Three-dimensional gravity interpretations of the Round Lake batholith, northeastern Ontario

1970 ◽  
Vol 7 (1) ◽  
pp. 156-163 ◽  
Author(s):  
R. A. Gibb ◽  
J. van Boeckel
Keyword(s):  
Volcanic Rocks ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Density Contrast ◽  
Upper Crust ◽  
Normal Faulting ◽  
Maximum Thickness ◽  
Dimensional Gravity ◽  
Facies Change ◽  
Basic Volcanic

Gravity surveys of the Timmins–Senneterre mining belt of northeastern Ontario and western Quebec were made by the Dominion Observatory during the period 1946–1964. The Round Lake batholith is one of several composite granitic plutons of the Algoman series which are outlined by intense negative gravity anomalies. The anomaly over the Round Lake batholith can be explained by the large density contrast (0.22 g/cm3) between the granite and surrounding Keewatin volcanic rocks.Two possible models of the batholith are presented which depend on different assumptions as to the composition of the upper crust. The first model involves normal faulting of the batholith to explain the variations in anomaly level within the batholith. In this model the granite is assumed to be homogeneous in density and extends to a maximum depth of 10 km. Alternatively density variations corresponding to a facies change within the pluton may be the major cause of the local internal anomaly variations. In this interpretation the true thickness of the granite cannot be evaluated as the whole region is assumed to be underlain by granite, but the maximum thickness of the surrounding basic volcanic rocks is 5 km.

Gravity anomalies of three-dimensional bodies with a variable density contrast

1989 ◽  
Vol 130 (4) ◽  
pp. 711-719 ◽  
Author(s):  
I. V. Radhakrishna Murthy ◽  
P. Rama Rao ◽  
P. Ramakrishna
Keyword(s):  
Three Dimensional ◽  
Gravity Anomalies ◽  
Variable Density ◽  
Density Contrast

Spectral correlation of magnetic and gravity anomalies of Ohio

Geophysics ◽  
1997 ◽  
Vol 62 (1) ◽  
pp. 365-380 ◽  
Author(s):  
Ralph R. B. von Frese ◽  
Michael B. Jones ◽  
Jeong Woo Kim ◽  
Wen Sheng Li
Keyword(s):  
Volcanic Rocks ◽  
Gravity Anomalies ◽  
Field Analysis ◽  
Positive Density ◽  
Upper Crust ◽  
Spectral Correlation ◽  
South Central ◽  
Basement Rocks ◽  
Positive Correlations ◽  
First Vertical Derivative

Geologic interpretation of Ohio's magnetic or gravity anomalies is hindered by the effects of anomaly superposition and source ambiguity inherent to potential field analysis. A common approach to minimizing interpretational ambiguities is to consider analyses of anomaly correlations. A spectral procedure is adapted which correlates anomaly fields in the frequency domain to produce filters separating positively and negatively correlated, as well as null correlated features. The correlation filter passes or rejects wavenumbers between coregistered fields based on the correlation coefficient between common wavenumbers as given by the cosine of their phase difference. This procedure is applied to reduced‐to‐pole magnetic and first vertical derivative gravity anomalies of Ohio for mapping correlative magnetization and density contrasts within the basement rocks. The analysis reveals predominantly positive correlations between anomaly maxima and minima. Correlative anomaly maxima may be generally modeled as mafic bodies of the upper crust. They map out a possible dike complex in northwestern Ohio, a batholith as a possible source of volcanic rocks in southwestern Ohio, and numerous mafic bodies related presumably to Keweenawan rifting and Grenville tectonics. Correlative anomaly minima include several isolated features that may define felsic terranes of the upper crust, and ringed features around some of the larger mafic bodies which also may contain significant edge‐effect components. A large circular feature in south‐central Ohio involves correlative minima of a possible anorthosite body that is ringed by an inversely correlative zone of positive density and negative magnetization contrasts. Another prominent negative correlation involves an extensive area of possible extrusive rocks with positive magnetization and negative density contrasts just north of the batholith in southwestern Ohio.

Three-dimensional gravity modelling applied to the exploration of uranium unconformity-related basement-hosted deposits: the Contact prospect case study, Kiggavik, northeast Thelon region (Nunavut, Canada)

2017 ◽  
Vol 54 (8) ◽  
pp. 869-882 ◽  
Author(s):  
Régis Roy ◽  
Antonio Benedicto ◽  
Alexis Grare ◽  
Mickaël Béhaegel ◽  
Yoann Richard ◽  
...  
Keyword(s):  
Gravity Data ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
3D Models ◽  
Low Density ◽  
Uranium Deposits ◽  
Geological Interpretation ◽  
Thelon Basin ◽  
Dimensional Gravity ◽  
Density Values

In unconformity-related uranium deposits, mineralization is associated with hydrothermal clay-rich alteration haloes that decrease the density of the host rock. In the Kiggavik uranium project, located in the eastern Thelon Basin, Nunavut (Canada), basement-hosted shallow deposits were discovered by drilling geophysical anomalies in the 1970s. In 2014, gravity data were inverted for the first time using the Geosoft VOXI Earth ModellingTM system to generate three-dimensional (3D) models to assist exploration in the Contact prospect, the most recent discovery at Kiggavik. A 3D unconstrained inversion model was calculated before drilling, and a model constrained by petrophysical data was computed after drilling. The unconstrained inversion provided a first approximation of the geometry and depth of a low-density body and helped to collar the discovery holes of the Contact mineralization. The constrained inversion was computed using density values measured on 315 core samples collected from 21 drill holes completed between 2014 and 2015. The constrained modelling highlights three shallower and smaller low-density bodies that match the geological interpretation and refines the footprint of the gravity anomalies in relation to the current understanding of the deposit. The 3D inversion of gravity data is a valuable tool to guide geologists in exploration of shallow basement-hosted uranium deposits associated with alteration haloes and to assess the deposit gravity geometry.

Correlation of gravity anomalies with Yellowknife Supergroup rocks, North Arm, Great Slave Lake

1980 ◽  
Vol 17 (11) ◽  
pp. 1506-1516 ◽  
Author(s):  
R. A. Gibb ◽  
M. D. Thomas
Keyword(s):  
Greenstone Belt ◽  
Gravity Data ◽  
Volcanic Rocks ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Rock Density ◽  
The North ◽  
Great Slave Lake ◽  
Western Shore ◽  
Gravity Map

A gravity map compiled from observations made on the frozen surface of Great Slave Lake shows that the positive gravity anomaly associated with the Yellowknife greenstone belt extends offshore into the North Arm of the lake. On the western shore of Yellowknife Bay the axis of the anomaly coincides with mafic volcanic rocks of the Kam Formation. Offshore the axis continues southwards for about 10 km to the West Mirage Islands where it takes a dramatic turn to the southeast and continues for a further 60 km to the Outer Whaleback Rocks. Using the geology and rock density determinations on land for control, a three-dimensional geological model comprising a large number of prismatic blocks was derived from the gravity anomalies. In the model the simplifying assumption has been made that the greenstone belt is everywhere floored by granodiorite similar to the adjacent Western and South-east granodiorites. According to the model, mafic volcanic rocks of the Kam Formation are generally 1–3 km thick with a maximum thickness of 7 km at the mouth of Yellowknife Bay. Greywacke and mudstone of the Burwash Formation vary in thickness from 1 to 3 km. Locally these sedimentary rocks attain a thickness of 8 km but this is probably an overestimated value as they may very well be underlain by volcanic rocks of the Kam Formation. The presence of a third pluton of granodiorite flanking the belt to the southwest is also inferred from the gravity data. Previous seismic work indicated a greenstone basin with an average thickness of about 10 km. However, reexamination of the seismic records suggests that weak arrivals interpreted as originating from the base of the greenstone belt are more likely to be pulses associated with earlier arrivals.

scholarly journals Testing of Permian – Lower Triassic stratigraphic data in a half-graben/tilt-block system: evidence for the initial rifting phase in Antalya Nappes

2019 ◽  
Vol 56 (11) ◽  
pp. 1262-1283 ◽  
Author(s):  
Nazif Şahin ◽  
Demir Altiner
Keyword(s):  
Hanging Wall ◽  
Volcanic Rocks ◽  
Lower Triassic ◽  
Normal Faulting ◽  
Triassic Boundary ◽  
Block System ◽  
Basement Rocks ◽  
Half Graben ◽  
Facies Change ◽  
Permian Triassic Boundary

Testing of Middle Permian – Lower Triassic stratigraphic data from the Antalya Nappes in a half-graben/tilt-block system has revealed the presence of episodic rifting events separated by periods of tectonic quiescence. Following a period of uplift during the Permian (Late Artinskian to Roadian), the basement rocks have been activated by displacement faults and several depocenters in half-graben-like asymmetrical basins began to be filled with Roadian to Wordian continental clastic deposits intercalated with coal and marine rocks. The Early Capitanian time was a period of tectonic quiescence. The second event occurred in Middle to Late Capitanian times and produced basaltic volcanic rocks intercalated in the shallow marine fossiliferous carbonate successions. Following the Lopingian (Wuchiapingian and Changhsingian) and Permian–Triassic boundary interval representing a long tectonic quiescence, the last rifting episode started with an abrupt facies change in the late Griesbachian. Variegated shales, limestones, volcanics, talus breccia, and debris flow deposits were laid down in a half-graben/tilt-block system. As normal faulting has become active, the deposition continued on the subsiding hanging wall side. The stratigraphic gap increased in magnitude as the erosional truncation has incised deeply the footwall side. This initial rifting phase in the Antalya Nappes is prior to the onset of a stronger and more continuous rifting event that occurred in the Anisian–Carnian interval including a variety of deepwater clastic and carbonate deposits, radiolarites containing sometimes blocks and clasts derived from the basin margins, and volcanic rocks carrying intraoceanic setting character.

Distribution of Paleogene and Cretaceous rocks around the Nazko River belt of central British Columbia from 3-D long-offset first-arrival seismic tomography

2014 ◽  
Vol 51 (4) ◽  
pp. 358-372 ◽  
Author(s):  
Draga Talinga ◽  
Andrew J. Calvert
Keyword(s):  
British Columbia ◽  
Seismic Velocity ◽  
Volcanic Rocks ◽  
Sedimentary Rocks ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Hydrocarbon Exploration ◽  
P Wave ◽  
Velocity Models ◽  
First Arrival

Across the Nechako–Chilcotin plateau of British Columbia, the distribution of Cretaceous sedimentary rocks, which are considered prospective for hydrocarbon exploration, is poorly known due to the surface cover of glacial deposits and Tertiary volcanic rocks. To constrain the subsurface distribution of these Cretaceous rocks, in 2008 Geoscience BC acquired seven long, up to 14.4 km, offset vibroseis seismic reflection lines across a north-northwest-trending belt of exhumed sedimentary rocks inferred to be part of the Taylor Creek Group. P-wave velocity models, which are consistent with sonic logs from nearby wells, have been estimated using three-dimensional first-arrival tomography to depths ranging from 1 to 4 km. Igneous basement can be identified on most lines using the 5.5 km/s isovelocity contour, which locates the top of the basement to an accuracy of ∼400 m where its depth is known in exploration wells. There is no general distinction on the basis of seismic velocity between Cretaceous sedimentary and Paleocene–Eocene volcanic–volcaniclastic rocks, both of which appear to be characterized in the tomographic models by velocities of 3.0–5.0 km/s. The geometry of the igneous basement inferred from the velocity models identifies north-trending basins and ridges, which correlate with exposed rocks of the Jurassic Hazelton Group. Identified Cretaceous sedimentary rocks occur beneath less negative Bouguer gravity anomalies, but the original distribution of these rocks has been disrupted by later Tertiary extension that created north-trending basins associated with the most negative gravity anomalies. We suggest that Cretaceous sedimentary rocks, if deposited, could be preserved within these basins if the rocks had not been eroded prior to Tertiary extension.

Three-dimensional gravity analysis of the Kiglapait layered intrusion, Labrador

1979 ◽  
Vol 16 (1) ◽  
pp. 24-37 ◽  
Author(s):  
Randell Stephenson ◽  
Michael D. Thomas
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
Three Dimensional ◽  
Layered Intrusion ◽  
Southeastern Part ◽  
Model Studies ◽  
Maximum Thickness ◽  
Granitic Intrusion ◽  
Dimensional Gravity ◽  
Southeastern Region

The Kiglapait basic layered intrusion in northern Labrador previously has been interpreted, on geological grounds, to be a lopolith with a maximum thickness of 8.7 km. It is associated with a large (~45 mGal) positive gravity anomaly that for the most part is very similar to a theoretical anomaly computed for the proposed lopolith model, except over the southeastern part of the exposure where the theoretical anomaly is significantly more positive. Model studies of the gravity data suggest that the form and dimensions of the lopolithic model predicted on geological criteria are essentially valid and that a granitic intrusion is present in the southeastern region of the lopolith causing the discrepancy between observed and theoretical anomalies. The latter intrusion is believed to be genetically related to the Manvers granite, which occurs throughout the southeastern region as dykes and small stocks. The presence of another buried granitic mass south of the Kiglapait intrusion is also suggested by the gravity data.

Three‐dimensional Fourier gravity inversion with arbitrary density contrast

Geophysics ◽  
1992 ◽  
Vol 57 (1) ◽  
pp. 131-135 ◽  
Author(s):  
F. Guspí
Keyword(s):  
Frequency Domain ◽  
Three Dimensional ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Variable Density ◽  
Density Contrast ◽  
Gravity Inversion ◽  
Linear Quadratic ◽  
Space Domain ◽  
Depth Functions

The use of variable‐density contrasts in gravity inversion has gained increasing importance in recent years due to the necessity of constructing more realistic models of geophysical structures such as sedimentary basins. Linear, quadratic, and exponential variations, either in the space or in the frequency domain, are the basis of several methods. See, among others, the papers by Granser (1987), Chai and Hinze (1988), Reamer and Ferguson (1989), and Rao et al. (1990). Guspí (1990) used polynomial density‐depth functions for inverting gravity anomalies into 2-D polygons in the space domain.

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