Geologic interpretation of magnetic and gravity anomalies in the Strait of Juan de Fuca, U.S.–Canada

1977 ◽  
Vol 14 (2) ◽  
pp. 223-238 ◽  
Author(s):  
N. S. MacLeod ◽  
D. L. Tiffin ◽  
P. D. Snavely Jr. ◽  
R. G. Currie
Keyword(s):  
Continental Shelf ◽  
Gravity Anomalies ◽  
Vancouver Island ◽  
Magnetic Data ◽  
Magnetic Survey ◽  
Olympic Peninsula ◽  
Basement Rocks ◽  
Gravity And Magnetic ◽  
Juan De Fuca ◽  
Magnetic And Gravity Anomalies

A gravity and magnetic survey of the Strait of Juan de Fuca and adjacent Pacific continental shelf was conducted to define the tectonic framework in this 20 to 35 km wide seaway and its relation to that of Vancouver Island and the Olympic Peninsula. The offshore extensions of large onshore faults are delineated by linear magnetic and gravity anomalies. One of these, the Leech River fault of southern Vancouver Island, marks the northern limit of oceanic-type basaltic basement present in western Washington and Oregon. This fault probably continues southeast-ward from Vancouver Island across the strait to near the northeastern coast of the Olympic Peninsula, and westward across the strait to the continental shelf off Cape Flattery. The Calawah fault, which extends northwestward from near Cape Flattery onto the Pacific shelf, terminates the Leech River fault. Northwest of the Leech River fault on the shelf, the Calawah fault probably is the contact between oceanic and continental crustal types. The gravity and magnetic data also indicate the location of folds, other faults, and areas of shallow basement rocks.

Integrated Geophysical Interpretation of Kerio Valley Basin Stratigraphy, Kenya Rift

2016 ◽  
Author(s):  
Godfred Osukuku ◽  
Abiud Masinde ◽  
Bernard Adero ◽  
Edmond Wanjala ◽  
John Ego
Keyword(s):  
Gravity Data ◽  
Gravity Anomalies ◽  
Fault System ◽  
Magnetic Data ◽  
Data Sets ◽  
Seismic Profiles ◽  
The West ◽  
Seismic Reflection Data ◽  
Basement Rocks ◽  
Western Side

Abstract This research work attempts to map out the stratigraphic sequence of the Kerio Valley Basin using magnetic, gravity and seismic data sets. Regional gravity data consisting of isotactic, free-air and Bouguer anomaly grids were obtained from the International Gravity Bureau (BGI). Magnetic data sets were sourced from the Earth Magnetic Anomaly grid (EMAG2). The seismic reflection data was acquired in 1989 using a vibrating source shot into inline geophones. Gravity Isostacy data shows low gravity anomalies that depict a deeper basement. Magnetic tilt and seismic profiles show sediment thickness of 2.5-3.5 Km above the basement. The Kerio Valley Basin towards the western side is underlain by a deeper basement which are overlain by succession of sandstones/shales and volcanoes. At the very top are the mid Miocene phonolites (Uasin Gishu) underlain by mid Miocene sandstones/shales (Tambach Formation). There are high gravity anomalies in the western and southern parts of the basin with the sedimentation being constrained by two normal faults. The Kerio Valley Basin is bounded to the west by the North-South easterly dipping fault system. Gravity data was significantly of help in delineating the basement, scanning the lithosphere and the upper mantle according to the relative densities. The basement rocks as well as the upper cover of volcanoes have distinctively higher densities than the infilled sedimentary sections within the basin. From the seismic profiles, the frequency of the shaley rocks and compact sandstones increases with depths. The western side of the basin is characterized by the absence of reflections and relatively higher frequency content. The termination of reflectors and the westward dip of reflectors represent a fault (Elgeyo fault). The reflectors dip towards the west, marking the basin as an asymmetrical syncline, indicating that the extension was towards the east. The basin floor is characterized by a nearly vertical fault which runs parallel to the Elgeyo fault. The seismic reflectors show marked discontinuities which may be due to lava flows. The deepest reflector shows deep sedimentation in the basin and is in reasonable agreement with basement depths delineated from potential methods (gravity and magnetic). Basement rocks are deeper at the top of the uplift footwall of the Elgeyo Escarpment. The sediments are likely of a thickness of about 800 M which is an interbed of sandstones and shales above the basement.

Late Pleistocene history and geomorphology, southwestern Vancouver Island, British Columbia

1979 ◽  
Vol 16 (9) ◽  
pp. 1645-1657 ◽  
Author(s):  
Neville F. Alley ◽  
Steven C. Chatwin
Keyword(s):  
British Columbia ◽  
Continental Shelf ◽  
Late Pleistocene ◽  
Vancouver Island ◽  
Glacial Lakes ◽  
Small Lakes ◽  
Strait Of Georgia ◽  
Coast Mountains ◽  
Juan De Fuca ◽  
Juan De Fuca Strait

The major Pleistocene deposits and landforms on southwestern Vancouver Island are the result of the Late Wisconsin (Fraser) Glaciation. Cordilleran glaciers formed in the Vancouver Island Mountains and in the Coast Mountains had advanced down Strait of Georgia to southeastern Vancouver Island after 19 000 years BP. The ice split into the Puget and Juan de Fuca lobes, the latter damming small lakes along the southwestern coastal slope of the island. During the maximum of the glaciation (Vashon Stade), southern Vancouver Island lay completely under the cover of an ice-sheet which flowed in a south-southwesterly direction across Juan de Fuca Strait, eventually terminating on the edge of the continental shelf. Deglaciation was by downwasting during which ice thinned into major valleys and the strait. Most upland areas were free of ice down to an elevation of 400 m by before 13 000 years BP. A possible glacier standstill and (or) resurgence occurred along Juan de Fuca Strait and in some interior upland valleys before deglaciation was complete. Glacial lakes occupied major valleys during later stages of deglaciation.

A GEOLOGICAL AND GEOPHYSICAL STUDY OF PILOT KNOB (SOUTH), TRAVIS COUNTY, TEXAS

Geophysics ◽  
1954 ◽  
Vol 19 (3) ◽  
pp. 438-454 ◽  
Author(s):  
Frederick Romberg ◽  
Virgil E. Barnes
Keyword(s):  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Complete Picture ◽  
Magnetic Data ◽  
Strong Gravity ◽  
Local Setting ◽  
Gravity And Magnetic ◽  
Geophysical Study ◽  
Gravity And Magnetic Data ◽  
First Time

Pilot Knob is an exhumed volcano of Cretaceous age, composed of “serpentinized” pyroclastics and minor amounts of basalt in both intrusive and extrusive masses. The geology of Pilot Knob was re‐examined, and gravity and magnetic observations made and interpreted, in order to present a complete picture of the feature itself, its history, its relation to the region and area surrounding it, and the resemblances between it and the serpentine plugs in the neighborhood, to which it is geologically related. Some of these plugs have been discovered by geophysical means, and some so discovered have produced oil; the application of gravity and magnetic data to such discoveries is analyzed. The extrusive masses are here reported for the first time, and other evidence is given for the age and volcanic nature of Pilot Knob. The observations reveal 1) strong gravity and magnetic anomalies over the central basalt mass, 2) a pattern of weaker anomalies probably caused by flows and dikes and suggesting that Pilot Knob is situated near the intersection of two sets of fractures, and 3) evidence that “serpentinized” pyroclastics show weak magnetic anomalies and (in the local setting) no visible gravity anomalies.

Paleomagnetism and counterclockwise tectonic rotation of the Upper Oligocene Sooke Formation, southern Vancouver Island, British Columbia

2008 ◽  
Vol 45 (4) ◽  
pp. 499-507 ◽  
Author(s):  
Donald R. Prothero ◽  
Elizabeth Draus ◽  
Thomas C. Cockburn ◽  
Elizabeth A. Nesbitt
Keyword(s):  
British Columbia ◽  
Recent Work ◽  
Marine Mammals ◽  
Vancouver Island ◽  
Southern Coast ◽  
Olympic Peninsula ◽  
Upper Oligocene ◽  
Basement Rocks ◽  
Molluscan Fauna ◽  
Tectonic Rotation

The age of the Sooke Formation on the southern coast of Vancouver Island, British Columbia, Canada, has long been controversial. Prior paleomagnetic studies have produced a puzzling counterclockwise tectonic rotation on the underlying Eocene volcanic basement rocks, and no conclusive results on the Sooke Formation itself. We took 21 samples in four sites in the fossiliferous portion of the Sooke Formation west of Sooke Bay from the mouth of Muir Creek to the mouth of Sandcut Creek. After both alternating field (AF) and thermal demagnetization, the Sooke Formation produces a single-component remanence, held largely in magnetite, which is entirely reversed and rotated counterclockwise by 35° ± 12°. This is consistent with earlier results and shows that the rotation is real and not due to tectonic tilting, since the Sooke Formation in this region has almost no dip. This rotational signature is also consistent with counterclockwise rotations obtained from the northeast tip of the Olympic Peninsula in the Port Townsend volcanics and the Eocene–Oligocene sediments of the Quimper Peninsula. The reversed magnetozone of the Sooke sections sampled is best correlated with Chron C6Cr (24.1–24.8 Ma) or latest Oligocene in age, based on the most recent work on the Liracassis apta Zone molluscan fauna, and also a number of unique marine mammals found in the same reversed magnetozone in Washington and Oregon.

Magnetic and gravity investigation of kimberlites in north-central Botswana

Geophysics ◽  
2020 ◽  
pp. 1-51
Author(s):  
Kitso matende ◽  
Kevin Mickus
Keyword(s):  
Gravity Anomaly ◽  
Kimberlite Pipe ◽  
Magnetic Data ◽  
Magnetic Survey ◽  
Density Anomaly ◽  
Gravity And Magnetic ◽  
Magnetic Inversion ◽  
Magnetic Analysis ◽  
Lateral Extent ◽  
The Difference

The Orapa Kimberlite field of Botswana is one of the world’s major diamond producing regions. Within this field, there are several small kimberlite pipes that have not been completely explored in terms of the lateral extent, depth and diamond potential. Two such pipes, BK54 and BK55, were found during a ground gravity and magnetic survey and subsequent drilling confirmed the presence of kimberlite material. To determine the physical properties of these pipes, their lateral extent, depth and thickness were estimated using a gravity and magnetic analysis and 2.5D and 3D modeling. Tilt derivatives of the magnetic data indicated that BK54 has a northwest-trending elliptical shape and BK55 has a roughly circular shape. Residual gravity anomaly maps indicate that BK54 does not have a density anomaly while BK55 is associated with a gravity maximum. 3D gravity and magnetic inversion modeling constrained by magnetic susceptibility measurements indicate that BK54 is smaller in volume than BK55 and neither pipe is thicker than 125 m. The difference in shape and the lack of a gravity anomaly over BK54 implies a different formation mechanism for each kimberlite pipe. While several mechanisms are suggested, BK54 may have formed by a more explosive eruption producing more tuffistic material in both the crater and diatreme facies. The gravity and magnetic analysis also found that the kimberlite pipes, while small, are larger in extent than determined by drilling and warrant additional drilling to determine their economic potential.

Poisson magnetization-to-density-ratio and magnetization inclination properties of banded iron formations of the Carajás mineral province from processing airborne gravity and magnetic data

Geophysics ◽  
2020 ◽  
Vol 85 (5) ◽  
pp. K1-K11
Author(s):  
Caio Alencar de Matos ◽  
Carlos Alberto Mendonça
Keyword(s):  
Iron Ore ◽  
Density Ratio ◽  
Iron Formation ◽  
Magnetic Data ◽  
Airborne Gravity ◽  
Magnetic Survey ◽  
Carajás Mineral Province ◽  
Uniform Magnetization ◽  
Gravity And Magnetic ◽  
Gravity And Magnetic Data

According to the Poisson theorem, gravity and magnetic fields arising from geologic bodies that share common sources, with a uniform magnetization-to-density ratio (MDR) and a uniform magnetization direction, are related by a linear transformation that allows each field to be calculated from the other. Provided that these conditions on the sources are met, when the gravity and magnetic data are available over an area, the Poisson theorem can be used to infer the MDRs and magnetization directions of sources from their associated gravity and magnetic anomalies. These conditions are partially met in many geologic structures but are expected in iron ore deposits, usually associated with strongly magnetic and highly dense formations. Due to the importance of iron ore as a global commodity, most mineral provinces of the world have been investigated by accurate gravity and magnetic sensors, providing a reliable database, but they have not yet been explored with joint interpretation based on Poisson’s relationships. We have interpreted a gravity-magnetic survey covering the Serra Sul of the Carajás Mineral Province, Brazil, where world-class iron deposits are found. We have adapted a formulation formerly developed to estimate the MDR and the magnetization inclination (MI) from profile data to process gridded data sets. Due to faulting and folding, the same density and magnetic structure may assume different strike directions, requiring corrections to improve MDR and MI estimates. Because the geomagnetic field inclination in the studied area is very low (−6.7°), a procedure for stable computation of the components of the anomalous magnetic field vector is applied. The inferences for Serra Sul MDR suggest minor variations for the entire 30 km long formation containing the mineralized bodies, the strong remanent magnetization showing reverse polarity for banded iron formation segments of the Carajás Serra Sul.

scholarly journals GRAVITY AND MAGNETIC INTEGRATED DATA INTERPRETATION OF THE CORRÉGO DOS BOIS COMPLEX, GOIÁS ALKALINE PROVINCE, CENTRAL BRAZIL

2016 ◽  
Vol 33 (4) ◽  
Author(s):  
Yára Regina Marangoni ◽  
Henglei Zhang ◽  
Herbert Jorge Ferreira
Keyword(s):  
Gravity Anomalies ◽  
Data Interpretation ◽  
Central Brazil ◽  
Gravity And Magnetic ◽  
Magnetic And Gravity Anomalies

ABSTRACT. Previous studies carried out in some alkaline provinces around Paran´a Basin, Brazil, show that most of the alkaline intrusions are related to high magnetic and gravity anomalies... RESUMO. Estudos anteriores em algumas províncias alcalinas ao redor da Bacia do Paraná, Brasil, indicaram que a maioria das intrusões alcalinas apresentam anomalias...

A Gravity Survey of the Continental Shelf south of Cape Sable, Nova Scotia

1974 ◽  
Vol 11 (9) ◽  
pp. 1329-1334 ◽  
Author(s):  
A. B. Watts
Keyword(s):  
Nova Scotia ◽  
Continental Shelf ◽  
Continental Margin ◽  
Gravity Anomalies ◽  
Granitic Rocks ◽  
Gravity Survey ◽  
Bouguer Gravity ◽  
Eastern Canada ◽  
Bouguer Gravity Anomalies ◽  
Basement Rocks

The western part of the continental shelf off Nova Scotia is associated with a region of relatively low Bouguer gravity anomalies. The gravity 'low' reaches a minimum of −23 mgal about 20 km south of Cape Sable Island, Nova Scotia. Computations suggest the 'low' can be explained by an outward sloping body about 40 km wide and up to 15 km thick. The gravity 'low' is interpreted as caused by granites or granitic rocks which intrude the pre-Mesozoic basement rocks underlying the shelf. The granites or granitic rocks may extend to within about 50 km of the eastern Canada continental margin.

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