The results of magnetotelluric studies on the profile of the Kuray depression – lake Teletskoye

Abstract The results of magnetotelluric studies (MTS) conducted in the western part of the Mountainous Altai region on the profile of the Kurai depression – lake Teletskoye are presented. The profile intersects two large tectonic units – the Mountainous Altai and Teletskaya, bounded by regional tectonic suturs of the Teletsko-Bashkausskiy, Northern Sayans, Shapshalskiy and other faults. The obtained data indicate a fairly fractional recent block divisibility of the Earth’s crust of the Mountainous Altai territory. It is shown that according to the characteristics of the electrical resistivity distribution within the studied profile, large blocks are distinguished that differ sharply in the features of the composition and structure of the Earth’s crust, as well as the manifestation intensity of deep processes. The sections constructed from magnetotelluric data allow us to trace the behavior of the main neotectonic disturbances, which are marked by subvertical zones with abnormally low resistivity values (1-5 Ohm·m).

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Crustal architecture of a metallogenic belt and ophiolite belt: implications for mineral genesis and emplacement from 3-D electrical resistivity models (Bayankhongor area, Mongolia)

Earth Planets and Space ◽

10.1186/s40623-021-01400-9 ◽

2021 ◽

Vol 73 (1) ◽

Author(s):

Matthew J. Comeau ◽

Michael Becken ◽

Alexey V. Kuvshinov ◽

Sodnomsambuu Demberel

Keyword(s):

Electrical Resistivity ◽

Crustal Structure ◽

Three Dimensional ◽

Suture Zone ◽

Fault System ◽

Magnetotelluric Data ◽

Low Resistivity ◽

Ophiolite Belt ◽

Geological Processes ◽

Metallogenic Belt

AbstractCrustal architecture strongly influences the development and emplacement of mineral zones. In this study, we image the crustal structure beneath a metallogenic belt and its surroundings in the Bayankhongor area of central Mongolia. In this region, an ophiolite belt marks the location of an ancient suture zone, which is presently associated with a reactivated fault system. Nearby, metamorphic and volcanic belts host important mineralization zones and constitute a significant metallogenic belt that includes sources of copper and gold. However, the crustal structure of these features, and their relationships, are poorly studied. We analyze magnetotelluric data acquired across this region and generate three-dimensional electrical resistivity models of the crustal structure, which is found to be locally highly heterogeneous. Because the upper crust (< 25 km) is found to be generally highly resistive (> 1000 Ωm), low-resistivity (< 50 Ωm) features are conspicuous. Anomalous low-resistivity zones are congruent with the suture zone, and ophiolite belt, which is revealed to be a major crustal-scale feature. Furthermore, broadening low-resistivity zones located down-dip from the suture zone suggest that the narrow deformation zone observed at the surface transforms to a wide area in the deeper crust. Other low-resistivity anomalies are spatially associated with the surface expressions of known mineralization zones; thus, their links to deeper crustal structures are imaged. Considering the available evidence, we determine that, in both cases, the low resistivity can be explained by hydrothermal alteration along fossil fluid pathways. This illustrates the pivotal role that crustal fluids play in diverse geological processes, and highlights their inherent link in a unified system, which has implications for models of mineral genesis and emplacement. The results demonstrate that the crustal architecture—including the major crustal boundary—acts as a first‐order control on the location of the metallogenic belt.

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Temporal variations in the electrical resistivity of the Earth's crust

Journal of Geophysical Research Atmospheres ◽

10.1029/jb091ib11p11618 ◽

1986 ◽

Vol 91 (B11) ◽

pp. 11618 ◽

Cited By ~ 9

Author(s):

H. F. Morrison ◽

R. Fernandez

Keyword(s):

Electrical Resistivity ◽

Temporal Variations ◽

Earth’S Crust ◽

Earth's Crust

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Geoelectric heterogeneities of the Kerch iron ore basin

Geofizicheskiy Zhurnal ◽

10.24028/gzh.v43i5.244078 ◽

2021 ◽

Vol 43 (5) ◽

pp. 165-180

Author(s):

I. Yu. Nikolaev ◽

T. K. Burakhovych ◽

A. M. Kushnir ◽

Ye. M. Sheremet

Keyword(s):

Electrical Conductivity ◽

Upper Mantle ◽

Iron Ore ◽

Earth’S Crust ◽

Crust And Upper Mantle ◽

Near Surface ◽

Low Resistivity ◽

Kerch Peninsula ◽

Mud Volcanism ◽

Earth's Crust

The three-dimensional geoelectric model of the Earth’s crust and upper mantle of the Kerch Peninsula has been built for the first time based on the results of experimental observations of the Earth’s low-frequency electromagnetic field, carried out in 2007—2013 by the Institutes of the National Academy of Sciences of Ukraine. Its physical and geological interpretation and detailing of the near-surface part were carried out according to the data of the audiomagnetotelluric sounding method to study the deep structure of the Kerch iron ore basin. To the east of the Korsak-Feodosiya fault along the southern part of the Indolo-Kuban trough (in the north of the South Kerch and almost under the entire North Kerch zones), a low-resistance anomaly (ρ=1 Ohm∙m) was found at depths from 2.5 km to 12 km about 20 km wide. Its eastern part is located in the consolidated Earth’s crust and is galvanically connected with surface sedimentary strata, while the western part is completely in sedimentary deposits. The anomaly covers the territory of the Kerch iron ore basin and occurrences of mud volcanism. The characteristics of the upper part of the layered section of the Kerch Peninsula in the interval of the first hundreds of meters were obtained from the results of one-dimensional inversion of the audiomagnetotelluric sounding data (frequency range 8—4000 Hz). It is shown that the first 15 m of the section, corresponding to Quaternary deposits, have resistivity values up to 1 Ohm∙m. Below, in the Neogene sediments, the electrical resistance increases to values of 5 Ohm∙m and more. Both horizontally and vertically, the distribution of resistivity values has a variable character, manifesting as a thin-layered structure with low resistivity values. Possibly, such areas have a direct connection with the channel for transporting hummock material and gases. A connection is assumed between the low-resistivity thin-layered near-surface areas, a deep anomaly of electrical conductivity in the upper part of the Earth’s crust, and the likely high electrical conductivity of rocks at the depths of the upper mantle with iron ore deposits, as well as the manifestation of mud volcanism. The heterogeneity of the crustal and mantle highly conductive layers may indicate a high permeability of the contact zones for deep fluids.

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Lower crustal low-resistivity zones caused by compaction-induced fluid localization and stagnation – recent results from electromagnetic data in an intracontinental setting

10.5194/egusphere-egu21-12413 ◽

2021 ◽

Author(s):

Matthew Joseph Comeau ◽

Michael Becken ◽

James A. D. Connolly ◽

Alexander Grayver ◽

Alexey V. Kuvshinov ◽

...

Keyword(s):

Electrical Resistivity ◽

Transition Zone ◽

Hydrodynamic Model ◽

Lower Crust ◽

Dislocation Creep ◽

Magnetotelluric Data ◽

Low Resistivity ◽

Vertical Extent ◽

Brittle Ductile Transition ◽

Lower Crustal

We investigate how a conceptual hydrodynamic model consisting of fluid localization and stagnation by thermally activated compaction can explain low-resistivity anomalies observed in the lower crust (>20 km depth). Electrical resistivity models, derived from magnetotelluric data collected across the intracontinental Bulnay region, a subset of a larger regional array across central Mongolia, are generated. They reveal low-resistivity (3 - 30 &#937;m) domains with a width of ~25 km and a vertical extent of <10 km in the lower crust, with their tops ~5 km below the brittle-ductile transition zone. In 3-D these features appear as laterally extended (tube-like) structures, 300 km long, rather than disconnected ellipsoids. The features are oriented parallel to the adjacent Bulnay fault zone segments and perpendicular to the far-field compressive tectonic stress (i.e., northward motion from China and Tibet). These low-resistivity domains are consistent with the presence of saline metamorphic fluids. Deeper features imaged with the data include a large upper mantle conductor that we attribute to an asthenospheric upwelling, and thin lithosphere, related to intraplate surface uplift and volcanism, in agreement with recent geodynamic modelling of lithospheric removal in this region.Based on the observed thermal structure of the crust, and assuming the mean stress at the brittle-ductile transition is twice the vertical load, the hydrodynamic model predicts that fluids would collect in zones <9 km below the brittle-ductile transition zone, and the zones would have a vertical extent of ~9 km, both in agreement with the resistivity models across the Bulnay region. The hydrodynamic model also gives plausible values for the activation energy for viscous creep (270 - 360 kJ/mol), suggesting that the mechanism is dislocation creep.From the electrical resistivity models, the lower crustal viscous compaction-length is constrained to be ~25 km - in this region. Within the conceptual model, this length-scale is entirely consistent with independent estimates for the specific hydraulic and rheological properties of this region. In fact, this can be used to independently constrain acceptable ranges for the lower crustal effective viscosity, which is found to be low (on the order of 10^18 Pas). Accordingly, the results indicate that low-salinity fluids (likely 1 - 0.01 wt% NaCl), and correspondingly low porosities (likely 5 - 0.1 vol%), are the most plausible. These key findings suggest partial melts are not favoured to explain the anomalies. Overall, the results of this contribution imply that it is tectonic and compaction processes that control lower crustal fluid flow, rather than lithological or structural heterogeneity.

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Imaging the magmatic system beneath the Krafla geothermal field, Iceland: A new 3-D electrical resistivity model from inversion of magnetotelluric data

Geophysical Journal International ◽

10.1093/gji/ggz427 ◽

2019 ◽

Vol 220 (1) ◽

pp. 541-567 ◽

Cited By ~ 4

Author(s):

Benjamin Lee ◽

Martyn Unsworth ◽

Knútur Árnason ◽

Darcy Cordell

Keyword(s):

Electrical Resistivity ◽

Volcanic Field ◽

Geothermal Field ◽

Fault System ◽

Impedance Tensor ◽

Magnetotelluric Data ◽

Near Surface ◽

Low Resistivity ◽

Rhyolite Magma ◽

Resistivity Model

SUMMARY Krafla is an active volcanic field and a high-temperature geothermal system in northeast Iceland. As part of a program to produce more energy from higher temperature wells, the IDDP-1 well was drilled in 2009 to reach supercritical fluid conditions below the Krafla geothermal field. However, drilling ended prematurely when the well unexpectedly encountered rhyolite magma at a depth of 2.1 km. In this paper we re-examine the magnetotelluric (MT) data that were used to model the electrical resistivity structure at Krafla. We present a new 3-D resistivity model that differs from previous inversions due to (1) using the full impedance tensor data and (2) a finely discretized mesh with horizontal cell dimensions of 100 m by 100 m. We obtained similar resistivity models from using two different prior models: a uniform half-space, and a previously published 1-D resistivity model. Our model contains a near-surface resistive layer of unaltered basalt and a low resistivity layer of hydrothermal alteration (C1). A resistive region (R1) at 1 to 2 km depth corresponds to chlorite-epidote alteration minerals that are stable at temperatures of about 220 to 500 °C. A low resistivity feature (C2) coincides with the Hveragil fault system, a zone of increased permeability allowing interaction of aquifer fluids with magmatic fluids and gases. Our model contains a large, low resistivity zone (C3) below the northern half of the Krafla volcanic field that domes upward to a depth of about 1.6 km b.s.l. C3 is partially coincident with reported low S-wave velocity zones which could be due to partial melt or aqueous fluids. The low resistivity could also be attributed to dehydration and decomposition of chlorite and epidote that occurs above 500 °C. As opposed to previously published resistivity models, our resistivity model shows that IDDP-1 encountered rhyolite magma near the upper edge of C3, where it intersects C2. In order to assess the sensitivity of the MT data to melt at the bottom of IDDP-1, we added hypothetical magma bodies with resistivities of 0.1 to 30 Ωm to our resistivity model and compared the synthetic MT data to the original inversion response. We used two methods to compare the MT data fit: (1) the change in r.m.s. misfit and (2) an asymptotic p-value obtained from the Kolmogorov–Smirnov (K–S) statistical test on the two sets of data residuals. We determined that the MT data can only detect sills that are unrealistically large (2.25 km3) with very low resistivities (0.1 or 0.3 Ωm). Smaller magma bodies (0.125 and 1 km3) were not detected; thus the MT data are not sensitive to small rhyolite magma bodies near the bottom of IDDP-1. Our tests gave similar results when evaluating the changes in r.m.s. misfit and the K–S test p-values, but the K–S test is a more objective method than appraising a relative change in r.m.s. misfit. Our resistivity model and resolution tests are consistent with the idea of rhyolite melt forming by re-melting of hydrothermally altered basalt on the edges of a deeper magma body.

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Analysis of the features of the spatio-temporal distribution of geoelectric inhomogeneities in the Earth’s crust and seismic events

EPJ Web of Conferences ◽

10.1051/epjconf/202125402003 ◽

2021 ◽

Vol 254 ◽

pp. 02003

Author(s):

Elena Bataleva

Keyword(s):

Temporal Distribution ◽

Physical Nature ◽

Tien Shan ◽

Earth’S Crust ◽

Seismic Events ◽

Magnetotelluric Data ◽

Geological Conditions ◽

Spatio Temporal ◽

Earth's Crust ◽

The Relationship

The paper presents the results of experiments carried out at the regime points of magnetotelluric monitoring both on the territory of the Bishkek geodynamic test site (Northern Tien Shan) and on a series of monitoring profiles in various geological conditions. Previous studies indicate the relationship of variations in the electromagnetic and seismic fields, lunisolar tidal effects, seismic regime with the processes of fracturing. The purpose of this work is to establish the features of the relationship between the spatio-temporal distribution of seismicity and the distribution of geoelectric inhomogeneities in the Earth’s crust (fault-block tectonics of the region). Based on the analysis of the results of the interpretation of magnetotelluric data (2D inversion) and new detailed seismotomographic constructions, the verification of geoelectric models was carried out, the analysis of the distribution of hypocenters of seismic events was carried out. Special attention was paid to the confinement of earthquakes to listric fault structures. The relationship between the distribution of the hypocenters of seismic events and the spatial position of the electrical conductivity anomalies is confirmed by the authors explanation of the physical nature of the identified conducting structures, based on hypotheses of fluidization and partial melt of the Earth’s crust.

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Electrical resistivity structure of the Flathead Basin in southeastern British Columbia, Canada

Canadian Journal of Earth Sciences ◽

10.1139/e90-110 ◽

1990 ◽

Vol 27 (8) ◽

pp. 1061-1073 ◽

Cited By ~ 9

Author(s):

Jagdish C. Gupta ◽

Alan G. Jones

Keyword(s):

British Columbia ◽

Electrical Resistivity ◽

Rocky Mountain ◽

Wide Band ◽

Trial And Error ◽

Magnetotelluric Data ◽

Low Resistivity ◽

Clastic Rocks ◽

Survey Line ◽

Geomagnetic Transfer Function

In June 1985, wide-band magnetotelluric data were acquired at 12 equally spaced sites along a 30 km profile crossing the Flathead (Kishenehn) Basin in southeastern British Columbia, Canada. These data have been modelled by both one-dimensional inverse techniques and two-dimensional forward trial-and-error fitting. The results indicate the presence in the area of the following three major zones of low electrical resistivity (10–500 Ω∙m):1. Sediments of the 10 km wide Flathead sedimentary basin, extending to a depth of about 2 km, dominate the responses in the middle of the profile.2. In the eastern part of the profile, in the area of the Lewis Range, a thin [Formula: see text] zone of low resistivity (35 Ω∙m) is imaged at a depth of some 3 km extending eastward from the edge of the basin. We associate this zone with the less dense thrusted Mesozoic clastic rocks lying directly below the Proterozoic rocks of the Lewis thrust sheet.3. Beneath the Flathead Basin is a third zone, of higher resistivity (500 Ω∙m), which extends to deep within the crust. This zone may originate from mantle upflow, as recently proposed to explain the existence of Cordilleran conductors in other localities. Additionally, to model the long-period geomagnetic transfer function responses, we are required to postulate the existence of a zone of low resistivity in the middle to lower crust, 50 km west of the survey line, corresponding to the location of the Rocky Mountain Trench.

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