely, to thermal uplift accompanying the passage of a spreading ridge to the south.

Seismological evidence for variable growth of crust along the Izu intraoceanic arc

The processes that create continental crust in an intraoceanic arc setting are a matter of debate. To address this issue, we conducted an active source wide‐angle seismic study to examine along‐arc structural variations of the Izu intraoceanic arc. The data used were acquired over a 550‐km‐long profile along the volcanic front from Sagami Bay to Tori‐shima. The obtained structural model showed the existence of felsic to intermediate composition middle crust with a P wave velocity (Vp) of 6.0–6.5 km s−1 in its upper part and 6.5–6.8 km s−1 in its lower part. The thickness of the middle crust varied markedly from 3 to 13 km. The underlying lower crust also consisted of two layers (Vp of 6.8–7.2 km s−1 in the upper part and Vp of 7.2–7.6 km s−1 in the lower part). The upper of these layers was interpreted to consist of plutonic gabbro, and the lower layer was interpreted to be mafic to ultramafic cumulates. Average crustal velocities calculated from our model showed remarkable lateral variation, which correlated well with arc volcanism. Low average crustal seismic velocities (∼6.7 km s−1), due to thick middle crust, were obtained beneath basaltic volcanoes (e.g., O‐shima, Miyake‐jima, Hachijo‐jima, Aoga‐shima), while higher average velocities (∼7.1 km s−1) were obtained beneath rhyolitic volcanoes (e.g., Nii‐jima, Kurose, South Hachijo caldera, Myoji knoll, and South Sumisu caldera). We concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the Izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution, or remelting of preexisting continental crust, or both.

Crustal Structure Features of the Leiqiong Depression in Hainan Province

AbstractSeismic data obtained from an about 210km long Baiyan‐Jianghong deep seismic sounding profile, which was completed in the Leiqiong area in 1984 and 1985, were processed by means of finite difference tomography, time term inversion and 2‐D dynamic ray tracing methods. And 2‐D inhomogeneous crustal structure images along the Baiyan‐Jianghong seismic line were determined. The results show that the crustal thickness in northern Hainan province and Leizhou Peninsula is about 25~26km. In northern Hainan Island embossment of the southern part of Qiongzhou Channel, the average crustal velocity is about 6.16km·s–1 and 5.70km·s–1 in the Leiqiong Depression area of northern segment of Qiongzhou Channel. The crustal velocity is particularly low and especially obvious in lower crust where it is only about 6.3~6.5km·s–1. The crystalline basement presents a velocity gradient belt, the thickness of which is about 2~4km and the velocity is approximately 5.5~6.0km·s–1. Moho, the velocities of which on its two sides are 6.5km·s–1 and 8.05km·s–1 respectively, shows one‐order velocity discontinuity, and there is no obvious crust‐mantle transitional zone to be found. Because the Baiyan‐Jianghong seismic line lies in the Leiqiong Depression area with basalt covering of the Pleistocene Period, it is suggested that the thinner crust and lower velocity in this area mainly reflect the multi‐erupting of volcanic magma from the Cenozoic till the late Quaternary as well as the underplating and delaminating of hot mantle to the crustal bottom which are interrelated with the crust being still in a comparatively hot state.

The major features of the crustal structure in north-eastern Venezuela from deep wide-angle seismic observations and gravity modelling

Venezuela is located on the plate boundary zone between the South American continent and the Caribbean plate. A relative movement of 2 cm/year is accommodated by a system of strike–slip faults running from the Andes to the Gulf of Paria. The Interior Range, a moderate-height mountain range, separates the Oriental Basin from the Caribbean. To the south, predominantly Precambrian rocks are outcropping in the Guayana Shield south of the Orinoco River. Results of deep wide-angle seismic measurements for the region were obtained during field campaigns in 1998 (ECOGUAY) for the Guayana Shield and in 2001 (ECCO) for the Oriental Basin. The total crustal thickness decreases from 45 km beneath the Guayana Shield, to 39 km at the Orinoco River, and 36 km close to El Tigre, in the center of the Oriental Basin. The average crustal velocity decreases in the same sense from 6.5 to 5.95 km/s. Detailed information was obtained on the velocity distribution within the Oriental Basin. Velocities are as low as 2.2 km/s for the uppermost 2 km, 4.5 km/s down to 4 km in depth, and a maximum depth of 13 km was derived for material with seismic velocities up to 5.9 km/s, interpreted as the base of the sedimentary basin. A gravimetric model confirms the structures derived from the seismic data. Discrete increases in sedimentary thickness along the basin may be associated to extension processes during the passive margin phase in the Cretaceous, or during earlier extension phases.

Crust–upper mantle seismic velocity structure across Southeastern China

One in-line wide-angle seismic profile was conducted in 1990 in the course of the Southeastern China Continental Dynamics project aimed at the study of the contact between the Cathaysia block and the Yangtze block. This 380-km-long profile extended in NW–SE direction from Tunxi, Anhui Province, to Wenzhou, Zhejiang Province. Five in-line shots were fired and recorded at seismic stations with spacing of about 3 km along the recording line. We have used two-dimensional ray tracing to model P- and S-wave arrivals and provide constraints on the velocity structure of the upper crust, middle crust, lower crust, Moho discontinuity, and the top part of the lithospheric mantle. P-wave velocity, S-wave velocity and V P/ V S ratio are mapped. The crust is 36-km thick on average, albeit it gradually thins from the northwest end to the southeast end (offshore) of the profile. The average crustal velocity is 6.26 km/s for P-waves but 3.6 km/s for S-waves. A relatively narrow low-velocity layer of about 4 km of thickness, with P- and S-wave velocities of 6.2 km/s and 3.5 km/s, respectively, marks the bottom of the middle crust at a depth of 23-km northwest and 17-km southeast. At the crust–mantle transition, the P- and S-wave velocity change quickly from 7.4 to 7.8 km/s (northwest) and 8.0 to 8.2 km/s (southeast) and from 3.9 to 4.2 km/s (northwest) and 3.9 to 4.5 km/s (southeast), respectively. This result implies a lateral contrast in the upper mantle velocity along the 140 km sampled by the profile approximately. The average V P/ V S ratio ranges from 1.68–1.8 for the upper crust to 1.75 for the middle and 1.75–1.85 for lower crust. With the interpretation of the wide-angle seismic data, Jiangshan–Shaoxin fault is considered as the boundary between the Yangtze and the Cathaysia block.

Composition and structure of the central Aleutian island arc from arc‐parallel wide‐angle seismic data

New results from wide‐angle seismic data collected parallel to the central Aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry. The 3‐D ray tracing/2.5‐D inversion of this sparse wide‐angle data set, which incorporates independent phase interpretations and new constraints on shallow velocity structure, produces a faster and smoother result than a previously published velocity model. Middle‐crustal velocities of 6.5–7.3 km/s over depths of ∼10–20 km indicate an andesitic to basaltic composition. High lower‐crustal velocities of 7.3–7.7 km/s over depths of ∼20–35 km are interpreted as ultramafic‐mafic cumulates and/or garnet granulites. The total crustal thickness is 35–37 km. This result indicates that the Aleutian island arc has higher velocities, and thus more mafic compositions, than average continental crust, implying that significant modifications would be required for this arc to be a suitable building block for continental crust. Lateral variations in average crustal velocity (below 10 km) roughly correspond to trends in major element geochemistry of primitive (Mg # &gt; 0.6) lavas. The highest lower‐crustal velocities (and presumably most mafic material) are detected in the center of an arc segment, between Unmak and Unalaska Islands, implying that arc segmentation exerts control over crustal composition.