Seismic Crustal Model Research Articles

Continental Hotspots Tracks From an Analysis of GOCE Gravity Gradients Data

AbstractThis study aims to detect residual thermal anomalies along the path of a hotspot beneath the continental lithosphere using gradiometry data. We calculate perturbations in gravity vectors and gradients by employing a simple thermal anomaly model. The results highlight that the horizontal components of Bouguer gravity gradients, sensitive to source structure and elongation, exhibit substantial magnitudes, exceeding current detection levels. To improve signals from sources of different sizes and orientations, we apply a filtering approach involving wavelet transforms and rotated local frames. We also assess the impact of lateral crustal variations on the gravity field by introducing random density anomalies and a seismic crustal model, testing the method's ability to distinguish between these anomalies and thermal sources. Using Gravity field and steady‐state Ocean Circulation Explorer satellite data, we generated scale‐orientation diagrams aiming to identify signals aligned with the direction of plate movement and with a spatial scale of a few hundred kilometers (scale of the plumes). Maps of filtered Bouguer gravity gradients aligned with hotspot trajectories are generated for all continental hot plumes, with the position and the age of the intraplate volcanism to support the interpretation. While we do not show a clear signal in regions that are too tectonically complex or with lateral variations in the crust or at the lithospheric boundary correlated with the hotspot trace, we found observed signals with expected sign and amplitude over Hoggar, Tibesti, Darfour and Cameroon tracks in Africa and over Iceland and Jan Mayen in Greenland. However, determining corresponding mass source depths remains a complex task.

Remaining non-isostatic effects in isostatic-gravimetric Moho determination—is it needed?

SUMMARY For long time the study of the Moho discontinuity (or Moho) has been a crucial topic in inferring the dynamics of the Earth's interior, and with profitable result it is mapped by seismic data, but due to the heterogeneous distribution of such data the quality varies over the world. Nevertheless, with the advent of satellite gravity missions, it is today possible to recover the Moho constituents (i.e. Moho depth; MD and Moho density contrast; MDC) via gravity observations based on isostatic models. Prior to using gravity observations for this application it must be stripped due to the gravitational contributions of known anomalous crustal density structures, mainly density variations of oceans, glacial ice sheets and sediment basins (i.e. stripping gravity corrections). In addition, the gravity signals related mainly with masses below the crust must also be removed. The main purpose of this study is to estimate the significance of removing also remaining non-isostatic effects (RNIEs) on gravity, that is, gravity effects that remain after the stripping corrections. This is carried out by using CRUST19 seismic crustal model and employing Vening Meinesz–Moritz (VMM) gravimetric-isostatic model in recovering the Moho constituents on a global scale to a resolution of 1° × 1°. To reach this goal, we present a new model, named MHUU22, formed by the SGGUGM2 gravitational field, Earth2014 topography, CRUST1.0 and CRUST19 seismic crustal models. Particularly, this study has its main emphasis on the RNIEs on gravity and Moho constituents to find out if we can modify the stripping gravity corrections by a specific correction of the RNIEs. The numerical results illustrate that the RMS differences between MHUU22 MD and the seismic model CRUST1.0 and least-squares combined model MOHV21 are reduced by 33 and 41 per cent by applying the NIEs, and the RMS differences between MHUU22 MDC and the seismic model CRUST1.0 and least-squares combined model MDC21 are reduced by 41 and 23 per cent when the above strategy for removing the RNIEs is applied. Hence, our study demonstrates that the specific correction for the RNIEs on gravity disturbance is significant, resulting in remarkable improvements in MHUU22, which more clearly visualize several crustal structures.

The uncertainty of CRUST1.0

Abstract As crustal structure models based on seismic and other data are frequently used as a-priori information for further geophysical and geological studies and interpretations (e. g., for gravity inversion), it is important to accurately document their qualities. For instance, the uncertainties in published crustal structures deeply affect the accuracies of produced Moho contour maps. The qualities in seismic crustal models arise from several factors such as the survey method, the spatial resolution of the survey (for example the spacing of the shot points and the recording stations), and the analytical techniques utilized to process the data. It is difficult to determine the uncertainties associated with seismic based crustal depth/Moho depth (MD) models, and even more difficult to use such data for estimating the Moho density contrast (MDC) and its accuracy. However, there is another important observable available today, namely global satellite gravitational data, which are fairly homogeneous v. r. t. accuracy and distribution over the planet. For instance, we find by simple error propagation, using the error covariance matrix of the GOCE TIM5 gravitational model, that this model can determine the MD to a global RMS error of 0.8 km with a resolution of about 1° for a known MDC of 200 kg / m 3 \text{kg}/{\text{m}^{3}} . However, the uncertainty in the MDC will further deteriorate the result. We present a new method for estimating the MD and MDC uncertainties of one model by comparing it with another (correlated or uncorrelated) model with known uncertainty. The method is applied in estimating the uncertainty for the CRUST1.0 MD model from four global models (CRUST19, MDN07, GEMMA1.0, KTH15C), yielding mean standard errors varying between 2 and 4.9 km in ocean regions and between 3.2 and 6.0 km on land regions with overall means of 3.8±0.4 and 4.8 ± 0.6 km 4.8\pm 0.6\hspace{0.1667em}\text{km} , respectively. Also, starting from the KTH15C MDC model, the mean standard error of CRUST1.0 MDC was estimated to 47.4 and 48.3 kg / m 3 \text{kg}/{\text{m}^{3}} for ocean and land regions, respectively.

Moho density contrast in Antarctica determined by satellite gravity and seismic models

SUMMARY As recovering the crust–mantle/Moho density contrast (MDC) significantly depends on the properties of the Earth's crust and upper mantle, varying from place to place, it is an oversimplification to define a constant standard value for it. It is especially challenging in Antarctica, where almost all the bedrock is covered with a thick layer of ice, and seismic data cannot provide a sufficient spatial resolution for geological and geophysical applications. As an alternative, we determine the MDC in Antarctica and its surrounding seas with a resolution of 1° × 1° by the Vening Meinesz-Moritz gravimetric-isostatic technique using the XGM2019e Earth Gravitational Model and Earth2014 topographic/bathymetric information along with CRUST1.0 and CRUST19 seismic crustal models. The numerical results show that our model, named HVMDC20, varies from 81 kg m−3 in the Pacific Antarctic mid-oceanic ridge to 579 kg m−3 in the Gamburtsev Mountain Range in the central continent with a general average of 403 kg m−3. To assess our computations, we compare our estimates with those of some other gravimetric as well as seismic models (KTH11, GEMMA12C, KTH15C and CRUST1.0), illustrating that our estimates agree fairly well with KTH15C and CRUST1.0 but rather poor with the other models. In addition, we compare the geological signatures with HVMDC20, showing how the main geological structures contribute to the MDC. Finally, we study the remaining glacial isostatic adjustment effect on gravity to figure out how much it affects the MDC recovery, yielding a correlation of the optimum spectral window (7≤ n ≤12) between XGM2019e and W12a GIA models of the order of ∼0.6 contributing within a negligible $ \pm 14$ kg m−3 to the MDC.

A New Moho Depth Model for Fennoscandia with Special Correction for the Glacial Isostatic Effect

In this study, we present a new Moho depth model in Fennoscandia and its surroundings. The model is tailored from data sets of XGM2019e gravitationl field, Earth2014 topography and seismic crustal model CRUST1.0 using the Vening Meinesz-Moritz model based on isostatic theory to a resolution of 1° × 1°. To that end, the refined Bouguer gravity disturbance is determined by reducing the observed field for gravity effect of topography, density heterogeneities related to bathymetry, ice, sediments, and other crustal components. Moreover, stripping of non-isostatic effects of gravity signals from mass anomalies below the crust due to crustal thickening/thinning, thermal expansion of the mantle, Delayed Glacial Isostatic Adjustment (DGIA), i.e., the effect of future GIA, and plate flexure has also been performed. As Fennoscandia is a key area for GIA research, we particularly investigate the DGIA effect on the gravity disturbance and gravimetric Moho depth determination in this area. One may ask whether the DGIA effect is sufficiently well removed in the application of the general non-isostatic effects in such an area, and to answer this question, the Moho depth is determined both with and without specific removal of the DGIA effect prior to non-isostatic effect and Moho depth determinations. The numerical results yield that the RMS difference of the Moho depth from our model HVMD19 vs. the seismic CRUST19 and GRAD09 models are 3.8/4.2 km and 3.7/4.0 km when the above strategy for removing the DGIA effect is/is not applied, respectively, and the mean value differences are 1.2/1.4 km and 0.98/1.4 km, respectively. Hence, our study shows that the specific correction for the DGIA effect on gravity disturbance is slightly significant, resulting in individual changes in the gravimetric Moho depth up to − 1.3 km towards the seismic results. On the other hand, our study shows large discrepancies between gravimetric and seismic Moho models along the Norwegian coastline, which might be due to uncompensated non-isostatic effects caused by tectonic motions.

Rock probes and seismic methods to constrain the structure, composition and evolution of the deep crust beneath North China Block

<p indent="0mm">Deep continental crust archives critical information on the bulk crustal composition as well as its formation. It is commonly the inherent zone for complex crust-mantle interaction. Unlike the shallow crust, the deep crust is generally inaccessible for direct geological sampling. Instead, knowledge of the deep crust can be obtained from a combination study of geophysical techniques and “lithoprobe” (deep crustal fragments entrained in magmatic outcrops). Wide-angle reflection/refraction seismic profiles have the highest resolution on crustal velocity structure and provide information on the averaged present-day nature of large deep crust domains; “lithoprobe”, instead, give detailed information of locality-specific deep crustal sections through determining the physical and chemical properties, dating the formation and alteration ages and constraining the melting and equilibration depths of the xenoliths. Here, the two methodologies are integrated to constrain the crustal structure on the North China Block. The seismic crustal structure model of the block is compiled from all available seismic wide-angle reflection/refraction profiles and receiver function results. The model parameters include Moho depth, <italic>V</italic>p velocity and thicknesses for four crustal layers (sedimentary cover, and the upper, middle and lower crust). The crust as a whole is thin (&lt; <sc>36 km)</sc> in the eastern part relative to the western part <sc>(38−44 km).</sc> In addition, the crust is shown to have a thick upper and middle crust and a relatively thin (typically <sc>&lt;5 km)</sc> lower mafic crust; no spatial variations in average crustal <italic>V</italic>p velocity and densities are observed. The average crustal <italic>V</italic>p and density for the whole North China Block are <sc>6.33−6.37 km/s</sc> and <sc>2.72−2.78 g/cm³,</sc> respectively. The overall absence of high P-wave velocity layer and the relatively low average <italic>V</italic>p and density indicate that the block has an intermediate to felsic bulk crustal composition. In contrast, regional crustal xenoliths include mafic to felsic granulites and amphibolites and rare eclogite xenoliths (e.g., Xinyang and Xuhuai), indicating the presence of thick lower crust (e.g., <sc>&gt;45 km)</sc> at that time. Meanwhile, mafic xenoliths in Hannuoba and Nüshan show strong mafic underplating at the lower crust in the Meso-Cenozoic while no high-velocity layer is shown in the seismic crustal model. These inconsistences can be explained by the fact that the two methodologies tap the crustal information at different timeslices and dimensions and be complementary to understand the chemical and physical structure of the deep crust. Therefore, the combination study of petrological and geochemical of the crustal xenoliths and geophysical observations can provide essential information on making and destroying the deep continental crust through time.

Identification of Cameroon's geological structures through a gravity separation and using seismic crustal models

Residual gravity data provide useful information for a geophysical interpretation of geological sources. Several techniques have been developed and applied to separate a different content in gravity data. Nevertheless, few methods actually incorporate factual geophysical or geological information in gravity data processing. Instead, most of these methods relay only on mathematical techniques that do not necessarily reflect the actual geological composition and problems inherent to gravity inversion, most notably a non-uniqueness of solutions to gravimetric inverse problems (i.e. infinity many geological configurations could theoretically provide the same gravity solution) as well as the fact that gravimetric inverse solutions are generally ill-posed problems that require employment of regularization techniques. To partially address this issue, we apply a gravimetric forward modelling based on available seismic crustal structure models to separate a residual gravity signal from observed gravity data in order to interpret more realistically detailed geological and tectonic features. This procedure involves the estimation of the uppermost mantle gravity contribution that is treated as a regional gravity data. The residual gravity data is then obtained by subtracting the regional gravity data from observed gravity data. In addition, observed gravity data are typically corrected for the gravitational contributions of topography and bathymetry (and possibly other known crustal density heterogeneities, such as sediments and density variations within the underlying consolidated crust). The application of these gravity corrections to observed (free-air) gravity data yields the Bouguer gravity data. We employ this forward modelling approach to the Bouguer gravity disturbances over Cameroon. Furthermore, we apply a regional-residual separation based on a spectral analysis and a robust polynomial fitting. The performance of these three techniques is compared in terms of their interpretational quality. The numerical analysis reveals that the residual gravity disturbance map obtained by applying the forward modelling technique resembles main known geological and tectonic configuration and closely reproduces even more detailed features. The results from using a spectral analysis could to some extent reproduce realistically tectonic and geological structures, except for more localized features otherwise seen in the forward modelling result. A robust polynomial fitting, on the other hand, fails to provide residual gravity disturbances that realistically reflect geological and tectonic configuration of the study area. The reason is that this method produces either only positive or only negative gravity disturbances, but not both. Moreover, this merely mathematical technique does not take into consideration available geological and geophysical information.

Updated Antarctic crustal model

We use seismic data together with a subglacial bedrock relief from the BEDMAP2 database to obtain a new three-layer model of the consolidated (crystalline) crust of Antarctica that locally improves the global seismic crustal model CRUST1.0. We collect suitable data for constructing crustal layers, analyse them and build maps of the crustal layer thickness and seismic velocities. We use the subglacial relief according to a tectonic configuration and then interpolate data using a statistical kriging method. The P-wave velocity information from old seismic profiles have been supplemented with the new shear-wave velocity models. We adjust the thickness of crustal layers by multiplying a total crustal thickness by a percentage ratio of each individual layer at each point. Our results reveal large variations in seismic velocities between different crustal blocks forming Antarctica. The most pronounced differences exist between East and West Antarctica. In East Antarctica, a high P-wave velocity (vP > 7 km/s) layer in the lower crust is absent. The P-wave velocity in the lower crust changes from 6.1 km/s beneath the Lambert Rift to 6.9 km/s beneath the Wilkes Basin. In West Antarctica, a thick mafic lower crust is characterized by large P-wave velocities, ranging from 7.0 km/s under the Ross Sea to 7.3 km/s under the Byrd Basin. In contrast, velocities in the lower crust beneath the Transantarctic and Ellsworth-Whitmore Mountains are ~6.8 km/s. The P-wave velocities in the upper crust in East Antarctica are within the range 5.5–6.4 km/s. The upper crust of West Antarctica is characterized by the P-wave velocities of 5.6–6.3 km/s. The P-wave velocities in the middle crust vary within 5.9–6.6 km/s in East Antarctica and within 6.3–6.5 km/s in West Antarctica. A low-velocity layer (5.8–5.9 km/s) is detected at depth of ~20–25 km beneath the Princes Elizabeth Land.

Crustal structure of the Azores Archipelago from Rayleigh wave ellipticity data

SUMMARYDetermining the crustal structure of ocean island volcanoes is important to understand the formation and tectonic evolution of the oceanic lithosphere and tectonic swells in marine settings, and to assess seismic hazard in the islands. The Azores Archipelago is located near a triple junction system and is possibly under the influence of a mantle plume, being at the locus of a wide range of geodynamic processes. However, its crustal structure is still poorly constrained and debated due to the limited seismic coverage of the region and the peculiar linear geometry of the islands. To address these limitations, in this study we invert teleseismic Rayleigh wave ellipticity measurements for 1-D shear wave speed (VS) crustal models of the Azores Archipelago. Moreover, we test the reliability of these new models by using them in independent moment tensor inversions of local seismic data and demonstrate that our models improve the waveform fit compared to previous models. We find that data from the westernmost seismic stations used in this study require a shallower Moho depth (∼10 km) than data from stations in the eastern part of the archipelago (∼13–16 km). This apparent increase in the Moho depth with increasing distance from the mid-Atlantic ridge (MAR) is expected. However, the rate at which Moho deepens away from the MAR is greater than that predicted from a half-space cooling model, suggesting that local tectonic perturbations have modified crustal structure. The 1-D VS models obtained beneath the westernmost seismic stations also show higher wave speeds than for the easternmost stations, which correlates well with the ages of the islands except Santa Maria Island. We interpret the relatively low VS profile found beneath Santa Maria Island as resulting from underplating, which agrees with previous geological studies of the island. Compared to a recent receiver function study of the region, the shallow structure (top ∼2 km) in our models shows lower shear wave speed, which may have important implications for future hazard studies of the region. More generally, the new seismic crustal models we present in this study will be useful to better understand the tectonics, seismicity, moment tensors and strong ground motions in the region.

Estimating a combined Moho model for marine areas via satellite altimetric - gravity and seismic crustal models

Isostasy is a key concept in geoscience in interpreting the state of mass balance between the Earth’s lithosphere and viscous asthenosphere. A more satisfactory test of isostasy is to determine the depth to and density contrast between crust and mantle at the Moho discontinuity (Moho). Generally, the Moho can be mapped by seismic information, but the limited coverage of such data over large portions of the world (in particular at seas) and economic considerations make a combined gravimetric-seismic method a more realistic approach. The determination of a high-resolution of the Moho constituents for marine areas requires the combination of gravimetric and seismic data to diminish substantially the seismic data gaps. In this study, we estimate the Moho constituents globally for ocean regions to a resolution of 1° × 1° by applying the Vening Meinesz-Moritz method from gravimetric data and combine it with estimates derived from seismic data in a new model named COMHV19. The data files of GMG14 satellite altimetry-derived marine gravity field, the Earth2014 Earth topographic/bathymetric model, CRUST1.0 and CRUST19 crustal seismic models are used in a least-squares procedure. The numerical computations show that the Moho depths range from 7.3 km (in Kolbeinsey Ridge) to 52.6 km (in the Gulf of Bothnia) with a global average of 16.4 km and standard deviation of the order of 7.5 km. Estimated Moho density contrasts vary between 20 kg m-3 (north of Iceland) to 570 kg m-3 (in Baltic Sea), with a global average of 313.7 kg m-3 and standard deviation of the order of 77.4 kg m-3. When comparing the computed Moho depths with current knowledge of crustal structure, they are generally found to be in good agreement with other crustal models. However, in certain regions, such as oceanic spreading ridges and hot spots, we generally obtain thinner crust than proposed by other models, which is likely the result of improvements in the new model. We also see evidence for thickening of oceanic crust with increasing age. Hence, the new combined Moho model is able to image rather reliable information in most of the oceanic areas, in particular in ocean ridges, which are important features in ocean basins.

Moho Density Contrast in Central Eurasia from GOCE Gravity Gradients

Seismic data are primarily used in studies of the Earth’s inner structure. Since large parts of the world are not yet sufficiently covered by seismic surveys, products from the Earth’s satellite observation systems have more often been used for this purpose in recent years. In this study we use the gravity-gradient data derived from the Gravity field and steady-state Ocean Circulation Explorer (GOCE), the elevation data from the Shuttle Radar Topography Mission (SRTM) and other global datasets to determine the Moho density contrast at the study area which comprises most of the Eurasian plate (including parts of surrounding continental and oceanic tectonic plates). A regional Moho recovery is realized by solving the Vening Meinesz-Moritz’s (VMM) inverse problem of isostasy and a seismic crustal model is applied to constrain the gravimetric solution. Our results reveal that the Moho density contrast reaches minima along the mid-oceanic rift zones and maxima under the continental crust. This spatial pattern closely agrees with that seen in the CRUST1.0 seismic crustal model as well as in the KTH1.0 gravimetric-seismic Moho model. However, these results differ considerably from some previously published gravimetric studies. In particular, we demonstrate that there is no significant spatial correlation between the Moho density contrast and Moho deepening under major orogens of Himalaya and Tibet. In fact, the Moho density contrast under most of the continental crustal structure is typically much more uniform.

Sensitivity analysis of crustal correction for calculation of lithospheric mantle density from gravity data

We investigate how uncertainties in seismic and density structure of the crust propagate to uncertainties in mantle density structure. The analysis is based on interpretation of residual upper-mantle gravity anomalies which are calculated by subtracting (stripping) the gravitational effect of the crust from the observed satellite gravity field data (GOCE Direct release 3). Thus calculated residual mantle gravity anomalies are caused mainly by a heterogeneous density distribution in the upper mantle. Given a relatively small range of expected compositional density variations in the lithospheric mantle, knowledge on uncertainties associated with incomplete information on crustal structure is of utmost importance for progress in gravity modelling. Uncertainties in the residual upper-mantle gravity anomalies result chiefly from uncertainties in (i) seismic VP velocity–density conversion for the crust and (ii) uncertainties in the seismic crustal structure (thickness and average VP velocities of individual crustal layers, including the sedimentary cover). We examine the propagation of these uncertainties into determinations of lithospheric mantle density and analyse both sources of possible uncertainties by applying different velocity-to-density conversions and by introducing variations into the crustal structure which correspond to typical resolution of high-quality and low-quality seismic models. We apply our analysis to Siberia (the West Siberian Basin and the Siberian Craton) for which a new regional seismic crustal model, SibCrust, has recently become available. For the same region, we also compute upper-mantle gravity and density anomalies based on three global crustal models (CRUST 5.1, CRUST 2.0 and CRUST 1.0) and compare the results based on four different crustal models. A large uncertainty in the VP-to-density conversion may result in the uncertainty in lithospheric mantle density anomalies of ca. 0.02–0.03 g cm−3 (i.e. 0.5–1 per cent, which is comparable to compositional density anomalies expected for continental lithosphere mantle). Similar values of uncertainties may be caused by a 0.2 km s−1 error in average crustal VP velocities or by a 2 km uncertainty in the Moho depth. One of the largest uncertainties is caused by errors in thickness of the sedimentary layer, and a 2 km error leads to ca. 0.03 g cm−3 error in lithospheric mantle densities. Large deviations (locally ±10 km) of the Moho depth in global crustal models (CRUST 5.1, CRUST2.0 and CRUST1.0) from the high-resolution regional seismic model of the crust, SibCrust, may produce artefact residual mantle gravity anomalies of up to ±150 mGal locally, caused by large errors in crustal gravity corrections. These errors in gravity anomalies produce up to ca. 0.04 g cm−3 (ca. 1.2 per cent) errors in density of the lithospheric mantle, which may well correspond to the amplitude of real density anomalies in the mantle. Our results demonstrate that gravity modelling alone cannot reliably constrain the crustal structure, including the Moho depth and thickness of sediments.

NACr14: A 3D model for the crustal structure of the North American Continent

Based on the large number of crustal seismic experiments carried out in the last decades we create NACr14, a 3D crustal model of the North American continent at a resolution of 1°×1°. We present maps of thickness and average velocities of the main layers that comprise the North American crystalline crust, obtained from the most recent seismic crustal models within the USGS crustal structure database. However, the crustal data are unevenly distributed and in some cases discrepancies exist between published models. In order to construct a consistent 3D crustal model with three layers in the crystalline crust, we refrained from a direct interpolation of the crustal seismic parameters in the database. Instead, we implemented the following sequence of steps: 1. Definition of the geometry of the main tectonic provinces of North America; 2. Selection and evaluation of the reliability of seismic crustal models in the database; 3. Estimation of the P-wave seismic velocity and thickness of the upper, middle and lower crust for each tectonic province; 4. Estimation of the interpolated Pn velocity distribution. The resulting average velocity of the crystalline crust is mostly consistent with that of the seismic points. The main variations of the structure of the crystalline crust of North America displayed in the model can be related to its tectonic evolution. The model, available in a digital form, can be used in various geophysical applications, such as the correction for the crustal effects in gravity and seismic tomography and models of dynamic topography, in order to detect heterogeneities characterizing the underlying upper mantle.

Seismic structures of the 154–160 Ma oceanic crust and uppermost mantle in the Northwest Pacific Basin

We present detailed P-wave velocity models for the Northwest Pacific Basin which were produced in 154–160 Ma at a high seafloor spreading half-rate of >8 cm/yr and have not been appreciably deformed by tectonic or igneous activity since then. We carried out wide-angle seismic experiments on two crossing survey lines which are respectively parallel and perpendicular to paleomagnetic lineations. The seismic crustal models for both lines are almost identical and homogeneous along these lines. The crust consists of an upper layer (Layer 2) with a P-wave velocity V p = 2.5–6.8 km/s and a thickness of 1.3–2.2 km, and a lower layer (Layer 3) with a velocity of 6.8–7.1 km/s and a thickness of 4.6–5.9 km. These characteristics indicate that the crust beneath the survey line has a standard oceanic crustal structure. The structure of the uppermost mantle of the line parallel to the seafloor spreading direction exhibits considerable V p heterogeneity within 5 km immediately below the Moho and shows an unusually high V p of 8.5–8.7 km/s. The P n velocity for the perpendicular line is 7.9 km/s, and the magnitude of the velocity anisotropy for the uppermost mantle amounts to a large value of 7–10%.

Crustal structure of the ocean-island arc transition at the mid Izu-Ogasawara (Bonin) arc margin

Abstract Wide-angle refraction experiments were conducted to reveal the crustal structure at the transition between the intra-oceanic island arc crust of the mid Izu-Ogasawara (Bonin) arc and the backarc oceanic crust of the Shikoku Basin. The island arc crust consists of an upper crust about 5 km thick with a P-wave velocity &lt;6.0 km/s, a middle crust 5 km thick with a P-wave velocity of 6.0–6.3 km/s, and a lower crust 10 km thick with a P-wave velocity of 6.8–7.2 km/s. The total crustal thickness is about 20 km. The thickness thins to approximately 6 km over a distance of 30 km at the western margin of the Izu-Ogasawara arc (IOA). These features are very similar to those of the northern IOA, which indicates that the crustal structure is relatively constant within 200 km at the northern and mid IOA. The Kinan Escarpment, a 500-km-long fault with a maximum offset of 800 m, characterizes the transition zone between the IOA and Shikoku Basin. The seismic crustal model indicates that the escarpment is a fault which tears the whole oceanic crust along the western margin of the IOA. However, no significant differences exist in the crustal structure on either side of the escarpment, and the Kinan Escarpment seems to be a zone of the structural weakness from its birth.

Last stage of the Japan Sea back‐arc opening deduced from the seismic velocity structure using wide‐angle data

The Japan Sea is one of the most well studied back‐arc basins in the northwestern Pacific. The seismic crustal model, however, has been inadequate to elucidate the detailed opening model of the Japan Sea. In 2002, to clarify the late stage of the formation style of the Japan Sea opening, a seismic experiment using 35 ocean bottom seismographs (OBSs), an air gun array, and a multichannel hydrophone streamer was undertaken in the areas from the southwestern Yamato Basin, the Oki Ridge, and the southwestern Oki Trough to the coast of the southwestern Japan Island Arc. The crusts beneath the southwestern Yamato Basin and the Oki Ridge are estimated as having approximately 13 km and 19.5 km, respectively. The upper and lower crusts of the southwestern Yamato Basin are approximately 3.2 km and 8 km thick, respectively. Those of the Oki Ridge are approximately 8.2 km and 10.5 km thick, respectively. The upper crust of the Oki Ridge thickens more steeply than that of the southwestern Yamato Basin; however, the lower crust thickens more gently. The crustal structure of the southwestern Yamato Basin shows the extended continental crust accompanied with the opening of the Japan Sea. A remarkable structural characteristic, the upper crust being thinner than the lower crust, caused by listric or complicated normal faults developed in the upper crust of the southwestern Yamato Basin. This deformed upper crust is a common structural characteristic in the southern Japan Sea, which includes the Yamato Basin. The southern Yamato Basin, including the southwestern Yamato Basin, has the thinnest upper and lower crusts in the Japan Sea. For that reason, it is suggested that the southern Yamato Basin had the strongest deformation by a back‐arc opening and that the period of the opening in the southern Yamato Basin had been longest in the southern Japan Sea. The formation process of the southern Yamato Basin is inferred to have two stages: rifting and extension of continental crust separating the northeastern and southwestern Japan Island Arcs from the Asian continent and, further, the extension affected by the rotation of the southwestern Japan Island Arc.

Deep seismic refraction and gravity crustal model and tectonic deformation in Tocantins Province, Central Brazil

Interpretation of seismic refraction data in the central sector of Tocantins Province, Central Brazil, has produced a seismic crustal model with well-defined upper, intermediate, and lower crust layers having smooth velocity gradient in each layer. The depths to Moho vary from 32 to 43 km, and mean crustal P velocity varies from 6.3 km/s, beneath Goiás magmatic arc on the western side, to 6.4 km/s, below Goiás massif in the central portion and the foreland fold-and-thrust belt on the eastern side. The behaviour of the lower crust layer allows an improved understanding of regional gravimetric features of the central and northern sectors of Tocantins Province and suggests subduction of the Amazon plate in Central Brazil. In the southeastern sector, the refraction experiment resulted in the detection of a thinner crust (38 km) below Brasília fold belt and a thicker crust (41 km) below Paraná basin and São Francisco craton (42 km). The upper crust beneath Paraná Basin is around 20 km thick, whereas it is less than 10 km thick below the craton. These results bring new insights into the geological history of the central and southeastern sectors of Tocantins Province. Gravimetric measurements in the central sector of Tocantins Province delineate a high and a low anomaly separated by a steep gradient with a NE direction. The axis of the gradient seems to bend still further to NE in the northern sector of that province, whereas the gravimetric high continues northwards, defining a separation between them. This suggests that those features belong to different tectonic processes that occurred during Tocantins Province orogenesis. The gravimetric model, which incorporates seismically resolved structure beneath Tocantins Province, better matches the observed gravimetric data. Although tectonic movements have only been monitored with high-precision GPS for short time interval (1999–2001), the results suggest observable deformations. The main seismicity of Central Brazil, the Goiás–Tocantins seismic belt, seems to be spatially associated with the large gravimetric high anomaly and with the observed tectonic deformation.

Gravity modeling of Bushveld Complex connectivity supported by Southern African Seismic Experiment results

Research Article| June 01, 2004 Gravity modeling of Bushveld Complex connectivity supported by Southern African Seismic Experiment results Susan J. Webb; Susan J. Webb School of Geosciences, Private Bag 3, University of the Witwatersrand, Wits, 2050, South Africa e-mail:webbs@geosciences.wits.ac.za and cawthorng@geosciences.wits.ac.za Search for other works by this author on: GSW Google Scholar R. Grant Cawthorn; R. Grant Cawthorn School of Geosciences, Private Bag 3, University of the Witwatersrand, Wits, 2050, South Africa e-mail:webbs@geosciences.wits.ac.za and cawthorng@geosciences.wits.ac.za Search for other works by this author on: GSW Google Scholar Teresia Nguuri; Teresia Nguuri School of Geosciences, Private Bag 3, University of the Witwatersrand, Wits, 2050, South Africa CTBTO, Preparatory Commission, Vienna International Centre, P.O. Box 1200, 1400 Vienna, Austria e-mail:teresia.nguuri@ctbto.org Search for other works by this author on: GSW Google Scholar David James David James Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Rd. N.W., Washington, D.C., 20015 USA e-mail: james@dtm.ciw.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Susan J. Webb School of Geosciences, Private Bag 3, University of the Witwatersrand, Wits, 2050, South Africa e-mail:webbs@geosciences.wits.ac.za and cawthorng@geosciences.wits.ac.za R. Grant Cawthorn School of Geosciences, Private Bag 3, University of the Witwatersrand, Wits, 2050, South Africa e-mail:webbs@geosciences.wits.ac.za and cawthorng@geosciences.wits.ac.za Teresia Nguuri School of Geosciences, Private Bag 3, University of the Witwatersrand, Wits, 2050, South Africa CTBTO, Preparatory Commission, Vienna International Centre, P.O. Box 1200, 1400 Vienna, Austria e-mail:teresia.nguuri@ctbto.org David James Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Rd. N.W., Washington, D.C., 20015 USA e-mail: james@dtm.ciw.edu Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2003 Geological Society of South Africa South African Journal of Geology (2004) 107 (1-2): 207–218. https://doi.org/10.2113/107.1-2.207 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Susan J. Webb, R. Grant Cawthorn, Teresia Nguuri, David James; Gravity modeling of Bushveld Complex connectivity supported by Southern African Seismic Experiment results. South African Journal of Geology 2004;; 107 (1-2): 207–218. doi: https://doi.org/10.2113/107.1-2.207 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Recent gravity modelling of the Bushveld Complex indicates that the western and eastern limbs of the Bushveld Complex are connected at depth. The model predicts a downwarp in the Moho beneath the Bushveld Complex, ensuring observed Airy isostatic balance is achieved. By constraining a new Bouguer gravity model with published Vibroseis results, crustal thicknesses determined using the receiver function method, and seismic velocity modelling of the crust from the Southern African Seismic Experiment; we demonstrate that the connected model of the Bushveld Complex is consistent with all available data. Crustal thicknesses determined from receiver functions indicate that the depth to the Moho thickens from a value of ~35 to ~40km in the southern Kaapvaal craton to ~50km beneath the central region of the Bushveld Complex. This seismologically determined Moho varies significantly from that calculated from Airy isostatic balance based solely on topography as a load in this region. The corresponding crustal velocity model, determined from inverting the receiver function results for Bushveld Complex stations, also indicates a thick crust and delimits a ~6km thick high velocity zone in the upper 10km of crust attributed to the presence of the Bushveld Complex. Comparison of the seismic crustal model with drill core data on the mafic rocks of the Bushveld Complex suggests a correspondence between high seismic velocities and high densities in the upper crust. Both the gravity model and the seismological results imply a density contrast of about 0.30mg/m3 at the crust/mantle boundary beneath the Bushveld Complex. We also find that the Moho transition beneath the Bushveld Complex is significantly broader than that beneath the rest of the Kaapvaal craton, outside of the Limpopo Belt. By constraining the modelling of the gravity data with these seismological results, outcropping geology and published Vibroseis profiles, we show that the dense mafic units of the western and eastern Bushveld Complex can be interpreted as having originally been emplaced as a connected sheet (or series of connected sheets), which has subsequently been deformed and faulted. The seismological results of the Kaapvaal project support the interpretation of a connected Bushveld Complex. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Färoe‐Iceland Ridge Experiment 1. Crustal structure of northeastern Iceland

Results from the Färoe‐Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (&lt;800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Seismic evidence for stretched continental crust in the Japan Sea

The Japan Sea is a major marginal sea in the western Pacific comprised of basins and bathymetric highs. Technically, these features were created by multiple‐rifting and spreading processes, as exemplified in the creation of the Japan Basin and Yamato Basin. However, there are smaller scale features for which seismic crustal models have been inadequate to infer or verify their origin and evolution. Recent advances in marine seismological surveys enable us to obtain heterogeneous seismic crustal structures in sufficient detail to characterize these features. In September of 1992, a seismic experiment was conducted to obtain the detailed seismic crustal structure in the southwestern part of the Japan Sea, including the basin and bank areas. Seventeen ocean bottom seismometers (OBSs) were deployed on two lines. Explosives and airguns were fired as controlled seismic sources. The model of the crust obtained in the northern Tsushima Basin is about 13 km thick including a 1.5‐km‐thick sedimentary layer. Beneath the bank area the crust is 22 km thick and the upper crust is relatively homogeneous with a P‐wave velocity of about 6 km/s. The bank is interpreted to be a stretched continental crust fragment.