Revisiting Toba Caldera: an insight from regional magnetotelluric data

2020 ◽  
Author(s):  
Lukman Sutrisno ◽  
Fred Beekman ◽  
Yunus Daud ◽  
Jan Diederik Van Wees
Keyword(s):  
Fault System ◽  
High Resistivity ◽  
Magnetotelluric Data ◽  
Recording Time ◽  
Magma Plumbing System ◽  
Basement Structures ◽  
Clay Cap ◽  
Surface Geology ◽  
Toba Caldera ◽  
Sumatran Fault

<p>Regional magnetotelluric (MT) survey had been conducted to image resistivity structures beneath Toba Caldera, Indonesia. A crustal-scale 2D inversion model is generated from ten MT stations with extended recording time, deployed along NE-SW regional line to cross perpendicularly both the Caldera and the nearby regional strike-slip fault system, the Sumatran Fault. High resistivity background is likely related to Palaeozoic rocks which is basement of the Tertiary sediments and the Quaternary volcanics. The most noticeable conductive anomaly is located between 10-20 km deep, interpreted as the main magma reservoir beneath the region. An intermediate, less than 10 km-deep, less conductive anomaly beneath the Caldera is interpreted as shallow magma chamber affected by the last major eruption. Shallow, less than 2 km-deep conductive layers are associated either with hydrothermal clay cap beneath the Caldera, or sedimentary formations of the nearby basins. Other conductive anomaly is spatially associated with the Sumatran Fault which located 15 km away from the Caldera. Parameter plots of some stations are consistent with the orientation of basement structures, while the others may be affected by more complex caldera structures. A conceptual model of magma plumbing system beneath the Caldera is then interpreted from the combination of regional resistivity structures, surface geology, and available seismic tomography.</p>

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

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.

Magnetotelluric observations in the western Ouachita Mountains, southeastern Oklahoma

Geophysics ◽  
1999 ◽  
Vol 64 (6) ◽  
pp. 1680-1688 ◽  
Author(s):  
Kevin L. Mickus
Keyword(s):  
Drill Hole ◽  
High Resistivity ◽  
Geologic Mapping ◽  
Gulf Coastal Plain ◽  
Ouachita Mountains ◽  
Arkoma Basin ◽  
Magnetotelluric Data ◽  
Low Resistivity ◽  
Northern Border ◽  
Seismic Reflection Profiles

The first magnetotelluric (MT) analysis of the Ouachita Mountains region is presented. Magnetotelluric data acquired at 19 sites along a 60-km profile in southeastern Oklahoma were used to image the western extension of the Ouachita Mountains and to determine the poorly known subsurface interaction between the Pennsylvanian Tishomingo‐Belton uplift and the subsurface extension of the exposed western Ouachita Mountains. Drill‐hole data, geologic mapping, seismic reflection profiles, and 1-D and 2-D MT-derived models indicate that lying beneath the low‐resistivity Gulf Coastal Plain sediments are 2–3 km of deep‐water lower Pennsylvanian (Jackfork Group) sediments and 6–8 km of Ouachita facies lithologies, mainly consisting of the Stanley Group. Beneath the profile’s northern section are 2–4 km of Atoka Formation sediments, probably deposited within the Arkoma basin, that underlie thrusted zones of the Stanley Group. The most unique feature is a high‐resistivity zone beneath stations 7–9, interpreted to be Precambrian/Cambrian granite similar to that exposed in the Tishomingo‐Belton uplift. A deep (5–6 km) low‐resistivity zone that may represent the northern border of the subsurface extension of the Broken Bow uplift is located along the Texas/Oklahoma border; however, this zone is not required by the MT data.

scholarly journals Co-seismic offsets due to two earthquakes (M w 6.1) along the Sumatran fault system derived from GNSS measurements

2016 ◽  
Vol 68 (1) ◽  
Author(s):  
Takeo Ito ◽  
Endra Gunawan ◽  
Fumiaki Kimata ◽  
Takao Tabei ◽  
Irwan Meilano ◽  
...  
Keyword(s):  
Fault System ◽  
Gnss Measurements ◽  
Sumatran Fault

scholarly journals Distribution of slip at the northern Sumatran fault system

2000 ◽  
Vol 105 (B12) ◽  
pp. 28327-28341 ◽  
Author(s):  
J. F. Genrich ◽  
Y. Bock ◽  
R. McCaffrey ◽  
L. Prawirodirdjo ◽  
C. W. Stevens ◽  
...  
Keyword(s):  
Fault System ◽  
Sumatran Fault

An application of the second derivative as a tool in tectonic analysis in the Qattara Depression area, Egypt

1986 ◽  
Vol 123 (3) ◽  
pp. 307-313
Author(s):  
A. El-Hussaini ◽  
M. Youssef ◽  
H. Ibrahim
Keyword(s):  
Gravity Anomalies ◽  
Horizontal Displacement ◽  
Fault System ◽  
Second Derivative ◽  
Vertical Movements ◽  
Very Old ◽  
First Order ◽  
Basement Structures

AbstractThe second derivative of gravity anomalies of the Qattara area was analysed and statistically studied for determining the tectonic elements. Zones of zero second derivative were considered as the locations of possible faults. The analysis of a constructed tectonic map portrays the predominance of N45°W, N85°E and N45°E fault trends in addition to less pronounced N15°E and N–S faults. The NW–SE faults are very old and inherited from the basement structures. They acted as first order right-lateral wrench faults during the Alpine tectonism. Second and higher orders of faults, developed as a consequence of these movements, are represented by the N85°E and other less abundant trends. Vertical movements along the existing fault system, in addition to the horizontal displacement, is supported by the analysis of the pronounced anomalies of the second derivative map. The subsurface structural picture of the area is composed of uplifted and downfaulted adjoining blocks.

scholarly journals Identification of "WS" geothermal field system by analyzing TE, TM, and TE-TM of 2D magnetotelluric inversion models

2019 ◽  
Vol 1 (2) ◽  
pp. 41
Author(s):  
Triana Triana ◽  
Tony Yulianto ◽  
Udi Harmoko ◽  
Iqbal Takodama
Keyword(s):  
Geothermal Field ◽  
Reservoir Rock ◽  
High Resistivity ◽  
Geothermal System ◽  
Magnetotelluric Data ◽  
Low Resistivity ◽  
A Value ◽  
Te Mode ◽  
Tm Mode ◽  
Representative Model

Magnetotelluric data has been carried out at the "WS" geothermal field to analyze the resistivity model resulting from 2D inversion of magnetotelluric data in TE, TM and TE-TM modes. Base on the three models produced, the mode is determined to produce the most representative model to assist in the interpretation of the "WS" geothermal system. There is a step of modes separation, namely TE (Tranverse Electric) and TM (Transverse Magnetic) modes in processing MT data. Each mode produces a 2D model with different conductivity properties. The analysis results of the three modes explain that TE mode is dominated by low resistivity with a range of values of 10-35 Ωm and medium resistivity with a value range of 35-250 Ωm and a vertical resistivity contrast. The TM mode describes the high resistivity in the Southwest and the center of the track with a value of more than 470 sehinggam resulting in lateral resistivity contrast. While the TE-TM mode produces a model that is not much different from TM mode, only the distribution of the resistivity value is a combination with TE mode. This mode describes the distribution of resistivity both vertically and laterally. Based on the analysis of the three modes, it can be concluded that the TE-TM mode is the mode that produces the most representative model. Interpretation model shows that from the TE-TM mode we have a low resistivity distribution (10-35 Ωm) represent a cap rock zone, reservoir rock with a medium resistivity distribution (35-380 Ωm), resistive zone with a high resistivity distribution (more than 380 Ωm), and the existence of the three of faults structures ro be a controller system of the "WS" geothermal.

scholarly journals Crustal Electrical Structure of the Zhaheba Complex Imaged by Magnetotelluric Data and Its Tectonic Implications

2021 ◽  
Vol 11 (21) ◽  
pp. 10013
Author(s):  
Pingchuan Zhang ◽  
Changqing Yu ◽  
Xiangzhi Zeng
Keyword(s):  
Junggar Basin ◽  
High Resistivity ◽  
Precambrian Basement ◽  
Structure And Properties ◽  
Uppermost Mantle ◽  
Magnetotelluric Data ◽  
Northern Margin ◽  
Electrical Structure ◽  
Low Resistivity ◽  
Intrusive Rocks

A Magnetotelluric profile stretching northward from the Wulungu Depression (on the northern margin of the Junggar Basin) to the Dulate arc (crossing the Zhaheba–Aermantai ophiolite belt) was carried out in an attempt to probe the crustal structure and properties of the East Junggar, NW China. Along the profile, the inversion model was used to determine the electrical structure of the crust and uppermost mantle. The results revealed that the crust of the eastern Junggar Basin is composed of the shallow low resistivity layer and underlying high resistivity bodies. There is a crustal detachment in the basement: the upper layer is a Hercynian folded basement and the lower is a Precambrian basement. The Zhaheba complex is characterized by relatively high resistivity, with a thickness of ~5 km, the bottom controlled by the Zhaheba–Aermantai fault. The crust of the Yemaquan arc is composed of the residual continental crust, characterized by stable resistance. The exposed intrusive rocks are characterized by irregular resistors. The crust of the Dulate arc is characterized by relatively low resistivity. The shallow low resistivity layers represent the Zhaheba depression composed of the Devonian-Permian volcanic and sedimentary rocks. The crustal conductive anomalies are related to the magmatism and mechanism of metal deposits in the post-collision period.

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

2019 ◽  
Vol 220 (1) ◽  
pp. 541-567 ◽  
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.

Geophysical characterization of the Menengai volcano, Central Kenya Rift from the analysis of magnetotelluric and gravity data

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. B187-B199 ◽  
Author(s):  
Antony Munika Wamalwa ◽  
Kevin L. Mickus ◽  
Laura F. Serpa
Keyword(s):  
Gravity Data ◽  
Gravity Models ◽  
High Resistivity ◽  
Geothermal System ◽  
Low Density ◽  
Fracture Density ◽  
Density Region ◽  
Magnetotelluric Data ◽  
Low Resistivity ◽  
2D Resistivity

In this study, we qualitatively analyze detailed gravity and broadband magnetotelluric data in and surrounding the Menengai volcano of the East African rift in Kenya to assess geothermal potential of the region. Three-dimensional gravity models obtained by inverting residual gravity anomalies and 2D resistivity models obtained by inverting the transverse electric and transverse magnetic magnetotelluric modes show several common features. Our models show that a low-resistivity zone above a higher resistivity zone correlates with a low-density region located 1–4 km beneath the volcano. These zones may be related to a high temperature gradient or hydrothermally altered, fractured rocks. Additionally, a low-resistivity ([Formula: see text]) and a low-density region located approximately 4–6 km below the volcano may be related to molten material that is the source of heat for the geothermal system. The low-resistivity ([Formula: see text]) regions that correlated with a denser ([Formula: see text]) region within the caldera are bounded by high-resistivity ([Formula: see text]), high-density ([Formula: see text]) volcanic units implying that the dense and electrically resistive volcanic material is relatively cool and lacks significant fluid content that can lower resistivity. At shallow depths, 0.5–1.5 km below the caldera, a low-resistivity and low-to-moderate density region is interpreted as a zone with high fracture density that consists of clay minerals resulting from hydrothermal alteration. These results agree well with the results from previous seismic studies on the depth of the suggested molten rocks.

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