Imaging the mantle lithosphere of the Precambrian Grenville Province: large-scale electrical resistivity structures (original) (raw)
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Canadian Journal of Earth Sciences, 2005
Magnetotelluric (MT) measurements to image the three-dimensional resistivity structure of the North American continent from an Archean core to a region of Tertiary assembly were recorded at almost 300 sites along 3200 km of profiles on the Lithoprobe Slave Northern Cordillera Lithospheric Evolution (SNORCLE) transect in northwestern Canada. At the largest scale, the MT results indicate significant lithospheric thickness variation, from 260 km at the southwest margin of the Slave craton to significantly < 100 km at the southwestern end of the SNORCLE transect in the Cordillera. At intermediate scale, the resistivity results allow broad terrane subdivisions to be made. Several anomalously conductive zones along the SNORCLE transect, in rocks ranging in age from Archean to Tertiary, are attributed to the introduction of either water or carbon into the crust and mantle during subduction processes. At the local scale, the MT data image two major faults crossing the study area, the G...
Journal of Geophysical Research: Solid Earth, 2015
Magnetotelluric (MT) responses at the Proterozoic Grenville Front in Canada have been interpreted as being caused by lithospheric electrical anisotropy, and the area is often noted as a classic example of lithospheric anisotropy. This study reevaluates evidence for the electrical anisotropy using 56 MT stations. The spatially uniform MT responses noted at the Grenville Front extend to~200 km southeast and for at least 400 km along strike and are associated with rocks at less than 150 km depth. Examination of induction arrows at longer periods shows arrows at high angle to the MT conductive direction consistent with the presence of macroscopic resistivity structures. New 2-D anisotropic inversions show that electrical anisotropy is not required to fit the MT data. The results indicate that in the resistive mantle lithosphere beneath the Grenville Front, and in conductive lithosphere in adjacent areas, the maximum horizontal resistivity anisotropy is <10%, much less than the factor of 15 determined in earlier 1-D studies. The results suggest that the upper lithospheric mantle in the area is devoid of significant electrical anisotropy and that the observed MT response directionality is due to large-scale resistivity structure. We interpret the spatially consistent MT responses observed at the Grenville Front as being associated with the resistive Archean lithosphere extending southeast beneath the Grenville Front. The obliquity between seismic and MT responses arises because the Superior fabric is oblique to the seismic fast direction. If dextral shearing occurred, it appears to have not caused any significant shape preferred electrical anisotropy.
Geophysical Journal International, 2014
The Precambrian basement rocks in southern Alberta are hidden beneath the Western Canada Sedimentary Basin, making studies of these rocks dependent on geophysical measurements. Magnetotelluric (MT) data were used to study the structure of these basement rocks through measurements of electrical resistivity. Long-period MT data collected in Southern Alberta during the Lithoprobe project were combined with new data to produce a grid of data that permitted a 3-D approach to data analysis. Dimensionality analysis suggested that data at periods less than 1000 s were relatively 2-D. However, 2-D inversion models of MT data in Alberta resulted in low resistivity features in the crust which moved dependant on the data included in the inversion. These features were previously attributed to crustal anisotropy. 3-D inversion yielded a resistivity model that fit the measured MT data and was well correlated with both the Precambrian domain boundaries and interpretations of other geophysical data. This MT data set defines a major upper-mantle conductor coincident with the Archean Loverna Block of the Hearne Domain. This anomaly is called the Loverna Conductor, and its southern boundary is defined by a pronounced increase in upper-mantle resistivity along the Vulcan Structure, which is an approximately 300-km-long linear potential field anomaly completely buried beneath the western Canada Sedimentary Basin. Since the lithosphere in this region was assembled ca. 1.9-1.8 Ga, the low resistivity anomaly in the upper mantle is not associated with recent tectonic activity. The Loverna Conductor was likely formed by the enrichment of the lithospheric mantle through subduction along the Vulcan Structure during the Proterozoic assembly of Laurentia. In particular, this model is consistent with recent interpretations which attribute the origin of the Vulcan Structure to collision along a north dipping subduction zone.
Journal of Geophysical Research: Solid Earth, 2014
New magnetotelluric soundings at 64 locations throughout the central Rae craton on mainland Nunavut constrain 2-D resistivity models of the crust and lithospheric mantle beneath three regional transects. Responses determined from colocated broadband and long-period magnetotelluric recording instruments enabled resistivity imaging to depths of > 300 km. Strike analysis and distortion decomposition on all data reveal a regional trend of 45-53°, but locally the geoelectric strike angle varies laterally and with depth. The 2-D models reveal a resistive upper crust to depths of 15-35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries. Surface projections of the conductive layer coincide with areas of high grade, Archean metasedimentary rocks. Tectonic burial of these rocks and thickening of the crust occurred during the Paleoproterozoic Arrowsmith (2.3 Ga) and Trans-Hudson orogenies (1.85 Ga). Overall, the uppermost mantle of the Rae craton shows resistivity values that range from 3000 Ω m in the northeast (beneath Baffin Island and the Melville Peninsula) to~10,000 Ω m beneath the central Rae craton, to >50,000 Ω m in the south near the Hearne Domain. Near-vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of Hudsonian granites. A regional decrease in resistivities to values of~500 Ω m at depths of 180-220 km, increasing to 300 km near the southern margin of the Rae craton, is interpreted as the lithosphere-asthenosphere boundary.
Journal of Geophysical Research, 2004
1] Two goals of Lithoprobe's geoscientific studies in the Phanerozoic accretionary cordillera of western North America were to define the subsurface geometries of the terranes and to infer the physical conditions of the crust. These questions were addressed in Canada's southern cordillera a decade ago and have more recently been addressed in the northern cordillera, of which one component of the new studies is magnetotelluric (MT) profiling from ancestral North American rocks to the coast. We present a resistivity cross section, and its interpretation, of the northern cordillera derived from modeling data from 42 MT sites along a 470-km-long NE-SW profile. Beneath the Coast Belt (southwestern end of the profile) a deep crustal low-resistivity layer dips inland; we interpret the crustal part of this conductor as being due to metasedimentary rocks emplaced and metamorphosed during Paleocene Kula plate subduction. A strong lateral transition in lithospheric mantle resistivity exists below the Intermontane Belt that is spatially coincident with changes in chemical and isotopic characteristics of Tertiary to recent alkaline lavas, suggesting that isotopically enriched lithosphere related to the Coast Belt basalts extends partly beneath the Intermontane Belt. The unusually high lower crustal resistivity in the Intermontane and Omineca Belts, similar in value to the resistivity found in the unextended part of central British Columbia, excludes the presence of fluids or conducting metasediments. Finally, our resistivity model displays strong lateral variation of the middle and lower crust between different terranes within the same belt, as a result of the complex structural evolution of the lithosphere.
Magnetotelluric soundings, structure, and fluids in the southern Canadian Cordillera
Canadian Journal of Earth Sciences, 1992
The Cordillera of western Canada lies in a region of oceanic and island-arc lithosphere accreted to North America during subductions of the last 200 Ma. Magnetometer arrays have shown the crust of the region to be highly conductive. Magnetotelluric (MT) soundings across the Intermontane and Omineca tectonic belts between 50°N and 54"N reveal structure in terms of electrical resistivity. Pseudosections of phase and apparent resistivity and preliminary resistivity -depth sections are shown for three transects. The resistivity range is from less than one ohm metre to several thousands of ohm metres. In old continental shields, crustal resistivities cover a similar four-decade range transposed up two decades, i.e., 1O2-lo6 Q . m. We show that the observed resistivities can be produced by water with NaCl and (or) C 0 2 in solution, at the high temperatures of the Cordilleran crust, in fractured rock of effective porosity 4-5%. The resistivity variations may represent varying fracture densities. By following structures from outcrops we infer that the more resistive rocks are probably granitoid plutons, with low fracture densities. The highly conductive basalts probably have higher fracture densities. Sections and phase maps indicate that granitoid plutons continue from the Coast Plutonic Complex, under a thin layer of basalt, across the southwestern half of the Intermontane Belt. Near the centre of the Intermontane Belt, in line with the Fraser fault system, highly conductive rock continues from the surface at least to midcrustal depths. Resistivities as low as 1 Q . m in the uppermost crust under the Cariboo Mountains, in the Omineca Belt, are ascribed to intense fracturing or mineralization. For the southernmost transect, between 50°N and 51°N, a phase pseudosection shows informative resemblances to the sections farther north. Resistivity-depth inversions at seven sites from six-decade MT data give penetration into the upper mantle, but some of these sites may be affected by static shift. All results fit the mantle upflow hypothesis advanced earlier by Gough.
Journal of Geophysical Research: Solid Earth, 1979
A combination of electrical sounding methods has been used to study the vertical resistivity structure of the crust on the southern extension of the Canadian Shield in northern Wisconsin. Direct current dipole-dipole resistivity soundings were made at transmitterreceiver separations of 1 m to 40 km. Electromagnetic transient soundings were made at ranges of 5 to 40 km and with frequency bandwidths of about 0.5 to 10 Hz. The soundings were made in a region where the gross subsurface structure is laterally uniform, so horizontal, plane-layered models were used to interpret the data. Layered earth models were randomly generated and tested against the observed data to determine the range of models that fit the data. A four-layer model with the following range for the resistivities and thicknesses fits the data: (1) A surface layer, comprised mainly of glacial till, has a few hundred ohm meters resistivity down to depths of a few tens of meters. (2) A bedrock layer has a resistivity in. the range of 3000 to 7000 ohm m down to depths of 4.5 to 11 kin. (3) A deeper, high-resistivity layer has resistivities of greater than 100,000 ohm m down to depths of 14 to 22 km. (4) A lower layer has resistivities of from 50 to 1500 ohm m. The Dowling [ 1968, 1970 ] made natural-source magnetotelluric (MT) measurements on the southern .1Now at Continental Oil Company, Exploration
Geophysical Journal International, 2014
Three magnetotelluric (MT) profiles in northwestern Canada cross the central and western segments of Great Slave Lake shear zone (GSLsz), a continental scale strike-slip structure active during the Slave-Rae collision in the Proterozoic. Dimensionality analysis indicates that (i) the resistivity structure is approximately 2-D with a geoelectric strike direction close to the dominant geological strike of N45 • E and that (ii) electrical anisotropy may be present in the crust beneath the two southernmost profiles. Isotropic and anisotropic 2-D inversion and isotropic 3-D inversions show different resistivity structures on different segments of the shear zone. The GSLsz is imaged as a high resistivity zone (>5000 m) that is at least 20 km wide and extends to a depth of at least 50 km on the northern profile. On the southern two profiles, the resistive zone is confined to the upper crust and pierces an east-dipping crustal conductor. Inversions show that this dipping conductor may be anisotropic, likely caused by conductive materials filling a network of fractures with a preferred spatial orientation. These conductive regions would have been disrupted by strike-slip, ductile deformation on the GSLsz that formed granulite to greenschist facies mylonite belts. The predominantly granulite facies mylonites are resistive and explain why the GSLsz appears as a resistive structure piercing the east-dipping anisotropic layer. The absence of a dipping anisotropic/conductive layer on the northern MT profile, located on the central segment of the GSLsz, is consistent with the lack of subduction at this location as predicted by geological and tectonic models.
Canadian Journal of Earth Sciences, 2014
Over the last 30 years, through Lithoprobe and other programmes, modern, high-quality magnetotelluric (MT) measurements probing deep into the lithosphere and underlying asthenosphere have been made at over 6000 sites across Canada in all provinces and territories, except Nova Scotia. Some regions are well covered, particularly Alberta, southern British Columbia, and western Ontario, whereas others remain poorly covered, such as Quebec and large swaths of Nunavut. Prior publications from individual studies have added significantly to the wealth of Canada's geoscience knowledge, and have demonstrated that MT can contribute significantly to understanding of the tectonic processes that have shaped Canada. However, to date no continent-scale maps of lithospheric electrical parameters have been constructed from the extensive MT database. Herein we review the contributions made by the MT components of Lithoprobe, and present new continental-scale maps of various electrical parameters at crustal and upper mantle depths for the whole of Canada. From those maps, combined with regional estimates of temperature, we develop derivative information on petrological-geophysical properties, including predictions of temperature and water content. We find that at 100 km depth the Canadian Shield is cold and dry, and the Cordillera is warmer but mostly dry, i.e., little water is present in the peridotite. Exceptions are beneath the Prairies, the Wopmay Orogen, and northeast Nunavut where there does appear to be water in the nominally anhydrous minerals. Also, southwest British Columbia appears colder than the rest of the Cordillera due to the subducting Juan de Fuca plate. In contrast, at 200 km depth almost all of Canada is dry.