The sub-lithospheric stress due to mantle convection can be computed from gravity data and propagated through the lithosphere by solving the boundary-value problem of elasticity for the Earth's ...lithosphere. In this case, a full tensor of stress can be computed at any point inside this elastic layer. Here, we present mathematical foundations for recovering such a tensor from gravitational tensor measured at satellite altitudes. The mathematical relations will be much simpler in this way than the case of using gravity data as no derivative of spherical harmonics or Legendre polynomials is involved in the expressions. Here, new relations between the spherical harmonic coefficients of the stress and gravitational tensor elements are presented. Thereafter integral equations are established from them to recover the elements of stress tensor from those of the gravitational tensor. The integrals have no closed-form kernels, but they are easy to invert and their spatial truncation errors are reducible. The integral equations are used to invert the real data of the gravity field and steady-state ocean circulation explorer (GOCE) mission, in November 2009, over the South American plate and its surroundings to recover the stress tensor at a depth of 35 km. The recovered stress fields are in good agreement with the tectonic and geological features of the area.
Continental rifts evolve along two possible paths. In one, a rift successfully evolves into seafloor spreading, leaving the rift structures buried beneath thick sedimentary and volcanic rocks at a ...passive continental margin. Alternatively, the rift fails and remains as a fossil feature within a continent. We consider insights into these processes from studies of North America's Midcontinent Rift (MCR). The MCR combines the linear geometry of a rift formed at a plate boundary and the huge igneous rock volume of a Large Igneous Province. The rift is a fault bounded basin filled with volcanics and sediments, which record a history of extension, volcanism, sedimentation, subsidence, and inversion. The MCR came close to evolving into an oceanic spreading center, but it instead failed and thus records a late stage of rifting. It thus preserves a snapshot of a stage of the process by which actively extending rifts, characterized by upwelling mantle and negative gravity anomalies, evolve either into failed and often inverted rifts without upwelling mantle and positive gravity anomalies or into passive continental margins. Many rifts can be viewed as following a generally similar evolutionary sequence, within which a complex combination of factors control the variability of structures within and among rifts. Study of the MCR also gives insight into passive continental margins. The MCR gives a snapshot of deposition of a thick, dense, and highly magnetized volcanic section during rifting. Surface exposures, seismic, and gravity data delineate a rift basin filled by inward dipping flood basalt layers, underlain by thinned and underplated crust. The fact that the MCR shows many features of a rifted volcanic margin suggests that it came close to continental breakup before it failed, and illustrates how many passive margin features form prior to breakup.
•The Midcontinent Rift (MCR) formed in a plate boundary reorganization at 1.1 Ga.•Its arms were likely the boundaries of a microplate between diverging major plates.•It experienced extension, volcanism, sedimentation, subsidence, and inversion.•It almost evolved to seafloor spreading and so records a late stage of rifting.•The MCR illustrates how many features of volcanic passive margins form.
SUMMARY
Joint inversion for the same or different geophysical parameters is proved to be an effective technique for obtaining high-resolution solutions. Thus, comprehensive geophysical interpretation ...based on joint inversion has been widely concerned and applied in recent years. To realize joint inversion conveniently and efficiently, we proposed a new inversion strategy based on the alternating direction method of multipliers. In this regard, three optimization algorithms were presented respectively to attain the joint inversion of body wave traveltime and surface wave dispersion data, to obtain the joint inversion of magnetotelluric and seismic data with cross-gradient constraints, and to acquire gravity constrained inversion. A complex model with inconsistent structures in terms of resistivity, velocity and density was designed to evaluate the accuracy and effectiveness of the multiparameter joint inversion algorithms. In our joint inversion processes, each method was optimized independently and the jointly inverted results were significantly more accurate than those of separate inversions. Finally, we applied the algorithms to the field data involving gravity anomaly data, magnetotelluric data and Rayleigh wave dispersion data. The reliable underground structure was achieved by the joint interpretation of density, resistivity and velocity profiles, which verified the practicality of the inversion strategy in the actual data.
The Kyushu-Palau Ridge (KPR), a remnant arc on the Philippine Sea Plate (PSP), is subducting beneath the Kyushu, southwest Japan. Influenced by the subducting KPR, the Kyushu subduction zone ...corresponding to the KPR is significantly different from Shikoku subduction zone in terms of gravity anomalies, seismicity, the stress state, and the subducting slab morphology. Significant negative free-air and Bouguer gravity anomalies are observed in a prolonged area of KPR, southeast of the Miyazaki Plain, indicating that this is where KPR overlaps the overriding plate. The gravity anomaly in this area is much lower than that in other areas where the inferred KPR extends, suggesting that the subduction of the buoyant KPR may cause the lower mantle density to decrease. More earthquakes have occurred in Hyuga-nada region where the KPR subducts than in Shikoku forearc and other areas in the Kyushu forearc, indicating that the subduction of the KPR enhances the local coupling between the subducting and overriding plates. The centroid moment tensor (CMT) mechanism of earthquakes shows that stress is concentrated in the accumulated crust beneath the Kyushu forearc corresponding to the KPR, and the shallow thrusting events in the obducting plate are caused by the KPR subduction. The buoyant KPR, with a large volume of low-density sediments, was responsible for the differences of the subduction depth and dip angle of the subducting Philippine Sea (PS) slab between northern Kyushu and Shikoku. The seismic gaps and the sudden change of the dipping angle of the subducting PS slab indicate that slab tear may have occurred along the west side of the KPR beneath southwest Kyushu. A two-tear model was proposed, and the subduction of the buoyant KPR was believed to play an important role in the slab tear.
We present a comprehensive view of the origin, significance and implications of topographic effects in gravimetry. These are gravitational effects of topographic masses that are present in any ...observable quantity of the earth's gravitational field. In most gravimetric applications and their computational realizations, when topography is not the focus of the study, these effects need to be properly treated as corrections (reductions). Some of them might not be obvious or intuitive, and may remain misunderstood and mistreated. First, we look at topographic effects in geodesy and focus on those that affect the determination of the geoid from terrestrial gravity data. We review the origin and role of both the direct and indirect, the primary and secondary topographic effects. Then, we review the Bouguer concept in geophysical applications. Finally, we take a look at the topographic effect induced by the deformation of the topographic surface and its importance in the interpretation of observed spatiotemporal gravity changes. We illustrate the sizes and shapes (spatial properties) of these effects, discuss their relevance and impacts in the areas of geodesy, geophysical structural studies (exploration and prospection), and in geodynamics with a focus on volcano geodesy.
Located in Northeast Asia, the Nadanhada Terrane contains the Paleozoic and Mesozoic accretionary complex that is an ideal record for studying the closure of the Paleo-Asian Ocean and the evolution ...of the Paleo-Pacific Ocean. To reveal the process of the Nadanhada Terrane's composition and formation we use geophysical and geological data. The characteristics reveal that the Nadanhada Terrane is composed of three distinct tectonic units ordered from southwest to northeast: the Yuejinshan Complex Belt, the Raohe Basin, and the Raohe Complex Belt. The electrical characteristics indicate that the Yuejinshan Complex Belt and Jiamusi Block share a common basement, while the Raohe Complex Belt is a complete accretionary terrane. The order of gravity anomalies from high to low of the three tectonic units is the Yuejinshan Complex Belt, followed by Raohe Basin and then the Raohe Complex Belt. High magnetic volcanic rocks are developed in the Yuejinshan and Raohe complexes, and low magnetic granite is developed in the Raohe Basin. The Nadanhada Terrane was formed via continuous accretion during the multi-stage subduction of oceanic and continental plates. The main formation process was as follows: 1) From the Late Carboniferous to the Late Triassic, the Paleo-Asian Ocean continued to subduct below the Jiamusi Block, resulting in the formation of the main body of the Yuejinshan Complex Belt. 2) From the Late Triassic to the Early Cretaceous, the Paleo-Pacific (Izanagi) Plate was subducted toward Northeast Asia, and the Mesozoic parts of the Yuejinshan Complex and the Raohe Complex were formed via collage and accretion. 3) During the Early Cretaceous, an extensional tectonic background was generally formed in Northeast Asia and the Raohe Basin began to form. 4) During the Late Cretaceous, the Pacific Plate was subducted toward Northeast Asia, and the Nadanhada Terrane was continuously squeezed and formed.
•We obtain a high resolution geophysical model of the Nadanhada Terrane.•The Nadanhada Terrane has three tectonic units with different geodynamics and sources.•The Nadanhada Terrane was influenced by the Paleo-Asian Ocean into the Paleo-Pacific Ocean.
Intraplate strain generally focuses in discrete zones, but despite the profound impact of this partitioning on global tectonics, geodynamics, and seismic hazard, the processes by which deformation ...becomes localized are not well understood. Such heterogeneous intraplate strain is exemplified in central Asia, where the Indo-Eurasian collision has caused widespread deformation while the Tarim block has experienced minimal Cenozoic shortening. The apparent stability of Tarim may arise either because strain is dominantly accommodated by pre-existing faults in the continental suture zones that bound it—essentially discretizing Eurasia into microplates—or because the lithospheric-scale strength (i.e., viscosity) of the Tarim block is greater than its surroundings. Here, we jointly analyze seismic velocity, gravity, topography, and temperature to develop a 3-D density model of the crust and upper mantle in this region. The Tarim crust is characterized by high density, vs, vp, and vp/vs, consistent with a dominantly mafic composition and with the presence of an oceanic plateau beneath Tarim. Low-density but high-velocity mantle lithosphere beneath southern (southwestern) Tarim underlies a suite of Permian plume-related mafic intrusions and A-type granites sourced in previously depleted mantle lithosphere; we posit that this region was further depleted, dehydrated, and strengthened by Permian plume magmatism. The actively deforming western and southern margins of Tarim—the Tien Shan, Kunlun Shan, and Altyn Tagh fault—are underlain by buoyant upper mantle with low velocity; we hypothesize that this material has been hydrated by mantle-derived fluids that have preferentially migrated along Paleozoic continental sutures. Such hydrous material should be weak, and herein strain focuses there because of lithospheric-scale variations in rheology rather than the pre-existence of faults in the brittle crust. Thus this world-class example of strain partitioning arises not simply from the pre-existence of brittle faults but from the thermo-chemical and therefore rheological variations inherited from prior tectonism.
•Lithospheric density from joint analysis of gravity, topography, velocity and temperature.•Tien Shan, Kunlun Shan and Altyn Tagh fault are underlain by buoyant upper mantle.•Low-density but high-velocity mantle lithosphere beneath southern Tarim.•Rheological variations dominate the strain partitioning.
The seafloor depths under the Cook Ice Shelf and Ninnis Glacier Tongue have not been directly measured, despite their importance for understanding ocean circulation and ice shelf change. We model the ...bathymetry underneath the floating ice and surrounding ocean using airborne gravity data. Our model is constrained by few ship‐based seafloor measurements near the ice front and by ice‐base measurements over areas of grounded ice from radar data. Localized basins (∼1,400 m deep) are found beneath both ice shelves. The shallowest modeled bathymetry (∼200 m) represents the offshore extension of Cape Freshfield. Near the grounding line, seafloor depths are found to be deeper than the observed depth of the modified Circumpolar Deep water in the region (<350 m), key factor for basal melt analyses. From transit flight gravity anomalies, we suggest the relocation of the mapped edge of the continental shelf and a narrowing of the Cook Shelf Depression.
Plain Language Summary
The knowledge of how deep the ocean floor is under the floating ice shelves that connect to grounded ice sheets, is crucial for understanding how ocean water circulates and interacts with the overlying ice. We present a new bathymetric model of the seafloor beneath two ice shelves located in East Antarctica: Cook Ice Shelf and Ninnis Glacier Tongue. Both ice shelves are inaccessible to ships due to heavy sea ice conditions, so the data used in our model were collected from airborne surveys. Our bathymetry model shows new information on the depth and shape of the seafloor that will help understanding the ocean circulation in the area and how this might impact ice thickness changes.
Key Points
High resolution bathymetry model of Cook Ice Shelf, Ninnis Glacier Tongue, and surrounding open ocean from airborne gravity inversion
New bathymetry model improves the understanding of water pathways between the ice shelves and the continental shelf edge
Transit flight gravity anomalies suggest relocation of the edge of the continental shelf northwards of the currently attributed position
The northwestern region of the Antarctic Peninsula is of main interest to the geosciences. Complex tectonic activity in this area is evidenced by the presence of the sinistral South Scotia ridge ...system and Shackleton plate contacts; As well as the opening of the Bransfield Strait leading to the formation of the Shetland Block, where the last remnant of the Phoenix convergent margin is located. Potential fields such as gravity and magnetism can be used to study crustal structure in these remote regions, where sedimentary basins are located. In this study, we utilized gravity data from the EIGEN-6C4 combined field model and magnetic data from the ADMAP2 magnetic anomaly model. We employed the Spectral decomposition of gravity anomalies to analyze sedimentary basement depth and directional derivatives of gravity and magnetic anomalies to derive structural features and magmatic activity. The gravimetric basement model was compared with previous interpretations of the acoustic basement and provides a new interpretation of basement morphology in regions that have not been studied previously in detail. The gravity/magnetic derivatives highlight evidence for ongoing magmatic activity in the Central Bransfield Basin which correlates with seismic catalogs, and crustal segmentation between King George and Gibbs Islands related to the unstable intersection of the South Scotia Ridge system and Shackleton Fracture Zone.
•Tectonic features near the Antarctic Peninsula highlight complex tectonism.•Gravity and magnetic models were used to analyze crustal structure and magmatism.•Our gravimetric basement model provides a new interpretation of basement morphology.•We identified evidence of crustal segmentation between King George and Gibbs Islands.•Magmatic activity near the South Shetland Islands was derived from magnetic anomalies.
In western Anatolia, the Anatolide domain of the Tethyan orogen is exposed in one of the Earth’s largest metamorphic core complexes, the Menderes Massif. The Menderes Massif experienced a two-stage ...exhumation: tectonic denudation in the footwall of a north-directed Miocene extensional detachment, followed by fragmentation by E–W and NW–SE-trending graben systems. Along the northern boundary of the core complex, the tectonic units of the Vardar–Izmir–Ankara suture zone overly the stage one footwall of the core complex, the northern Menderes Massif. In this study, we explore the structure of the upper crust in the northern Menderes Massif with cross-gradient joint inversion of gravity and aeromagnetic data along a series of 10-km-deep profiles. Our inversions, which are based on gravity and aeromagnetic measurements and require no geological and petrophysical constraints, reveal the salient features of the Earth’s upper crust. We image the northern Menderes Massif as a relatively homogenous domain of low magnetization and medium to high density, with local anomalies related to the effect of interspersed igneous bodies and shallow basins. In contrast, both the northern and western boundaries of the northern Menderes Massif stand out as domains where dense mafic, metasedimentary and ultramafic domains with a weak magnetic signature alternate with low-density igneous complexes with high magnetization. With our technique, we are able to delineate Miocene basins and igneous complexes, and map the boundary between intermediate to mafic-dominated subduction–accretion units of the suture zone and the underlying felsic crust of the Menderes Massif. We demonstrate that joint gravity and magnetic inversion are not only capable of imaging local and regional changes in crustal composition, but can also be used to map discontinuities of geodynamic significance such as the Vardar–Izmir–Ankara suture and the West Anatolia Transfer Zone.
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