Overview of regional geophysical studies in Manitoba and northwestern Ontario

1982 ◽  
Vol 19 (11) ◽  
pp. 2049-2059 ◽  
Author(s):  
D. H. Hall ◽  
W. C. Brisbin
Keyword(s):  
Greenstone Belt ◽  
Seismic Velocities ◽  
Crustal Layer ◽  
Northwestern Ontario ◽  
Reflection And Refraction ◽  
Gravity And Magnetic ◽  
Winnipeg River ◽  
Lower Crustal ◽  
Gravity And Magnetic Anomaly

This paper presents an overview of six geophysical projects (seismic reflection and refraction, gravity and magnetic anomaly interpretation, specific gravity and magnetic property measurements) carried out in an area in Manitoba and northwestern Ontario bounded by 93 and 96°W longitude, and 49 and 51°N latitude.The purpose of the surveys was to define crustal structure in the Kenora–Wabigoon greenstone belt, the Winnipeg River batholithic belt, the Ear Falls – Manigotagan gneiss belt, and the Uchi greenstone belt. The following conclusions emerge.In all of the belts, a major discontinuity divides the crust into the commonly found upper and lower crustal sections. At the top of the lower crust, a seismically distinct layer (the mid-crustal layer) occurs. Seismic velocities in this layer suggest either intermediate to basic igneous rocks or metamorphic rocks of the amphibolite facies.Crustal geophysical characteristics vary sufficiently among the four belts to justify the classification of all four as distinct subprovinces of the Superior Province.Cet article présente une vue générale sur six projets de géophysique (réflexion et réfraction sismique, interprétation d'anomalies de gravité et magnétiques, déterminations de densité et de propriétés magnétiques) réalisés dans une région du Manitoba et du nord-ouest de l'Ontario encadrée par les longitudes 93 et 96°O et les latitudes 49 et 51°N.

An expanding spread seismic reflection survey across the Snake Bay–Kakagi Lake greenstone belt, northwestern Ontario

1979 ◽  
Vol 16 (8) ◽  
pp. 1599-1612 ◽  
Author(s):  
A. G. Green ◽  
N. L. Anderson ◽  
O. G. Stephenson
Keyword(s):  
Greenstone Belt ◽  
Seismic Reflection ◽  
Lower Layer ◽  
Signal To Noise Ratio ◽  
Middle Layer ◽  
High Signal ◽  
Total Charge ◽  
Crustal Layer ◽  
Northwestern Ontario ◽  
Long Bay

An expanding spread seismic reflection survey has been conducted across the Snake Bay–Kakagi Lake greenstone belt in northwestern Ontario. Receiver and shot arrays with multiple shots per location helped to maintain a high signal to noise ratio in most of the data. Distances between the shots and receivers ranged from 1.04–8.48 km and the total charge per shot location varied from 26–86 kg. After computer processing the data, numerous coherent reflections were observed from near vertical and near horizontal discontinuities.Prominent early reflections were used to map a granite–greenstone contact to the south of the profile and a section of the Long Bay fault zone to the northeast of the profile. A noticeable absence of reflections from the Aulneau granite batholith–greenstone contact suggests that this contact dips westwards, towards the centre of the batholith.From the later reflections a model of the deep crust beneath the Snake Bay–Kakagi Lake greenstone belt was derived. This model, which represents a lateral extension of the Aulneau crustal model, consists of a three-layered crust. The top crustal layer is 19 km thick with Pg and Sg velocities of 6.2 and 3.5 km/s respectively, the middle layer is 3 km thick, and the lower layer extends to the Mohorovicic discontinuity at 38 km depth.

Crustal structure in Northwestern Ontario: Regional magnetic anomalies

1969 ◽  
Vol 6 (1) ◽  
pp. 101-107 ◽  
Author(s):  
Peter H. McGrath ◽  
Donald H. Hall
Keyword(s):  
Crustal Structure ◽  
Upper Mantle ◽  
Magnetic Anomalies ◽  
Two Dimensional ◽  
Crustal Layer ◽  
Smoothing Operator ◽  
Northwestern Ontario ◽  
Map Series ◽  
Lower Crustal ◽  
Seismic Discontinuity

A regional aeromagnetic map, portraying the regional magnetic anomaly system in Northwestern Ontario west of longitude 92 °W and south of latitude 55 °N and extending westward into Manitoba to longitude 97 °W (with an additional block bounded by latitudes 54° N and 56 °N and longitudes 97° W and 102 °W) is presented. The map was prepared by multiple application of a two-dimensional smoothing operator applied to data digitized at 3 km intervals from the 1-inch-to-1-mile aeromagnetic map series published by the Geological Survey of Canada. Comparison was made with previous maps overlapping on portions of the area, which had been made by various techniques, including Fourier analysis, fitting of 6th-order polynomials, and photographic reduction. The general features of the anomaly system were found to be similar for all of these techniques. The regional anomaly system is found to be related in some cases to the thickness of the upper crustal layer (defined as lying above the Intermediate seismic discontinuity) and to structure within it, but not to the lower crustal layer or to the upper mantle.

Crustal velocity structure from SAREX, the Southern Alberta Refraction Experiment

2002 ◽  
Vol 39 (3) ◽  
pp. 351-373 ◽  
Author(s):  
Ron M Clowes ◽  
Michael JA Burianyk ◽  
Andrew R Gorman ◽  
Ernest R Kanasewich
Keyword(s):  
Velocity Structure ◽  
Data Interpretation ◽  
Crustal Thickening ◽  
Tectonic Zone ◽  
Crustal Layer ◽  
Crustal Velocity ◽  
Crustal Velocity Structure ◽  
Southern Alberta ◽  
Magmatic Underplating ◽  
Lower Crustal

Lithoprobe's Southern Alberta Refraction Experiment, SAREX, extends 800 km from east-central Alberta to central Montana. It was designed to investigate crustal velocity structure of the Archean domains underlying the Western Canada Sedimentary Basin. From north to south, SAREX crosses the Loverna domain of the Hearne Province, the Vulcan structure, the Medicine Hat block (previously considered part of the Hearne Province), the Great Falls tectonic zone, and the northern Wyoming Province. Ten shot points along the profile in Canada were recorded on 521 seismographs deployed at 1 km intervals. To extend the line, an additional 140 seismographs were deployed at intervals of 1.25–2.50 km in Montana. Data interpretation used an iterative application of damped least-squares inversion of traveltime picks and forward modeling. Results show different velocity structures for the major blocks (Loverna, Medicine Hat, and Wyoming), indicating that each is distinct. Wavy undulations in the velocity structure of the Loverna block may be associated with internal crustal deformation. The most prominent feature of the model is a thick (10–25 km) lower crustal layer with high velocities (7.5–7.9 km/s) underlying the Medicine Hat and Wyoming blocks. Based on data from lower crustal xenoliths in the region, this layer is interpreted to be the result of Paleoproterozoic magmatic underplating. Crustal thickness varies from 40 km in the north to almost 60 km in the south, where the high-velocity layer is thickest. Uppermost mantle velocities range from 8.05 to 8.2 km/s, with the higher values below the thicker crust. Results from SAREX and other recent studies are synthesized to develop a schematic representation of Archean to Paleoproterozoic tectonic development for the region encompassing the profile. Tectonic processes associated with this development include collisions of continental blocks, subduction, crustal thickening, and magmatic underplating.

Three-dimensional numerical modeling of gravity and magnetic anomaly in a mixed space-wavenumber domain

Geophysics ◽  
2019 ◽  
Vol 84 (4) ◽  
pp. G41-G54 ◽  
Author(s):  
Shikun Dai ◽  
Dongdong Zhao ◽  
Shunguo Wang ◽  
Bin Xiong ◽  
Qianjiang Zhang ◽  
...  
Keyword(s):  
Numerical Modeling ◽  
Fourier Transform ◽  
Differential Equations ◽  
Numerical Solution ◽  
Magnetic Anomaly ◽  
Large Scale ◽  
Vertical Component ◽  
Wavenumber Domain ◽  
Gravity And Magnetic ◽  
Gravity And Magnetic Anomaly

Fast and accurate numerical modeling of gravity and magnetic anomalies is the basis of field-data inversion and quantitative interpretation. In gravity and magnetic prospecting, the computation and memory requirements of practical modeling is still a significant issue, which leads to the difficulty of using efficient and detailed inversions for large-scale complex models. A new 3D numerical modeling method for gravity and magnetic anomaly in a mixed space-wavenumber domain is proposed to mitigate the difficulties. By performing a 2D Fourier transform along two horizontal directions, 3D partial differential equations governing gravity and magnetic potentials in the spatial domain are transformed into a group of independent 1D differential equations wrapped with different wavenumbers. Importantly, the computation and memory requirements of modeling are greatly reduced by this method. A modeling example with 4,040,100 observations can be finished in approximately 28 s on a desktop using a single core, and the independent differential equations are highly parallel among different wavenumbers. The method preserves the vertical component in the space domain, and thus a mesh for modeling can be finer at a shallower depth and coarser at a deeper depth. In general, the new method takes into account the calculation accuracy and the efficiency. The finite-element algorithm combined with a chasing method is used to solve the transformed differential equations with different wavenumbers. In a synthetic test, a model with prism-shaped anomalies is used to verify the accuracy and efficiency of the proposed algorithm by comparing the analytical solution, our numerical solution, and a well-known numerical solution. Furthermore, we have studied the balance between computational accuracy and efficiency using a standard fast Fourier transform (FFT) method with grid expansion and the Gauss-FFT method. A model with topography is also used to explore the ability of modeling topography with our method. The results indicate that the proposed method using the Gauss-FFT method has characteristics of fast calculation speed and high accuracy.

Tectonic significance of potential-field anomalies in western Canada: results from the Lithoprobe SNORCLE transect

2005 ◽  
Vol 42 (6) ◽  
pp. 1239-1255 ◽  
Author(s):  
C Elissa Lynn ◽  
Frederick A Cook ◽  
Kevin W Hall
Keyword(s):  
Cross Sections ◽  
Upper Mantle ◽  
Potential Field ◽  
Precambrian Rocks ◽  
Reflection And Refraction ◽  
Gravity And Magnetic ◽  
Tectonic Significance ◽  
Geometrical Constraints ◽  
Subsurface Geometry ◽  
Local Anomalies

Potential-field anomalies within the Lithoprobe SNORCLE (Slave – Northern Cordillera Lithospheric Evolution) transect area provide geometrical constraints for regional crustal and lithospheric structures, as well as for local anomalies when coupled with subsurface geometry visible on nearly 2500 km of deep seismic reflection and refraction profiles. Areal distribution of gravity and magnetic anomalies permit structures to be projected away from seismic cross sections, and forward modelling provides tests of different interpretations of deep (crustal and upper mantle) density structures. In a key result from modelling, a Paleoproterozoic subduction zone beneath the Wopmay orogen probably consists of high-density rocks, such as eclogite, within the upper mantle. This result supports the concept of moderate- to low-angle intra-lithospheric sutures. On an even larger scale, applications of bandpass and directional filters assist in detecting anomalies according to wavelength or azimuthal orientation and thus provide means to track patterns across structural grain. For example, gravity and magnetic trends that are associated with Precambrian rocks of the Canadian Shield can, in some cases, be followed across much of the Cordillera. This result is consistent with North American Precambrian rocks composing much of the crust in the Cordillera and thus that the addition of "new" lithosphere during Mesozoic – early Tertiary accretion has been relatively minor.

U–Pb ages constraining structural development of an Archean terrane boundary in the Lake of the Woods area, western Superior Province, Canada

2006 ◽  
Vol 43 (7) ◽  
pp. 967-993 ◽  
Author(s):  
M Melnyk ◽  
D W Davis ◽  
A R Cruden ◽  
R A Stern
Keyword(s):  
Greenstone Belt ◽  
Thermal Ionization Mass Spectrometry ◽  
Magmatic Zircon ◽  
Ionization Mass ◽  
Structural Development ◽  
Ion Microprobe ◽  
Widespread Occurrence ◽  
Lake Of The Woods ◽  
Second Stage ◽  
Winnipeg River

Layered gneisses in the Winnipeg River subprovince contain magmatic zircon with U–Pb ages of 3317 ± 9 and 3055 ± 4 Ma at Tannis Lake, and ~3170 and 3255 ± 5 Ma at Cedar Lake, indicating widespread occurrence of Mesoarchean crust. This is in contrast to the well-documented Neoarchean age of the western Wabigoon subprovince. Further geochronology using both SHRIMP (sensitive high resolution ion microprobe) and ID-TIMS (isotope dilution thermal ionization mass spectrometry), combined with structural observations, in the Kenora area and Lake of the Woods greenstone belt show the effects of juxtaposition of these two terranes. Isoclinally folded gneiss north of the subprovince boundary zone near Kenora gives a magmatic age of 2882 ± 2 Ma with 3051 ± 6 Ma inheritance. Ages of syntectonic dykes show that asymmetric refolding of these gneisses occurred between 2717 ± 2 and about 2713 ± 1 Ma. Subsequent regional vertical flattening and horizontal extension are dated at 2708 ± 2 Ma by syntectonic tonalite sheets. These events are broadly coeval with deposition of orogenic sediments in the Warclub Group and a first stage of regional folding (age brackets of 2716–2709 Ma) in the Lake of the Woods greenstone belt to the south. A second stage of folding and regional faulting in the greenstone belt occurred about 2695 ± 4 Ma and is approximately coeval with open upright folding in the Winnipeg River subprovince. These observations are consistent with overthrusting and collapse of a Mesoarchean continental terrane by a juvenile Neoarchean arc terrane over the time span 2717–2695 Ma.

scholarly journals The fate of fluids released from subducting slab in northern Cascadia

2011 ◽  
Vol 3 (2) ◽  
pp. 943-962 ◽  
Author(s):  
K. Ramachandran ◽  
R. D. Hyndman
Keyword(s):  
Poisson’S Ratio ◽  
Subduction Zones ◽  
Poisson's Ratio ◽  
Crustal Layer ◽  
Collision Tectonics ◽  
Significant Addition ◽  
Increasing Temperature ◽  
Lower Crustal ◽  
Forearc Region ◽  
Forearc Mantle

Abstract. Large amounts of water carried down in subduction zones are driven upward into the overlying forearc upper mantle and crust as increasing temperature and pressure dehydrate the subducting crust. Through seismic tomography velocities we show that, (a) the overlying forearc mantle in Northern Cascadia is hydrated to serpentinite, and (b) the low Poisson's ratio at the base of the forearc lower crust that may represent silica deposited from the rising fluids. From the velocities observed in the forearc mantle, the volume of serpentinite estimated is ~30 %. This mechanically weak hydrated forearc region has important consequences in limits to great earthquakes and to collision tectonics. An approximately 10 km thick lower crustal layer of low Poisson's ratio (σ = 0.22) in the forearc is estimated to represent a maximum addition of ~14 % by volume of quartz (σ = 0.09). If this quartz is removed from rising silica-saturated fluids over long times it represents a significant addition of silica to the continental crust and an important contributor to its average composition.

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