Three-dimensional Gravity Models Research Articles

Lithosphere structure of European sedimentary basins from regional three-dimensional gravity modelling

A three-dimensional (3D) density model, approximated by two regional layers—the sedimentary cover and the crystalline crust (offshore, a sea-water layer was added), has been constructed in 1° averaging for the whole European continent. The crustal model is based on simplified velocity model represented by structure maps for main seismic horizons—the “seismic” basement and the Moho boundary. Laterally varying average density is assumed inside the model layers. Residual gravity anomalies, obtained by subtraction of the crustal gravity effect from the observed field, characterize the density heterogeneities in the upper mantle. Mantle anomalies are shown to correlate with the upper mantle velocity inhomogeneities revealed from seismic tomography data and geothermal data. Considering the type of mantle anomaly, specific features of the evolution and type of isostatic compensation, the sedimentary basins in Europe may be related into some groups: deep sedimentary basins located in the East European Platform and its northern and eastern margins (Peri-Caspian, Dnieper–Donets, Barents Sea Basins, Fore–Ural Trough) with no significant mantle anomalies; basins located on the activated thin crust of Variscan Western Europe and Mediterranean area with negative mantle anomalies of −150 to −200×10 −5 ms −2 amplitude and the basins associated with suture zones at the western and southern margins of the East European Platform (Polish Trough, South Caspian Basin) characterized by positive mantle anomalies of 50–150×10 −5 ms −2 magnitude. An analysis of the main features of the lithosphere structure of the basins in Europe and type of the compensation has been carried out.

Deep magmatic structures of Hawaiian volcanoes, imaged by three-dimensional gravity models

Deep magmatic structures of Hawaiian volcanoes, imaged by three-dimensional gravity models

Three-dimensional subsidence analysis and gravity modelling of the continental margin offshore Namibia

SUMMARY Seismic re£ection pro¢les and gravity anomaly data have been used to determine the structure and evolution of the Namibian continental margin. In comparison to other margins, the gravity anomaly at the Namibian margin shows a number of distinctive features. It lacks an oishore gravity ‘low’, and, despite the presence of up to 9 km of sediments, the gravity ‘high’ is displaced landwards of the maximum sediment thickness. In an attempt to explain these features, the Early Cretaceous^Recent stratigraphic record is analysed using a combined 3-D backstripping and gravity modelling technique that enables constraints to be placed on the long-term mechanical properties of the lithosphere. A neutral depth of necking and a relatively high £exural rigidity (or equivalent elastic thickness, Te ,o f*25 km) can explain part of the anomaly, at least in the south of the margin. However, large residual anomalies exist that can be reduced by the presence of pre-rift low-density sediments within the zone of maximum stretching, south of 22 0 S, high-density volcanic material at the eastern pinch-out of the lower rift sequence and a lateral sediment density variation across the shelf, slope and rise. In addition, the presence of a magmatic body at the base of the crust, provided that the £exural strength of the margin is high, signi¢cantly improves the ¢t between observed and calculated gravity anomalies. Recovered stretching factors enable predictions of the geometry of the Moho to be made. Palaeobathymetry is estimated along the entire margin through time by comparing the subsidence determined from backstripping to that predicted by simple models of rifting. These estimates are within the errors of observed palaeowater depths where well control exists. The requirement of high Te rather than Te that increases with time suggests that stretched continental crust extends oishore for some distance and the continent^ocean boundary occurs in relatively deep water near magnetic anomaly M4. Reconstruction of the gravity ¢eld of the South Atlantic at 100 Ma illustrates that, in terms of their gravity signature, the South Atlantic margins are asymmetric.

Structure of the Mygdonia basin (N. Greece) inferred from MT and gravity data

The area of the Mygdonia basin (N. Greece) comprises the European Seismological Test Site. For that reason, a large number of different geophysical methods have been applied in the area. However, information on the structure below the sediments has not been revealed. MT and gravity data were interpreted in order to identify the deeper structure of the region where the basin is in detail. Gravity data were also used in order to reveal information about the extent of the magmatic intrusions in the area. A three-dimensional gravity model for the broader area of the Mygdonia basin was constructed, based on the interpretation of the gravity data. A buried body of a density less than that of the hosting metamorphic rocks is suggested and is considered as the continuation of the Arnea granite, which outcrops mainly southwards of the studied area. Eight MT soundings were performed in the Mygdonia basin. The frequency range was 128–0.004 Hz, and nine channels were employed for each sounding, with four used as a reference station. The data were processed using both least-square and remote-reference least-square methods. Different types of dimensionality indicators were calculated, resulting in a N65E regional strike. A profile perpendicular to the strike was concluded, and a vertical grid along this line was created using information from d.c.-resistivity measurements for the first 400 m depth. The grid was used as an input to a two-dimensional inversion algorithm (Occam's 2D). The geological model proposed for the basement consists of schists with a thickness of 200 m, and gneisses, which reach a depth of 1200 m. Below these formations, the existence of the granite is suggested from the results of both the MT and gravity data. The depth of the bottom of the granitic body reaches 5 km.

Deep magmatic structures of Hawaiian volcanoes, imaged by three-dimensional gravity models

A simplified three-dimensional model for the island of Hawai9i, based on 3300 gravity measurements, provides new insights on magma pathways within the basaltic volcanoes. Gravity anomalies define dense cumulates and intrusions beneath the summits and known rift zones of every volcano. Linear gravity anomalies project southeast from Kohala and Mauna Kea summits and south from Hualālai and Mauna Loa; these presumably express dense cores of previously unrecognized rift zones lacking surface expression. The gravity-modeled dense cores probably define tholeiitic shield–stage structures of the older volcanoes that are now veneered by late alkalic lavas. The three-dimensional gravity method is valuable for characterizing the magmatic systems of basaltic oceanic volcanoes and for defining structures related to landslide and seismic hazards.

Large-scale three-dimensional gravity analysis of the lithosphere below the transition zone from Western Europe to the East European Platform

Large-scale three-dimensional gravity modelling using data averaged on a 1° grid has been carried out for the transition area from Western Europe to the East European craton. The model consists of two regional layers of variable thickness — the sedimentary cover and the crystalline crust — with lateral variation of average density. Removal of the calculated gravity effect of the crustal model from the observed gravity field reveals residual anomalies of mantle origin. This is confirmed by the correlation between mantle gravity anomalies and upper mantle velocity heterogeneities imaged by seismotomographic studies. The Tornquist–Teisseyre Zone (TTZ), marked in general by a gradient zone of mantle gravity anomalies, is considered as a deep density section traced in the upper mantle down to a depth of 200km. Across this suture zone a dramatic change of the structure, physical properties and composition of the lithosphere of the continent occurs. The different characters of the mantle gravity anomalies over the two main parts of the TTZ — the Danish–Scandinavian and Polish segments — are explained by the peculiarities of their evolution and the different character of the prevailing tectonic regimes during the Proterozoic–Cenozoic.

Gravity and aeromagnetic constraints on the structure of the Woods Mountains volcanic center, southeastern California

The Woods Mountain volcanic center is a well-exposed, mildly alkaline volcanic center that formed during the Miocene in southeastern California. Detailed geologic mapping and geochemical studies have distinguished three major volcanic phases: precaldera, caldera forming, and postcaldera. Geologic mapping indicates that caldera formation occurred incrementally during eruptions of three large ignimbrites and continued into a period of voluminous intracaldera lava-flow eruptions. Rhyolitic ignimbrites and lava flows within the caldera are associated with large amplitude, circular gravity, and magnetic minima that are among the most prominent gravity and magnetic anomalies in southeastern California. Analysis of a Bouguer gravity anomaly map, reduced-to-the-pole magnetic intensity map, and three-dimensional gravity and magnetic models indicates that there is a single, funnel- to bowl-shaped caldera approximately 4 km thick and approximately 10 km wide at the surface. This model is consistent with other siliceous, pyroclastic-filled calderas on continental crust, except that most siliceous volcanic centers associated with more than one eruption are characterized by more than one caldera.

Three-dimensional simplicial gravity and degenerate triangulations

I define a model of three-dimensional simplicial gravity using an extended ensemble of triangulations where, in addition to the usual combinatorial triangulations, I allow degenerate triangulations, i.e. triangulations with distinct simplices defined by the same set of vertices. I demonstrate, using numerical simulations, that allowing for this type of degeneracy substantially reduces the geometric finite-size effects, especially in the crumpled phase of the model; in other respects the phase structure of the model is not affected.

Fracture control of late Archean pluton emplacement in the northern Slave Province, Canada

The structural and geophysical characteristics of 2590–2580 Ma leucogranites in the northern Slave Province have been studied to determine the role and conditions of late Archean regional deformation during their emplacement. The irregularly shaped, straight sided, c. 10 km long Ulu pluton is located in the southern part of the High Lake greenstone belt, northern Slave Province. Fabrics in the pluton and its wall rocks indicate that it was emplaced at mid- to upper crustal levels during regional E–W late Archean compression. A gravity survey and resulting three-dimensional gravity model of the pluton show that it has several linear, deep (>6 km) root zones, which feed a relatively thin (<2 km) tabular body. The orientations of the major root zone, along the pluton's western side, and three subsidiary roots correspond to those of the pluton contacts to within 5°. Landsat TM data reveal several sets of lineaments, interpreted to represent fractures formed during the early assembly of the Slave Province, which cut both the greenstone belt and the younger plutons. Analysis of these lineaments defines several maxima whose orientations correspond with those of both the feeder zones and the contacts of the Ulu pluton. The pluton therefore appears to have been emplaced via an intersecting set of vertical fractures. Magma was arrested by and spread along an unknown horizontal structure. Subsequent inflation of the pluton was also facilitated by vertical translation on pre-existing fractures.

Phase structure of dynamical triangulation models in three dimensions

The dynamical triangulation model of three-dimensional quantum gravity is shown to have a line of transitions in an expanded phase diagram which includes a coupling μ to the order of the vertices. Monte Carlo renormalization group and finite size scaling techniques are used to locate and characterize this line. Our results indicate that for μ < μ 1 ∼ −1.0 the model is always in a crumpled phase independent of the value of the curvature coupling. For μ < 0 the results are in agreement with an approximate mean field treatment. We find evidence that this line corresponds to first-order transitions extending to positive μ. However, the behavior appears to change for μ > μ 2 ∼ 2.0−4.0. The simplest scenario that is consistent with the data is the existence of a critical end point.

A three-dimensional gravity model of the southern contact of the Sebago pluton, Maine

The Sebago pluton is a two-mica granite that intruded the metasedimentary rocks of the Central Maine Terrane around 292 Ma. In recent years, geologists have raised an increasing number of questions related to the overall thickness of the Sebago pluton and the position of its subsurface contact with the underlying metasedimentary rocks. Past studies have shown the Sebago pluton to be a thin, 1-2 km thick, subhorizontal sheet dipping 3° to the northeast. This study examines anomalies in the Earth's gravitational field related to the southern portion of the Sebago pluton, specifically to determine the thickness of the pluton and to locate the subsurface contact between the pluton and the underlying metasedimentary rocks. A three-dimensional model shows the thickest portions of the pluton (~1.8 km) to occur at the bottom of a bowl hape along the southwestern contact. Moreover, the model shows the pluton to thin toward the northern and eastern regions of the study area, where the average thickness is less than 0.5 km. The pluton appears to extend southward below the cover of the metasedimentary rocks along the southwestern contact. Thus, contrary to previous models, the Sebago pluton is not a northeasterly dipping sheet of uniform thickness, but rather an arched sheet with an irregular thickness extending beneath the metasedimentary rocks along both its northern and southern contacts. Based on this new geometry, either the relationship of the pluton to the surrounding metamorphic zones must be modified, or the possibility must be considered that the Sebago pluton is actually a composite batholith, composed of a younger (Permian) granite to the north and an older (Carboniferous) granite to the south.

Tectonic controls on magmatism in The Geysers–Clear Lake region: Evidence from new geophysical models

In order to study magmatism and geothermal systems in The Geysers–Clear Lake region, we developed a detailed three-dimensional tomographic velocity model based on local earthquakes. This high-resolution model resolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The Geysers–Clear Lake region occur in the steam production area, where high velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick section of Mesozoic argillites. In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, but at a lower resolution than the local model. The regional model outlines low velocities at depths of 8–12 km in The Geysers–Clear Lake area, which extend eastward to the Coast Range thrust. These low velocities are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high velocities in the lower crust beneath the Clear Lake volcanic field, which we interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic models. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5–7 km of the crust. Two-dimensional magnetotelluric models of data from a regional, east-west profile indicate high resistivities associated with the granitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluric data. On the basis of heat flow and geologic evidence, Holocene intrusive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysical data provide no conclusive evidence for such activity, but the detailed velocity model is suggestive of intrusive activity near Mount Hannah similar to that in the “felsite” of The Geysers production area. The geophysical models, seismicity patterns, distribution of volcanic vents, heat flow, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.

A three-dimensional gravity model of the southern contact of the Sebago pluton, Maine

A three-dimensional gravity model of the southern contact of the Sebago pluton, Maine

Three-dimensional gravity modelling of the European Mediterranean lithosphere

SUMMARY Residual gravity anomalies characterizing the density heterogeneities in the upper mantle of the Alpine belt of Western Europe are determined. Residual anomalies were calculated on a 1 x 1 grid by subtracting the gravity effect of the density model for the Earth's crust from the observed gravity field. Our 3-D density model consists of two regional layers of varying thicknesses with a lateral variation in average density: the sedimentary cover and the crystalline crust. Offshore, the model is supplemented by a sea-water layer. This 3-D density model is based on a generalized velocity model represented by structure maps of the main seismic horizons (the 'seismic' basement and the Moho boundary) and a map of the average P-wave velocity in the consolidated crust. The density distribution within the model layers was obtained using the correlation functions between P-wave velocity and density. For sediments, sediment consolidation with depth was taken into account. The gravity effect of the model, approximated by parallelepipeds 1 x 1 in planar size, was calculated by a program designed for solving 3-D gravity problems. The program takes into account the spherical configuration of the Earth. This method also permits the estimation of the isostatic state of the crust. A mantle origin of residual gravity anomalies is confirmed by their close correlation with upper-mantle velocity heterogeneities, established by both seismic-tomography and thermal-regime data. The residual gravity field is inverted into the distribution of anomalous density within the uppermost mantle layer (between the Moho and a bottom level at 200 km depth). The most important anomalies are the high-density domains caused by the thick lithosphere of the Adriatic plate and by the lithosphere 'roots' beneath the Alps and the Calabrian Arc. Negative density anomalies over the Pannonian Basin and the Western Mediterranean basins reach -0.04 to -0.05 x lo3 kg m-3 due to thermal expansion of the asthenosphere.

On a Three-Dimensional Gravity Model with Higher Derivatives

The purpose of this work is to present a model for 3D massive gravity with topological and higher-derivative terms. Causality and unitarity are discussed at tree-level. Power-counting renormalizability is also contemplated.

Deep structure and evolution of the Harz Mountains: results of three-dimensional gravity and finite-element modeling

A new Bouguer anomaly map is presented for the region of the entire Harz Mountains based on more than 60,000 gravity values. The various gravity anomalies are discussed and interpretation is carried out by high-resolution 3-D gravity modeling. One of the main subjects of interest in the investigation is the northern boundary fault zone of the Harz Mountains, separating the Mesozoic sediments in the north from the Palaeozoic rocks of the Harz in the south. Dip and vertical displacement are determined for this fault zone; mean values are 3400 m and 70°, respectively. Gravity modeling shows that the Brocken and the Ramberg Granites are distinctly different. The Brocken Granite is shallow, whereas the Ramberg Granite has a maximum depth of 8.5 km and a NS dimension of 37 km. The prominent Benneckenstein Gravity High is explained by two different models, one based on a granodioritic intrusion (2900 kg/m 3) with a center-depth of 14 km and the other based on phyllites (2740 kg/m 3) on a depth of 3–4 km. Studies on the geodynamic evolution of the Harz Mountains are carried out using the finite-element method. On the basis of a 3-D model, vertical displacements that can be related to horizontal forces are computed. For the period of the Variscan Orogeny an uplift of 600 m in the Harz area is calculated, for Late Cretaceous and Tertiary 400 m are determined. The total amount of 1000 m is about 1 2 of the vertical displacement of the northern boundary fault zone of the Harz Mountains shown by the gravity modeling. These results do not contradict geological ideas.

An Interactive 2D and 3D gravity modeling program for IBM-compatible personal computers

An Interactive Graphic System for three-dimensional (3D) gravity modeling is presented. The system uses a highly flexible algorithm for the calculation of theoretical anomalies, which permits accurate 3D simulation of most types of geological bodies, and the simultaneous introduction of several bodies. This ability to combine several objects, together with features such as topographic relief, faults, and erosional process, adds a higher degree of realism to the model.

Geometry of the allochthonous Daly Bay Complex, Northwest Territories: a model constrained by geology and a three-dimensional gravity interpretation

The Daly Bay Complex is one of several metamorphic complexes making up the Aqxarneq gneisses north of Chesterfield Inlet in central District of Keewatin. Granulite-facies metamorphism (0.55 GPa, 750 °C) and ductile deformation have affected all of the rocks in the complex. A 1–15 km wide, inward-dipping, ductile shear zone forms the outer part of the complex and contains strongly deformed equivalents of rocks in the core. Mesoscopic structures and metamorphic mineralogy suggest the Daly Bay Complex was emplaced into the surrounding lower grade rocks by northward-directed thrusting. A three-dimensional gravity model, constrained by structural observations and 1091 surface density measurements, shows that the relatively dense rocks of the complex form a spoon-shaped structure with a long axis trending northwest–southeast. It is approximately 50 km by 120 km in lateral extent and reaches a maximum depth of about 9 km. The thin-skinned geometry of the Daly Bay Complex supports the notion that the crust in central Keewatin between the Daly Bay Complex and Baker Lake comprises a series of undulating imbricated gneiss sheets of middle and lower crustal material, which were juxtaposed by a major tectonic event sometime between 2.5 and 1.9 Ga. The interpreted basal décollement is comparable to seismic features in many orogens, and a predictable consequence of increased ductility with depth in the crust.

Entropy and the approach to the thermodynamic limit in three-dimensional simplicial gravity

We present numerical results supporting the existence of an exponential bound in the dynamical triangulation model of three-dimensional quantum gravity. Both the critical coupling and the number of nodes per unit volume show a slow power law approach to the infinite volume limit.

Upper-crustal structure beneath the Columbia River Basalt Group, Washington: Gravity interpretation controlled by borehole and seismic studies: Discussion and reply

A three-dimensional gravity model based on deep-borehole stratigraphy, densities from borehole gravimetry, and crustal structure from deep seismic and electrical surveys puts limits on sub-basalt crustal structure in the northwest quadrant of the Columbia Plateau, Washington. The gravity model indicates that the Miocene Columbia River Basalt Group (CRBG) overlies a broad basin filled with sedimentary rocks that reach thicknesses in excess of 5 km; thickest sub-basalt sedimentary rocks are beneath the late Cenozoic Yakima basin. Analysis of residual gravity shows that the Eocene Chiwaukum graben does not continue beneath the CRBG and that sub-basalt sedimentary rocks are not thrust into the cores of the basalt anticlines in the Yakima fold belt. A gravity high that trends north-south in the center of the CRBG is probably caused by a broad basement ridge marked by thinner sedimentary rocks beneath the Columbia River basalts; the western edge of the north-south basement high is traced at the surface by the Hog Ranch-Naneum Ridge. A major crustal feature, the Olympic- Wallowa lineament, is expressed in the gravity field; the lineament crosses the north-south basement high but does not offset it, suggesting that any strike- slip motion on the lineament preceded development of the north-south basement high. An alternative interpretation of the north-south gravity high, based on a previous seismic study, relates it to a high-density structure in the crystalline basement, perhaps associated with a failed continental rift.