3D Density Model Research Articles

Distribution of Temperature and Strength in the Central Andean Lithosphere and Its Relationship to Seismicity and Active Deformation

AbstractWe present three‐dimensional (3D) models of the present‐day steady‐state conductive thermal field and strength distribution in the lithosphere beneath the Central Andes. Our primary objective was to investigate the influence that the structure of the Central Andean lithosphere has on its thermal and rheological state, and the relationship between the latter and the active deformation in the region. We used our previous data‐driven and gravity‐constrained 3D density model as starting point for the calculations. We first assigned lithology‐derived thermal and rheological properties to the different divisions of the density model and defined temperature boundary conditions. We then calculated the 3D steady‐state conductive thermal field and the maximum differential stresses for both brittle and ductile behaviors. We find that the thickness and composition of the crust are the main factors affecting the modeled thermal field, and consequently also the strength distribution. The orogen is characterized by a thick felsic crust with elevated temperatures and a low integrated strength, whereas the foreland and forearc are underlain by a more mafic and thinner crust with lower temperatures and a higher integrated strength. We find that most of the intraplate deformation coincides spatially with the steepest strength gradients and suggest that the high potential energy of the orogen together with the presence of rheological lateral heterogeneities produce high compressional stresses and strong strain localization along the margins of the orogen. We interpret earthquakes within the modeled ductile field to be related to the weakening effect of long‐lived faults and/or the presence of seismic asperities.

Gravity modeling of the Alpine lithosphere affected by magmatism based on seismic tomography

Abstract. The southern Alpine regions were affected by several magmatic and volcanic events between the Paleozoic and the Tertiary. This activity undoubtedly had an important effect on the density distribution and structural setting at lithosphere scale. Here the gravity field has been used to create a 3D lithosphere density model on the basis of a high-resolution seismic tomography model. The results of the gravity modeling demonstrate a highly complex density distribution in good agreement with the different geological domains of the Alpine area represented by the European Plate, the Adriatic Plate and the Tyrrhenian basin. The Adriatic-derived terrains (Southalpine and Austroalpine) of the Alps are typically denser (2850 kg m−3), whilst the Alpine zone, composed of terrains of European provenance (Helvetic and Tauern Window), presents lower density values (2750 kg m−3). Inside the Southalpine, south of the Dolomites, a well-known positive gravity anomaly is present, and one of the aims of this work was to investigate the source of this anomaly that has not yet been explained. The modeled density suggests that the anomaly is related to two different sources; the first involves the middle crust below the gravity anomaly and is represented by localized mushroom-shaped bodies interpreted as magmatic intrusions, while a second wider density anomaly affects the lower crust of the southern Alpine realm and could correspond to a mafic and ultramafic magmatic underplating (gabbros and related cumulates) developed during Paleozoic extension.

Subsurface structure investigation of the United Arab Emirates using gravity data

AbstractThe crustal structure beneath the United Arab Emirates (UAE) is still relatively unknown. Here, we use regional gravity data to constrain the subsurface density distribution and structure of the crust of the UAE by applying diverse gravity derivatives methods such as horizontal derivative (HDR), analytic signal (AS), and tilt angle (TA) to analyze the subsurface structure and perform three-dimensional (3D) gravity inversion for imaging crustal structure from the surface down to 35 km depth. The results are compared with known geological regional structures and the location of the petroleum fields.The Bouguer anomalies range from −100.8 to 113.5 mGal. The 3D gravity inversion results and the maximum Bouguer values coincide with the ophiolitic Hajar mountains in the east and the successive anticlines (uplifted basement rocks) and synclines in different parts of UAE, which could be promising sites for future mining and petroleum exploration. Also, the 3D density model results and the minimum Bouguer anomalies are located over the Aruma Basin, eastern UAE Platform, and Low Central UAE Platform, which can be the places for deep groundwater aquifers. These new results from HDR, AS, and TA successfully identify known geological structures, especially in the eastern part of UAE.

Upgraded GROWTH 3.0 software for structural gravity inversion and application to El Hierro (Canary Islands)

GROWTH 3.0 is a software package to determine the subsurface 3D density distribution that adjust discrete gravity anomaly values determined from gravity observations. The software uses very general assumptions and quite automatically determines the 3D density structures described by the aggregation of thousands of small rectangular prisms. A previous version of the software was published in 2011. However, methodological developments and computational technical evolution made it necessary to upgrade the software to include new advances and recent improvements in the inversion methodology (automatic determination of density contrasts, background medium with downward density increase, etc.), and a change in the computer operating system (from Windows XP, which was compatible with the 2011 version, to Windows 10 Pro for the current version). Here we describe the new upgraded version of the software package (which can be downloaded free), its use and application, using the study of El Hierro (Canary Islands) as a test case from which to obtain a 3D density model of its crustal structure. We also give a brief geological and volcanological interpretation.

Characteristics of gravity and magnetic fields and deep structural responses in the southern part of the Kyushu-Palau Ridge

The southern part of the Kyushu-Palau Ridge (KPR) is located at the conjunction of the West Philippine Basin, the Parece Vela Basin, the Palau Basin, and the Caroline Basin. This area has extremely complex structures and is critical for the research on the tectonic evolution of marginal seas in the Western Pacific Ocean. However, only few studies have been completed on the southern part, and the geophysical fields and deep structures in this part are not well understood. Given this, this study finely depicts the characteristics of the gravity and magnetic anomalies and extracts information on deep structures in the southern part of the KPR based on the gravity and magnetic data obtained from the 11th expedition of the deep-sea geological survey of the Western Pacific Ocean conducted by the Guangzhou Marine Geological Survey, China Geological Survey using the R/V Haiyangdizhi 6. Furthermore, with the data collected on the water depth, sediment thickness, and multichannel seismic transects as constraints, a 3D density model and Moho depths of the study area were obtained using 3D density inversion. The results are as follows. (1) The gravity and magnetic anomalies in the study area show distinct zoning and segmentation. In detail, the gravity and magnetic anomalies to the south of 11°N of the KPR transition from high-amplitude continuous linear positive anomalies into low-amplitude intermittent linear positive anomalies. In contrast, the gravity and magnetic anomalies to the north of 11°N of the KPR are discontinuous and show alternating positive and negative anomalies. These anomalies can be divided into four sections, of which the separation points correspond well to the locations of deep faults, thus, revealing different field-source attributes and tectonic genesis of the KPR. (2) The Moho depth in the basins in the study area is 6–12 km. The Moho depth in the southern part of KPR show segmentation. Specifically, the depth is 10–12 km to the north of 11°N, 12–14 km from 9.5°N to 11°N, 14–16 km from 8.5°N to 9.5°N, and 16–25 km in the Palau Islands. (3) The KPR is a remnant intra-oceanic arc with the oceanic-crust basement.which shows noticeably discontinuous from north to south in geological structure and is intersected by NEE-trending lithospheric-scale deep faults. With large and deep faults F3 and F1 (the Mindanao fault) as boundaries overall, the southern part of the KPR can be divided into three zones. In detail, the portion to the south of 8.5°N (F3) is a tectonically active zone, the KPR portion between 8.5°N and 11°N is a tectonically active transition zone, and the portion to the north of 11°N is a tectonically inactive zone. (4) The oceanic crust in the KPR is slightly thicker than that in the basins on both sides of the ridge, and it is inferred that the KPR formed from the thickening of the oceanic crust induced by the upwelling of deep magma in the process of rifting of remnant arcs during the Middle Oligocene. In addition, it is inferred that the thick oceanic crust under the Palau Islands is related to the constant upwelling of deep magma induced by the continuous northwestward subduction of the Caroline Plate toward the Palau Trench since the Late Oligocene. This study provides a scientific basis for systematically understanding the crustal attributes, deep structures, and evolution of the KPR.©2021 China Geology Editorial Office.

ГЕОДИНАМІКА

This study aims to derive the Earth’s temporally varying Earth’s tensor of inertia based on the dynamical ellipticity , the coefficients , from UT/CSR data. They allow to find the time-varying Earth’s mechanical and geometrical parameters during the following periods: (a) from 1976 to 2020 based on monthly and weekly solutions of the coefficient ; (b) from 1992 to 2020 based on monthly and weekly solutions of the non zero coefficients , related to the principal axes of inertia, allowing to build models their long-term variations. Differences between and , given in various systems, represent the average value , which is smaller than time variations of or , characterizing a high quality of UT/CSR solutions. Two models for the time-dependent dynamical ellipticity were constructed using long-term variations for the zonal coefficient during the past 44 and 27.5 years. The approximate formulas for the time-dependent dynamical ellipticity were provided by the additional estimation of each parameter of the Taylor series, fixing at epoch =J2000 according to the IAU2000/2006 precession-nutation theory. The potential of the time-dependent gravitational quadrupole according to Maxwell theory was used to derive the new exact formulas for the orientation of the principal axes , , via location of the two quadrupole axes. Hence, the Earth’s time-dependent mechanical and geometrical parameters, including the gravitational quadrupole, the principal axes and the principal moments of inertia were computed at each moment during the past 27.5 years from 1992 to 2020. However, their linear change in all the considered parameters is rather unclear because of their various behavior on different time-intervals including variations of a sign of the considered effects due to a jump in the time-series during the time-period 1998 – 2002. The Earth’s 3D and 1D density models were constructed based on the restricted solution of the 3D Cartesian moments inside the ellipsoid of the revolution. They were derived with conditions to conserve the time-dependent gravitational potential from zero to second degree, the dynamical ellipticity, the polar flattening, basic radial jumps of density as sampled for the PREM model, and the long-term variations in space-time mass density distribution. It is important to note that in solving the inverse problem, the time dependence in the Earth's inertia tensor arises due to changes in the Earth's density, but does not depend on changes in its shape, which is confirmed by the corresponding equations where flattening is canceled.

Seismic Density Model of the White Sea’s Crust

Study of the deep structure of the White Sea region is relevant to active geodynamics, manifestations of kimberlite magmatism, and the prospects of oil and gas searches. The aim of this work was to model the velocity and density structure of the earth’s crust in the White Sea region. Modelling was carried out using the known data of instrumental observations and the software complex “Integro”. With the help of 2D models based on deep seismic sounding (DSS) profiles and a digital map of the anomalous gravity field, the density structures of local areas of the region’s crust were refined. A 3D density model was built. Within the framework of this model, the positions of the density layers were determined. The relief of the Mohorovichich (Moho or M) discontinuity reflects the anomalies of the gravity field. Depression of the Moho boundary in the bottleneck of the White Sea indicates the vertical structure of the earth’s crust associated with manifestations of kimberlite magmatism.

Скоростная сСтруктура и плотностные неоднородности земной коры Белого моря

The study area is the White Sea basin and adjacent territories. The relevance of the work carried out here is determined by active geodynamics, kimberlite magmatism, and prospects for the hydrocarbon search. The authors set the goal to model the velocity structure of the region’s crust using data from instrumental observations and the Integro software package. A comprehensive interpretation of gravimetric, magnetometric, seismic, petrophysical and geological data has been carried out. With the help of 2D models based on the DSZ profiles and digital maps of geophysical fields, refined density structures of local sections of the earth’s crust have been specified. The developed 3D density model gives a general picture of the deep structure of the region’s crust. Within its framework, the spatial positions of the layers of the velocity reference model are determined and their connections with density inhomogeneities and geophysical anomalies are established.

Mapping New IOCG Mineral Systems in Brazil: The Vale do Curaçá and Riacho do Pontal Copper Districts

The Vale do Curaçá and Riacho do Pontal copper districts are located within the northern part of the Archaean São Francisco Craton and represent two pulses of mineralization. The copper districts have been identified as Iron-Oxide-Copper-Gold (IOCG) classes of deposits. An older metallogenic event associated with the Caraíba copper deposit, which is located in the Vale do Curaçá district, is related to Palaeoproterozoic (ca. 2 to 2.2 Ga) hydrothermal processes. A younger Neoproterozoic (ca. 750 to 570 Ma) episode of volcanism and associated plutonism is represented by the Riacho do Pontal mineral district. Seismic tomography data from across east-central Brazil show that the multiage Carajás province and Vale do Curaçá and Riacho do Pontal copper districts sit along either side of a prominent NW-trending upper lithospheric high-velocity zone. The edges of the high-velocity zone point to long-lived subparallel transcrustal structures that have been the focus of multiple reactivations and copper mineralization events. Regional gravity and magnetic maps show that the Vale do Curaçá copper district extends over an area greater than 110 km by 22 km. The magnetic and gravity values show significant variations correlated with this area. The district includes high gravity values associated with the Caraíba copper mine (>−35 mGal), which has a greater density (3.13 g/cm3) than the nonmineralized host rock density (2.98 g/cm3). The gravity anomaly signature over the Riacho do Pontal copper district is characterized by a 40-km long NW–SE trending Bouguer gravity low. The Ria4 occurrences of the Riacho do Pontal copper district are situated in these regional low-gravity domains. Data from regional airborne magnetic and ground gravity surveys were inverted to obtain a 3D magnetic susceptibility and density model, respectively, for the known districts. The results show that the Caraíba deposit is characterized by a both dense and magnetic source showing structural control by thrust shear zones. The 2D and 3D geological models show two main NNW prospective trends. Trends I and II have a sigmoidal shear shape and are positioned in the contact zone between domains with high magnetic susceptibility (SI > 0.005) and density > 0 g/cm3). Trend I is 40 km × 10 km in size and hosts the Caraíba, Surubim, and Vermelho copper mines and other minor deposits. The results obtained from the 3D magnetic inversion model for the region of the Riacho do Pontal district show weak magnetic anomaly highs extending along a NW–SE magnetic gradient trend. The gradient is related to mapped shear zones that overprint older and deeper NE–SW features of the São Francisco cratonic root. The area includes high gravity values associated with the Caraíba copper deposit, which has a greater density (3.13 g/cm3) than the nonmineralized host rock density (2.4 g/cm3).

Three-dimensional seismic characterization and imaging of the Soda Lake geothermal field

Accurate characterization of subsurface geophysical properties and detection of the fault system are essential for geothermal energy exploration and production. The Soda Lake geothermal field is in western Nevada with a complex fault system. Previous seismic characterization only produced a low-resolution, smooth velocity model along with a simple, conceptual fault model. Using optimized correlation-based full-waveform inversion, wavefield-separation-based reverse-time migration, and automatic fault detection techniques, we present 3D seismic characterization for the Soda Lake geothermal field using 3D surface seismic data acquired with Vibroseis sources. We obtain 3D high-resolution velocity, density, and acoustic impedance models, 3D seismic images with different grid spacings, and a high-resolution fault system. Consistency check between the constructed faults and currently active injection and production geothermal wells verifies that our seismic inversion and imaging results and detected faults are reliable. These results can provide valuable information for optimizing well placement and geothermal energy production at the Soda Lake geothermal field.

3D gravity modelling of Colorado and Claromecó basins: new evidences for the evolution of the southwestern margin of Gondwana

Although cratons have long been recognized as an important part of continental tectonic processes, we still have much to learn about their structural features and their relation to the genesis of basins, sedimentary thickness distributions, fold and thrust belts and surface processes. Contributing to a better knowledge of the crustal state and configuration of the southernmost part of the Rio de la Plata Craton, could shed light on the still controversial tectonic processes, which were responsible for the deformation of the southwestern margin of Gondwana during the Late Paleozoic. In particular, the deformation and uplift of the Sierras Australes, which are part of the Claromeco Basin (Buenos Aires, Argentina), would be closely related to the crustal structure of the southern limit of the Rio de la Plata Craton. Therefore, it is of crucial importance to investigate the possible existence of crustal heterogeneities underneath the Claromeco Basin, the Sierras Australes and the Colorado Basin, which can be related to structural features and weakness zones that could have played a major role in the tectonic evolution of the study area. For this purpose, we developed a 3D lithospheric-scale density model integrating various data, such as geological information, global gravity models, well data (thicknesses and lithologies), seismic tomography data (Moho depth), and pre-existing 3D density models of the Colorado Basin. Our model includes layers of sediments, crystalline crust and lithospheric mantle and therefore predicts the thickness variation of the upper and lower crust in the study area and of the main sedimentary sequences infilling the basins. Moreover, by analysing the model results, we propose a tentative location of the southern limit of the Rio de la Plata Craton and its possible tectonic relationship with transfer zones identified in the Atlantic platform southwards. Our results suggest that the southern boundary of the craton is located along the northernmost limit of the Colorado Basin, which is in contrast with the boundary proposed in previous works. Therefore, we propose that the Colorado Basin rifting process could have occurred along this weakness domain.

Comment on “Crustal structure and erosion of the Lofoten/Vesterålen shelf, northern Norwegian margin. Tectonophysics 776, 228318” by A.J. Breivik, J.I. Faleide, R. Mjelde, E.R. Flueh, Y. Murai

• Moho depth beneath the Lofoten-Vesterålen region is questionable according to new deep seismic profiles • Presence of the low-velocity upper mantle beneath the Lofoten-Vesterålen region is supported by deep seismic tomography • Presence of the low-density/velocity upper mantle beneath the Lofoten-Vesterålen region is supported by 3D density modelling

The Efficient 3D Gravity Focusing Density Inversion Based on Preconditioned JFNK Method under Undulating Terrain: A Case Study from Huayangchuan, Shaanxi Province, China

Since polymetallic ores show higher anomalies in gravity exploration methods, we usually obtain the position and range of ore bodies by density inversion of gravity data. The three-dimensional (3D) gravity focusing density inversion is a common interpretation method in mineral exploration, which can directly and quantitatively obtain the density distribution of subsurface targets. However, in actual cases, it is computation inefficient. We proposed the preconditioned Jacobian-free Newton-Krylov (JFNK) method to accomplish the focusing inversion. The JFNK method is an efficient algorithm in solving large sparse systems of nonlinear equations, and we further accelerate the inversion process by the preconditioned technique. In the actual area, the gravity anomalies are distributed on the naturally undulating surface. Nowadays, the gravity inversion under undulating terrain was mainly achieved by discretizing the ground into unstructured meshes, but it is complicated and time-consuming. To improve the practicality, we presented an equivalent-dimensional method that incorporates unstructured meshes with structured meshes in gravity inversion, and the horizontal size is determined by the gradient of observed gravity and terrain data. The small size meshes are adopted at the position where the terrain or gravity gradient is large. We used synthetic data with undulating-terrain to test our new method. The results indicated that the recovered model obtained by this method was similar to the inversion method of unstructured meshes, and the new method computes faster. We also applied the method to field data in Huayangchuan, Shaanxi Province. The survey area has complicated terrain conditions and contains multiple polymetallic ores. Based on the high-density characteristics of polymetallic ore bodies in the area, we calculate the field data into 3D density models of the subsurface by the preconditioned JFNK method and infer six polymetallic ores.

A geophysical perspective on the lithosphere–asthenosphere system from Periadriatic to the Himalayan areas: the contribution of gravimetry

The structure of the lithosphere–asthenosphere system as imaged from the complementary inversion of seismological and gravimetric data can provide significant clues to understand the processes of orogeny and cratonization and to better formulate the theory of plate tectonics within a sound geodynamic context. In this paper, we review some studies sharing a common synergic methodology that uses the geometry of cellular shear-wave velocity models of the crust and uppermost mantle to constrain a 3D density model obtained by means of linear inversion of gravimetric data. These studies, which were applied to selected areas along an ideal stream running from the Periadriatic region to the Himalayas, despite having different resolution, show some basic common features in the density distribution of the upper mantle whose unveiling may be precluded to gravity models achieved by assuming velocity–density relationships. In fact, one of the main conclusions of the discussed studies is that lithospheric slabs under the investigated orogenic belts, as identified by seismic wave velocity, are, as a rule, not marked by high density. This fact casts serious doubts about the effectiveness of slab-pull and confirms one of the key issues of polarized plate tectonics, mostly fuelled by tidal drag.

Технология построения плотностной 3D-модели земной коры в ГИС INTEGRO

The work is devoted to the construction of a three-dimensional density model of the territory, which would not contradict the information obtained by other methods (drilling, seismic, etc.). A generalized scheme for constructing such a model is given, its stages and ways of matching with the results of other methods are described. The technology described in the work is based on the tools included in the GIS INTEGRO software package (FSBI "VNIGNI"). The article is illustrated with examples of the technology application in the construction of density models for various parts of the Volga-Ural oil and gas province. Key words: density modeling, 3D density model, inverse problem of gravity exploration, GIS INTEGRO.

Методические и технологические аспекты исключения гравитационного эффекта нижней части земной коры при изучении осадочного чехла нефтегазоносных территорий

In the case of seismic density modeling of sedimentary basins, it is necessary to exclude from the observed gravitational field the effect created by inhomogeneities of the lower part of the crustal section. The article offers one of the approaches to the geological field reduction, implemented through the construction of a 3D density model for the entire thickness of the earth's crust and upper mantle. A fragment within the study area is selected from the constructed 3D model and its gravitational effect is calculated. Various options for implementing this approach are considered, depending on the amount of a priori information. The technological base of the method is GIS INTEGRO. Keywords: Gravitational field, geologic reduction, density model, GSS profiles, inversion, structural framework of the model.

Structure of the Lithosphere in the Zone of the Continent–Ocean Transition in the Southern Part of the Russian Far East: Evidence from the Density Modeling

The quantitative processing of gravimetric data is performed and the 3D density model is calculated for the southern part of the Russian margin of Eurasian continent. Based on the density model, new patterns of disjunctive tectonics at different depth levels are revealed and the position of the region at the global tectonic level is shown. It is established that the transition zone is an integrated lithospheric structure of the northeastern orientation, in which there are mantle blocks with the boundaries corresponding to the borders of depressions and major tectonic objects. The data obtained may be applied for refinement of the deep structure and paleogeodynamics of the studied area.

Density Structure of the Von Kármán Crater in the Northwestern South Pole-Aitken Basin: Initial Subsurface Interpretation of the Chang'E-4 Landing Site Region.

The Von Kármán Crater, within the South Pole-Aitken (SPA) Basin, is the landing site of China’s Chang’E-4 mission. To complement the in situ exploration mission and provide initial subsurface interpretation, we applied a 3D density inversion using the Gravity Recovery and Interior Laboratory (GRAIL) gravity data. We constrain our inversion method using known geological and geophysical lunar parameters to reduce the non-uniqueness associated with gravity inversion. The 3D density models reveal vertical and lateral density variations, 2600–3200 kg/m3, assigned to the changing porosity beneath the Von Kármán Crater. We also identify two mass excess anomalies in the crust with a steep density contrast of 150 kg/m3, which were suggested to have been caused by multiple impact cratering. The anomalies from recovered near surface density models, together with the gravity derivative maps extending to the lower crust, are consistent with surface geological manifestation of excavated mantle materials from remote sensing studies. Therefore, we suggest that the density distribution of the Von Kármán Crater indicates multiple episodes of impact cratering that resulted in formation and destruction of ancient craters, with crustal reworking and excavation of mantle materials.

Unrest at Domuyo Volcano, Argentina, Detected by Geophysical and Geodetic Data and Morphometric Analysis

New volcanic unrest has been detected in the Domuyo Volcanic Center (DVC), to the east of the Andes Southern Volcanic Zone in Argentina. To better understand this activity, we investigated new seismic monitoring data, gravimetric and magnetic campaign data, and interferometric synthetic aperture radar (InSAR) deformation maps, and we derived an image of the magma plumbing system and the likely source of the unrest episode. Seismic events recorded during 2017–2018 nucleate beneath the southwestern flank of the DVC. Ground deformation maps derived from InSAR processing of Sentinel-1 data exhibit an inflation area exceeding 300 km2, from 2014 to at least March 2018, which can be explained by an inflating sill model located 7 km deep. The Bouguer anomaly reveals a negative density contrast of ~35 km wavelength, which is spatially coincident with the InSAR pattern. Our 3D density modeling suggests a body approximately 4–6 km deep with a density contrast of –550 kg/m3. Therefore, the geophysical and geodetic data allow identification of the plumbing system that is subject to inflation at these shallow crustal depths. We compared the presence and dimensions of the inferred doming area to the drainage patterns of the area, which support long-established incremental uplift according to morphometric analysis. Future studies will allow us to investigate further whether the new unrest is hydrothermal or magmatic in origin.

Imaging Yellowstone magmatic system by the joint Gramian inversion of gravity and magnetotelluric data

Gravity and long-period magnetotelluric data are combined in a joint Gramian inversion to obtain structurally-similar 3D density and electrical resistivity models of the Yellowstone magmatic feeding system. Structural constraints are enforced through a correlation of the model gradients. Gravity data are sensitive to upper-crustal structure, whereas long-period magnetotelluric data are more sensitive to deeper structure. By combining these complementary data, the jointly inverted models provide a consistent image of the partially molten structure underlying the Yellowstone Caldera from the surface to the Moho. Discrete zones of inferred partial melt in the upper and lower crust are fed by a southwest trending plume, and satisfy expectations from modern mantle plume models.