John A. Stodt

Fluid generation and pathways beneath an active compressional orogen, the New Zealand Southern Alps, inferred from magnetotelluric data

[1] Forty-one wideband magnetotelluric (MT) soundings were collected in a 150-km-long transect across the Southern Alps of the central South Island of New Zealand, an active compressional orogen. Decomposed MT impedance tensors, vertical magnetic field relations, and reconnaissance soundings at two locations off line imply an approximately two-dimensional geometry here with average regional geoelectric strike of ∼N40°E, similar to surface geologic trends. Two independent, two-dimensional inversion algorithms were applied to the MT data, and both imply a concave-upward (U-shaped), middle to lower crustal conductive zone beneath the west central portion of the island. The average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along-strike conductive strands narrower than the transverse magnetic (cross-strike) mode MT data can resolve. The deep crustal conductor under the Southern Alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust. Fluid interconnection and electrical conduction are promoted by shear deformation. The conductor rises northwestward toward the trace of the Alpine Fault but attains a near-vertical configuration at a depth of ∼10 km and reaches close to the surface 5-10 km inland of the fault trace itself. The transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle-ductile transition in uplifting schist and approach the surface through induced hydrofractures. The high-grade schist becomes resistive after depletion of fluids and continues to extrude toward the Alpine Fault. Shallow extensions of the deep high conductivity are coincident with modern, hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance. To the southeast, high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent Southern Alps orogen, which also exhibits evidence for expulsion of high-temperature fluids. The higher conductivity inferred along strike (possible anisotropy) could reflect more efficient fluid interconnection in this higher-strain direction, as well as possible contributions by sheared, fluid-deposited graphite. Conductivity of the uppermost mantle of the South Island is low, consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by P wave delay studies.

Dormant state of rifting below the Byrd Subglacial Basin, West Antarctica, implied by magnetotelluric (MT) profiling

During the 1994–1995 austral summer field season, we collected twelve, high-quality MT soundings over the Byrd Subglacial Basin of central West Antarctica (82°36′S lat., 118°14′W long, approx.) in the period range 0.01 s to 400 s. Ten equispaced sites in a 54 km profile cross regional aeromagnetic trends and complement seismic reflection and refraction results collected by others. Our purposes were to prove such measurements were feasible over the 2 km thick interior ice sheet, and to show from deep electrical resistivity whether the Byrd Basin comprises an active rift environment. The difficult acquisition of electric field data on ice was overcome using a custom electrometer system, with preamplifiers located at the electrode sites to buffer the high contact impedances of the ice as close to the source as possible. Two-dimensional modeling of the profile shows that resistivity of the deep crust and upper mantle is about 2000–3000 ohm-m to 100 km depth or more. This is much higher than observed in active extensional regimes, suggesting that the current state of rifting, at least in this part of central West Antarctica, is dormant.

SUBDUED STATE OF TECTONISM OF THE GREAT BASIN INTERIOR RELATIVE TO ITS EASTERN MARGIN BASED ON DEEP RESISTIVITY STRUCTURE

Abstract Lower-crustal fluid content and physical state of the central and eastern subprovinces of the Great Basin are compared on the basis of their deep electrical resistivity structure as derived from two, detailed east-west profiles of magnetotelluric (MT) soundings. A simple, but powerful and effective transformation of each profiles data allows derivation of a single characteristic MT sounding for each subprovince which is essentially free of the effects of middle- to upper-crustal lateral heterogeneity. The characteristic soundings are amenable to layered Earth inversion, yielding broad-scale, model resistivity profiles into the uppermost mantle. A striking difference in lower-crustal electrical resistivity is apparent between the two subprovinces. The tectonically more active eastern domain has a lower-crustal conductance of ∼ 3000 siemens (S) at a model depth around 15 km, while that of the relatively subdued Great Basin interior is only ∼ 750 S with a depth around 22 km. Thicknesses of the two subprovince conductors are resolvable independent of their resistivity (∼ 20 km at 7 Ω-m in the east and ∼ 15 km at 20 Ω-m in the interior). This allows estimates of fluid-filled porosity in the deep crust assuming highly saline brines and predominantly grain-edge interconnection. Nominal values are 0.4 vol% for the eastern subprovince but only ∼ 0.15 vol% for the central region. These porosities are consistent with fluid percolation models for ductile crustal rocks, assuming on-going fluid replenishment by basaltic underplating in the eastern Great Basin and with little fluid input occurring in the central Great Basin in the last 5–10 Ma. Resistivity of the uppermost mantle in both regions is greater than that of the lower crust. This implies relatively lesser fluid/melt contents or poorer interconnection in the shallow mantle.

Anatomy of the southern Cordilleran hingeline, Utah and Nevada, from deep electrical resistivity profiling

To address outstanding questions in Mesozoic‐Cenozoic structure and present‐day deep physicochemical state in the region of the southern Cordilleran hingeline, a detailed, east‐west profile of magnetotelluric (MT) soundings 155 km in length was acquired. From these soundings, a resistivity interpretation was produced using an inversion algorithm based on a structural parameterization. In the upper ten kilometers of the transect, the interpretation shows two segments of low resistivity lying beneath allochthonous rocks of the Late Mesozoic, Sevier thrust sheet. Subsequent industry drilling motivated in part by our surveying confirms the existence and position of the eastern subthrust conductor and, more spectacularly, identifies the presence of yet deeper, autochthonous Mesozoic rocks. The conductors cannot be specified uniquely with present public data, because their electrical characteristics appear consistent with Paleozoic, pyrolized graphitic strata of either Late Devonian‐Mississippian or Middle Ordo...

Mapping hydraulic fractures using a borehole-to-surface electrical resistivity method

Abstract A series of shallow-depth hydraulic fracturing experiments was carried out in the summer of 1988 at the Elda landfill near Cincinatti, Ohio and mise-a-la-masse (MLM) borehole-to-surface resistivity measurements were obtained in an attempt to detect the fracturing. The well casing of an injection borehole was energized and potentials were measured at various points on the surface near the borehole before and after hydraulic fracturing was performed with a conductive fluid. Forward and inverse modeling algorithms based on the DC alpha center method were developed and tested with synthetic data to create a tool for interpretation of the experimental data. The alpha center forward algorithm incorporates a vertical line source of current to model an energized steel well casing. The advantages of the alpha center method are its speed and simplicity, and its ability to handle 3D geometry and indicate positions of conductive inhomogeneities. The forward solution is incorporated into an iterative least-squares inversion algorithm, with constraints applied to the alpha center parameters to facilitate modeling of fractures.

The telluric‐magnetotelluric method in two‐ and three‐dimensional environments

The assumption of spatial uniformity of the horizontal magnetic field, which is an implicit assumption made in straightforward applications of the telluric‐magnetotelluric (T-MT) method, is not always valid near conductivity inhomogeneities. For a two‐dimensional (2-D) case, the transverse electric mode horizontal magnetic field may vary more than a factor of three. The spatial variation of the horizontal magnetic field is not as great over three‐dimensional (3-D) inhomogeneities, but it may still contribute significantly to impedance magnitude and phase over shallow inhomogeneities at higher frequencies. Spatial variation of the horizontal magnetic field can cause T-MT impedances to differ significantly from magnetotelluric (MT) impedances. Consequently, MT modeling of T-MT field data could result in a misleading interpretation of conductivity structure. To avoid erroneous interpretation, numerical modeling programs should calculate actual T-MT responses, rather than MT responses, for comparison with T-M...

Verification of five magnetotelluric systems in the mini‐EMSLAB experiment

Given the degree of complexity of modern magnetotelluric (MT) instrumentation, comparison of the total performance for two or more systems is an important verification test. This paper compares the processed data from five MT systems which were designed and constructed separately, and which employ different electrode types, electrode separations, magnetometers, and methods of signal processing. The comparison shows that there is a high degree of agreement among the data from the different systems. The study also demonstrates the compatibility and reliability of the MT systems employed as part of EMSLAB Juan de Fuca (Electromagnetic Sounding of the Lithosphere and Asthenosphere Beneath the Juan de Fuca Plate). This project, proposed by a consortium of institutions, involves not only magnetotellurics studies but also studies of magnetic variation, on land and on the sea bottom. The project calls for the real-time MT systems to occupy stations along segments of a profile in Oregon. A composite profile will be created from the segments. Prior to commencing the main MT profiling phase, one week was set aside in August, 1984, for all groups to record and process MT data sequentially at six sites in diverse geologic terrains; this experiment was called mini-EMSLAB.

Author Correction: Uplift of the central transantarctic mountains

The original version of this Article incorrectly referenced the Figures in the Supplementary Information. References in the main Article to Supplementary Figure 7 through to Supplementary Figure 20 were previously incorrectly cited as Supplementary Figure 5 through to Supplementary Figure 18, respectively. This has now been corrected in both the PDF and HTML versions of the Article.