Deep electrical structure of northern Alberta (Canada): implications for diamond exploration

2009 ◽  
Vol 46 (2) ◽  
pp. 139-154 ◽  
Author(s):  
Erşan Türkoğlu ◽  
Martyn Unsworth ◽  
Dinu Pana
Keyword(s):  
Subduction Zone ◽  
Upper Mantle ◽  
Lithospheric Mantle ◽  
Three Dimensional ◽  
Diamond Formation ◽  
Magnetotelluric Data ◽  
Diamond Exploration ◽  
Upper Mantle Structure ◽  
Resistivity Model ◽  
Northern Alberta

Geophysical studies of upper mantle structure can provide constraints on diamond formation. Teleseismic and magnetotelluric data can be used in diamond exploration by mapping the depth of the lithosphere–asthenosphere boundary. Studies in the central Slave Craton and at Fort-à-la-Corne have detected conductors in the lithospheric mantle close to, or beneath, diamondiferous kimberlites. Graphite can potentially explain the enhanced conductivity and may imply the presence of diamonds at greater depth. Petrologic arguments suggest that the shallow lithospheric mantle may be too oxidized to contain graphite. Other diamond-bearing regions show no upper mantle conductor suggesting that the correlation with diamondiferous kimberlites is not universal. The Buffalo Head Hills in Alberta host diamondiferous kimberlites in a Proterozoic terrane and may have formed in a subduction zone setting. Long period magnetotelluric data were used to investigate the upper mantle resistivity structure of this region. Magnetotelluric (MT) data were recorded at 23 locations on a north–south profile extending from Fort Vermilion to Utikuma Lake and an east–west profile at 57.2°N. The data were combined with Lithoprobe MT data and inverted to produce a three-dimensional (3-D) resistivity model with the asthenosphere at 180–220 km depth. This model did not contain an upper mantle conductor beneath the Buffalo Head Hills kimberlites. The 3-D inversion exhibited an eastward dipping conductor in the crust beneath the Kiskatinaw terrane that could represent the fossil subduction zone that supplied the carbon for diamond formation. The low resistivity at crustal depths in this structure is likely due to graphite derived from subducted organic material.

Electromagnetic images of the Trans-Hudson orogen: the North American Central Plains anomaly revealed

2005 ◽  
Vol 42 (4) ◽  
pp. 457-478 ◽  
Author(s):  
Alan G Jones ◽  
Juanjo Ledo ◽  
Ian J Ferguson
Keyword(s):  
Electrical Properties ◽  
North American ◽  
Upper Mantle ◽  
Lithospheric Mantle ◽  
Three Dimensional ◽  
Conductivity Anomaly ◽  
Central Plains ◽  
Slave Craton ◽  
The North ◽  
Kimberlite Field

Magnetotelluric studies of the Trans-Hudson orogen over the last two decades, prompted by the discovery of a significant conductivity anomaly beneath the North American Central Plains (NACP), from over 300 sites yield an extensive database for interrogation and enable three-dimensional information to be obtained about the geometry of the orogen from southern North Dakota to northern Saskatchewan. The NACP anomaly is remarkable in its continuity along strike, testimony to along-strike similarity of orogenic processes. Where bedrock is exposed, the anomaly can be associated with sulphides that were metamorphosed during subduction and compression and penetratively emplaced deep within the crust of the internides of the orogen to the boundary of the Hearne margin. A new result from this compilation is the discovery of an anomaly within the upper mantle beginning at depths of ~80–100 km. This lithospheric mantle conductor has electrical properties similar to those for the central Slave craton mantle conductor, which lies directly beneath the major diamond-producing Lac de Gras kimberlite field. While the Saskatchewan mantle conductor does not directly underlie the Fort à la Corne kimberlite, which is associated with the Sask craton, the spatial correspondence is close.

Upper mantle structure of the northern Cascadia subduction zone

1995 ◽  
Vol 32 (1) ◽  
pp. 1-12 ◽  
Author(s):  
M. G. Bostock ◽  
J. C. Vandecar
Keyword(s):  
British Columbia ◽  
Subduction Zone ◽  
Upper Mantle ◽  
Puget Sound ◽  
Cascadia Subduction Zone ◽  
Mantle Structure ◽  
Data Set ◽  
Juan De Fuca Plate ◽  
Upper Mantle Structure ◽  
Juan De Fuca

Previous knowledge of the structure of the Cascadia subduction zone north of the Canada–United States border has been derived from a variety of geophysical studies that accurately delineated the downgoing Juan de Fuca plate from the offshore deformation front to depths of ~50–60 km beneath south-central Vancouver Island and the Georgia Strait. Little is known, however, of the structure of the Cascadia subduction zone farther westward and to greater depths in the upper mantle. We have assembled a set of some 1100 teleseismic traveltimes from events recorded on the Western Canadian Telemetered Network to augment a previously existing data set recorded on the Washington Regional Seismograph Network. The composite data set is inverted for upper mantle structure below Washington, Oregon, and southwestern British Columbia. We analyze the new northern portion of the model between 48.5–50°N and 118–127°W, which provides the first images of the deep slab structure in this region. The model is parameterized using splines under tension over a dense grid of knots. The nonlinearity of the inverse problem is treated by iteratively performing three-dimensional ray tracing and linear inversion. Resolution tests performed with a synthetic slab model indicate that the deep structure is resolved by the data north to at least 50°N. The inversions are characterized by a quasi-planar, high-velocity body inferred to represent the thermal and compositional anomaly of the subducted Juan de Fuca plate. This body exhibits velocity deviations of up to 3% from the background reference model and extends to depths of at least 400–500 km. The depth contours of the slab in the upper mantle mimic those of the shallow slab by changing strike, in the latitude range 48.0–48.5°N, from north–south in Washington to northwest–southeast in southern British Columbia. This forces the development of two arch-type structures: a main arch observed in previous studies trending east–west over Puget Sound and a possible second arch extending northeasterly from the Georgia Strait into the British Columbia interior. A steepening of the deep slab dip from British Columbia south towards Puget Sound and complexity in the evolution of the arches in depth may be the result of a change in plate motions at 3.5 Ma associated with the detachment of the Explorer plate.

scholarly journals Fluid Transport and Storage in the Cascadia Forearc inferred from Magnetotelluric Data

2021 ◽  
Author(s):  
Gary Egbert ◽  
Bo Yang ◽  
Paul A. Bedrosian ◽  
Kerry Key ◽  
Dean Livelybrooks ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Subduction Zone ◽  
Fluid Transport ◽  
Critical Role ◽  
Three Dimensional ◽  
Oceanic Lithosphere ◽  
Cascadia Subduction Zone ◽  
Plate Interface ◽  
Magnetotelluric Data ◽  
And Storage

Abstract Subduction of hydrated oceanic lithosphere can carry water deep into the Earth, with important consequences for a range of tectonic and magmatic processes. Most fluid is released at relatively shallow depths in the forearc where it is thought to play a critical role in controlling mechanical properties and seismic behavior of the subduction megathrust. Here we present results from three-dimensional inversion of nearly 400 long-period magnetotelluric sites, including 64 offshore, to provide new insights into the distribution of fluids in the forearc of the Cascadia subduction zone. Our amphibious dataset provides new constraints on the geometry of the electrically resistive Siletzia terrane, a thickened section of oceanic crust accreted to North America in the Eocene, and the conductive accretionary complex, which is being underthrust all along the margin. Fluids accumulate, over time-scales likely exceeding 1 My, above the plate interface in metasedimentary units, while the mafic rocks of Siletzia remain dry. Fluids in metasediments tend to peak at fixed slab-depths of 17.5 and 30 km, suggesting control by metamorphic processes, but also concentrate around the edges of Siletzia, suggesting that this mafic block is impermeable, with dehydration fluids escaping up-dip along the megathrust. Our results demonstrate that lithology of the overriding crust can play a critical role in controlling fluid transport and sequestration in a subduction zone, with potentially important implications for mechanical properties.

Seismogenic structures in western Yunnan revealed by three-dimensional magnetotelluric imaging

2020 ◽  
Author(s):  
Qinghua Huang ◽  
Tao Ye ◽  
Xiaobin Chen
Keyword(s):  
Three Dimensional ◽  
Magnetotelluric Data ◽  
Block Boundary ◽  
Strong Earthquakes ◽  
Yunnan Region ◽  
Continental Block ◽  
Western Yunnan ◽  
Resistivity Model ◽  
Seismogenic Structures ◽  
Magnetotelluric Imaging

<p>Influenced by the extrusion of Tibetan blocks and Indo-Burmese collision, the region in western Yunnan is associated with active seismicity and Quaternary volcanoes. Based on broadband magnetotelluric data collected in western Yunnan, we obtain a three-dimensional crustal electrical resistivity model after various data processing and three-dimensional inversion test. The above resistivity model reveals the seismogenic structures of the moderate and strong earthquakes in this tectonic region. We investigate the possible relationship between the seismicity and the electrical structure in western Yunnan region. The results indicate that earthquakes in this region tend to occur in the transition zone between the resistive and conductive structures. Our results also show that one resistive body imaged at the mid-lower crust may have blocked the previously proposed crustal channel flow along this intra-continental block boundary to the east of Tibetan Plateau. Our resistivity model suggests a bifurcation of the crustal flow in western Yunnan. This bifurcated crustal flow structure may play an important dynamical role in the seismogenesis of the earthquakes in western Yunnan.</p>

scholarly journals Magnetotelluric multiscale 3-D inversion reveals crustal and upper mantle structure beneath the Hangai and Gobi-Altai region in Mongolia

2020 ◽  
Vol 221 (2) ◽  
pp. 1002-1028 ◽  
Author(s):  
J S Käufl ◽  
A V Grayver ◽  
M J Comeau ◽  
A V Kuvshinov ◽  
M Becken ◽  
...  
Keyword(s):  
Electrical Conductivity ◽  
Data Acquisition ◽  
Upper Mantle ◽  
Spatial Scales ◽  
Mantle Structure ◽  
Central Mongolia ◽  
Altai Region ◽  
Upper Mantle Structure ◽  
Resistivity Model ◽  
And Inversion

SUMMARY Central Mongolia is a prominent region of intracontinental surface deformation and intraplate volcanism. To study these processes, which are poorly understood, we collected magnetotelluric (MT) data in the Hangai and Gobi-Altai region in central Mongolia and derived the first 3-D resistivity model of the crustal and upper mantle structure in this region. The geological and tectonic history of this region is complex, resulting in features over a wide range of spatial scales, which that are coupled through a variety of geodynamic processes. Many Earth properties that are critical for the understanding of these processes, such as temperature as well as fluid and melt properties, affect the electrical conductivity in the subsurface. 3-D imaging using MT can resolve the distribution of electrical conductivity within the Earth at scales ranging from tens of metres to hundreds of kilometres, thereby providing constraints on possible geodynamic scenarios. We present an approach to survey design, data acquisition, and inversion that aims to bridge various spatial scales while keeping the required field work and computational cost of the subsequent 3-D inversion feasible. MT transfer functions were estimated for a 650 × 400 km2 grid, which included measurements on an array with regular 50 × 50 km2 spacing and along several profiles with a denser 5–15 km spacing. The use of telluric-only data loggers on these profiles allowed for an efficient data acquisition with a high spatial resolution. A 3-D finite element forward modelling and inversion code was used to obtain the resistivity model. Locally refined unstructured hexahedral meshes allow for a multiscale model parametrization and accurate topography representation. The inversion process was carried out over four stages, whereby the result from each stage was used as input for the following stage that included a finer model parametrization and/or additional data (i.e. more stations, wider frequency range). The final model reveals a detailed resistivity structure and fits the observed data well, across all periods and site locations, offering new insights into the subsurface structure of central Mongolia. A prominent feature is a large low-resistivity zone detected in the upper mantle. This feature suggests a non-uniform lithosphere-asthenosphere boundary that contains localized upwellings that shallow to a depth of 70 km, consistent with previous studies. The 3-D model reveals the complex geometry of the feature, which appears rooted below the Eastern Hangai Dome with a second smaller feature slightly south of the Hangai Dome. Within the highly resistive upper crust, several conductive anomalies are observed. These may be explained by late Cenozoic volcanic zones and modern geothermal areas, which appear linked to mantle structures, as well as by major fault systems, which mark terrane boundaries and mineralized zones. Well resolved, heterogeneous low-resistivity zones that permeate the lower crust may be explained by fluid-rich domains.

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