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Hypothetical one-dimensional (1-D) layered resistivity model (log depth) of stratigraphic groups with magnetotelluric apparent resistivity and phase response. Upper Santa Fe Group (SNTFu) is stratigraphic group model top. LOG RHO is log base 10 resistivity. LOG DEPTH is log base 10 depth. LOG Frequency is log base 10 frequency . Black solid line is stratigraphic group resistivity model layer in A and resistivity and phase response in B and C, respectively. Black dashed lines separate the layers of the resolvable resistivity model. See Table 2 for geologic symbol key. Mz—Mesozoic; Pz—Paleozoic. (A) Log-depth resistivity model. (B) Apparent resistivity. (C) Phase. Dotted line is resolvable resistivity model in A and its resistivity and phase response in B and C.
Source publication
Two- and three-dimensional electrical resistivity models derived from the magnetotelluric method were interpreted to provide more accurate hydrogeologic parameters for the Albuquerque and Española Basins. Analysis and interpretation of the resistivity models are aided by regional borehole resistivity data. Examination of the magnetotelluric respons...
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Citations
... It does so by measuring Earth's natural electric and magnetic fields, with worldwide lightning activity and geomagnetic micropulsations providing the energy and main frequency bands. The MT method has been used successfully to help define basin geometry in the Albuquerque and Española basins (e.g., Rodriguez and Sawyer, 2013). ...
Interpretation of gravity, magnetotelluric, and aeromagnetic data in conjunction with geologic constraints reveals details of basin geometry, thickness, and spatiotemporal evolution of the southern San Luis Basin, one of the major basins of the northern Rio Grande rift. Spatial variations of low-density basin-fill thickness are estimated primarily using a 3D gravity inversion method that improves on previous modeling efforts by separating the effects of the low-density basin fill from the effects of pre-rift rocks. The basin is found to be significantly narrower—and more complex in the subsurface—than indicated or implied by previous modeling efforts. The basin is also estimated to be significantly shallower than previously estimated. Five distinct subbasins are recognized within the broader southern San Luis Basin. The oldest and shallowest subbasin is the Las Mesitas graben along the northwestern basin margin, formed during the Oligocene transition from Southern Rocky Mountain volcanic field magmatism to rifting. In this subbasin, sediments are estimated to reach a maximum thickness of ~400 m within a north–south elongated structural depression. Other subbasins that likely initially developed during the Miocene are the dominant tectonic features in the southern San Luis Basin. This includes the Tres Orejas subbasin, which formed in the southwestern portion of the basin by the Embudo fault zone and a hypothesized fault zone along its western margin. This subbasin reaches a maximum thickness of ~2 km, as indicated by magnetotelluric and gravity modeling. The Sunshine Valley, Questa, and Taos subbasins occupy the eastern part of the southern San Luis Basin. The southern Sangre de Cristo fault zone is the dominant tectonic feature that controlled their development after ~20 Ma. The east-down Gorge fault zone controlled the western margins of significant parts of these eastern subbasins, although much of the Taos subbasin may be superimposed on the Tres Orejas subbasin. Maximum low-density basin-fill thicknesses are estimated to be 1.2 km for the Sunshine Valley subbasin, 800 m for the Questa subbasin, and 1.8 km for the Taos subbasin. Subbasin-forming tectonic activity along the Gorge fault zone and within the Tres Orejas subbasin ceased by the end of the development of the largely Pliocene Taos Plateau volcanic field. After that, rift-related subsidence became more narrowly centered on the eastern margin of the basin, controlled mainly by the linked Embudo and southern Sangre de Cristo fault zones.
... Unfortunately, a narrow station distribution and the lack of 3-D MT modeling capabilities at the time prevented further characterization of the geometry of the conductor, hindering its interpretation in a regional tectonic context. The U.S. Geological Survey has conducted numerous MT studies of the rift basins in southern Colorado and northern New Mexico (e.g., Rodriguez & Sawyer, 2013). Those studies have generally focused on interpreting stratigraphic relationships and shallow geologic structure rather than large-scale tectonic features. ...
We present electrical resistivity models of the crust and upper mantle from two‐dimensional (2‐D) inversion of magnetotelluric (MT) data collected in the Rio Grande rift, New Mexico, USA. Previous geophysical studies of the lithosphere beneath the rift identified a low‐velocity zone several hundred kilometers wide, suggesting that the upper mantle is characterized by a very broad zone of modified lithosphere. In contrast, the surface expression of the rift (e.g., high‐angle normal faults and synrift sedimentary units) is confined to a narrow region a few tens of kilometers wide about the rift axis. MT data are uniquely suited to probing the depths of the lithosphere that fill the gap between surface geology and body wave seismic tomography, namely the middle to lower crust and uppermost mantle. We model the electrical resistivity structure of the lithosphere along two east‐west trending profiles straddling the rift axis at the latitudes of 36.2 and 32.0°N. We present results from both isotropic and anisotropic 2‐D inversions of MT data along these profiles, with a strong preference for the latter in our interpretation. A key feature of the anisotropic resistivity modeling is a broad (~200‐km wide) zone of enhanced conductivity (<20 Ωm) in the middle to lower crust imaged beneath both profiles. We attribute this lower crustal conductor to the accumulation of free saline fluids and partial melt, a direct result of magmatic activity along the rift. High‐conductivity anomalies in the midcrust and upper mantle are interpreted as fault zone alteration and partial melt, respectively.
... A comparatively low-resistivity layer (170-400 X m) of thickness about 2 km (Fig. 5c) is seen below 7-10 km from surface (from west to east) might indicate presence of the dolomitic limestone of Udaipur Formation followed by Paleoproterozoic carbonate rocks of Babarmal Formation (Table 1) with resistivity (100-170 X m). The carbonate rocks are generally moderate to highly resistive (hundreds to thousands of X m), with the range dependant upon their fluid content, porosity and fracturing (Rodriguez and Sawyer 2013). In the present study, the resistivity range of the carbonate rocks has been found in hundreds. ...
Magnetotelluric (MT) data have been acquired at 40 locations in Tuwa and its surrounding region (200 km east of Ahmedabad and 15 km north–northwest of Godhra) in the Mainland Gujarat with an average station spacing of 1.5 km. MT impedance tensors have been estimated in the period range of 0.001–100 s. The data have been modeled using non-linear conjugate gradient scheme taking both apparent resistivity and phase into account. From the 2D models of the MT data, the weathered granite with Quaternary sediments (with resistivity of < 700 Ω m) have been inferred up to a depth of 500 m followed by Godhra granite (having resistivity up to 105 Ω m) with a thickness 6.5 km. The Aravalli supergroup has been inferred below Godhra granite. The Lunavada group of rocks have been inferred in the eastern part of the study area (having resistivity value ranging from 103 to 104 Ω m) separated from the Godhra granite by a contact zone. The comparatively very low-resistivity rocks (< 400 Ω m) of Udaipur formation followed by Paleoproterozoic carbonate rocks with fluid have been inferred below 8–10 km depth. The percolation of water from the surface through the contact zone of Lunavada and Champaner groups has been suggested. The presence of hot water springs in 10 km SW from the center of the study area (at the contact zone of Godhra granite and basalt) might be due to the western trending lithostratigraphic slope, hydrostatic pressure generated due to heat produced from interaction of water with the carbonate rocks at deeper depth and high subsurface temperature due to high geothermal gradient. The segmented nature of Himmatnagar Fault (HnF) is identified in the central portion of the study area.
... Multiple geological (e.g., Kelley, 1977) and geophysical studies targeted the northern and central Albuquerque basin. These include active-source seismic (e.g., Brown et al., 1980), gravity (e.g., Birch, 1982), magnetotelluric (e.g., Jiracek et al., 1987), resistivity (e.g., Rodriguez and Sawyer, 2013), paleomagnetic (e.g., Hudson and Grauch, 2003), and aeromagnetic (Grauch and Hudson, 2007) surveys to understand continental rifting, as well as to explore for water and petroleum resources. Two competing models describe the evolution and geometry of the Albuquerque basin. ...
The Albuquerque–Belen and Socorro basins reside in the central Rio Grande rift and partially overlie the midcrustal Socorro magma body (SMB), the inflation of which contributes to ongoing seismicity and localized uplift of the region. Rayleigh‐wave dispersion measurements extracted from ambient noise cross correlation of seismic data collected from the ∼800 station Sevilleta seismic array in the southern Albuquerque and northern Socorro basins were inverted to obtain Rayleigh‐wave phase‐velocity maps at 3–7 s. Tomography results indicate large (±6%) lateral variations in Rayleigh‐wave phase velocities at this period range. Rayleigh‐wave velocities were inverted utilizing a nonlinear Monte Carlo Markov chain method to obtain a 3D S‐wave velocity in the uppermost 10 km of the crust. At shallow depths (<5 km), S‐wave velocity models show low‐velocity (2.90–3.10 km/s) anomalies in the Rio Grande rift axis, mainly due to Cenozoic rift sediments, and relatively higher (3.20–3.40 km/s) velocities in the local mountain ranges could be due to older or volcanic rocks. At 8–10 km depth, this relationship is inverted, and higher S‐wave velocities (3.7–3.85 km/s) are generally more present beneath the rift axis than beneath the surrounding ranges. We suggest that the elevated S‐wave velocities (3.7–3.85 km/s) could be related to a large (30 km by 12 km) granitic pluton found below the sediment cover in Consortium for Continental Reflection Profiling (COCORP) reflection profiles in the Abo Pass survey (line 2A). The low S‐wave velocities could be related to structural changes in moderate‐to‐high‐grade metamorphism of midcrustal rocks and upper‐crustal extension from ongoing uplift of the northern part of midcrustal SMB.
Discrepancies among previous models of the geometry of the Albuquerque Basin motivated us to develop a new model using a comprehensive approach. Capitalizing on a natural separation between the densities of mainly Neogene basin fill (Santa Fe Group) and those of older rocks, we developed a three-dimensional (3D) geophysical model of syn-rift basin-fill thickness that incorporates well data, seismic-reflection data, geologic cross sections, and other geophysical data in a constrained gravity inversion. Although the resulting model does not show structures directly, it elucidates important aspects of basin geometry. The main features are three, 3-5-km-deep, interconnected structural depressions, which increase in size, complexity, and segmentation from north to south: the Santo Domingo, Calabacillas, and Belen subbasins. The increase in segmentation and complexity may reflect a transition of the Rio Grande rift from well-defined structural depressions in the north to multiple, segmented basins within a b
Interpretation of gravity, aeromagnetic, and magnetotelluric (MT) data reveals patterns of rifting, rift-sediment thicknesses, distribution of pre-rift volcanic and sedimentary rocks, and distribution of syn-rift volcanic rocks in the central San Luis Basin, one of the northernmost major basins that make up the Rio Grande rift. Rift-sediment thicknesses for the central San Luis Basin determined from a three-dimensional gravity inversion indicate that syn-rift Santa Fe Group sediments have a maximum thickness of ~2 km in the Sanchez graben near the eastern margin of the basin along the central Sangre de Cristo fault zone, and reach nearly 1 km within the Monte Vista graben near the western basin margin along the San Juan Mountains. In between, Santa Fe Group thickness is negligible under the San Luis Hills and estimated to reach ~1.1 km under the Costilla Plains (although no independent thickness constraints exist, and a range of thicknesses of 600 m to 2 km is geophysically reasonable). From combined geophysical and geologic considerations, pre-rift, dominantly sedimentary rocks appear to increase in thickness from none in the Sanchez graben on the east to perhaps 800 m under the San Luis Hills on the west. The pre-rift rocks are most likely early Tertiary in age, but the presence of Mesozoic and Paleozoic sedimentary rocks cannot be ruled out. Geophysical data provide new evidence that an isolated exposure of Proterozoic rocks on San Pedro Mesa is rooted in the Precambrian basement. This narrow, north-south–trending basement high has ~2 km of positive relief with respect to the base of the Sanchez graben, and separates the graben from the structural depression beneath the Costilla Plains. A structural high composed of pre-rift rocks, long inferred to extend from under the San Luis Hills to the Taos Plateau, is confirmed and found to be denser than previously believed, with little or no overlying Santa Fe Group sediments. Major faults in the study area are delineated by geophysical data and models; these faults include significant vertical offsets (≥1 km) of Precambrian rocks along the central and southern zones of the Sangre de Cristo fault system. Other faults with similarly large offsets of the Santa Fe Group include a fault bounding the western margin of San Pedro Mesa, and other faults that bound the Monte Vista graben in an area previously assumed to be a simple hinge zone at the western edge of the San Luis Basin. A major north-south–trending structure with expression in gravity and MT data occurs at the boundary between the Costilla Plains and the San Luis Hills structural high. Although it has been interpreted as a down-to-the-east normal fault or fault zone, our modeling suggests that it also is likely related to pre-rift tectonics. Aeromagnetic anomalies over much of the area are interpreted to mainly reflect variations of remanent magnetic polarity and burial depth of the 5.3–3.7 Ma Servilleta Basalt of the Taos Plateau volcanic field. Magnetic-source depth estimates are interpreted to indicate patterns of subsidence following eruption of the basalt, with maximum subsidence in the Sanchez graben.
