Positive Gravity Anomalies Research Articles

Subsurface complexity controls the aquifer heterogeneity: A case study from the Al-Hassa oasis, Eastern Saudi Arabia

Al-Hassa region in the eastern part of Saudi Arabia is well-known for its geological and hydrogeological importance since it has historically hosted over 280 natural springs, which were used to irrigate the largest oasis in the world. Al-Hassa is located near the renowned Ghawar oil field, the largest conventional oil field globally, which represents a potential pollution source. This study utilizes and integrate hydrochemical investigations and geophysical gravity surveys to understand and reconstruct the subsurface heterogeneity in the Al-Hassa area. The dataset encompasses 113 groundwater wells distributed across the Al-Hassa Oasis which have been analyzed for salinity major ions, and isotopic (oxygen and hydrogen) compositions. A total of 571 gravity stations covering the broader oasis area (approximately 350 km2) are collected, processed, and modeled. The combined hydrochemical and geophysical results show a good agreement between groundwater quality and density (gravity anomalies) distribution within the study area. The southeastern part of the study area exhibits distinctive positive gravity anomalies, indicating denser rock formations alongside high total dissolved solids (TDS) in groundwater, reflecting poor water quality. Conversely, the southwest displays significant negative gravity anomalies, suggesting basins filled with loose sediments and low TDS values, signifying good water quality. Furthermore, the study reveals a certain pattern in groundwater temperature distribution, with cooler waters in the areas characterized by negative gravity anomalies (basins), and hotter waters emerging from areas with positive gravity anomalies. These findings suggest that groundwater quality differences may stem from varying sub-basins and interactions with distinct geological substrates. Temperature variations may also be attributed to differing subsurface flow pathways. This study attempts to explain the controlling factors for groundwater heterogeneity in the Al-Hassa Oasis area, emphasizing the role of geological, tectonic, and hydrogeological elements in shaping the Oasis's hydrological and hydrochemical pattern.

Plate interface geometry complexity and persistent heterogenous coupling revealed by a high-resolution earthquake focal mechanism catalog in Mentawai, Sumatra

Geometry and frictional states of the megathrust in subduction zones play critical roles on the rupture extent, hence the magnitude, of great earthquakes. However, their details are often difficult to obtain, primarily due to limited offshore geophysical observations. Here we overcome this challenge by precisely determining source parameters (i.e., location and focal mechanism) of medium-sized earthquakes using global broadband seismic waveform data. The dip angles of 108 Mw 5.0+ plate interface events from 1990 to 2017 in the central Sumatran subduction zone are tightly constrained by waveform inversion, and their locations are refined with surface-wave relocation and depth phases modeling. Our results reveal compact trench parallel earthquake belts at the up-dip and down-dip boundaries of the ruptures of the Mentawai 2007 Mw 8.4 and Mw 7.9 earthquakes. The down-dip seismicity belt marks ∼8º bending of the plate interface, suggesting that the change of fault geometry may have played a role in halting the megathrust rupture along with thermal effects. The up-dip seismicity belt shows a clear spatial complementarity to the largest events’ slip distribution. This belt is quite narrow (∼20 km) for the Mw 8.4 rupture but much broader (∼50 km) for the Mw 7.9 event. The combination of the up-dip seismicity belt and slip distributions matches well with the positive residual bathymetry and gravity anomalies, suggesting that these events resulted from long-term coupling on the plate boundary. Our findings suggest the importance of considering both up-dip and down-dip seismicity belts, along with residual bathymetry and gravity data, in understanding megathrust rupture behaviors and assessing the associated hazards.

An Integrated Microgravity and Soil Analysis Methods to Assist the Resistivity Profiles Interpretation in an Engineering Site Southern Baghdad, Iraq

A 2D Electrical Resistivity Imaging survey was conducted at the Al-Jadiriya Campus of Baghdad University. The 2D resistivity survey was achieved on two profiles using the Wenner- Schlumberger configuration. The length of each profile is 80 meters, and the spacing between adjacent electrodes is 1 mr. The obtained resistivity data were interpreted using the RES2DINV program. Both profiles show high resistivity anomaly at depths more than 6 meters, extending from a distance of 32-44 m and 37-44 mrs for profiles 1 and 2, respectively. In addition to the deep anomalies, there are many shallow high-resistivity anomalies. A microgravity survey was carried out for a distance of 20- 60 m along the two profiles to check the nature of the deep high resistivity anomaly. The gravity profiles showed many positive gravity anomalies reflected the effect of shallow sources, which coincide with the shallow relatively high resistivity anomalies. Many soil samples were analyzed chemically and sedimentologically to investigate soil homogeneity. It is found that the soil is homogenous, composed mainly of silt and clay. According to the integrated interpretation of resistivity, gravity, and soil analyses, it is found that the high resistivity anomaly is related to dry compacted soil mass, which is indicated in the gravity study. This study confirms the importance of integrated research to access better geophysical interpretation. The water table level in the study sites ranges 5.6–6.2 ms.

Crustal architecture of the Dharwar craton and Southern Granulite Terrane, Southern India, from the analysis of gravity-magnetic data

The potential field (gravity & magnetic) data, along with results of available seismic and magnetotelluric studies over the Southern India Shield comprising the Dharwar craton (DC), Southern Granulite Terrane (SGT) and southern part of the Eastern Granulite Mobile Belt (EGMB), were synthesized. The study attempts to elucidate the geometry and continuity of greenstone belts/shear zones, isostatic condition and deeper crustal architecture. We observed a characteristic elevation-gravity relationship for each of the tectonic domains. The greenstone belts of the DC are characterized by N-S to NW-SE trending positive residual gravity anomalies and continue beneath the Cuddapah sediments. The basement of the Cuddapah basin is characterized by NW-SE trending greenstone belts with low-density intrusive rocks. The gravity anomaly suggests occurrence of a high-density rock unit occurs at mid-crustal depth beneath the ‘Closepet granite’ and ‘Chitradurga schist belt’. The potential field signatures of the DC and SGT are distinctly different. The Cauvery Shear Zone is characterized by positive residual gravity and magnetic anomalies with ∼E-W to ENE-WSW trend that continues further east under sediment covers of the east coast. The residual gravity signatures and horizontal gravity gradient signatures suggest that the orogenic systems encompassing the Cauvery Shear Zone and the eastern part of the EGMB may be a part of a common structural belt or tectonic regime associated with the assembly of the Gondwana supercontinent. The potential field modelling reveals the presence of a thick, high-density material resting over the base of the crust in the entire stretch of the east-coast of India.

Geological Structure Model for Recharge Area in Patuha Geothermal Field

Geothermal sustainability requires geological structure analysis using gravity methods for groundwater management. Geological structure analysis reviews fault data. Geological structure as a pathway for fluid movement for groundwater management in the research area, especially the Patuha Geothermal Field, West Java. This study uses global gravity model, terrestrial gravity, DEM, and geological data. The gravity method applied in this research involves calculations for latitude correction, free air correction, bouguer correction and terrain correction. The final results of this research are free air anomaly, complete Bouguer anomaly, and model evaluation (MRSE, MRE, and MAE). Free air anomaly results from free air correction of global gravity model data. DEM topography analyzes free air anomaly. The FAA data model evaluated RMSE at 1.27, MRE at 0.004, and MAE at 0.83. 138.02–280.26 mgal is the FAA range. The research area’s elevation range is 836–2431 m. Topography and free air anomaly correlate 0.75. Positive high connection. Mountains and plateaus may have positive gravity anomalies. Free air, bouguer, terrain, and CBA are applied to global gravity model data. The CBA model evaluation indicates quality. RMSE is 1.47, MRE is 0.012, and MAE is 1.11. CBA fluctuation implies fault spread. Faults serve as water flow conduits and channels, increasing the risk of infiltration. Meanwhile, cracks are being distributed as conduits and for infiltration. This research forms the basis for developing and increasing geothermal production so that further studies are needed regarding the influence of geological structures for reservoir in geothermal areas.

Gravity imaging of sub-Zechstein geological structures in the UK sector of the North Sea using the gravity layer stripping method

Extensive geophysical databases, covering the UK sector of the North Sea, have been used to gravity layer strip the sedimentary layers down to the base Zechstein so that the gravity response of the Carboniferous and deeper strata can be identified and structurally interpreted. To achieve this, the average bulk density grids for each layer were derived using Gardner's functions derived from well velocity and density logs. The resulting residual gravity response of each layer and the Moho response were then removed from the Free air gravity anomaly to generate the isostatic gravity response of the crust below the base Zechstein. This gravity response was used to re-evaluate the British Geological Survey interpretation over the Mid North Sea High (MNSH) and was able to identify the same crustal structures. Using the tilt derivative method, a positive gravity anomaly was found to parallel the central fracture zone that forms a northern extension of the Dowsing fault zone. This anomaly can be traced north across the MNSH with offsets coinciding with the WSW–ENE basement lineaments. To the south, the southern North Sea basin is well defined by the stratigraphic layer depth and thickness maps as well as the residual gravity maps which identify the structures associated with the low-density Carboniferous Coal Measures.

Geophysical investigation of the Mado Megamullion oceanic core complex: implications for the end of back-arc spreading

Detachment faulting is one of the main styles of seafloor spreading at slow to intermediate mid-ocean ridges. However, we have limited insight into its role in back-arc basin formation. We surveyed a remnant back-arc spreading center in the Philippine Sea and determined the detailed features and formation processes of the Mado Megamullion (Mado MM) oceanic core complex (OCC). This was undertaken in the context of back-arc evolution, based on the shipborne bathymetry, magnetics, and gravity with radiometric age dating of the rock samples collected. The Mado MM OCC has a typical OCC morphology with prominent corrugations on the domed surface and positive gravity anomalies, suggesting that there has been an exposure of the lower crust and mantle materials by a detachment fault. The downdip side of the detachment continues to the relict axial rift valley, which has indicated that the Mado MM OCC was formed at the end of the back-arc basin opening. The spreading rate of the basin decreased once when the spreading direction changed after six million years of stable trench perpendicular spreading. The rate then further decreased immediately prior to the end of the spreading when the Mado MM OCC was formed. The existence of other OCC-like structures in the neighboring segment and the previously reported OCCs along the Parece Vela Rift have indicated that the melt-poor, tectonic-dominant spreading is a widespread phenomenon at the terminal phase of back-arc spreading. The decrease in spreading rate in the later stage is consistent with the previous numerical modeling because of the decrease in trench retreat. In the Izu–Bonin–Mariana arc trench system, the rotation of the spreading axis and the resultant axis segmentation have enhanced the lithosphere cooling and constrained mantle upwelling, which caused the tectonic-dominant spreading at the final phase of the basin evolution.

Decratonised lithosphere associated with mantle plume upwelling in the Singhbhum craton, eastern India based on geophysical and geodynamic perspective

The mode of crustal and lithospheric mantle evolution of cratonic blocks and their subsequent deformation and decratonisation due to regional plate tectonics and continued intraplate geodynamic process, are not well understood. This is especially true for the highly complex Eoarchean Singhbhum craton, located in the eastern part of the Indian shield, which underwent prolonged episodes of intraplate volcanism, mafic dyke intrusions, rifting, collision and widespread sedimentation lasting for over two billion years, leaving behind noticeable imprints in the crust and mantle lithosphere. Our detailed analysis of the geophysical characteristics indicates high crustal velocities and presence of a pronounced metasomatised zone between the depth 14 km and 30 km, which is characterised by drop in Vs from 3.93 km/s to 3.76 km/s. This zone is further followed by 14 km thick high velocity magmatic crust above the Moho. A substantial part of the upper crust has been eroded away from this craton due to thermal-induced uplifting and massive crustal exhumation, caused by the multiple hot mantle plume interactions during Precambrian period, in which mantle potential temperature and pressure reached as high as 1700 oC and 4.4 GPa respectively. This craton is conspicuously associated with highly positive gravity anomalies (up to +10mGal), which are well correlated with high elevation and the location of mafic volcanic suites. It is also characterised by an unusually thin lithosphere (∼ 60-128 km), high Moho temperatures (910°C) and high dose of heat flow (47mW/m2) emanating from the mantle, which conforms with the estimated low effective elastic thickness of ∼30 km. Transient effects of the paleo-thermal anomalies possibly still persist due to periodic rejuvenation of surrounding rift valleys and suture zones. Eventually, this craton now shows geophysical characters of more or less a mobile belt, not of a typical craton. Since, almost 150-200 km thick mantle lithosphere from the bottom has been eroded away from this region due to persistent thermo-geodynamic activities and mantle upwelling induced rise of mantle isotherms, our study does not favour the idea of cratonic stability and preservation of deep continental roots.

A review of the geophysical knowledge of the Borborema Province, NE-Brazil, and tectonic implications

This review summarizes the results of nearly 50 years of application of geophysical data to study tectonic problems in Borborema Province (BP), Northeastern Brazil. Much information has been obtained from the interpretation of magnetic, gamma-ray spectrometry, gravity, seismological, magnetotelluric, deep seismic and heat flow data. The resulting picture is that, in comparison with other Brazilian geological domains, BP has a thin crust, a lithosphere with small effective elastic thickness, the presence of predominantly positive free-air gravity anomalies, besides the most expressive geoid anomaly, in addition to relatively high heat flow and intraplate seismicity. These facts reinforce the hypothesis that the BP lithosphere suffered a strong impact in Gondwana fragmentation. Predominantly positive free-air gravity anomalies reveal that part of the province has not yet reached isostatic equilibrium. Gravity data established the boundaries of the BP crustal block with neighboring blocks; in particular, the crustal boundary with the São Francisco Craton is located in the innermost regions of the marginal fold belt, tens of kilometers at the north of the conventional limit. Magnetic data allowed to infer the continuity of basement structures beneath sedimentary basins, in particular that BP continues to the Transbrasiliano Lineament, underneath the sedimentary of the Parnaíba Basin. In addition, enhanced magnetic maps made it possible assessing the amount of deformation associated with the major shear zones and identified a continental-scale Cretaceous dike system crossing the BP. Gamma-ray spectrometry data allowed extensive geological mapping of metasedimentary sequences, granitic intrusions and basement complexes. Seismological data allowed to recognize that major shear zones in BP have lithospheric significance. Deep seismic data evidenced a well-defined contrast between the upper and lower crust in BP (Conrad discontinuity). Magnetotelluric data revealed several deep geoelectrical anomalies in BP, some of them interpreted as relics of subduction zones. Intraplate seismicity studies allowed inferring that the current stress state in the BP upper crust is predominantly transcurrent. The list of open problems in BP is also expressive. Notable examples are: a) it has not yet been possible to define which type of orogeny (accretionary or intracontinental?) was dominant in the Neoproterozoic; b) the cause of the expressive geoid anomaly is unknown; c) the tectonic significance of the Transbrasiliano Lineament (collisional suture? Just an intracontinental shear zone?) is also unknown. Regarding economic applications, the use of geophysical data to prospect new ore deposits is still relatively poor. We hope that this review can contribute to increase the familiarity and acceptance of geophysical results by the geological community.

From the Lebombo Monocline to the Mozambique Deep Basin, using combined wide-angle and reflection seismic data

The North Natal valley (NNV), South Mozambique margin, is a key area for the understanding of the SW Indian Ocean history since the Gondwana break-up as its crustal nature and geometry strongly impacted the reconstruction of the paleogeography before the rifting. It is also of considerable importance for the understanding of the evolution of a margin system as the NNV is situated at the transition between divergent and strike-slip segments and at the conjunction of Oxfordian-Kimmeridgian Indian Ocean and the Valanginian-Aptian Atlantic one. As one part of the PAMELA project (PAssive Margins Exploration Laboratories), the NNV and the East Limpopo margin have been investigated during the MOZ3/5 cruise (2016), through the acquisition of 7 intersecting wide-angle profiles and coincident marine multichannel (720 traces) seismic as well as potential field data. Simultaneously, land seismometers were deployed in the Mozambique coastal plain (MCP), extending six of those profiles on land for about 100 km in order to provide information on the onshore-offshore transition. Wide-angle seismic data are of major importance as they can highlight constraints on the crustal structure of the margin and the position of the continent-ocean boundary in an area where the crustal nature is poorly known and largely controversial. The MOZ3/5 data set therefore reveals new essential constraints for kinematic reconstructions. This work presents results on the crustal structure from P-waves velocity modeling along two E-W wide-angle profiles (MZ1 and MZ2) through the NNV, from the Lebombo Monocline to the Mozambique Basin (MB), and crossing the Mozambique Fracture Zone (MFZ).The new geophysical data reveals an upper sedimentary sequence characterized by low velocities generally not exceeding 3 km/s, and up to 3 km thick where a major contouritic structure was observed. This feature formes together with several other contouritic structures, a N-S alignment just west of the MFZ, which produces high positive gravity anomalies, previously thought to be related to the magmatism that built the Galathea and Dana Plateaus. High velocity lenses are locally identified through the sedimentary layers and interpreted as inter-bedded volcanic sills. Furthermore, from the NNV to the MFZ, the underlying sequence is formed of a 3.0–3.5 km thick volcano-sedimentary sequence presenting important lateral changes in its seismic signature and characterized by a large velocity range (4.4 to 5.8 km/s), which partly reflects variations in the volcanic/sedimentary ratio laterally and with depth. At depth, an initially smoother and reduced eastward thinning of the crust occurs to the West below the continental shelf, from 34 to 31 km thick. The crustal thickness remains relatively constant of about 28–29 km along the Central Domain (CD), whereas a second and major region of thinning (26 to 12 km thick) is imaged West of the MFZ, in the southward prolongation of the Limpopo Corridor (LC). By contrast, as the eastern extremity, the crust is <10 km thick when reaching the MB. Crustal velocities reveal low velocity gradients, with atypical high velocities, increasing to 7.3 to 7.6 km/s at the base of the crust, and globally in the whole crust in the LC, just West of the MFZ. We interpreted the velocity architecture combined with the evidence of volcanism at shallower depths as indicative of an intensively intruded continental crust in the NNV, and discuss the particular segmentation of the longest profile (MZ1) in the kinematic context of both divergent and strike-slipe segments offshore Mozambique.Combining wide-angle and reflection seismic observations along these two profiles and the other MOZ3/5 lines, the data shows a coherent segmentation of the E-W crustal architecture off South Mozambique. These results along MZ1 and MZ2 profiles, combined with the previously published profiles, gives a 3D-view of the NNV, which becomes one of the most passive margins covered in the world by deep wide-angle seismic data.

2022 Joint Barringer Medal for Kai Wünnemann and Gareth S. Collins

2022 Joint Barringer Medal for Kai Wünnemann and Gareth S. Collins

Bouguer gravity anomalies and the three-dimensional density structure of a thick mudstone area: A case study of southwestern Taiwan

Gravity surveying is one of the best methods to deduce the subsurface structures in areas covered by thick mudstone in southwestern Taiwan where seismic data analysis is limited. We obtained regional and residual gravity anomalies from the Bouguer anomaly map using the wavelength filtering method to image the gravitational effects of subsurface sources at different depths. The distribution of the residual gravity anomalies corresponds well with the locations of faults and anticlines. Through the change of residual gravity anomalies due to sources at different depth, we estimated the inclination and extension of the anomalous bodies at different depths. Then, the three-dimensional density structure was inferred from inversion of the gravity data to reveal the distribution of mud diapirs and changes of depth to the basement. The positive residual gravity anomalies of approximately 1–2 mGal indicate that the material density of the diapirs is 0.1–0.2 g/cm3 higher than the surrounding rock. The origin of positive residual gravity anomalies can be roughly divided into three types corresponding to the possible subsurface structures. The depth of the positive gravity anomaly below the major anticlines is approximately 4–5 km, corresponding to the bottom of the mudstone formation, from which the mud diapirs may originate. The residual gravity anomaly pattern also indicates a connection of the Chegualin and Youchang faults and the nearly vertical Zuochen fault above 10 km depth. Finally, the low gravity in the Pingtung Plain results from the high-density contrasts on both sides of the two major faults and the deepening of the basement in the plain. The high-angle Chauchou fault, one of two major faults, was also identified at shallow depths based on analysis of the gravity data.

Geodynamic factors in the formation of large gold-bearing provinces with Carlin-type deposits on continental margins in the North Pacific

&lt;abstract&gt; &lt;p&gt;Several similar indicators in Nevada (USA) and South Yakutia (Russia) gold-bearing provinces have been identified based on modern tectonic, geophysical and seismic tomography observations, followed by the analysis of the main geodynamic factors of the formation and distribution of large gold-bearing provinces in the North Pacific. One of the significant metallogenic peculiarities is a wide variety of formational and mineral deposits concentrated in the areas. Both provinces are situated at active margins surrounded by fold-thrust belts. In South Yakutia, a combination of sublatitudinal Baikal-Elkon-Ulkan and submeridional Seligdar-Verkhnetimpon gravity field gradient zones is recorded. In contrast, significant positive gravity anomalies of the Northern Nevada Rift and higher-order gradient zones are presented in Nevada. Large pluton and transform fault zones in both provinces support a conclusion about the fundamental role of geodynamic factors in developing ore-magmatic systems in the regions. Significant differences in the scale of the gold mineralization in the considered provinces are explained by the existence under the North American continent not only of the Mendocino transform fault zone but also of the Juan de Fuca paleo-spreading center. In contrast, the Inagli-Konder-Feklistov magmatic-metallogenic belt alone controls mineralization under the Asian continent.&lt;/p&gt; &lt;/abstract&gt;

Searching for buried craters on Mars based on gravity potential field separation method

There are many buried craters on Mars invisible in imaging or topographic data; however, the residual gravity anomaly separated from the Mars Bouguer gravity anomaly by minimum curvature separation method can reveal these buried craters. In our research, gravity separation results exposed 39 residual gravity anomaly highs (RGAHs), which may represent deeply buried impact craters with 300–500 ​km diameter. The RGAHs are widely distributed across the surface of Mars, but are especially concentrated in volcanic areas like Tharsis and Elysium, which indicates that there might be more craters buried in these regions. Older craters in volcanic regions were likely buried by subsequent volcanism coming from surrounding volcanoes, so the retention age of craters in all areas where RGAHs are located will be older than reported in the existing research based on topographic data alone. This study will help improve the chronological model of Mars. • The impact craters of Mars with diameter of 300–500 km appear central positive residual gravity anomalies. • The gravity separation results exposed 39 anomaly highs, that is inferred to be buried impact craters. • The most RGAHs are concentrated in volcanic areas just like Tharsis and Elysium.

Extensional structures and Cenozoic magmatism in the northwestern South China Sea

This study investigates the crustal structure and Cenozoic magmatism in the northwestern South China Sea (SCS), based on two long-cable multi-channel seismic reflection profiles, together with gravity and magnetic data, and adjacent wide-angle refraction profiles. Basins/sags are bounded by large listric-normal faults (fault throws ≥ 0.5 km) and massifs are cut off by normal faults with small offsets (fault throws < 0.5 km) in the northwestern SCS. These structures are penetrated by magmatic edifices showing positive gravity and magnetic anomalies. syn-Rift magmatic intrusions/extrusions were intense in the basins/sags and continent-ocean transition zone while post-rift magmatism was widespread from basins/sags to massifs with the most intense stage occurring from 5.5 to 2.6 Ma. Based on previous geophysical and geochemical results, we suggest that syn-rift mantle upwelling from partial melting initiated seafloor spreading magmatic activities, whereas plume-related mantle upwelling contributed to the magmatism during and after seafloor spreading in the northwestern SCS. Stretching factors show that the upper and lower crusts have experienced differential extension from basins/sags to massifs. The non-uniform crustal extension resulted from upper crustal faulting and lower crustal flow. Particularly, the lower crustal flow was probably linked with the combined action of magmatic heating, mantle flow shearing stresses, and sediment loading, resulting in crustal boudinage and reestablishment of an equilibrium state over long distances.

Breakup mechanism of the northern South China Sea: Evidence from the deep crustal structure across the continent-ocean transition

To improve constraints on breakup processes that eventually lead to seafloor spreading and the subsequent formation of new oceanic crust in the northern margin of the South China Sea (SCS), we collect a 172-km-long deep-penetration wide-angle seismic data OBS2018-L5. In the continental domain, the results present a gradually thinning crust with thickness ranging from ∼17 km to ∼8 km, with a slight thickening crust at the Outer Margin High. The attenuated continental crust is fairly rough and highly offset by distinctly different scale faults system. In the oceanic domain, the top of the basement is highly reflective with weak faulting and relatively constant crust thickness ranging from 5 km to 8 km (average 6 km). The continent-ocean boundary is characterized by an abrupt change in crustal thickness accompanied by a specific boundary between mostly positive gravity anomalies in the oceanic basin and grossly negative anomalies in the thinned continental crust, and between low amplitude, out-of-shape magnetic anomalies in the thinned continental domain and high amplitude, linear anomalies in the oceanic domain. A 0–5 km thick high-velocity layer (7.0–7.5 km/s) is present in the distal margin, which might represent mafic magma underplating. We also identify other magmatic features such as distinct buried volcanic edifices overprinting the original velocity structure. We propose that three mechanisms control the extension and breakup of the SCS: (1) the rapid transition from continental to oceanic crust, (2) elevated potential mantle temperature, and (3) the ductile lower crustal necking. Their interactions lead to the intense thinning of the continental lithosphere, decompression melting of the enriched asthenosphere followed by fast upwelling of mafic melt, relatively strong volcanic activities during the final stage of syn-rift and breakup, and finally generating a margin with intermediately rich magma, referred to as in intermediate-type margin.

Role of an advanced gravity data analysis in improving the geologic understanding of the northern Tebessa region, Northeastern Algeria

The Tebessa region in the extreme northeastern part of the Saharan Atlas of Algeria is characterized by a succession of anticline and syncline structures, diapirs, grabens, and a thick sedimentary cover underlying the Paleozoic basement. The region constitutes a complex structural system. The extensional structures (i.e., grabens and basins) in the Tebessa region belong to the foreland grabens of the North Africa Alpine Belt and their evolution is related to the tectonic history of the western Mediterranean region. The Tebessa region has been the subject of several geologic studies in the last decades. However, the subsurface structural configuration remains unknown, resulting in uncertainty about the exact origin of the grabens present in the Tebessa region.The current research involves gravity data analysis in order to understand the geologic subsurface system. Accordingly, gravity data are used to delineate the fault systems and the tectonic framework by using the 2-D Source Edge Detection (SED) technique, Centre for Exploration Targeting (CET) grid analysis, Euler Deconvolution, multiscale edge detection method (Worming), and 3-D modeling.Results indicate that the main tectonic trends within the study area are NE-SW, E-W, ENE-WNW, and NW-SE directions, making the northern Tebessa a complex structural region. The eastern part of the Tebessa region is characterized by a basement uplift which contributes to the positive gravity anomalies of the diapirs. In addition, the grabens have a sedimentary deposit thickness of about 2–8 km and underlain by Triassic evaporite layers that facilitated their opening by providing rheological weakness.The grabens in the Tebessa region have different mechanisms that have been responsible for their formation. However, the Early Mesozoic extensional tectonic regime initiated some grabens by creating the bounding normal faults, followed by Cenozoic strike-slip motions, when grabens opened as pull-apart basins.

Two-dimensional electrical resistivity structure of the crust and upper mantle across the North-South Gravity Lineament in NE China

The North-South Gravity Lineament (NSGL) is a large anomalous gravity zone that traverses several tectonic units with different properties in northeastern China. Its deep structural features and formation mechanism have long been a topical area of deep geophysical research. This study presents a magnetotelluric profile that extends from the Dongwuqi to Tongliao in Inner Mongolia and passes through geological structures such as the NSGL. Dimensional analysis of the profile data reveals good overall 2D features. 2D nonlinear conjugate gradient inversion of the profile data produced a model of the deep electrical structure. Horizontally, the electrical structure along the profile displays different characteristics in different blocks. The NSGL serves as a boundary for the change in the depth of the Moho discontinuity. Specifically, the Moho discontinuity is deep on the west side of the NSGL and shallow on its east side. Overall, the thickness of the lithosphere varies nonsignificantly, except in the western part of the profile, where the presence of a large low-resistivity anomaly in the shallow layer obscures the boundary of the lithosphere. The large high-resistivity anomaly in the center of the profile may be an indication that the partial delamination of the lithosphere caused by the convection of mantle materials and the upwelling of high-density mantle materials under the action of the westward subduction of the Pacific plate were the major causes of large positive gravity anomalies on the east side of the NSGL. The west side of the NSGL was predominantly controlled by the closure of the Mongol-Okhotsk Ocean, resulting in an increase in the thickness of the crust, which led to the formation of large negative gravity anomalies. These two factors collectively led to the formation of the NSGL.

Environmental geophysical study for ideal locations of landfill within Iraqi Southern Desert

Solid wastes and toxic chemicals are the main contributors, which influence the environment and quality of life. Demographic population, economic and building structures considered as major factors that are controlling such pollution. Therefore, it is very important to find proper areas to dispose of these pollutants. The current study aims to find best locations for wastes disposal areas that comply with the national and international standards. The study area, the Iraqi Southern Desert had been subjected to comprehensive study for geology, gravity and magnetic investigations. The existence of impermeable beds, tectonic, seismology and structural features show that the area understudy is located in the stable shelf that has no folding and faulting complexity; however, the saline groundwater lies at about 150 m depth. The gravity field data were analyzed using three methods: Trend Surface Analysis, Average Smooth Matrix and the First Vertical Derivative. Negative and positive gravity anomalies were noticed and interpreted. Remote sensing is also carried out in order to locate linear features, drainage patterns and watersheds within the Southern Desert depending on the interpretation of satellite images of the study area. Finally, a comparison of natural and environmental factors with remote sensing and both gravity and magnetic analyses gives thirteen locations of different coordinates. Only sites 1, 2, 3,10 and 13 yield good environment satisfaction agreement that permits us to choose them as a good locations for future landfill waste purposes.

The origin of mascons on Ceres as constrained by crater morphology

Positive, superisostatic gravity anomalies (also known as mass concentrations or “mascons”) are found beneath some large impact basins on Ceres, including Kerwan and Yalode. Such anomalies may be due to high-density mantle material that has been uplifted towards the surface, a process comparable to that on rocky bodies. Alternatively, the anomalies may be caused by impact energy devolatilizing the icy crust beneath the impact site, leaving a denser, ice-poor portion of material below the surface. The two hypotheses would result in different simple-to-complex transition diameters of craters superposing the larger basins. We analyzed crater morphology in order to deduce transition diameters of superposed craters in the Kerwan and Yalode basins. The surveyed locations produced transition diameters between 7 and 14 km. This transition diameter is consistent with that of average cerean crust composed of a mixture of both rock and ice, and not of a purely rocky crust which would produce a transition diameter of about 30 km. Our results have two possible interpretations. First, positive gravity anomalies beneath the basins are due to mantle uplift, rather than devolatilization of local crust. Second, impact-induced devolatilization is an incomplete process, removing enough ice to substantially increase crustal density but leaving enough ice to cause later impact processes to be ice-controlled. We favor the former explanation as the simpler interpretation, which would imply that mascons formed by impact-induced mantle uplift is a common process in the Solar System, and not just confined to rocky planets.