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.

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.

Ambient seismic noise tomography in west-central and Southern Brazil, characterizing the crustal structure of the Chaco-Paraná, Pantanal and Paraná basins

2019 ◽  
Vol 220 (3) ◽  
pp. 2074-2085
Author(s):  
Taghi Shirzad ◽  
Marcelo Assumpcao ◽  
Marcelo Bianchi
Keyword(s):  
Surface Wave ◽  
Crustal Structure ◽  
Seismic Noise ◽  
Velocity Model ◽  
Dispersion Curves ◽  
Ambient Seismic Noise ◽  
Inversion Method ◽  
Crustal Layer ◽  
West Central ◽  
Lower Crustal

SUMMARY Surface wave analysis provides important information on crustal structure, but it is challenging to obtain accurate/robust models in aseismic regions because of the lack of local earthquake records. In this paper, interstation empirical Green's functions retrieved by ambient seismic noise in 75 broad-band stations from 2016 January to 2018 September were used to study crustal structure in west-central Brazil. Fast marching method was applied to calculate the 2-D surface wave tomographic maps, and local dispersion curves were estimated in the period range of 4–80 s for each geographic cell. 1-D damped least squares inversion method was then conducted to obtained shear wave velocity model. Finally, the average ($\tilde{\rm V}$S) of the calculated VSV and VSH quasi 3-D models were used to characterize the crustal structure. Besides the checkerboard test resolution, a stochastic test with the effect of errors in the dispersion curves and choice of inversion parameters were carried out to better evaluate model uncertainties. Our results show a clear relation between the sedimentary thickness and geological units with the shorter period tomographic maps. Agreement has also been observed in longer periods such as the clear N–S anomaly along the Asuncion and Rio Grande Arches representing the boundary between the Chaco-Paraná and the Paraná basins. A 3-D composite velocity model shows a crustal structure consisting of three main layers. Some differences in lower crustal properties were found between the Paraná and Chaco-Paraná basins, consistent with a recently postulated, gravity-derived Western Paraná suture zone. However, no high velocities along the SW–NE axis of the Paraná basin were found to confirm proposed underplating. At the eastern edge of the Pantanal basin, the thin crust seems to be associated with a very thin (or lack of) lower crustal layer, consistent with a recently proposed crustal delamination hypothesis for the formation of the Pantanal basin.

scholarly journals Complex rift patterns, a result of interacting crustal and mantle weaknesses, or multiphase rifting? Insights from analogue models

2021 ◽  
Author(s):  
Frank Zwaan ◽  
Pauline Chenin ◽  
Duncan Erratt ◽  
Gianreto Manatschal ◽  
Guido Schreurs
Keyword(s):  
Upper Mantle ◽  
Upper Crust ◽  
Crustal Layer ◽  
Tectonic History ◽  
Lithospheric Extension ◽  
Analogue Models ◽  
History Of ◽  
Lower Crustal ◽  
Degree Of Coupling ◽  
Over Time

Abstract. During lithospheric extension, localization of deformation often occurs along structural weaknesses inherited from previous tectonic phases. Such weaknesses may occur in both the crust and mantle, but the combined effects of these weaknesses on rift evolution remains poorly understood. Here we present a series of 3D brittle-viscous analogue models to test the interaction between differently oriented weaknesses located in the brittle upper crust and/or upper mantle. We find that crustal weaknesses usually express first at the surface with the formation of graben parallel to their orientation; then, structures parallel to the mantle weakness overprint them and often become dominant. Furthermore, the direction of extension exerts minimal control on rift trends when inherited weaknesses are present, which implies that present-day rift orientations are not always indicative of past extension directions. We also suggest that multiphase extension is not required to explain different structural orientations in natural rift systems. The degree of coupling between the mantle and upper crust affects the relative influence of the crustal and mantle weaknesses: low coupling enhances the influence of crustal weaknesses, whereas high coupling enhances the influence of mantle weaknesses. Such coupling may vary over time due to progressive thinning of the lower crustal layer, as well as due to variations in extension velocity. These findings provide a strong incentive to reassess the tectonic history of various natural examples.

Crustal structure of the Nova Scotian margin in the Laurentian Channel region

1987 ◽  
Vol 24 (9) ◽  
pp. 1859-1868 ◽  
Author(s):  
I. Reid
Keyword(s):  
Crustal Structure ◽  
Seismic Velocity ◽  
Structural Information ◽  
Seismic Refraction ◽  
Ocean Bottom ◽  
Eastern Canada ◽  
Crustal Layer ◽  
Air Gun ◽  
Lower Crustal

A seismic-refraction study on the outer Scotian Shelf of eastern Canada, carried out using large air-gun sources and ocean bottom seismograph receivers, has provided structural information on the entire crustal column. A thick (about 13 km) sedimentary sequence is characterized by significant lateral variation in this area, and a marked increase in seismic velocity around 8 km depth may delineate the synrift–postrift transition. Beneath the sediments is highly attenuated continental crust, about 11 km thick, with some evidence for a lower crustal layer of velocity around 7 km/s, which may be partly due to under-plating during rifting. Determination of the complete crustal structure, including the tentative delineation of the rift–drift transition, in a region of large crustal extension provides a useful test for models of continental rifting, and a simple uniform extension–subsidence model is found to produce an adequate fit to the interpreted structure.

Crustal structure of the Canadian polar margin: results of the 1985 seismic refraction survey

1989 ◽  
Vol 26 (5) ◽  
pp. 853-866 ◽  
Author(s):  
I. Asudeh ◽  
D. A. Forsyth ◽  
R. Stephenson ◽  
A. Embry ◽  
H. R. Jackson ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Seismic Refraction ◽  
Thick Layer ◽  
Parallel Line ◽  
Inner Shelf ◽  
Lower Paleozoic ◽  
Crustal Layer ◽  
Interval Velocity ◽  
Upper Paleozoic ◽  
Lower Crustal

The 1985 refraction survey based on Ice Island covered a northern transition zone along the Canadian polar margin north of Axel Heiberg Island. The refraction survey included a 60 km line along the inner shelf, a 180 km parallel line along the outer shelf, and a 60 km connecting line. Shotpoints offset from the line ends recorded upper mantle observations to a distance of 240 km.Along the inner shelf, the upper 700 m, with an interval velocity of 3.7 km/s, is interpreted as Tertiary–Cretaceous strata. The underlying 4 km thick layer has a starting velocity of 5 km/s and a gradient of 0.2 s−1. It is thought to consist of mainly deformed lower Paleozoic strata capped by upper Paleozoic – Triassic clastics and carbonates and (or) Cretaceous volcanics. Sequentially, the lower unit, with a starting velocity of 5.8 km/s, most likely consists of Proterozoic – lower Paleozoic rocks.Beneath the offshore line, up to 5 km of strata with a starting velocity of 2.2 km/s and a gradient of 0.5 s−1 probably represents Tertiary–Cretaceous elastics. The underlying material with a starting velocity of 4.5 km/s and a gradient of 0.1 s−1 is interpreted as a sedimentary succession of either Cretaceous–Tertiary elastics or upper Paleozoic to Cretaceous strata. Beneath this section, a probable Proterozoic – lower Paleozoic lower crustal layer with a starting velocity of 6.2 km/s extends to about 25 km. Apparent upper mantle velocities in the 8.0–8.2 km/s range are observed.Beneath the transitional onshore–offshore line, a Neogene sedimentary basin is interpreted as being floored by faulted blocks of probably deformed Proterozoic to lower Paleozoic rocks on the landward side and possibly Cretaceous to lower Tertiary rocks on the seaward side.

scholarly journals Complex rift patterns, a result of interacting crustal and mantle weaknesses, or multiphase rifting? Insights from analogue models

Solid Earth ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 1473-1495
Author(s):  
Frank Zwaan ◽  
Pauline Chenin ◽  
Duncan Erratt ◽  
Gianreto Manatschal ◽  
Guido Schreurs
Keyword(s):  
Upper Mantle ◽  
Upper Crust ◽  
Crustal Layer ◽  
Tectonic History ◽  
Lithospheric Extension ◽  
Analogue Models ◽  
History Of ◽  
Lower Crustal ◽  
Degree Of Coupling ◽  
Over Time

Abstract. During lithospheric extension, localization of deformation often occurs along structural weaknesses inherited from previous tectonic phases. Such weaknesses may occur in both the crust and mantle, but the combined effects of these weaknesses on rift evolution remain poorly understood. Here we present a series of 3D brittle–viscous analogue models to test the interaction between differently oriented weaknesses located in the brittle upper crust and/or upper mantle. We find that crustal weaknesses usually express first at the surface, with the formation of grabens parallel to their orientation; then, structures parallel to the mantle weakness overprint them and often become dominant. Furthermore, the direction of extension exerts minimal control on rift trends when inherited weaknesses are present, which implies that present-day rift orientations are not always indicative of past extension directions. We also suggest that multiphase extension is not required to explain different structural orientations in natural rift systems. The degree of coupling between the mantle and upper crust affects the relative influence of the crustal and mantle weaknesses: low coupling enhances the influence of crustal weaknesses, whereas high coupling enhances the influence of mantle weaknesses. Such coupling may vary over time due to progressive thinning of the lower crustal layer, as well as due to variations in extension velocity. These findings provide a strong incentive to reassess the tectonic history of various natural examples.

Crustal structure of the central Sverdrup Basin

1979 ◽  
Vol 16 (8) ◽  
pp. 1581-1598 ◽  
Author(s):  
D. A. Forsyth ◽  
J. A. Mair ◽  
I. Fraser
Keyword(s):  
Crustal Structure ◽  
Upper Mantle ◽  
Seismic Energy ◽  
Major Change ◽  
Compressional Wave ◽  
Geological Structure ◽  
Significant Loss ◽  
The Arctic ◽  
Crustal Layer ◽  
Sverdrup Basin

A synthesis of refraction data recorded in 1972 and 1973 in the central Sverdrup Basin with other geophysical data shows major features which correlate well with the regional geological structure. The record sections from the Arctic Archipelago show little coherent secondary energy compared with those from other areas of Canada. Normalization of the sections to remove effects of varying shot size and instrument gain has revealed a significant loss of amplitude and coherence of the upper and mid-crustal phases of the seismic energy on traversing a major northeast-trending structure between Melville and Lougheed Islands. The upper mantle phase (Pn), however, is not abnormally attenuated in its travel beneath the area. The aeromagnetic data reveal a major series of dykes or minor graben, a likely cause of scattering and attenuation of the seismic energy travelling within the crust. These seismic effects and the focal depths of earthquakes suggest that lateral heterogeneities in the crust may extend to near-mantle depths in this area. The age dates available suggest fracture or dyke development progressed from south to north beginning in the Early Cretaceous. The correlation of the recorded seismicity with these structures provides one of the better examples of an active, intraplate tectonic feature.East of King Christian Island (KCI) the refraction results concur with gravity and regional geology in suggesting a major change in crustal and upper mantle structure. Models derived using ray theory indicate a crust which thins from near 40 km beneath the eastern Sabine Peninsula to 32 km west of KCI. East of KCI the Moho may lie at 40 km beneath a complex crustal structure. The average crustal compressional wave velocity is between 5.9 and 6.4 km s−1 and the mean upper mantle velocity is 8.2 km s−1. The present study does not support the existence of a distinct mid-crustal layer with a velocity of about 7.3 km s−1.

Plumbing and architecture of a crustal mineralizing system in the Archean Superior Province, Canada

2021 ◽  
Author(s):  
Eric Roots ◽  
Graham Hill ◽  
Ben M. Frieman ◽  
James A. Craven ◽  
Richard S. Smith ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Spatial Association ◽  
Three Dimensional ◽  
3D Models ◽  
Superior Province ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Abitibi Subprovince ◽  
Lower Crustal

<p>The role of melts and magmatic/metamorphic fluids in mineralization processes is well established. However, the role of crustal architecture in defining source and sink zones in the middle to lower crust remains enigmatic. Integration of three dimensional magnetotelluric (MT) modelling and seismic reflection data across the Archean Abitibi greenstone belt of the Superior Province, Canada, reveals a ‘whole-of-crust’ mineralizing system and highlights the controls by crustal architecture on metallogenetic processes. Electrically conductive conduits in an otherwise resistive upper crust are coincident with truncations and offsets of seismic reflections that are mostly interpreted as major brittle-ductile fault zones. The spatial association between these features and low resistivity zones imaged in the 3D models suggest that these zones acted as pathways through which fluids and melts ascended toward the surface. At mid-crustal levels, these ‘conduit’ zones connect to ~50 km long, north-south striking conductors, and are inferred to represent graphite and/or sulphide deposited from cooling fluids. At upper mantle to lower crustal depths, east-west trending conductive zones dominate and display shallow dips. The upper mantle features are broadly coincident with the surface traces of the major deformation zones with which a large proportion of the gold endowment is associated. We suggest that these deep conductors represent interconnected graphitic zones perhaps augmented by sulphides that are relicts from metamorphic fluid and melt emplacement associated primarily with the later stages of regional deformation.  Thus, from the combined MT and seismic data, we develop a crustal-scale architectural model that is consistent with existing geological and deformational models, providing constraints on the sources for and signatures of fluid and magma emplacement that resulted in widespread metallogenesis in the Abitibi Subprovince.</p>

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