Article,
Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation
Journal of Volcanology and Geothermal Research, 68 (1-3): 29--58 (October 1995)
DOI: 10.1016/0377-0273(95)00007-H
Abstract
The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by extensive
volcanism and by high rates of magma production. Associated with
this volcanism are numerous high-temperature (> 250C) geothermal
systems through which the natural heat output of 4200 +- 500 MW is
channelled. Outside the geothermal fields the heat flow is negligible.
The average heat flux from the central 6000 km2 of the TVZ, which
contains most of the geothermal fields, is 700 mW/m3. This heat flux
appears to be more concentrated along the eastern margin of the TVZ.
Schlumberger resistivity measurements (AB/2 of 500 m and 1000 m)
have identified 17 distinct geothermal fields with natural heat outputs
greater than 20 MW. An additional six, low-heat-output geothermal
fields also occur, and may represent formerly more active systems
now in decline. Two extinct fields have also been identified. The
average spacing between fields is 10-15 km. The distribution of geothermal
fields does not appear to be directly associated with individual
volcanic features except for the geothermal system that occurs within
Lake Taupo and which occupies the vent of the 1800 yr.B.P. Taupo
eruption. The positions of the geothermal fields do not appear to
have varied for at least the last 200,000 years. These data are consistent
with a model of large-scale convection occurring throughout the TVZ,
in which the geothermal fields represent the upper portion of the
rising, high-temperature, convective plumes. The majority of the
recharge to the convection system is provided by the downward movement
of cold meteoric water between the fields which suppresses the heat
flow in these regions. Gravity measurements indicate that to a depth
of about 2.5 km the upper layers of the TVZ consist of low-density
pyroclastic infill. A seismic refraction interface with velocity
change from 3.2 km/s to 5.5 km/s occurs at a similar depth. The cross-sectional
area of the convection plumes (identified electrically) appears to
increase at depths of 1-2 km, consistent with a decrease in permeability
at the depth at which the velocity and density increase.The seismicity
is dominated by swarm activity which accounts for about half of all
earthquakes and is highly variable in both space and time. The small
number of seismic events (and swarms) that have well determined depths
show a cut off of seismicity at depths of 7-9 km. The depth of the
transition from brittle to ductile behaviour of the rocks is identified
with the transition from a regime where heat is transported by (hydrothermal)
convection and pore pressures are near-hydrostatic to a regime where
heat transport is dominantly conductive and pore pressures are lithostatic.
Within the convective region, temperatures are moderated by the circulation
of water so that the depth of the transition from convective to conductive
heat transfer can be linked to the bottom of the seismogenic zone.
Rocks must become ductile within about 1 km of the bottom of the
overlying convective zone.Seismic refraction studies suggest that
the crust beneath the TVZ is highly thinned with a seismic velocity
of about 7.5 km/s, typical of the upper mantle, occurring at depth
of 15 km. Seismological studies indicate the upper mantle is highly
attenuating beneath the TVZ. Conductive heat transfer between the
bottom of the convective system, at about 8 km, and the base of the
material with crustal velocities, at 15 km, is not able to provide
all the heat that is discharged at the surface. Repeated intrusion
from the mantle may provide the additional heat transport required.
