T. Grant Caldwell

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.

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.

Wide‐band magnetotelluric measurements across the Taupo Volcanic Zone, New Zealand‐Preliminary results

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by intensive geothermal activity and frequent rhyolitic volcanism. Sixteen wide-band (0.01-1,800 s) magnetotelluric soundings were measured along a 110 km-long profile approximately perpendicular to the strike of the TVZ. A model obtained from 2D inversion of the soundings shows two near-surface regions of high conductance which correspond to low density volcaniclastic sediments, up to 3 km thick, which infill a sequence of collapse calderas. At deeper levels (approximately 5-10 km) a resistive layer underlies the entire TVZ. Modelling shows other conductive zones occur beneath the TVZ, with the shallowest lying below the central part at a depth of 10 -15 km. Given the high heat flux and volcanic history of the TVZ, the high conductivity at depth may indicate the presence of connected melt. At greater depth (20-30 km) the upper mantle beneath the TVZ appears to be anomalously conductive, consistent with observed high seismic attenuation.

Fault zone fluids and seismicity in compressional and extensional environments inferred from electrical conductivity:the New Zealand Southern Alps and U. S. Great Basin

Seismicity in both compressional and extensional settings is a function of local and regional stresses, rheological contrasts, and the distribution of fluids. The influence of these factors can be illustrated through their effects on electrical geophysical structure, since this structure reflects fluid composition, porosity, interconnection and pathways. In the compressional, amagmatic New Zealand South Island, magnetotelluric (MT) data imply a concave-upward (“U”-shaped), middle to lower crustal conductive zone beneath the west-central portion of the island due to fluids generated from prograde metamorphism within a thickening crust. Change of the conductor to near-vertical orientation at middle-upper crustal depths is interpreted to occur as fluids cross the brittle-ductile transition during uplift, and approach the surface through induced hydrofractures. The central South Island is relatively weak in seismicity compared to its more subduction-related northern and southern ends, and the production of deep crustal fluids through metamorphism may promote slip before high stresses are built up. The deep crustal conductivity is highly anisotropic, with the greater conductivity along strike, consistent with fault zone models of long-range interconnection versus degree of deformation. The central Great Basin province of the western U.S. by contrast is extensional at present although it has experienced diverse tectonic events throughout the Paleozoic. MT profiling throughout the province reveals a quasi one-dimensional conductor spanning the lower half of the crust which is interpreted to reflect high temperature fluids and perhaps melting caused ultimately by exsolution from crystallizing underplated basalts. The brittle, upper half of the crust is generally resistive, but also characterized by numerous steep, narrow conductors extending from near-surface to the middle crust where they contact the deep crustal conductive layer. These are suggested to represent fluidised/altered fault zones, with at least some fluids contributed from the deeper magmatic exsolution. The best-known faults imaged geophysically before this have been the listric normal faults bounding graben sediments as imaged by reflection seismology. However, the major damaging earthquakes of the Great Basin appear to nucleate near mid-crustal depths on near-vertical fault planes, which we suggest are being imaged with the MT transect data, and where triggering fluids from the ductile lower crust are available. In both compressional and extensional examples, the fluidised fault zones are hypothesized to act to concentrate slip, with major earthquakes resulting in asperities along the fault surface.

Tensor time domain electromagnetic resistivity measurements at Ngatamariki geothermal field, New Zealand

Abstract Experimental measurements in the Ngatamariki geothermal field, North Island, New Zealand were made to test the applicability of the time domain electromagnetic method for detailed investigation of the resistivity structure within a geothermal field. Low-frequency square wave signals were transmitted through three grounded bipole current sources sited about 8 km from the measurement lines. Despite high levels of electrical noise, transient electric field vectors could be determined reliably for times between 0.02 and 3.3 s after each step in the source current. Instantaneous apparent resistivity tensors were then calculated. Apparent resistivity pseudosections along the two measurement lines show smooth variations of resistivity from site to site. Over most of the field the images consistently show a three-layer resistivity structure with a conductive middle layer (3–10 Ωm) representing the conductive upper part of the thermal reservoir. A deep-seated region of low resistivity in the northwest of the field may indicate a conductive structure at about 1 km associated with a deeper diorite intrusion. Measurements sited closer than about 100 m to drillholes appear to have been disturbed by metallic casing in the holes. A change in resistivity structure in the east of the field may indicate a major geological or hydrothermal boundary.

A Ground-Motion Transfer Function Matrix between Two Nearby Rock and Soil Sites: A System Identification Problem

Abstract Standard single-component spectral ratios used to estimate localized site effects may be generalized by using a 3×3 matrix transfer function between the three-component motion observed at a reference (rock) site and that at a nearby soil site. This complex matrix ( G matrix) represents the complete three-dimensional site-response within the linear regime. No prior knowledge of the geology at the soil site is required, and G may be computed directly from the spectral responses of earthquakes recorded at both sites. Earthquakes from different hypocentral locations may be used but must be selected so that the body-wave incidence at both stations is nearly vertical and must be time-windowed to avoid surface waves. G is calculated using a stochastic inverse of the observed response spectra and corresponding noise covariance for each response. This method of analysis depends on the availability of a good quality reference (rock) site, as do standard spectral ratio techniques that compare rock and soil site records. We have applied this method to determine site effects to data recorded by strong ground motion seismographs near Wellington, North Island, New Zealand. Magnitudes of the diagonal elements of G are similar to standard single-component spectral ratios, and large values of the off-diagonal matrix elements are seen at some locations. At these locations the maximum response is better estimated from the principal values of the transfer function matrix. The orientation at which the maximum amplification (up to 14 in one case) occurs varies widely according to location.