Gravity Moho Research Articles

Crustal structure of the Tian Shan Orogen and its adjacent areas inferred from EIGEN-6C4 gravity field data

The study of crustal structure plays a crucial role in understanding the tectonic development and seismicity of orogens. In this contribution, we analyzed the gravitational responses caused by the subsurface structure of the Tian Shan orogen using global EIGEN-6C4 gravity field data. The quality of the EIGEN-6C4 gravity data in the eastern Tian Shan area was evaluated, and we employed the wavelet multi-scale analysis method in combination with an existing P-wave velocity model to reveal the structures at different scales. The gravity Moho of the study area that best fits the seismological Moho was obtained from the filtered third-order wavelet approach component after several tests using different reference depths and density contrasts. The results reveal significant lateral discontinuities in the lower crust and upper mantle structures across the eastern and western Tian Shan orogens. The Moho beneath the western Tian Shan is about 9 km deeper compared to the eastern Tian Shan. Towards the Tarim and Junggar basins, the Moho decreases to about 45 km in depth. The analysis of the isostatic anomaly suggests that the eastern and western Tian Shan possess varying capabilities for isostatic adjustment. Furthermore, it indicates a gradual weakening of mantle upwelling in the Tian Shan orogen from west to east, implying that different stages of mantle dynamics may have occurred in the western and eastern Tian Shan. The north Tarim fault, Nikolaev-North Nalati fault, and Kazakh fault serve as channels that control magma upwelling in the Tian Shan orogen. The distribution of earthquakes differs between the eastern and western Tian Shan, with the western Tian Shan exhibiting greater gravitational potential energy. These variations between the eastern and western Tian Shan may be attributed to the clockwise rotation of the Tarim craton.

Crustal structure and isostatic compensation beneath the South China Sea using satellite gravity data and its implications for the rifting and magmatic activities

Opening of the South China Sea (SCS) was triggered by the breakup of south-eastern Eurasia and the southward drifting of the Palawan-Reed Bank microcontinent, as various prior studies have suggested. The young, moderately magmatic, rifted northern margin of the SCS is vital for investigating the relationship among magmatism, rheology, and structural evolution. This paper integrates satellite gravity anomaly, elevation/bathymetry, geoid, and seismic velocities to investigate continental breakup, magmatism, and rifting beneath the South China Sea. Source depths of 146, 31.5, and 8 km derived from the spectrum of Bouguer gravity anomalies suggest the average depths of the lithospheric base, continental, and oceanic crustal bases, respectively. Correspondingly, gravity Moho ranges from 8 km beneath the rifting center of SCS to 42 km in the Indo-China block. It is worth mentioning that the isostatic Moho from the Airy and flexural models were highly correlated with gravity Moho with correlation coefficients of ∼1. From the ratio between geoid and topography and our estimate of the vertical tectonic stress, the seamounts and reefs (Shuangfeng Basin: SB, Reed Bank: RB, Macclesfield Bank: MB) have a deep compensation depth. In contrast, the other parts (Manila Trench: MT, Phu Khan Basin: PK, East Sub Basin: ESB) have a smaller depth of compensation. 2D gravity modelling suggests the crustal thinning of the oceanic basin, and the thickening of the continental boundary implies the opening of the SCS, which is connected and happened at the same time as the northern subduction of the proto-SCS. The Gravity modelling also suggests a large rifting event within the lithosphere that favors mantle upwelling.

Determination of gravity and isostatic Moho: Implications for the evolution of rifting in the central Main Ethiopian Rift

We use gravity anomalies derived from GGMplus2013 gravity accelerations to generate a Moho undulation map of the Main Ethiopian Rift (MER) and surrounding plateau. The regional Bouguer anomaly is estimated by applying a low pass filters to different cutoff wavelengths and the resulting anomaly map is inverted using Parker Oldenburg method to generate a gravity Moho map. The isostatic Moho is computed using Airy model. Our results show that gravity Moho is in agreement with previous receiver function studies in the region. Based on the results on gravity and isostatic Moho, we estimate the magnitude and orientation of the vertical stress in the rift and surrounding region. In general, the vertical stress agrees very well with the regional tectonic and geomorphic features, where present day rifts are characterized by negative vertical stress except the central MER, which is characterized by positive stress. We argue that the crust in the central MER is in a state of isostatic adjustment. Our study provides an alternative explanation for why the central MER is less developed compared to the northern MER.

Origin, composition and relative timing of seaward dipping reflectors on the Pelotas rifted margin

Abstract The mechanism by which seaward dipping reflectors (SDRs) are formed is a topic of debate. Two contrasting models exist for their formation, the volcanic-faulting model and the volcanic-loading model. Each of these models has important implications for the processes which control the structure and formation of magma-rich rifted continental margins. We have examined high-quality deep-seismic reflection data across the Pelotas Basin, offshore Brazil. These data reveal a remarkable set of SDRs, for which we have investigated the likely nature of their formation. The total package of SDRs has an across-strike width of ~200 km and a variable vertical thickness of ~10–17 km, previously interpreted as volcanic flows. Detailed observations, however, show changes in seismic character and geometry within the SDR package, which suggest a complex and varied evolution. We have used gravity anomaly inversion and seismic observations together to investigate the likely composition of the SDRs by determining the proportion of basaltic material to sedimentary/volcaniclastic material (basalt fraction) within the SDRs. This has been achieved by minimising the difference between the depth of the gravity Moho and seismic Moho in order to quantify the lateral variation in basalt fraction, taken to be proportional to the bulk density of the package. The density of the SDR package together with seismic interpretation is then used to infer the composition, depositional environment, source and time of formation relative to breakup. Our analysis suggests that the overall SDR basalt fraction and bulk density decrease oceanwards, possibly due a change in the type of volcanic deposits from predominantly subaerial to volcaniclastics, possibly deposited subaqueously. The SDRs can be split into three sub-packages. The two inner SDR packages are interpreted to consist of lava flows sourced from syn-tectonic, subaerial eruptions, associated with the onshore Parana Large Igneous Province, flowing eastwards into an extensional basin. The outer SDR package shows reflector geometries that progressively offlap oceanwards, interpreted as extrusives sourced from an eastwards-migrating, newly formed ocean ridge. Our analysis suggests that both the volcanic-faulting and volcanic-loading models for SDR formation are applicable to the Pelotas rifted margin, recording distinct syn-rift and syn-breakup magmatic events. We show that both SDR formation models can be recognised in a naturally occurring example and can coexist on the same margin.

Determination of the isostatic and gravity Moho in the East China Sea and its implications

The East China Sea (ECS) is located among the Eurasian, Pacific and Philippine Sea plates, whose crustal structure and evolution are very complex. Elevation, geoid and gravity anomaly data are used in this study to determine the isostatic and gravity Moho topography of the ECS and surroundings. The gravity effects of sediments and subducting slabs are computed using recent datasets during estimation of the gravity Moho, and the results show good agreement between the gravity Moho and seismic results. The crustal isostatic implications of the difference between the isostatic and gravity Moho are discussed, and the decreasing Moho depth from the coast of the mainland China to the western margin of the Okinawa Trough suggests continental crust-type features, similar to the Yellow Sea. A combined isostatic analysis, which takes the difference between the isostatic and gravity Moho into account, indicates that the shallower Moho beneath the Okinawa Trough and the depressed Moho beneath the Ryukyu Arc may have implications for rifting in the Okinawa Trough. The steep Moho along the Ryukyu Trench marks the continent-ocean boundary at the crustal scale, which is consistent with the subduction of the Philippine Sea Plate. Taking the eastward extension of the ECS into account, it is expected that this would be steeper if it was generated by the isostatic over- and under-compensation beneath the Ryukyu Arc and Trench, respectively.

Effective elastic thickness in the Central Andes. Correlation to orogenic deformation styles and lower crust high-gravity anomaly

Global studies have assessed the importance of elastic thickness (Te) on orogenic evolution, showing that the style and nature of upper crustal shortening are influenced by the inherited lithospheric strength. Thus, pioneer works have identified that the upper crustal deformation style in the easternmost sector of the Central Andes in South America are related to the elastic thickness (Te). There, the thick-skinned and pure-shear style of Santa Bárbara system was initially related to the existence of low Te values. In contrast, the thin-skinned and simple-shear style of deformation in the Subandean system involves high Te values. However, more recent Te studies in the Central Andes present conflicting results which lead to question this straightforward relation. Results from these studies show a strong dependence on the applied methodology hampering the general understanding of the lithospheric thermo-mechanical state of the Central Andes. To contribute to this issue, we perform a high-resolution Te map, using forward modeling by solving flexural equation of infinite plate model in two dimensions. To achieve this, the crust-mantle interface was calculated using a high-resolution gravity anomaly dataset which combines satellite and terrestrial data, and an average density contrast. Additionally, the gravity anomaly and the foreland basin depth in the Central Andes were best predicted by considering that lower crustal rocks fill the space deflected downward in the plate model. The obtained Te values show an inverse correlation with previous heat flow studies, and a strong spatial correlation with the styles and mechanisms of deformation in the easternmost sector of the Central Andes. In the Santa Bárbara system Te values less than 10 km predominate, whereas in the Subandean system high Te values were observed. Such high values correlate with the orogenic curvature and with an shallower gravity Moho zone, which breaks the regional trend of the Central Andes. This shallower gravity Moho is linked to a high-gravity anomaly located in the east part of the Eastern Cordillera and Subandean system. These results are also correlated with a high-velocity zone in the upper mantle previously found by receiver functions studies. This correlation could indicate changes in the properties of the lower crustal rocks that justify the shallower gravity Moho zone and explain in part the highest Te values.

Moho depth model for the Central Asian Orogenic Belt from satellite gravity gradients

AbstractThe main purpose of this study is to construct a new 3‐D model of the Central Asian Orogenic Belt (CAOB) crust, which can be used as a starting point for future lithospheric studies. The CAOB is a Paleozoic accretionary orogen surrounded by the Siberian Craton to the north and the North China and Tarim Cratons to the south. This area is of great interest due to its enigmatic and still not completely understood geodynamic evolution. First, we estimate an initial crustal thickness by inversion of the vertical gravity component of the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) and DTU10 models. Second, 3‐D forward modeling of the GOCE gravity gradients is performed, which determines the topography of the Moho, the geometry, and the density distribution of the deeper parts of the CAOB and its surroundings, taking into account the lateral and vertical density variations of the crust. The model is constrained by seismic refraction, reflection, and receiver function studies and geological studies. In addition, we discuss the isostatic implications of the differences between the seismic Moho and the resulting 3‐D gravity Moho, complemented by the analysis of the lithostatic load distribution at the upper mantle level. Finally, the correlation between the contrasting tectonic domains and the thickness of the crust reveals the inheritance of Paleozoic and Mesozoic geodynamics, particularly the magmatic provinces and the orocline which preserve their crustal features.

The gravity and isostatic Moho in North China Craton and their implications to seismicity

The Bouguer gravity is the combination of field sources in different depths. Based on the multi-scale analysis of the Bouguer gravity, we can get the gravity anomaly caused by the Moho undulation. This study presents the various orders of approximation of gravity anomaly in North China Craton (NCC), the possible source depths with radial logarithmic power spectrum, and the relationship between the deep structure and gravity anomaly. Furthermore, we discuss the isostatic compensation about the Moho depth from gravity and deep seismic sounding profiles (DSS). The results show that: (1) the fourth approximation could have resulted from the Moho undulation, (2) in contrast to the isostatic Moho, the inverted gravity Moho and the DSS Moho show that most of NCC has been isostatically compensated, and (3) the isostatic compensation rate has some close relation to the seismicity.

A Comparative Analysis of Seismological and Gravimetric Crustal Thicknesses below the Andean Region with Flat Subduction of the Nazca Plate

A gravimetric study was carried out in a region of the Central Andean Range between28∘and32∘south latitudes and from72∘and66∘west longitudes. The seismological and gravimetrical Moho models were compared in a sector which coincides with the seismological stations of the CHARGE project. The comparison reveals discrepancies between the gravity Moho depths and those obtained from seismological investigations (CHARGE project), the latter giving deeper values than those resulting from the gravimetric inversion. These discrepancies are attenuated when the positive gravimetric effect of the Nazca plate is considered. Nonetheless, a small residuum of about 5 km remains beneath the Cuyania terrane region, to the east of the main Andean chain. This residuum could be gravimetrically justified if the existence of a high density or eclogitized portion of the lower crust is considered. This result differed from the interpretations from Project “CHARGE” which revealed that the entire inferior crust extending from the Precordillera to the occidental “Sierras Pampeanas” could be “eclogitized”. In this same sector, we calculated the effective elastic thickness (Te) of the crust. These results indicated an anomalous value of Te = 30 km below the Cuyania terrane. This is further conclusive evidence of the fact that the Cuyania terrane is allochthonous, for which also geological evidences exist.

Crustal modeling of the North Atlantic from spectrally correlated free-air and terrain gravity

We investigate the use of the correlations between free-air and terrain gravity for extending critical, but spatially restricted seismic constraints on the crust of the North Atlantic region. In this tectonically complicated area, we spectrally correlate spherical coordinate free-air gravity anomalies against computed gravity effects of the terrain at 20 km altitude for regional crustal thickness variations. However, at the Mid-Atlantic Ridge, thermal plume, and rift zones of Iceland, our gravity Moho estimates are deeper than the seismic estimates to suggest local reductions of upper mantle density. Invoking thermal expansion, partial melting, and related mantle depletion effectively accounts for the reductions that range up to a few percent of mantle density. The resultant North Atlantic crustal model conforms well to regional seismic constraints. Crustal thickness estimates for Iceland ranging from 25 to 35 km are consistent with seismic predictions of a thickened crust. However, about the plume and rift zones, temperatures approaching the basalt solidus may reach depths up to about 20 km. Continental crust extending from the Jan Mayen Platform to the eastern Iceland Plateau appears to have rifted off eastern Greenland by magnetic chron 7 (ca. 26–28 Ma). A now obscure transform boundary on the Iceland Plateau that may include a rift or fault separating the southeastern Iceland Plateau from the rest of the plateau transformed Reykjanes Ridge spreading from the extinct Aegir Ridge. Thickened crust extends up to 500 km across the Greenland–Scotland Ridge. Prominent necking of our crustal thickness estimates for the Greenland–Iceland Ridge immediately west and east of the Iceland Plateau may reflect the reduction in the strength of the Icelandic Plume at ca. 26–28 Ma. However, our results also map the broad, thickened crust of the Iceland Plateau and prominent Reykjanes Ridge that developed with the resurgence of the Icelandic Plume at ca. 25 Ma.

Topography of the Moho undulations in France from gravity data: their age and origin

The complete gravity data set from France and part of the neighboring countries has been analyzed to compute the topography of the Moho undulations. This work is based on an improved filtering technique and an appropriate assumed density contrast between the crust and the upper mantle. Comparison with deep seismic refraction data reveals that this relief map expresses the continuity and geometry of the Moho undulations better than the sparsely distributed seismic refraction data in France. This gravity Moho map, though may not give absolute depths at places, provides a far better correlation with surface geology than the result from other geophysical techniques. Four domains have been recognized: (a) the Alpine domain where all the Moho undulations are concentric with the Alps; (b) the Armorican domain in which all the undulations are north-west/south-east oriented; (c) the Pyrenean domain, in which the undulations are parallel with the Mountain chain; and (d) the Massif Central Domain which does not show clear structural orientation because of the influence of the strong heat flow located at the lower crust/upper mantle interface. Study of the topography and of the superficial structures associated with these undulations reveals that the undulations delineated in the Alpine Domain result from the Tertiary compression which shaped the Alps. The Armorican Domain was first created during the Lower to Middle Cretaceous opening of the Bay of Biscay. It is now slightly affected by the Tertiary to Quaternary closure of this Bay. The Pyrenean Domain was successively shaped by the Lower Cretaceous oblique opening of the Bay of Biscay and by the Upper Cretaceous to Eocene northward displacement of Spain. Comparison between the Moho undulations map and the stress map of France reveals that most of the undulations are perpendicular to the actual shortening directions. This observation suggests that the Mesozoic, Cenozoic and Quaternary stress directions were roughly the same. Massif Central is characterized by the convergence of these three sets of undulations. Its Post-Oligocene uplift was probably the result of the converging stresses recognized in the three surrounding domains. When the Moho undulations and the topography are compared, two types of periodic crustal instabilities can be recognized. One corresponds to the buckling of the crust developed under compression, the other to boudinage which was associated with extension. Both phenomena show a typical wavelength of 200–250 km which is in agreement with the results of the actual physical and numerical modeling currently available.

Relation of Isotope Geochemical Steep Zones with Geophysical Fields and Tectonics in the Junction Area of the Cathaysian, Yangtze and Indochina Plates

AbstractThrough lead isotope geochemical mapping in the Yunnan‐Guizhou area geochemical steep zones (GSZ) have been established, which clearly reveal the junction relationship of the Cathaysian, Yangtze and Indo‐China plates. GSZ are closey related to gravity Moho gradient zones and lithospheric thickness. The GSZ between the Yangtze and Cathaysian plates is consistent with the Shizong‐Mile tectonic belt, where island are basalts are well developed. The Yangtze‐Indo‐China GSZ is parallel to the Jingdong‐Mojiang volcanic belt in rift‐island are environments. The evidence of geology, geophysics and geochemistry all indicates that Cathaysia was subducted towards the Yangtze plate and that the Yangtze plate was underthrust beneath the Indo‐China, which took place from the Early Carboniferous to the Early Triassic.

Tectonic evolution of the Alboran Sea basin

AbstractThe Alboran Sea is an extensional basin of Neogene age that is surrounded by highly arcuate thrust belts. Multichannel seismic (MCS) reflection profile data suggest the basin has a complex tectonic fabric that includes extensional, compressional and strike‐slip structures. The early Miocene history appears to be dominated by graben formation with border faults that are in large part contemporaneous with thrust movements in the external zones of the Betic and Rif mountains. Extension appears to have continued into the late Miocene although the main movements were probably completed by the time of the Messinian ‘salinity crisis’. The Pliocene and younger history of the basin is dominated by infilling of the Messinian topography, gentle subsidence, and extensional, compressional and strike‐slip movements. There is evidence from the sea‐floor morphology and seismicity patterns that the basin is actively deforming in response to present‐day plate motions. Backstripping of well data in the basin margin suggests that the initial extensional event was accompanied by crustal and lithospheric thinning. The depth to Moho inferred from backstripping is greater than the depth expected based on seismic and gravity modelling, suggesting that backstripping underestimates the true amount of thinning. One explanation is that some of the thinning occurred while the crust was above sea level, perhaps as a result of either crustal thickening, or a period of lithospheric heating and thinning, prior to rifting. We found that a model with a ‘normal’ crustal thickness of 31.2 km, a lithospheric thickness of 50 km, and β= 1.4 predicts 0.8 km of initial uplift. These parameters fit the well subsidence data and bring the backstripped Moho into better agreement with the seismic and gravity Moho. The origin of such a thin lithosphere is not constrained by the data, but we believe that it may be a result of the detachment of a cold lithospheric ‘root’ that formed during pre‐Neogene collisional orogeny in the region.