Showing papers on "Bouguer anomaly published in 1990"

An evaluation of some systematic error sources affecting terrestrial gravity anomalies

Abstract: Terrestrial free-air gravity anomalies form a most essential data source in the framework of gravity field determination. Gravity anomalies depend on the datums of the gravity, vertical, and horizontal networks as well as on the definition of a normal gravity field; thus gravity anomaly data are affected in a systematic way by inconsistencies of the local datums with respect to a global datum, by the use of a simplified free-air reduction procedure and of different kinds of height system. These systematic errors in free-air gravity anomaly data cause systematic effects in gravity field related quantities like e.g. absolute and relative geoidal heights or height anomalies calculated from gravity anomaly data.

Tomographic images of P wave velocity variation at Parkfield, California

Abstract: Tomographie inversion is applied Io delay times from local earthquakes to image three dimensional velocity variations near Parkfield, California. The 25×20 square km region is represented by nearly cubic blocks of 0.5 km per side. Arrival times of P waves from 551 local earthquakes, with depths of 0 to 15 km, were used as sources producing 3135 rays covering the target region. The data were recorded on low-noise downhole seismographs. A conjugate gradient method is used to invert the resulting sparse system of simultaneous equations. To diminish the effects of noisy data, the Laplacian of the model parameters is constrained to be small within horizontal layers, providing smoothing of the model. The resolution of the model is estimated by calculating point spread functions at blocks of interest. Estimates of standard errors of the model parameters are calculated by the jackknife statistical procedure. The results of the inversion show correlation with some of the local geological and geophysical features. Station corrections removed the long-wavelength anomaly associated with the contrast of the Salinian block southwest of the San Andreas fault versus the Franciscan to the northeast. A velocity low located a few kilometers northwest of Parkfield (depth 2.5–3.5 km), appears to lie along the gradient of the large Bouguer gravity anomaly associated with the Parkfield syncline. The south-southeastward extension of the low velocities may relate to reflections observed on the Parkfield, Consortium for Continental Reflection Profiling (COCORP) lines. We speculate on the geological meaning of these features and interpret them either as part of the local strike slip tectonics or a shallow crustal detachment. The correlation of higher-velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower-velocity, less competent rocks.

Heat flow in the Oregon Cascade Range and its correlation with regional gravity, Curie point depths, and geology

Abstract: New heat flow data for the Oregon Cascade Range are presented and discussed. Heat flow measurements from several deep wells (up to 2500 m deep), as well as extensive new data from industry exploration efforts in the Breitenbush and the Santiam Pass-Belknap/Foley areas are described. The regional heat flow pattern is similar to that discussed previously. The heat flow is about 100 mW m−2 in the High Cascade Range and at the eastern edge of the Western Cascade Range, It is about 40–50 mW m−2 to the west in the outer arc block of the subduction zone. In the high heat flow zone the heat flow is low at shallow depths in young volcanic rocks due to the high permeability of the rocks and the resultant rapid groundwater flow. Below a depth of 200–400 m much of the area appears to be dominated by conductive heat transfer at least to 2–2.5 km depth. There are perturbations to the regional heat flow in the vicinity of the hot springs where values are up to twice the background. The gravity field in the Cascade Range has characteristics that can be closely related to the heat flow pattern. The relationship may be causal, and to examine the relationship in more detail, earlier two-dimensional modeling is extended to three dimensions. Consideration of the effects of a midcrustal density anomaly, such as might be associated with a region with at least areas of partial melt, has two major consequences. The first of these is that a high-frequency gravity gradient near the Western Cascade Range/High Cascade Range boundary is explained. Second, the negative gravity anomaly associated with the north half of the High Cascade Range can be removed, and as a result, the prominent northeast/southwest striking regional Bouguer gravity anomaly associated with the north edge of the Blue Mountains becomes continuous across the Cascade Range with a similar feature along the north side of the Klamath Mountains. Apparently, this zone is a major crustal feature upon which the negative gravity anomaly coincident with the high heat flow is superimposed. The correlation, or lack thereof, of the heat flow, depth to Curie point, gravity field, crustal electrical resistivity, crustal seismic velocity, and geology in the High/Western Cascade Ranges is summarized. Many of the data show aspects that can be interpreted in relation to possible high temperatures in the midcrust of the Cascade Range. The High Cascade Range midcrust has unusually high temperatures and contains a zone of magma staging at 10±2 km depth that can also be identified in subdued form in the Cascade Range in Washington and British Columbia.

Bouguer gravity trends and crustal structure of the Palmyride Mountain belt and surrounding northern Arabian platform in Syria

Abstract: Copyright 1990, Geological Society of America. See also: http://dx.doi.org/10.1130%2F0091-7613%281990%29018%3C1235%3ABGTACS%3E2.3.CO%3B2 http://atlas.geo.cornell.edu/syria/best_1990.html

Generation and interpretation of geophysical images with examples from the Rae Province, northwestern Canada shelf

Abstract: Different types of images generated from gravity, magnetic, and gamma ray spectrometry data from the Rae Province of the Canadian shield were compared with each other and geologic maps to evaluate their effectiveness for displaying the geologically relevant content of the data sets.Shading methods were useful for enhancing weak directional anomalies in the aeromagnetic data. Multidirectional, shaded-relief images produced by overlaying three colored, shaded-relief images are useful for analysis of anomalies associated with structure. Vertical gravity derivative images display a continuous gravity feature linking the Wager Bay and Amer Lake shear zones that is obscured on the Bouguer gravity intensity image. Detailed vertical magnetic derivative images of the shear zone clearly displayed anomalies associated with the internal structure.Composite images generated using three different geophysical parameters show correlations between the magnetic, gravity, and radiometric data which can be related to the geology. Subtle variations in uranium, thorium, and potassium concentrations determined by gamma ray spectrometry can be effectively displayed using ternary radioelement images

Inversion for sources of crustal deformation and gravity change at the Yellowstone caldera

Abstract: The 3100 km2 Yellowstone caldera, Yellowstone National Park, was formed in the latest of three explosive eruptions of rhyolites and ash flow tuffs totalling 3700 km3 at 2, 1.2, and 0.6 m.y. before present. Its youthful volcanic history, widespread hydrothermal activity, intense seismicity, and extremely high heat flow, in excess of 30 times the continental average, marks the Yellowstone volcanic system as a giant caldera at unrest Orthometric height increases of the caldera of up to 76 cm, measured from precise leveling surveys from 1923 to 1975–1977, were inverted to determine volume expansion source models for the caldera-wide deformation. For the 1923 to 1977 uplift episode, two regions of expansion were found: (1) in the northern part of the caldera near the Sour Creek resurgent dome of ∼0.37 km3, and (2) in the southern part of the caldera, near the Mallard Lake resurgent dome of ∼0.41 km3. Both bodies occur in the upper crust from near-surface depths to 6.0 km, but the largest volume expansions were found in the 3.0–6.0 tan depth range. The southern caldera source volume, near the Mallard Lake dome, may extend down to 9.0 km. The data are, however, unable to resolve if these two resurgent domes, separated laterally by ∼40 km, are connected. Prom 1976 to 1987, nearly simultaneous measurements of elevation and gravity changes were made on a profile across the northern caldera during a period of net uplift. Models of the temporal gravity variation infer that the volume increase for the northern caldera source must lie above 9.0 km and involved a density perturbation greater than +0.002 g/cm3. The modeled volumetric sources are in the same general locations as bodies of low P wave velocities, high seismic attenuation, and large negative Bouguer gravity anomalies. In view of the intense Quaternary volcanism, the anomalously high heat flow, and the correlation of the source volumes with the regional geophysical anomalies, it is likely that the modeled volumetric increases were caused by migration of magmas and/or the introduction of large volumes of hydrothermal fluids into the upper crust.

Free-Air Gravity Map of Taiwan and Its Applications

Abstract: An island-wide gravity survey in Taiwan was conducted by the Institute of Earth Sciences, Academia Sinica, between 1980 and 1987.The 603 stations at which the gravity values were determined included308 points in the 500 m or higher mountain range where few readings were available previously. The average spacing of the stations in the present survey is about 7km apart. A new Free-air gravity anomaly map has been constructed based on these values. The map is dominated by a NNE-SSW gravity high trend with a maximum value of 300 mgal, that follows closely the Central Range, a folded and faulted mountain belt with many peaks 3000m or higher. The magnitude of the Free-airanomaly in the Taiwan area is quite large compared to that elsewhere in the world. The good correlation between the Free-air anomaly and elevation suggests that the Taiwan area is not in isostatic equilibrium. An average surface rock density of 2.57gcm^(-3) is estimated from the Free-air gravity data by using the least-squares method. This value can be used for both terrain and Bouguer corrections. The undulation of the geoid and the deflections of the vertical in the Taiwan area are also calculated by using the Free-air anomaly data. The geoid undulation is not rugged over the Taiwan area. The maximum difference is about 5m. And the deflection of the vertical seems mainly to be affected by both land and submarine topographies.

Bouguer density determination by fractal analysis

Abstract: Density values for the Bouguer reduction of two gravity data sets from Iceland are determined using a new method based on minimization of the roughness of the Bouguer anomaly surface. The fractal dimension of the surface is used as a gauge of the roughness. The analysis shows the size of topographic features supported by crust without isostatic compensation to be 25 to 30 km in southwest Iceland and 9 to 10 km inside the active rifting zone. The densities selected for these areas are 2490 and 2730kg/m3, respectively.

Regional gravity anomalies, depth of the Foreland Basin and isostatic compensation of the New Guinea Highlands

Abstract: The New Guinea Highlands form an active thrust belt advancing southward over the Australian continental margin. Surface geology indicates thin-skinned deformation of sediments and continental basement, building mountains 2–4 km in average elevation. The depth of the adjacent foreland basin is no more than 1500 m in Papua New Guinea and in many places only 100–200 m, indicating little deflection of the Australian lithosphere beneath the load of the mountains. Calculations of elastic loading, in which the topographic load of the entire mountain range is supported by Australian lithosphere of uniform flexural rigidity (D), predict a basin 2–10 times deeper than observed in eastern New Guinea for a wide range of flexural rigidities (1021–1025 Nm). Bouguer gravity anomalies in the eastern highlands between 143°E and 144°E are consistent with local Airy compensation of the mountains, and preclude a continuous plate with D > 1022 Nm underlying any significant portion of the mountains. In contrast, the observed broad foredeep flexure in central New Guinea near 141°–142°E requires a strong lithosphere (D = 1024–1025 Nm), and limited gravity and sediment thickness observations farther west indicate similar flexural rigidities. Bouguer gravity measurements and observed foreland basin depths throughout New Guinea can be explained by a strong and continuous lithosphere beneath the foreland basin extending no more than 20–60 km north of the range front, implying that the plate has weakened beneath the thrust belt. Collateral evidence for plate weakening beneath the highlands may be found in earthquakes at 10–25 km depth with high-angle thrust mechanisms and in widespread Quaternary volcanism throughout the eastern thrust belt. A 50-mGal positive residual anomaly over the northern 100 km of the eastern highlands cannot be explained by local compensation or mechanical support from the south, indicating either a large excess mass at depth or some additional form of support for the northeastern highlands. The gravity observations in eastern New Guinea require 10–15 km of crustal thickening, but observed sediment thicknesses can only account for 0–5 km, necessitating 5–15 km of thickening in the basement.

Gravity field of Kuwait and its relevance to major geological structures

Abstract: Regional gravity surveys were done in the state of Kuwait during 1986-1988. The new gravity maps show a good correlation with geomorphological features as well as with deeper geological structures. The free-air anomaly map clearly reflects the topography of the Jal Az Zor and Ahmadi ridges, and Wadi Al-Batin. The Bouguer anomaly map is dominated by two prominent gravity highs correlatable with subsurface structural arches. The north-trending gravity high in eastern Kuwait represents the effect of a major structure, the Kuwait arch, along which many important oil fields are located. A smaller northwest-trending high mapped in western Kuwait indicates the presence of a second subsurface arch, which in this paper is named the Dibdibba arch. The two gravity highs are separate by a wedge-shaped gravity low presumably caused by thicker sediments of the Dibdibba basin. Magnetic measurements along selected profiles show the two arches to be associated with 100-200-nT (nannoTesla) anomalies apparently reflecting the positive subsurface relief of the crystalline basement. Along the length of the Kuwait arch, magnetic data also indicate lateral susceptibility variations possibly related to lithological variations within the basement.

Mafic-ultramafic intrusives and their gravity field: Lac des Iles area, northern Ontario

Abstract: The Bouguer anomaly map of the Lac des Iles area constitutes a number of distinct gravity highs and lows that are associated, respectively, with the mafic-ultramafic and granitic components of a late Archean plutonic terrane. The apparent density and vertical derivative maps were computed to assist in the identification and the delineation of lithologies. From the apparent density map, several distinct density zones are recognized within the mafic-ultramafic intrusions. The identification of the density zones and their apparent densities are in good agreement with the mapped lithological groups and measured densities. Gravity models in 2½-D were computed along six selected profiles. The Mafic-ultramafic Lac des Iles Complex with a mean density of 3.13 g/cm 3 is characterized by a 30-mgal gravity high. The complex is modeled as a flattened funnel-shaped, steeply inward-dipping, intrusive body, part of which extends to a depth of about 4.5 km. The Tib Gabbro, represented by a 16-mgal gravity high, is modeled as a tilted, steeply inward-dipping, basin-like intrusive body extending to a depth of 3 km. Gravity maxima for most of the mafic intrusions are located in proximity to faults in tonalite gneiss country rocks. These faults may have acted as conduits, allowing the emplacement of discrete pulses of mantle-derived magma. Gravity models show that mafic magma locally underplated granitic plutons and that mafic plutons typically have limited depth extents relative to their horizontal dimensions. These models indicate that intraplating by mafic magmas contributed to crustal thickening in the late Archean.

3d gravity inversion of the cheshire basin1

Abstract: Three-dimensional inverse gravity modelling is used to determine the structure of the Carboniferous sediments underlying the Jurassic and Permo-Triassic formations in the Cheshire Basin. The gravity effect of the Jurassic and Permo-Triassic rocks was first removed from the observations and the resulting residual gravity field was then inverted to map variations in depth of the Carboniferous formations. The inversion technique is performed in the frequency domain, enabling rapid computation of solutions for large gridded data sets. Evaluation of results shows that there is good agreement between calculated and observed values, with interpreted depths to the base of the Carboniferous in excess of 6 km. Gravity inversion results also delineate areas of faulting on the south-western and eastern edges of the basin.

The inverse Vening Meinesz problem in isostasy

Abstract: SUMMARY Assuming an Airy-Vening Meinesz type of isostatic compensation with constant density contrast between crust and mantle and postulating zero isostatic anomaly, it is possible to calculate the depth of the MohoroviEiC discontinuity from Bouguer gravity anomalies. The author gives an explicit solution of this uniquely solvable gravimetric inverse problem by means of integral formulae.

Crustal structure of the Valhalla complex, British Columbia, from Lithoprobe seismic-reflection and potential-field data

Abstract: The Valhalla complex, situated in the Omineca crystalline belt in southeastern British Columbia, is a Cordilleran metamorphic core complex bordering the suture zone between Quesnellia and North American rocks. The region is tectonically interposed between a convergent plate margin along Canada's west coast and the stable North American craton, and is characterized by a crustal thickness of ~ 35 km, high surface heat flux, and elevated lower crustal electrical conductivity. In this study, Lithoprobe deep-crustal seismic-reflection data, potential-field data, and geological constraints have been used to gain a better understanding of crustal structure in the vicinity of the Valhalla complex. Analysis of Bouguer gravity and total-field aeromagnetic data indicates that mafic oceanic rocks and various syn- and post-accretionary granitoid plutonic rocks are not major constituents of the upper crust underlying the complex. The seismic data reveal a moderately reflective upper crust and image several fault zones,...

Major circular structure beneath southern Lake Huron defined from potential field data

Abstract: Newly compiled and merged Canadian and United States potential field data for the area of southern Lake Huron reveal a large circular structure that has a diameter that may extend to 100 km. The aeromagnetic image shows a circular central high with a diameter of 6 km located at fat 43°14.21′N, long 82°19.88′W, surrounded by a principal ring with a diameter of 50 km and indications of conformable, arcuate structure extending to a diameter of about 100 km. The prevailing, northeast-striking aeromagnetic signature of the Grenville front and adjacent Grenville front tectonic zone is interrupted and may be overprinted by the circular feature. The Bouguer gravity field shows a strong positive anomaly that extends southeast from the Grenville front and terminates in an arcuate complex of positive anomalies that correlate with the aeromagnetic field. The gravity field illustrates the circular structure with an annular low and central high, as well as positive extensions that follow the foliate pattern of magnetic anomalies marking the outer area of the feature. There is no strong evidence of the feature in the bathymetry data of southern Lake Huron, although the general coastline of the southern end of Lake Huron closely follows the outline of the main circular feature. The interruption of the Grenville trends, the clear, regular, circular signature of the feature, and the interpreted magnetic source depths at the Precambrian surface, suggest an event possibly as young as early Paleozoic. In the absence of basement samples, the origin of the feature is unconfirmed. However, the scale and potential field anomaly pattern of the feature are comparable to several of the larger impact structures exposed on the Canadian Shield.

Structural cross sections based on a gravity survey of parts of the Quetico and Wawa subprovinces near Thunder Bay, Ontario

Abstract: In this study, gravity data from 350 new gravity stations are combined with those from 50 previously surveyed stations in a detailed Bouguer anomaly map of a portion of the Quetico and Wawa subprovinces north and west of Thunder Bay, Ontario.In general, high gravity values characterize the southern and southwestern part of the area where metavolcanic rocks of the Wawa subprovince dominate. Much of the Quetico subprovince forms a broad gravitational low, reflecting extensive exposures of gneisses, schists, and migmatites. Well-defined gravity lows are associated with several granitic intrusive bodies.Three- and -dimensional gravity models of subsurface configuration of the density contrasts, representative of major rock units, indicate a trough-like structure for the metavolcanic rocks of the Wawa subprovince. This trough-like structure is flanked by a domical feature in the granitoid rocks to the south. North of the metavolcanic rocks, a succession of low-grade greywackes and slates occupies a basinal str...