George R. Jiracek

Origin of High Mountains in the Continents: The Southern Sierra Nevada

Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

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.

Near-surface and topographic distortions in electromagnetic induction

The most revealing description of electromagnetic (EM) distortions due to near-surface inhomogeneities and topography is in terms of galvanic and inductive effects. In either case, the distorted electric and magnetic fields can be best visualized as a vectorial sum of primary and secondary fields. Secondary electric fields due to electric charge build-up in the galvanic case persist to the longest periods. In contrast, the secondary electric and magnetic fields due to inductive, vortex currents disappear at long periods. The static shift of magnetotelluric (MT) apparent resistivity sounding curves is a classic example of the galvanic effect.Methods to correct for unwanted distortions such as the static shift can be classified into six categories: use of invariant response parameters, curve shifting, statistical averaging, spatial filtering, use of distortion tensors, and computer modeling. Although invariant impedance calculations are simple to make, they cannot, in general, recover the undistorted impedance. Short period curve shifting is best done with auxiliary soundings such as time domain EM; however, this requires multiple surveys. The shifting of long period MT sounding branches is useful if a standard curve is known and can be matched. Statistical averaging of neighboring MT soundings that are conformal but static shifted has proven very effective at removing random distortions if adaquate data are available. The new EMAP (Electromagnetic Array Profiling) method combats the inherent spatial high pass characteristics of EM distortions by low pass operations in data collection and processing. EMAP proposes the continuous, in-field measurement of electric field dipoles to avoid spatial aliasing. Distortion tensor stripping of topographic distortions is possible since terrain is deterministic but stripping the effects of uncertain subsurface inhomogeneities may be misleading. A new decomposition of the MT impedance tensor under the assumption of surficial three-dimensional (3-D) galvanic effects imposed on a one- or two-dimensional (1-D and 2-D) regional setting promises a way to recover the regional structure. There is a continual need for 3-D computer modeling to test new methods and to calculate topographic and regional effects. Computer modeling has established the value of 2-D modeling of the data identified as transverse magnetic (TM) in some 3-D environments. Ideally, EM distortion correction requires continuous, or at least many, data and the application of more than one correction-modeling scheme.

Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand

Newly forming subduction zones on Earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes. The transpressional subduction regime of New Zealand, which is advancing laterally to the southwest below the Marlborough strike–slip fault system of the northern South Island, is an ideal setting in which to investigate these processes. Here we acquired a dense, high-quality transect of magnetotelluric soundings across the system, yielding an electrical resistivity cross-section to depths beyond 100 km. Our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops. Massive fluid release just inland of the trench induces fault-fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates. Narrow strike–slip faults in the shallow brittle regime of interior Marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones. Deep subduction-generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high-angle thrusts that are poorly oriented for failure under dry conditions. The fluid-deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models.

Preliminary results from a geophysical study across a modern continent-continent collisional plate boundary - the Southern Alps, New Zealand

Abstract The Southern Alps of South Island, New Zealand, is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement. A joint US-NZ geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three-dimensional model of the deformation. The measurements undertaken include active source and passive seismology, magnetotelluric and electrical studies, and petrophysics. Preliminary models for the active source seismic measurements across South Island confirm, in general terms, a thickened crust under the Southern Alps, a high-velocity lower crustal layer, and a major crustal discontinuity associated with the Alpine fault. The anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements, shear wave splitting and resistivity. The mid-crust under the Southern Alps coincides with a major electrical conductivity high, which possibly corresponds to fluid in the crust. The top lies at about 15 km, close to the base of shallow seismicity east of the Alpine fault. Offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to South Island. These data are showing complex structure, particularly off western and southeastern South Island. The complexity in structure, high-quality data and consistency in results from several techniques indicates that the South Island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular, and the continental lithosphere in general.

Three-dimensional terrain corrections in resistivity surveys

A three‐dimensional finite‐element computer algorithm which can accommodate arbitrarily complex topography and subsurface structure has been developed to model the resistivity response of the earth. The algorithm has undergone extensive evaluation and is believed to provide accurate results for realistic earth models. Testing included comparison to scale‐model measurements, analytically calculated solutions, and results calculated numerically by other independent means. Computer modeling experiments have demonstrated that it is possible to remove the effect of topography on resistivity data even under conditions where such effects are large with respect to the subsurface responses. This can be done without resorting to lengthy and costly trial‐and‐error computer modeling. After correction, the data can be interpreted as if the anomalies are due only to subsurface structure. The results of case studies on field data measured in high‐relief topography indicate the following. (1) Pre‐ or postsurvey 3-D compu...

Magnetotelluric evidence of lithospheric mantle thinning beneath the southern Sierra Nevada

A wideband (0.01–1000 s) magnetotelluric survey across the southern Sierra Nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10–20 km. The eastern zone underlies the highest topography of the range and extends eastward. This eastern conductive zone extends to depths in excess of 100 km, based on model sensitivities, and lies well below the Moho depth of 32–38 km from seismological studies. Therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt. Such an interpretation is consistent with gravitational, seismological, and geologic evidence. Estimates of the partial melt fraction range from 2 to 5% at depths of 40–70 km, which are consistent with fractions of melt inclusions observed in xenoliths from the eastern Sierra Nevada. This partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations. The other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes. The western conductive zone also straddles the Moho and extends into the mantle beneath western foothills of the Sierra Nevada and the Great Valley sediments to the west. However, xenoliths from this region indicate high-velocity crustal rocks to depths in excess of 60 km, and we therefore attribute the enhanced conductivity to graphitic metasediments and/or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the Sierran plutons.

Magnetotelluric results opposing magma origin of crustal conductors in the rio grande rift

Although conductive intracrustal layers appear to be characteristic of continental rift zones, they also have been detected in stable crustal environments. The low-resistivity layers are often explained by free water or crustal magma; however, alternative explanations are possible. Purely geometrical effects can be mistaken as conductive layers at depth in the interpretation of magnetotelluric or geomagnetic depth soundings. Certain minerals such as graphite and hydrated minerals e.g. serpentine, may be important. Thermally activated processes associated with ductile flow mechanisms may also enhance conductivity. Two-dimensional analysis of a portion of 25 new magnetotelluric stations in the central Rio Grande rift supports the presence of a conductive zone at 10 km depth or less. However, very significant differences are apparent in the crustal sections at two close localities. Mid-crustal magma lenses, well-defined by seismic data, cannot explain these differences. In fact, the crust is modeled to be more conductive by at least an order of magnitude where magma is not detected compared to where it occurs at shallow and intermediate levels. Only the possible effects of regional three-dimensional current channeling could conceivably alter this conclusion. To explain the unexpected result, it is suggested that a conductive horizon occurs where an impermeable, ductile cap traps pore fluids beneath. The cap may be nearly an order of magnitude more conductive (~100s Ωm) than the dry, brittle crust above; the zone of trapped pore fluids much more conductive by more than an order of magnitude (~10 Ωm). Where active magma injection destroys the integrity of the ductile cap, trapped fluids may escape and produce an overall decrease in conductivity. The final electrical signature with this dynamic concept depends on the thermal gradient, the relative impermeability of the cap, the extent of pore fluids beneath, and the amount (and frequency) of magma intrusion. The temporal and spatial distribution of earthquake foci in our study area supports the existence of a ductile layer at about 10 km depth and the hypothesis of magma injection.

Implications of magnetotelluric modeling for the deep crustal environment in the Rio Grande rift

Abstract Block forward modeling and smooth Backus-Gilbert linearized inversion have independently been used to two-dimensionally model two east-west magnetotelluric profiles in the central Rio Grande rift. A crustal conductive zone of at least 1500 S has been detected at about 10 km depth along the northern line. However, along a line 40 km to the south where shallow, active magma is inferred and microseismicity is highest, no high conductivity zone is found. The conductive zone beneath the northern line is hypothesized to occur where water is trapped beneath an undisturbed ductile cap. Under the southern line, magma injection through the cap may release the water, thereby resulting in a relatively more resistive crust with no conductive, midcrustal layer. The resulting paradox is that where there is active magma injection into the upper crust of the Rio Grande rift, the crust is more resistive rather than more conductive.

Application of the Rayleigh-FFT technique to magnetotelluric modeling and correction

Abstract A modification of the original 1896 Rayleigh scattering theory has been implemented using the fast Fourier transform (FFT) to calculate magnetotelluric (MT) responses for complex Earth structures including topography. The method has been tested for three-dimensional Earth models with arbitrary irregular layering and topography, although results are presented for two-dimensional (2-D) Earth models only. As layers can be pinched-out, isolated surface and sub-surface bodies can be included. An inherent error in the Rayleigh method limits the maximum surface and sub-surface slopes that can be accurately modeled. Valid 2-D results are obtained for surface slopes of 53° in the TE mode and 26° in the TM case; however, sub-surface interface slopes can exceed 60° for both modes. The Rayleigh-FFT approach enables simultaneous calculation of many surface points, typically 32 or 64 in 2-D. Model input requires only the digitized values of surface and sub-surface interfaces and the electrical properties of the various media. The point TM topographic distortions are slope dependent and are nearly height independent, e.g., maximum TM distortions for topography of 1 m and 1 km are nearly equal if the surface slopes are equal. However, since measured electric fields are averaged over finite length dipoles, the point-by-point small-scale effects are greatly smoothed in practice. TE topographic distortions are typical frequency-dependent inductive effects which are much smaller than TM effects as manifested in the apparent resistivity calculations. Use of the Rayleigh-FFT algorithm to remove the unwanted effects of topography and near-surface inhomogeneities is demonstrated by 2-D distortion tensor stripping of both theoretical and actual field MT results.