Geophysical interpretation of the Torngat orogen along the North River – Nutak transect, Labrador

1994 ◽  
Vol 31 (4) ◽  
pp. 722-727 ◽  
Author(s):  
Tomas Feininger ◽  
Ingo Ermanovics
Keyword(s):  
Gravity Anomaly ◽  
Gravity Anomalies ◽  
Canadian Shield ◽  
Bouguer Gravity ◽  
Dimensional Model ◽  
Prismatic Body ◽  
Western Margin ◽  
Geological Interpretation ◽  
The North ◽  
Geophysical Interpretation

A 19.3 mGal (1 Gal = 1 cm/s2) positive Bouguer gravity anomaly and a broad aeromagnetic low coincide with the Tasiuyak domain of the early paleo-Proterozoic Torngat orogen. The domain evolved during the collision of the western margin of the Archean Nain Province with an inferred, eastward-facing, platform-to-rise, sedimentary wedge sited on a proto-continent of Churchill (Rae) Province.A density contrast of +0.065 g∙cm−3 was measured between rocks of the Tasiuyak domain and those of the flanking Lac Lomier complex of the orogen and Nain Province. Using this contrast, a two and one-half dimensional model shows that rocks of Tasiuyak domain constitute a triangular prismatic body with maximum thickness of 13 km adjacent to Nain Province, which thins westward to a feather edge. The model is compatible, qualitatively, with the aeromagnetic anomaly and consonant with geological interpretation of an eastward-facing and thickening continental slope deposit. Deep exhumation of the Torngat orogen may account for the absence of paired gravity anomalies characteristic of many sutures elsewhere in the Canadian Shield.

THE CORRELATION BETWEEN PRE‐CAMBRIAN ROCK DENSITIES AND BOUGUER GRAVITY ANOMALIES NEAR PARRY SOUND, ONTARIO

Geophysics ◽  
1954 ◽  
Vol 19 (1) ◽  
pp. 76-88
Author(s):  
C. H. G. Oldham
Keyword(s):  
Gravity Anomaly ◽  
Gravity Anomalies ◽  
Canadian Shield ◽  
Gravity Survey ◽  
Uniform Density ◽  
Bouguer Gravity ◽  
Geological Evidence ◽  
Bouguer Gravity Anomaly ◽  
Bouguer Gravity Anomalies

A gravity survey and a survey of rock densities have been carried out over an area of two thousand square miles near Parry Sound. A closed positive Bouguer gravity anomaly of thirty milligals was delineated, and a considerable variation was found to exist in the densities of pre‐Cambrian gneisses. In most previous interpretations of gravity over the Canadian Shield the gneisses have been assumed to possess a uniform density and anomalies have been attributed to changes in the thickness of horizontal crustal layers. In this paper it is shown that the Parry Sound anomaly can be explained in terms of structures within the crust taking the form of projections downward of the density variations found at the surface. The postulated structure is a nearly circular basin of dense gneisses. The shape is reasonable and agrees with such geological evidence as is available.

Empirical Determination of the Gravity Anomaly Covariance Function in Mountainous Areas

1980 ◽  
Vol 34 (3) ◽  
pp. 251-264 ◽  
Author(s):  
Gerard Lachapelle ◽  
K. P. Schwarz
Keyword(s):  
Gravity Anomaly ◽  
Covariance Function ◽  
Physical Geodesy ◽  
Gravity Anomalies ◽  
Mountainous Areas ◽  
The North ◽  
Regression Parameters ◽  
Free Air ◽  
Vening Meinesz ◽  
Free Air Gravity

An evaluation of the empirical gravity anomaly covariance function using over 95 000 surface gravity anomalies in the North American Western Cordillera was carried out. A regression analysis of the data exhibits a strong and quasi-linear correlation of free air gravity anomalies with heights. This height correlation is removed from the free air anomalies prior to the numerical evaluation of the gravity anomaly covariance function. This covariance function agrees well with that evaluated previously by the authors for the remainder of Canada. A possible use for such a covariance function of ‘height independent’ gravity anomalies in mountainous areas is described. First, the height independent gravity anomaly at a point of known height is evaluated by least squares prediction using neighboring measured height independent gravity anomalies. Secondly, the part caused by the height correlation is calculated using linear regression parameters estimated previously and added to the predicted height independent gravity anomaly to obtain a predicted standard free air anomaly. This technique can be used to densify the coverage of free air anomalies for subsequent use in integral formulas of physical geodesy, e.g., those of Stokes and Vening Meinesz. This method requires that point topographic heights be given on a grid.

scholarly journals On the topographic effects by Stokes’ formula

2014 ◽  
Vol 4 (1) ◽  
Author(s):  
L.E. Sjöberg
Keyword(s):  
Gravity Anomaly ◽  
Bouguer Anomaly ◽  
Gravity Anomalies ◽  
Geoid Height ◽  
Topographic Effects ◽  
Gravimetric Geoid ◽  
Bouguer Gravity ◽  
The Third ◽  
Stokes Formula ◽  
The Difference

AbstractTraditional gravimetric geoid determination relies on Stokes’ formula with removal and restoration of the topographic effects. It is shown that this solution is in error of the order of the quasigeoid-to-geoid difference, which is mainly due to incomplete downward continuation (dwc) of gravity from the Earth’s surface to the geoid. A slightly improved estimator, based on the surface Bouguer gravity anomaly, is also biased due to the imperfect harmonic dwc the Bouguer anomaly. Only the third estimator,which uses the (harmonic) surface no-topography gravity anomaly, is consistent with the boundary condition and Stokes’ formula, providing a theoretically correct geoid height. The difference between the Bouguer and no-topography gravity anomalies (on the geoid or in space) is the “secondary indirect topographic effect”, which is a necessary correction in removing all topographic signals.

Regional‐residual gravity anomaly separation using the minimum‐curvature technique

Geophysics ◽  
1991 ◽  
Vol 56 (2) ◽  
pp. 279-283 ◽  
Author(s):  
K. L. Mickus ◽  
C. L. V. Aiken ◽  
W. D. Kennedy
Keyword(s):  
Gravity Anomaly ◽  
Gravity Anomalies ◽  
Bouguer Gravity ◽  
Bouguer Gravity Anomaly ◽  
Residual Gravity ◽  
Residual Gravity Anomaly ◽  
Gravity Interpretation ◽  
Minimum Curvature

One of the most difficult problems in gravity interpretation is the separation of regional and residual gravity anomalies from the Bouguer gravity anomaly. This study discusses the application of the minimum‐curvature method to determine the regional and residual gravity anomalies.

Geophysical interpretation of the geology of the northeast Gander Terrane, Newfoundland

1988 ◽  
Vol 25 (8) ◽  
pp. 1161-1174 ◽  
Author(s):  
H. G. Miller
Keyword(s):  
Gravity Data ◽  
Geochemical Data ◽  
Ultramafic Rocks ◽  
Thrust Belt ◽  
Mafic Rocks ◽  
Geological Interpretation ◽  
The North ◽  
Geophysical Interpretation ◽  
Lateral Extent ◽  
North And South

New gravity data from the northeastern portion of the Gander Terrane of Newfoundland are analysed in association with existing gravity data. These are combined with the digitized and filtered aeromagnetic and geochemical data to produce an interpretation of the subsurface geology.Interpretation of these data suggests that there are two extensive areas underlain at depth by rocks similar to the Dunnage Terrane mafic and ultramafic rocks that outcrop at the Gander River ultrabasic belt. These regions of ultramafic and mafic rocks extend in two north–south belts throughout the study area, and both may have tongues continuing seaward beneath the Deadman's Bay pluton. The western belt, the Ocean Pond belt, probably consists of a series of granitic plutons underlain by mafic and ultramafic rocks. Geophysical modelling corroborates an earlier geological interpretation that the eastern belt, the Indian Bay Big Pond thrust belt, is a thrust sheet. The lateral extent of the thrust belt is accurately determined by new geophysical data. The presence of these two subsurface units composed of material similar to typical Dunnage Terrane rocks demonstrates that the Gander River ultrabasic belt is neither the most eastward extent of the Dunnage Terrane nor the sole thrust upon which Dunnage Terrane material was transported eastward. The two belts have associated diagnostic geochemical signatures and are bounded on the north and south by linear patterns in both the geophysical and geochemical patterns. The block defined by these geophysical and geochemical patterns is the same as that upon which the classic Gander Terrane was defined. The evidence presented in this paper suggests that this block may be allochthonous, which implies that the nature of the Gander Terrane may need to be reconsidered.

scholarly journals Analysis on Bouguer Gravity Anomaly Characteristics and Boundary Identification in China and Surrounding Regions

2020 ◽  
Author(s):  
Zhixin Xue ◽  
Dongmei Guo ◽  
Panpan Zhang
Keyword(s):  
Stress Field ◽  
Gravity Anomaly ◽  
Eastern China ◽  
Gravity Anomalies ◽  
Tectonic Block ◽  
Bouguer Gravity ◽  
Satellite Gravity ◽  
Principal Compressive Stress ◽  
Active Tectonic ◽  
Active Tectonic Block

Abstract China is located in the southeast of the Eurasian Plate and is subject to the effects of subducting, squeezing and collision by the Pacific Plate to the east, Philippine Plate to the southeast and Indian Ocean Plate to the southwest. It has exceptional geotectonic structure. Based on the satellite gravity data with high precision, high resolution and ample geophysical information, combined with geological data, by using satellite gravity potential field and its full tensor gradient, this paper studies the distribution characteristics of gravity anomalies and the identification of tectonic boundaries in China and surrounding regions. Results suggest that the Bouguer gravity anomaly in eastern China reduces gradually from east to west, mostly in the direction of NNE; in the western, it reduces gradually in a wave mode from north to south, mainly in the directions of NW and NWW. In general, the stress field reduces gradually from west to east, and the tectonic of stress field in western China is complex. The maximum principal compressive stress in Xinjiang exists in SN direction and that in Qinghai-Tibet Plateau mostly changes gradually from NNE to SSE; the change in eastern China is relatively simple, and the maximum principal compressive stress direction gradually changes from NE to WE and then to SE. In addition to the above study results, by comprehensively referencing the previous studies by other people and by using the boundary identification methods based on the satellite gravity full-tensor gradient data and its combinations, we update the extension route of Red River fault zone and deduce the tectonic unit boundary between the North China and South China active tectonic block regions. This paper identifies in China and surrounding regions 6 primary active tectonic blocks, 22 secondary active tectonic blocks, 3 tertiary active tectonic blocks and the 20 active tectonic block boundary zones constituted of deformation belts and active tectonic belts with various geometric structures and width variations. The results of this study can improve the understanding of gravity anomalies and boundary structures in China and surrounding regions, and provide certain geophysical supports for geological structure analysis and crustal dynamic process.

Bathymetry Inversion Using Vertical Deflections: a Case Study in South China Sea

2021 ◽  
Author(s):  
Xiaoyun Wan ◽  
Bo Liu ◽  
Xiaohong Sui ◽  
Richar Fiifi Annan ◽  
Yijun Min
Keyword(s):  
South China Sea ◽  
Gravity Anomaly ◽  
South China ◽  
Additional Data ◽  
Gravity Anomalies ◽  
Local Area ◽  
China Sea ◽  
The North ◽  
East Component

Abstract As an alternative method, an algorithm for bathymetry inversion using vertical deflections is proposed. Firstly, the formulas for the bathymetry inversion from north and east components of vertical deflections are derived and the data processing is introduced. Then a local area in the South China Sea is selected as an example to experiment the method. The bathymetry inversion based on gravity anomaly was also conducted for a comparison. The results show that the bathymetry derived from the north component of the vertical deflections have almost the same accuracy as that derived from gravity anomalies and the results derived from the east component have the poorest accuracy. The experiment’s results also show that accuracy of the derived bathymetry can be improved if the fitting parameters are adjusted according to the water depths. In summary, among the gravity field products used in this study, although the gravity anomaly yielded the best performance in the bathymetry inversion, the vertical defections can still be used as supplements, especially in areas where accurate vertical deflections exist. This is because deriving gravity anomaly from altimetry observations needs additional data and calculation efforts.

Bouguer gravity anomalies and occurrence patterns of kimberlite pipes in Narayanpet-Maddur Regions, Andhra Pradesh, India

Geophysics ◽  
2005 ◽  
Vol 70 (1) ◽  
pp. J13-J24 ◽  
Author(s):  
A. Vasanthi ◽  
K. Mallick
Keyword(s):  
Andhra Pradesh ◽  
Gravity Anomalies ◽  
Capital City ◽  
Bouguer Gravity ◽  
Residual Gravity ◽  
The North ◽  
Bouguer Gravity Anomalies ◽  
Pass Through ◽  
Occurrence Patterns ◽  
Gravity Map

The Narayanpet Kimberlite field, that lies southwest of Hyderabad, the capital city of Andhra Pradesh, India, hosts a number of kimberlite pipes. These pipes appear to be randomly positioned. However, based on regional geologic structures revealed by Bouguer gravity anomalies, especially in a regional gravity map, their locations form a definite pattern. In the Narayanpet-Maddur region, regional Bouguer gravity contours exhibit some features of geologic interest: (1) the eastward convex regional contours show an increase in convexity from the Maddur and Kotakonda area on the east to Narayanpet on the west, (2) convexity is maximum in the vicinity of Narayanpet, where a large number of Kimberlite pipes occur nearly parallel to the regional contour, and (3) between Narayanpet and the Maddur-Kotakonda region, kimberlite pipes occur at intersections of three eastward, convex concentric zones with four lineaments, one trending northeast-southwest and the other three nearly east-west. These linear trends are believed to be radial, extensional, deep-fracture zones, through which kimberlite magma erupted about 1100 Ma. Modeling the residual gravity anomaly over one of the four profiles shows fairly good agreement between observed and computed fields. Based on analysis of Bouguer gravity anomalies and modeling of the residual gravity field, likely locations for kimberlite pipes are the contact zones between granite plutons and the country rocks that coincide with the northeast-southwest–trending radial faults that pass through Narayanpet and Kotakonda to the south and through Kazipur to the north.

scholarly journals Gravity anomalies of the United Arab Emirates: Implications for basement structures and infra-Cambrian salt distribution

GeoArabia ◽  
2014 ◽  
Vol 19 (1) ◽  
pp. 143-158
Author(s):  
Mohammed Y. Ali ◽  
Anthony B. Watts ◽  
Asam Farid
Keyword(s):  
Gravity Anomaly ◽  
United Arab Emirates ◽  
Gravity Anomalies ◽  
Abu Dhabi ◽  
Fault System ◽  
Bouguer Gravity ◽  
Thrust Belt ◽  
Bouguer Gravity Anomaly ◽  
Fold And Thrust Belt ◽  
Semail Ophiolite

ABSTRACT Gravity measurements onshore and offshore of the United Arab Emirates (UAE) have been used to construct a new Bouguer gravity anomaly map of the region. The gravity data, which has been gridded at 2,700 m × 2,700 m interval, has been used to constrain the tectonic elements, major lineation trends and structures of the Neoproterozoic basement of the Arabian Plate and the distribution of infra-Cambrian salt basins. Advanced transformation techniques (including first vertical derivative, total horizontal derivative, tilt derivative and Euler deconvolution) were applied to identify gravity source edges as an aid to structural interpretation and geological modelling of the study area. Three major structural provinces (fold-and-thrust belt, foreland and salt tectonic provinces) were identified based on the residual Bouguer gravity anomaly field. The eastern fold-and-thrust belt province is associated with short-wavelength positive gravity anomalies, which are attributed to the allochthonous series of the Semail Ophiolite and its related thrust sheets. The central foreland basin province is characterised by NNW-oriented negative gravity anomalies associated with deepening of the basement and thickening of Aruma and Pabdeh sediments in the foredeep basins and flexure of the top and base of the crust by the load of the Semail Ophiolite. The western salt tectonic province displays well-defined local gravity lows superimposed on a regional gravity high, which probably reflects the swelling of infra-Cambrian salt above a shallowing of the basement and thinning of the foredeep sediments. In addition, gravity modelling constrained by seismic and well data indicates the presence of substantial infra-Cambrian salt bodies in all basins of the UAE both onshore and offshore including the southern area of the Rub’ Al-Khali Basin. An extensive array of previously unmapped N-S, NW- and SW-trending lineaments affecting the basement and possibly overlying sediments are mapped in the UAE. The N-S Arabian trending lineament represents the effect of a major structure, along which many important oilfields are located (e.g. Bu Hasa). The SW trend has regular spacing and is dominant in the southern and central part of Abu Dhabi, east of the Falaha syncline. The NW-SE lineament is the most striking and includes two well-defined trends that cross Abu Dhabi Emirate, which in this paper are named as the Abu Dhabi Lineaments. These lineaments are associated with a linear gravity high extending from the southwestern border with Oman to the offshore close to Zakum oilfield. They are probably related to the Najd Fault System.

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