Magnetovariational soundings across the South Island of New Zealand: difference induction arrows and the Southern Alps conductor

Pringle, Daniel; Ingham, M.; McKnight, Don; Chamalaun, F. H.

doi:10.1016/s0031-9201(99)00173-9

Cited by 8 publications

(5 citation statements)

References 30 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In short, all inversions including TE data show an increased conductance in the orogen's lower crust to along‐strike current flow, whether this is represented by narrow conductive heterogeneity or intrinsic anisotropy. Pringle et al [2000] also inferred high conductivity in the strike direction from geomagnetic variation data along a profile ∼100 km to the northeast, somewhat higher, in fact, than in our models. Broad‐scale increases in conductivity to the northeast could explain Re( K zx ) < 0 for T > 50 s across the island (Figure 7a) and contribute to the current bottleneck effect suggested above.…”

Section: Resistivity Model Constructionsupporting

confidence: 63%

“…There is a low‐amplitude but island‐wide anomaly of Re( K zx ) < 0 for T > 50 s resembling similar data of Chamalaun and McKnight [1993]. Conductors offshore do not explain this [ Pringle et al , 2000], and a large‐scale conductivity increase to the northeast is implied. Lack of a clear reversal in Re( K zx ) between coasts is consistent with our assumed strike lying between average trends of offshore conductivity to the northwest and southeast, indicating that N40°E is a reasonable strike approximation.…”

Section: Observed Magnetotelluric Datamentioning

confidence: 64%

“…MT surveying was concentrated along the Rangitata‐Whataroa line. Because of the high measurement density and good quality data, this survey offers considerably improved resolution compared to previous conductivity studies of the Southern Alps [ Ingham , 1996; Ingham and Brown , 1998; Pringle et al , 2000].…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

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

2002

View full text Add to dashboard Cite

[1] Forty-one wideband magnetotelluric (MT) soundings were collected in a 150-kmlong 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 nearvertical 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.

show abstract

Section: Resistivity Model Constructionsupporting

confidence: 63%

Section: Observed Magnetotelluric Datamentioning

confidence: 64%

See 1 more Smart Citation

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

2002

View full text Add to dashboard Cite

show abstract

“…Their TSC model incorporated a representation of the surrounding bathymetry, with conductances ranging from 3,300 to 16,500 S and used a uniform conductance of 0.1 S for land areas. Pringle et al () used a similar model to investigate the effect on induction arrows of a high conductance region associated with the Alpine Fault. However, New Zealand sits on the boundary between the Pacific and Australian tectonic plates, and resulting from the tectonic setting, significant conductivity variations exist across both the North and South Islands.…”

Section: Electric Field and Gic Modeling Methodsmentioning

confidence: 99%

Modeling Geoelectric Fields and Geomagnetically Induced Currents Around New Zealand to Explore GIC in the South Island's Electrical Transmission Network

et al. 2017

View full text Add to dashboard Cite

Transformers in New Zealand's South Island electrical transmission network have been impacted by geomagnetically induced currents (GIC) during geomagnetic storms. We explore the impact of GIC on this network by developing a thin‐sheet conductance (TSC) model for the region, a geoelectric field model, and a GIC network model. (The TSC is composed of a thin‐sheet conductance map with underlying layered resistivity structure.) Using modeling approaches that have been successfully used in the United Kingdom and Ireland, we applied a thin‐sheet model to calculate the electric field as a function of magnetic field and ground conductance. We developed a TSC model based on magnetotelluric surveys, geology, and bathymetry, modified to account for offshore sediments. Using this representation, the thin sheet model gave good agreement with measured impedance vectors. Driven by a spatially uniform magnetic field variation, the thin‐sheet model results in electric fields dominated by the ocean‐land boundary with effects due to the deep ocean and steep terrain. There is a strong tendency for the electric field to align northwest‐southeast, irrespective of the direction of the magnetic field. Applying this electric field to a GIC network model, we show that modeled GIC are dominated by northwest‐southeast transmission lines rather than east‐west lines usually assumed to dominate.

show abstract

“…Even though several sea‐effect correction methods have been suggested, the sea effect has not yet been satisfactorily resolved for land MT data. Most studies on the sea‐effect correction are based on geomagnetic depth sounding (Weaver & Agarwal 1991; Bapat et al 1993; Dosso et al 1996; Shimoizumi et al 1997; Pringle et al 2000; Yang 2006). In contrast, Nolasco et al (1998) firstly tried to correct the sea effects for MT data acquired on the seafloor of Society Island by employing electrical and magnetic distortion tensor.…”

Section: Introductionmentioning

confidence: 99%

Sea effect correction in magnetotelluric (MT) data and its application to MT soundings carried out in Jeju Island, Korea

Yang¹,

Min²,

Yoo³

2010

Geophysical Journal International

View full text Add to dashboard Cite

S U M M A R YWhen magnetotelluric (MT) data are obtained in the vicinity of the coast, the surrounding sea makes it difficult to interpret subsurface structures, especially at the deep parts of the subsurface. We apply an iterative method to remove the sea effect. The iterative method was originally developed to remove the distortion due to topographic changes from MT data recorded on the seafloor. The iterative sea-effect correction method is carried out in two steps. The first corrects the sea effect, whereas the second inverts the sea-effect-corrected responses. The two steps are alternatively carried out, until the criterion for either the inversion or the seaeffect correction is satisfied. Because the surrounding 3-D sea bathymetry is only incorporated into forward modelling for the sea-effect correction, it can be more robust than the method that directly incorporates 3-D sea bathymetry into a model space for inversion. The synthetic examples show that the sea-effect correction method yields an inverted model comparable to the true model. By applying the sea-effect correction method to real field data acquired in Jeju Island, Korea, we also demonstrate that the sea-effect correction method effectively removes the sea effects from the 1-D and 2-D real field data, which helps enhance the inversion results. On the basis of these results, it may be concluded that the iterative sea-effect correction method can be used as a promising technique for recovering the true response of the subsurface in MT data suffering from sea effects.

show abstract

Magnetovariational soundings across the South Island of New Zealand: difference induction arrows and the Southern Alps conductor

Cited by 8 publications

References 30 publications

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

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

Modeling Geoelectric Fields and Geomagnetically Induced Currents Around New Zealand to Explore GIC in the South Island's Electrical Transmission Network

Sea effect correction in magnetotelluric (MT) data and its application to MT soundings carried out in Jeju Island, Korea

Contact Info

Product

Resources

About