Hugh M. Bibby

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.

Shallow seismicity of the central Taupo Volcanic Zone, New Zealand: Its distribution and nature

Abstract A deployment of 87 seismometers, including 23 broadband instruments, for a 5‐month period in 1995 yielded a detailed view of the distribution and nature of the shallow seismicity (depth <20 km) within the central part of the Taupo Volcanic Zone (TVZ), New Zealand. On a broad scale, the pattern of shallow seismicity observed during this study was similar to that recorded by the permanent National Seismograph Network between 1987 and 1994. The distribution of seismicity was not uniform in either time period. Rather, it was scattered throughout the currently active portion of the Taupo Fault Belt, with a number of distinct clusters of events near the northern end of the fault belt. Specifically, in 1995, there did not appear to be any correlation between the seismicity and individual faults. With the exception of a cluster of events near Rotorua, little seismicity occurred on the western side of the TVZ. Similarly, on the southeastern margin of the TVZ, the Taupo‐Reporoa Depression was characterised...

Seismic velocity structure of the central Taupo Volcanic Zone, New Zealand, from local earthquake tomography

Abstract The 3-D distribution of P-wave velocity (Vp) and the P-wave/S-wave velocity ratio (Vp/Vs) are derived for the crust in the central Taupo Volcanic Zone (TVZ), New Zealand, by tomographic inversion of P- and S-wave arrival time data from local earthquakes. Resolution in the seismogenic mid-crust (4–6 km) is good, but poorer above and below these depths. The 3-D velocity model has several Vp anomalies as large as ±5% in the mid–lower crust (4–10 km) and more than ±10% in the upper crust (0–4 km). The model achieves a 55% reduction in data variance from an initial 1-D model. Young caldera structures, Okataina, Rotorua, and Reporoa, are characterised by low Vp anomalies at a depth of about 4 km and these coincide with large negative residual gravity anomalies. We attribute these anomalies to large volumes of low Vp, low-density, volcaniclastic sediments that have filled these caldera collapse structures. Although there are no Vp anomalies which suggest the presence of molten or semi-molten magma beneath the TVZ, a large, high Vp anomaly of more than +15% and a high Vp/Vs anomaly are observed coincident with a diorite pluton beneath the Ngatamariki geothermal field. However, Vp anomalies cannot be seen beneath the largest geothermal fields, Waimangu, Waiotapu, and Reporoa, and, consequently, if such anomalies exist, they must be below the resolution of our data. A prominent Vp contrast of 5–10% occurs at a depth of about 6 km beneath the boundary between the Taupo–Reporoa Depression and the Taupo Fault Belt (TFB), coincident with the eastern limit of the seismic activity beneath the TFB. We interpret this velocity contrast as being caused by the presence of extensive, non-molten, intrusives beneath the Taupo–Reporoa Depression. We suggest that the high-velocity material beneath the Taupo–Reporoa Depression is isolated from regional extension in the TVZ, and from the resulting faulting and seismicity, which occurs preferentially within the weaker material of the TFB. We are unable to determine whether greywacke, which forms the basement beneath the eastern most part of the TVZ continues further west, or is replaced by a volcanic rock such as andesite which has similar Vp and density. Vp/Vs anomalies are much smaller than Vp anomalies and generally have little spatial relationship to the Vp pattern. There is a widespread decrease in Vp/Vs, from >1.76 to 1.70–1.73, between 4 and 6 km depth over much of the study area and Vp/Vs is high southwest of the Okataina caldera, where Vp is low. Hypocentres calculated using the 3-D velocity model differ little from those obtained using a 1-D model with station terms, however, some groups of earthquakes are more tightly clustered. Following relocation, there is a slight decrease in the estimated thickness of the seismogenic zone, with 73% of hypocentres between 4 and 7 km depth and a slight increase in the depth of the brittle–ductile transition from 6 to 6.5 km.

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.

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.

Long offset tensor apparent resistivity surveys of the Taupo Volcanic Zone, New Zealand

Abstract The Taupo Volcanic Zone (TVZ) is the centre of rhyolite volcanism in New Zealand and is characterised by numerous calderas and active geothermal systems. Three long-offset multiple-source (tensor) bipole–dipole surveys, which together span the width of the TVZ, have been used to determine the resistivity structure down to depths of 5–8 km. These data have been interpreted using the constraints provided by extensive shallow ( The deep resistivity structure defines three distinct zones within the TVZ, each with boundaries approximately parallel to the eastern margin. The eastern zone is nearly 20 km wide and contains highly conductive rocks ( 4000 MW) occurs through this eastern zone. Along the centre of the TVZ lies the Taupo Fault Belt in which resistivities are an order of magnitude higher (>200 Ωm to depths of at least 7 km) than in the eastern zone. The Taupo Fault Belt also has low heat-flux, high seismicity and is undergoing rapid tectonic extension. A third zone along the western side of the TVZ contains conductive rocks below 500 m depth. Drilling shows that the high conductivities occur in thick ignimbrite sheets with ages greater than 1 Ma. These low resistivities are caused by clay minerals produced by low-temperature alteration.