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Seismic studies of the crust under the hydrothermal areas of the Taupo Volcanic Zone, New Zealand
Journal of Volcanology and Geothermal Research, 9 (1981) 253--267
253
Elsevier Scientific Publishing Company, Amsterdam -- Printed in Belgium
SEISMIC STUDIES OF THE CRUST U N D E R THE HYDROTHERMAL A R E A S OF THE TAUPO VOLCANIC ZONE, NEW ZEALAND
R. ROBINSON, E.G.C. SMITH and J.H. LATTER
Geophysics Division, D.S.I.R., P.O. Box 1320, Wellington, (New Zealand) (Received January 28, 1980; revised and accepted June 13, 1980)
ABSTRACT
Robinson, R., Smith, E.G.C. and Latter, J.H., 1981. Seismic studies of the crust under the hydrothermal areas of the Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res., 9: 253--267. Observations of relative P-wave travel-time residuals for local mantle earthquakes and P/S amplitude ratios for more distant events have been used to investigate the nature of the crust under the hydrothermal areas of the central Taupo Volcanic Zone. In contrast to some other regions of similar activity, such as the Yellowstone and Geysers geothermal areas, U.S.A., there is no evidence in the data for an extensive region of semi-molten or molten rock in the crust. Thus the large amounts of mafic intrusive rock and associated crustal melt inferred to be present on geological grounds must be now largely at temperatures below the appropriate solidus. The data do provide further evidence for previously suspected lateral inhomogeneity in the underlying mantle.
INTRODUCTION
The area of major hydrothermal activity in the North Island, New Zealand, coincides with the region of strong recent volcanic activity usually termed the Taupo Volcanic Zone (Healy, 1962; Fig.l). However, at present our understanding of the nature of the heat source driving the economically important and geophysically interesting hydrothermal activity is limited. The purposes of this study have been to investigate the seismic properties of the crust under the central Taupo Volcanic Zone, to compare these results with those from other active geothermal areas, and to place some constraints on geological and thermal models for the region. The area investigated, immediately north of Lake Taupo, includes several areas of strong hydrothermal upwelling, including the Wairakei geothermal field which is used for electric power generation. Also included in the area is the Maroa Volcanic Centre, thought to have been one of the major sites of past rhyolitic eruptions (Healy, 1962). The seismic properties determined are: (1) the difference in travel-time between P-waves through the region under the Taupo Volcanic Zone and through regions to either sid.e for local mantle earthquakes; (2) re-
0377-0273/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
254 [
Q
'PO0 k m
TAUPO VOLCANIC ZONE
+
/
/
/
~*S
d" / / / / / Fig. 1. The North Island, New Zealand. The solid circles represent large active andesite volcanoes within the Taupo Volcanic Zone; the solid triangles represent seismograph stations used to locate the mantle earthquakes indicated by crosses; the area shown in Fig.2 is indicated by the rectangle.
lative values of the P/S amplitude ratio across the Taupo Volcanic Zone for deep earthquakes to the north o f New Zealand. The first observation reflects a combination of compositional and thermal effects, the second depends largely on thermal effects, especially the degree of partial melt present (if any). Hopefully, these results will lead to a better understanding of the heat source for the near-surface hydrothermal convection. GEOLOGIC SETTING OF THE CENTRAL TAUPO VOLCANIC ZONE The volcanism in the North Island, New Zealand, forms the southern termination o f the volcanic activity of the Tonga-Kermadec island-arc, where the Pacific plate is subducted under the Indian plate. The present activity has evolved from a series o f volcanic arcs present in the North Island over the last 20 m.y., which have apparently migrated east and south during this time (Kear, 1959; Calhaem, 1973). The area of recent volcanic activity, the
255 Taupo Volcanic Zone ("TVZ" from here on) lies above mantle earthquakes of depth 100 km and coincides with the region of strong present-day shallow hydrothermal convection. Large active andesite volcanoes are present at each end of the 200 km length of the TVZ but the much more extensive central region has been characterised by intense rhyolitic activity within what is now a topographic depression 25--50 km wide (Healy, 1962). Within this central region volcanic rocks thought to represent some of the earliest activity (Te Kopia ignimbrite) have a radiometrically determined age of 0.6 m.y. (G.W. Grindley, personal communication). This is approximately twice the age of the currently active geothermal systems (Grindley, 1965; Lloyd, 1972). The youngest eruptions have deposited a thin layer of pumice ashes over a large area surrounding the TVZ itself. It is usually considered that the rhyolitic rocks of the central TVZ have been derived by melting of parts of the upper crust (Mesozoic greywacke grading downward to schist) due to intrusion of molten mafic rock at depth, a view supported by detailed petrological studies (Ewart et al., 1971). Consistent with this view, mafic intrusive material and mixtures of it with more acidic greywacke melt are present in small-scale basalt and andesite eruptions. Subsidence of the crust is thought to have followed the eruption of rhyolitic magma, resulting in a basement depression, some two to three kilometres deep on gravity evidence (Modriniak and Studt, 1959; Hochstein and Hunt, 1970), filled with a variety of volcanic rocks and debris. Drilling at Wairakei has reached a depth of 2.2 km without encountering greywacke basement although 20 km to the northeast at Broadlands, nearer the eastern boundary of the TVZ, greywacke has been found at various depths ranging from 1.0 to 2.3 km. The structural model that follows from these ideas will be referred to as the batholith model: when fully crystallised the intrusive material and associated crustal melt would form a batholith grading down in composition from granitic to mafic. This model is similar, although on a smaller scale, to that proposed by Pitcher (1978) for the coastal batholith in Peru, also a region of subduction of oceanic lithosphere under continental. Recently a different sort of model for the formation of the central TVZ has been proposed by Calhaem (1973). He considers the TVZ as the southern extension into a continental environment of the Lau-Havre Trough, a region in which Karig (19.70) has proposed that a back-arc spreading process is taking place. It is thus suggested that greywacke basement is not continuous across the central TVZ but that new crust is being created by rifting and concurrent intrusion of molten mafic rock. The geographic pattern of the ages of older volcanic rocks to the north and west of the TVZ is cited to support the contention that recently the rate of spreading has been sufficiently great to require the creation of entirely new crust rather than isolated igneous intrusions into the pre-existing crust as in the past. These ideas result in a structural model that will be referred to as the spreading model: when the intrusive rocks are fully crystallised, the result should be similar to but
256 thicker than newly created oceanic crust, with a thin veneer of rhyolitic volcanic rocks. In this model the rhyolitic rocks are accounted for b y melting of greywacke crust spalling from the edges of the intrusive region. In the central TVZ there is a very high regional heat flux o f a b o u t 0.8 W/m 2 (Studt and Thompson, 1969) which can be assumed to be representative of the flux for at least the age o f the present geothermal areas. In the case of either the batholith or spreading models, this is explained b y the upward transport of molten mafic rocks coupled with an efficient near-surface hydrothermal circulation system. The amount of intrusive material needed to maintain this heat o u t p u t is very dependent on its intrusive history and present temperature. The greater part o f the regional heat flux takes place in the areas of hydrothermal upwelling which are distributed throughout the central TVZ at a spacing o f roughly 15 km. This distribution has led to the concept of a regional " h o t plate" (e.g., Elder, 1965). It is generally thought, however, that the age o f the geothermal fields decreases eastwards. SEISMIC O B S E R V A T I O N S
IN S O M E O T H E R G E O T H E R M A L
REGIONS
Both the geologic models outlined above postulate that the crust under the central TVZ is quite different from that on either side due to the intrusion of large amounts o f magma, a process which may still be taking place and which provides the heat source to drive the near-surface geothermal activity. If substantial amounts o f this intrusive rock, or resulting crustal melt, are still molten or semi-molten (temperature > the solidus), then they should have a significant effect on the observations to be reported here. In this regard it is pertinent to examine some results obtained in other active ~eothermal regions. One such region is the Yellowstone area in the U.S.A. where substantial hydrothermal activity is associated with recent volcanism. The extent and petrology of the volcanic activity, there is similar to that in the central TVZ (Eaton et al., 1975) although the tectonic setting is different. Iyer (1979) has made a detailed study of teleseismic P-wave travel-time residuals in the Yellowstone region. He finds delays in the caldera region of up to 2 s. A three-dimensional inversion o f his data to obtain a velocity model indicates that relatively low-velocity material extends up to shallow depths in an area a b o u t 80 km in diameter under the caldera. This material has a P-wave velocity 15--20% less than the surrounding rocks in the crust b u t a lesser contrast (about 5%) at deeper levels. As to S-wave attenuation, there is some evidence that short-period S-waves from shallow local earthquakes are severely attenuated by propagation through some, b u t not all, parts o f the caldera region (Eation et al., 1975). The Geysers-Clear Lake geothermal field, California, has also been investigated seismically in a similar way (Iyer et al., 1979). Although the surface thermal and volcanic manifestations are less than at Yellowstone, P-wave delays of up to 1.0 s are observed within an area a b o u t 25 km in diameter.
257
The data require a region of low velocity (15-25% contrast) centred under the anomalous area but probably restricted to crustal depths. Iyer and Stewart (1977) have discussed the possible causes of a region of relatively low velocity in the crust. These include lateral variations in temperature, composition, fabric, and the nature of any fluid inclusions (e.g., partial melt). In the two cases discussed above the only viable explanation of the large P-wave delays appears to be presence of partial melt. This is not unexpected in such areas, of course. The data then provide very important restraints on the extent and thermal state of the heat sources responsible for the surface geothermal activity. Several other smaller geothermal areas have been investigated using the P-wave delay technique (e.g., Reasenberg et al., 1980; Robinson and Iyer, 1981). Generally, some evidence for local regions of partial melt is found, although their areal extent can be small and the resulting delays small, on the order of 0.3 s. However, given the extent of the geothermal activity in the TVZ, it would be natural to expect effects of similar magnitude to those at Yellowstone and the Geysers. The present experiment was designed with these results in mind. P-WAVE TRAVEL-TIME RESIDUALS IN THE CENTRAL TAUPO VOLCANIC ZONE
The direct application of the teleseismic method used in the Yellowstone and Geysers regions to the study.of the crust under the central TVZ would be difficult. Seismic recording conditions within the TVZ are quite noisy and the resulting low gains yield poor detection of teleseisms. Moreover, the teleseismic arrivals must pass through the whole of the subducted Pacific plate lithosphere thus introducing large travel-time perturbations (Robinson, 1976) that can not yet be estimated to the required accuracy of about + 0.2 s. In contrast to the problems that would be involved with a study of teleseismic P-wave arrivals, the mantle earthquakes occurring under and to the west of the TVZ, presumably in the top region of the subducted lithosphere, provide good sources. Events of sufficient size (M L > 3.5) are fairly frequent, produce sharp P-wave arrivals and can be accurately enough located by using observations from the permanent seismograph stations in New Zealand. Portable seismographs can then be used at temporary sites on either side and within the central TVZ to observe P-wave arrivals from these deep events. Sites used for the observation of deep earthquakes were chosen to extend to areas on or very near greywacke on either side of the TVZ (Fig.2). Site 17 was directly on greywacke, while sites 1, 15, and 16 were a short distance above it. The distance from the easternmost site, 1, to the westernmost, 17, is 90.4 km. Site 2 was near what is usually taken as the eastern boundary of the TVZ (the "Kaingaroa Fault") where resistivity data suggest that there may be about 250--300 m of volcanic rock overlying greywacke (H.M. Bibby, pets. commun.). Not all sites were occupied simultaneously (see Table 1), there being only five portable seismographs available. Site 3, however, was
258
~]/la /
12
f
~
I i
~ee
/ ~
o
arood,onds)/ '~
./-k~ ~
?'---/ (
/
Fig.2. The section o f the central Taupo Volcanic Zone investigated in this study. Solid triangles represent temporary seismograph sites used; open circles indicate sites used for the reconnaissance refraction survey. The line o f cross-section used in later figures is shown by an east-west dashed line; the boundaries o f the Taupo Volcanic Zone are indicated by the two northerly dashed lines
TABLE
1
Stationdata Station No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Relative P-wave residual
Relative P/S amplitude
number
average
std. dev.
number
average
std. dev.
6 6 17 5 5 6 3 4 2 6 3 2 2 5 4 2 1
--0.59 --0.29 0.00 0.22 0.02 0.18 0.13 0.33 0.11 0.47 0.49 0.24 0.07 0.46 0.43 0.50 0.69
0.18 0.23 0.00 0.24 0.26 0.38 0.19 0.20 0.06 0.29 0.44 0.11 0.03 0.23 0.19 0.14 --
5 6 9 3
1.5 1.9 1.0 2.1
0.6 0.8 0.0 0.4
1
3.5
2 2 1 2 2 1 3
3.1 6.7 3.5 1.0 5.9 3.7 4.6
1.2 3.2 0.2 1.6 0.8
259
occupied during all recording periods (a total of 45 days) and is used as a reference site for the calculation of relative residuals. Recordings were made with five portable smoked-paper seismographs (Kinemetrics models PS-1 and PSI-A) equipped with 1-Hz vertical seismometers {Mark Products L-4C). The recording speed was 60 mm/minute and radio time signals were recorded each day to ensure a timing accuracy of + 0.1 s. All instruments had the same frequency response, peaked at 2.5 Hz and falling by 12 db at 1 Hz and 12 Hz. Displacement gains ranged from 12,700 to 93,750. Seventeen deep earthquakes under the central North Island that were well recorded at the reference site were located using readings from the permanent stations of the New Zealand seismograph network (Fig.l). The events were located jointly using the method of Douglas (1967), which minimises relative location errors. A comprehensive study by Smith (1977) of New Zealand deep earthquakes and the means of locating them has shown that mantle inhomogeneity requires that the Jeffreys-Bullen travel-times used be scaled according to the position of the station in question. For Pwaves a percentage correction of - 9 % (higher awrage velocity) was used for stations in the east of the North Island and 0% or - 4 % for stations in the west. Corresponding S-wave corrections w e r e - 7 % and +6%. In addition, individual station corrections as determined by Smith (1977) were used. From the hypocentres of the events of interest, travel-times were calculated for those of our temporary stations which recorded the event. These were not used in the location procedure. Residuals were calculated, and the residual at site 3 was subtracted from those at the other sites for each individual event to minimise the effects of errors in origin time of the source events, and to reduce those due to spatial mislocation. The resulting relative residuals reflect the difference between paths to a station and to the reference site. Site 3 was used as the reference site because of its central position, and because of its relatively high gain which ensured that all events would be satisfactorily recorded. It has the disadvantage of being within the possibly anomalous region. If the structure under the central TVZ is so complex that paths from the different source earthquakes to site 3 encounter substantially different velocity structures, then this choice produces some complications in interpretation. For the purpose of detecting P-wave delays due to a large region of anomalous structure, however, this problem is not critical. The average relative residual for each site is given in Table 1, together with the number of observations (1--6) and, where possible, the standard deviation of the relative residual. The average standard deviation is 0.22 s. In order to estimate the effects of the near-surface, low-velocity volcanic rocks within the TVZ, a reconnaissance seismic refraction survey was carried out using the portable seismographs. This survey consisted of two explosions of 50 kg in Lakes Taupo and Ohakuri, recorded at sites between the shot points (Fig.2). Another shot was fired at the Lake Taupo shot point and recorded at sites to the northwest.
260
The results of this survey, summarised in Figs.3 and 4 can be interpreted in several ways, depending on the sort o f structural model used as a guide. Using the batholith model, the results are consistent with a uniform fiat basement with P-wave velocity o f 5.45 km/s (typical o f greywacke) overlain by 2.0 km of volcanic rocks with velocities as shown in Fig.4. Presumably this basement would be somewhat bumpy as found at Broadlands. The 2-
+/
10
F-
0
I 5
I 10
I 15
l 20
I 25
I 30
I 35
i L0
L5
DISTANCE, km
Fig.3. Travel-time data obtained in the reconnaissance refraction survey. First arrivals are shown by solid circles, strong later arrivals by crosses.
vp, km/s 0
,4.0
2.0
6.0
I
\ 1.0
[2. W
I
I
\ \
\
\ \\ \\
E
_-£
I
\
2.0
3.0
Fig.4. Velocity-depth models derived from the data shown in Fig.3. The solid line represents that m o d e l used for calculation o f corrections to the P-wave travel-time residuals. The other t w o curves represent alternative interpretations o f the first-arrival data.
261
km average depth to " b a s e m e n t " is only approximate due to the probable complexities in the velocity structure that are not well represented by a simple three-layer model. The layer o f velocity 3.2 km/s, determined primarily b y strong second arrivals, is taken as representing ignimbrite or rhyolite while the shallower material m a y be pumice, ash, etc. The velocities at depths less than 0.5 km are not well determined as there are no close-in observations to constrain them. The shallow velocities shown are similar to those measured b:~ Hochstein and H u n t (1970) at Broadlands. For the purposes o f calculating a correction to the observed relative residuals, the vertical travel-time through the upper 2 km was compared with that for a "normal" greywacke crust found near Wellington (Garrick, 1968). At sites situated on ignimbrite or rhyolite (as most were), the 3.2 km/s layer was taken as extending to the surface, giving a delay o f 0.17 s. At the few sites situated on pumice or ash the delay is 0.32 s. These corrections have been applied to the observed relative residuals at all sites within the TVZ. In another interpretation o f the refraction results, the first-arrival data can be explained by a gradual increase in velocity with depth, either linear d o w n to a final velocity o f 5.45 km/s at a depth of 2.5 km or hyperbolic as shown in Fig.4. In any case, the corrections to vertical P-wave travel-times, down to a depth of 2.5 or 3 km, respectively, are nearly the same as in the previous interpretation. The resulting relative P-wave travel-time residuals are shown in Fig.5, data from stations outside the TVZ being shown b y solid circles, from those inq )-~
0.6
\
0.~ 0
0 0.2 u~
~x O\ 0
o.o C3
0
0 \
0 0
Q.2
\Q
?1'
i
i
2O
L
i
i
4O
i
6O
~0
DISTANCE, km
Fig.5. Average P-wave travel-time residuals from local mantle earthquakes as a function of distance along the cross-section line shown in Fig.2 (site 17 equals 0.0 distance). Solid circles represent data for sites outside the Taupo Volcanic Zone, open circles represent data for sites within it. The latter have been corrected for the near-surface (< 2 kin) structure.
262
side by open circles. The major feature o f the data is the approximately linear increase in residual from east to west fo a b o u t 1.3 s, the trend being well defined b y the data from either side o f the TVZ. The configuration of o u r experiment is shown in Fig.6, drawn to scale for a typical source event. From this it is clear that a large region (on the order of 20 km in size) of relatively low velocity local to the crust under the TVZ is not indicated by the results. There are no significant positive departures (low velocity) from the regional trend in residuals at sites within the TVZ, the largest being 0.21 s which is within the expected scatter due to experimental and observational errors. The largest departure from the regional trend is negative, --0.41 s, at site 13, based on t w o consistent observations. This would imply a local region of relatively high velocity there. TVZ 17
8
1
!
I°
50 km
~v
Fig.& Simplified diagram of the configuration of our experiment drawn to scale. Triangles are seismograph sites.
Despite the apparent lack of a significant P-wave delay associated with the central TVZ itself, the substantial regional variation must reflect lateral variations in velocity, most probably in the mantle. A part of the explanation m a y be the relatively greater proportion of path length within the high-velocity subducted Pacific lithosphere for paths to the east. However, it could be argued that we are really seeing the effect of a crustal low-velocity region that extends to the west b e y o n d the boundary o f the TVZ and that if our observations were extended still further west, the relative residuals would drop again. The fact that there is little in the way of recent volcanic activity to the west, where t h e low-velocity region would have to be most well-de-
263 veloped, is then very puzzling and thus we do not think this suggestion is very likely to be true although it can not yet be ruled o u t completely. The above observations are not sufficient in themselves, however, to rule out the possibility o f a region o f partial melt in the crust under the TVZ since structural and compositional changes could offset the decrease in velocity that would result. For example, if the intrusive rocks thought to exist under the TVZ have a velocity higher, when cooled, than the surrounding crustal rocks, they could be partially molten (perhaps b y 5%) and still produce no large relative travel-time delays. The S-wave attenuation data to be discussed below are important in this regard and also in regard to the cause of the regional trend in P-wave residuals. P/S AMPLITUDE RATIOS IN THE CENTRAL TAUPO VOLCANIC ZONE The a m o u n t of inelastic attenuation of P- and S-waves in the crust seems to be quite variable. Under normal conditions, however, the value of Qp/Qs appears to be close to 2.0 with recent determinations of Qp on the order of 1000 (Hashizume, 1979; Brocher, 1979). At temperatures high enough to cause partial melting, however, theoretical (e.g., O'Connell and Budiansky, 1977) and laboratory (e.g., Murase and McBirney, 1973) results indicate that attenuation will increase substantially (low Q values). For example, taking a 4% melt fraction and a frequency of 3 Hz, the results of O'Connell and Budiansky (1977) give a Qp o f a b o u t 50, a Qs of about 10. The important point here is that the ratio Qp/Qsbecomes much larger at temperatures above the solidus than at temperatures below. Field evidence for this includes the use of S-wave "shadow zones" to map concentrations of magma beneath volcanoes (e.g., Matumoto, 1971), a zone where Qs is thought to be a b o u t 10 beneath the Mid-Atlantic Ridge (Solomon, 1973), and the attenuation of S-waves propagating through parts of the Yellowstone caldera (Eaton et al., 1975). It would seem, then, that observations of the relative attenuation of P- and S-waves for paths through and to either side of the TVZ would provide information on the presence or absence of a local region of partial melt. The values of Qp and Qs q u o t e d above for a 4% partial-melt region are sufficient to effectively cause the disappearance of 3-Hz S-waves relative to Pwaves for propagation through a region on the order of 10 km thick. Examination of the records from the deep earthquakes used above showed that strong arrivals at approximately the expected time for S-waves were sometimes recorded. Permanent stations of the New Zealand network commonly record S-arrivals of quite large amplitude, often with quite sharp onsets, from similar events. An a t t e m p t to evaluate the nature of these S-waves qualitatively produced no clear pattern of presence or absence of strong Swaves. Stations to the east of the TVZ, however, seemed to record strong Swaves more often than those within or to the west. Quantitative data on S-wave attenuation can be obtained b y study of distant events so that the probability of significant changes in radiation
P
Fig.7. Records of an earthquake in the Kermadec Islands area, north of New Zealand, at sites 3 and 15. Note the much smaller S-wave amplitude at site 15.
SITE 15
SITE 3
S
bo
265
pattern among rays to the observation points is small. However, at distances b e y o n d a b o u t 10°--15 ° short-period S-waves are effectively absent due to normal mantle inelastic attenuation, probably in the low-velocity zone o f the upper mantle. Fortunately a fair number of usable deep focus events occur to the north of New Zealand in the Kermadec region. An example is shown in Fig.7. F o r these events that were well recorded, the P/S amplitude ratios at our temporary sites have been measured and normalised in each case to the value observed at the reference site. Amplitude ratios are used rather than just amplitudes, in order to compensate for instrument and site gain factors and geometric spreading. Average ratios, based on from one to six observations, are shown in Fig.8 and Table 1; the higher the ratio, the higher the attenuation of S-waves relative to P-waves. Although there is considerable scatter, the majority o f the values define a westward increase in the ratio. Sites within the central TVZ do not exhibit abnormally high values relative to this regional trend, except perhaps at site 12. However, the two observations at site 12 have a very high variance (Table 1) so that not t o o much can be inferred from them. Mooney (1970), using observations of predominant frequency in recordings of deep earthquakes at permanent stations of the New Zealand seismograph network, has suggested that the whole o f the northwest North Island is underlain b y a relatively attenuative region in the mantle. This region is centred at a b o u t 100 km depth and is a b o u t 50 km thick. It possibly represents an intensification of the normal upper mantle low-velocity zone. It seems likely that we are observing across the eastern edge of this attenuative region. Moreover, the correspondence in trends o f the P-wave residuals and of the P/S amplitude ratios suggests a c o m m o n explanation, at least in part: that the mantle attenuative zone is also a region of relatively low P-wave velocity.
O .I)
o o L
J
20
~
~ o - ~
o i
I
~
40
I
60
I
I
t
80
DISTANCE, km
Fig.8. Average normalisecl P/S a m p l i t u d e r a t i o s as a f u n c t i o n o f d i s t a n c e along the line o f cross-section s h o w n in Fig.2. Solid circles are d a t a f r o m sites o u t s i d e t h e T a u p o Volcanic Zone, o p e n c ~ c l e s are data f r o m w i t h i n it.
266 CONCLUSION T h e c r u s t u n d e r t h e h y d r o t h e r m a l areas o f t h e c e n t r a l T V Z d o e s n o t prod u c e significant P-wave travel-tinge d e l a y s o r high S-wave a t t e n u a t i o n relative t o t h e areas i m m e d i a t e l y o n e i t h e r side. T h e s e results s e e m t o rule o u t t h e p r e s e n c e o f e x t e n s i v e regions o f s e m i - m o l t e n or m o l t e n r o c k local t o t h e crust b e n e a t h t h e region. This c o n t r a s t s to t h e s i t u a t i o n at s o m e o t h e r areas o f s t r o n g h y d r o t h e r m a l a c t i v i t y . In o t h e r words, e x c e p t f o r t h e p r o b a b i l i t y o f small localised h o t spots, t h e large v o l u m e o f intrusive r o c k s a n d associated crustal m e l t i n f e r r e d t o b e p r e s e n t o n geological g r o u n d s are n o w at t e m p e r a t u r e s b e l o w t h e solidus. It is possible t h a t this is indicative o f a m o r e active intrusive p e r i o d in t h e p a s t (as m i g h t be e x p e c t e d o n t h e b a t h o l i t h m o d e l ) so t h a t s u f f i c i e n t t i m e has elapsed f o r s u b s e q u e n t cooling t o t a k e place. O n t h e o t h e r h a n d , it c o u l d i n d i c a t e t h a t i n t r u s i o n i n t o t h e crust f r o m d e e p e r levels ( t h e m a n t l e a t t e n u a t i v e zone?) occurs~only in a q u i t e limited area, in c o n t r a s t to t h e large ar@as involved at t h e Y e l l o w s t o n e and G e y s e r s areas. This limited i n t r u s i o n m i g h t o c c u r a l o n g t h e eastern edge o f t h e m a n t l e a t t e n u a t i v e zone, w h i c h w o u l d c o r r e s p o n d t o t h e eastern T V Z , a n d a p p e a r s t o be m o r e c o m p a t i b l e w i t h t h e s p r e a d i n g m o d e l o f t h e region t h a n w i t h t h e b a t h o l i t h model.
REFERENCES •
= 2~'~
T.M., 1979. Q-~ in the Rio Grande Rlf~ n ~ r Socono, New Mexmo, using COCORP seismic refraction data. In: Abstractsof Conference on Selsmm Wave Attenuation. Stanford Univ. Publ. Geol. Sci., 17. Calhaem, I.M., 1973. Heat flow measurements under some lakes in North Island, New Zealand. Ph.D. Thesis, Victoria University, Wellington. Douglas, A., 1967. Joint epicentre determination. Nature, 215: 47--48. Eaton, G.P., Christiansen, R.L., Iyer, H.M•, Pitt, A.M., Mabey, D.R., Blank, H.R., Zietz, I. and Gettings, M.E., 1975• Magma beneath Yellowstone National Park. Science, 188: 787--796. Elder, J.W., 1965. Physical processes in geothermal areas• In: Terrestrial Heat Flow• Am. Geophys. Union, Geophys. Monogr., 8: 211--239. Ewart, A., Green, D.C., Carmichael, I_S. and Brown, G.H., 1971. Voluminous low-temperature rhyolitic magmas in New Zealand. Contrib. Mineral. Petrol., 33: 128--144. Garrick, R.A., 1968. A reinterpretation of the Wellington crustal refraction profile. N.Z. J. Geok Geophys., 11: 1280-1294. Grindley, G.W., 1965• The geology, structure and exploitation of Wairakei geothermal field, New Zealand. N.Z. Geol. Surv. Bull., 75. Hashizume, M., 1979• Q of the crust beneath southwestern Honshu, Japan, derived from explosion seismic waves. Phys. Earth Planet. Inter., 20: 25--32. Healy, J., 1962. Structure and volcanism in the Taupo Volcanic Zone, New Zealand. In: Crust of the Pacific Basin. Am. Geophys. Union, Geophys. Monogr., 6: 151--157. Hochstein, M.P. and Hunt, T.M., 1970• Seismic, gravity, and magnetic studies, Broadlands geothermal field, New Zealand. Geothermics, Spec• Issue, 2: 333--346. Iyer, H.M., 1979. Deep structure under Yellowstone National Park, U.S.A. :a continental "hot spot". Tectonophysics, 56: 165--197. Brocher,
267 Iyer, H.M. and Stewart, R.M., 1977. Teleseismic technique to locate magma in the crust and upper mantle. In: Proceedings of the Chapman Conference on Partial Melting in the Upper Mantle. Oregon State Department of Geology and Mineral Industries, pp. 281--299. Iyer, H.M., Oppenheimer, D.H. and Hitchcock, T., 1979. Abnormal P-wave delays in the Geysers-Clear Lake geothermal area, California. Science, 204: 495--497. Karig, D.E., 1970. Ridges and basins o f the Tonga-Kermadex Arc System. J. Geophys. Res., 75: 239--254. Kear, D.E., 1959. Stratigraphy of New Zealand's Cenozoic volcanism northwest of the volcanic belt. N . Z . J . Geol. Geophys., 2: 578--589. Lloyd, E.F., 1972. Geology and hot springs of Orakeikorako. N.Z. Geol. Surv. Bull., 85. Matumoto, T., 1971. Seismic b o d y waves observed in the vicinity of Mount KatmaiAlaska, and evidence for the .existence of molten chambers. Geol. Soc. Am. Bull., 82: 2905--2920. Modriniak, N. and Studt, F.E., 1959. Geologic structure and volcanism in the TaupoTarawera district. N . Z . J . Geol. Geophys., 2: 654--684. Mooney, H.M.~ 1970. Upper mantle inhomogeneity beneath New Zealand: seismic evidence. J. Geophys. Res., 75: 285--309. Murase, T. and McBirney, A.R., 1973. Properties o f some c o m m o n igneous rocks and their melts at high temperatures. Geol. Soc. Am. Bull., 84: 3563--3592. O'Connell, R.J. and Budiansky, B., 1977. Visco-elastic properties of fluid-saturated cracked solids. J. Geophys. Res., 82: 5719--5735. Pitcher, W.S., 1978. The a n a t o m y of a batholith. J. Geol. Soc. London, 135: 157--182. Reasenberg, P., Ellsworth, W. and Walter, A., 1980. Teleseismic evidence for a low-velocity b o d y under the Coso geothermal area. J. Geophys. Res., 85: 2471--2483. Robinson, R., 1976. Relative teleseismic travel-time residuals, North Island, New Zealand, and their relation to upper-mantle structure. Tectonophysics, 31 : T41--T48. Robinson, R. and Iyer, H.M., 1981.1Delineation of a low-velocity body under the Roosevelt Hot Springs geothermal area, Utah, using teleseismic P-wave data. Geophysics (in press). Smith, E.G.C., 1977. The theory o f multi-earthquake location by least squares and applications to groups of North Island, New Zealand, mantle earthquakes. Ph.D. Thesis, Victoria University, Wellington. Solomon, S.C., 1973. Shear wave attenuation and melting beneath the Mid-Atlantic Ridge. J. Geophys. Res., 78: 6044--6059. Studt, F.E. and Thompson, G.E.K., 1969. Geothermal heat flow in the North Island of New Zealand. N . Z . J . Geol. Geophys., 12: 673--683.
253
Elsevier Scientific Publishing Company, Amsterdam -- Printed in Belgium
SEISMIC STUDIES OF THE CRUST U N D E R THE HYDROTHERMAL A R E A S OF THE TAUPO VOLCANIC ZONE, NEW ZEALAND
R. ROBINSON, E.G.C. SMITH and J.H. LATTER
Geophysics Division, D.S.I.R., P.O. Box 1320, Wellington, (New Zealand) (Received January 28, 1980; revised and accepted June 13, 1980)
ABSTRACT
Robinson, R., Smith, E.G.C. and Latter, J.H., 1981. Seismic studies of the crust under the hydrothermal areas of the Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res., 9: 253--267. Observations of relative P-wave travel-time residuals for local mantle earthquakes and P/S amplitude ratios for more distant events have been used to investigate the nature of the crust under the hydrothermal areas of the central Taupo Volcanic Zone. In contrast to some other regions of similar activity, such as the Yellowstone and Geysers geothermal areas, U.S.A., there is no evidence in the data for an extensive region of semi-molten or molten rock in the crust. Thus the large amounts of mafic intrusive rock and associated crustal melt inferred to be present on geological grounds must be now largely at temperatures below the appropriate solidus. The data do provide further evidence for previously suspected lateral inhomogeneity in the underlying mantle.
INTRODUCTION
The area of major hydrothermal activity in the North Island, New Zealand, coincides with the region of strong recent volcanic activity usually termed the Taupo Volcanic Zone (Healy, 1962; Fig.l). However, at present our understanding of the nature of the heat source driving the economically important and geophysically interesting hydrothermal activity is limited. The purposes of this study have been to investigate the seismic properties of the crust under the central Taupo Volcanic Zone, to compare these results with those from other active geothermal areas, and to place some constraints on geological and thermal models for the region. The area investigated, immediately north of Lake Taupo, includes several areas of strong hydrothermal upwelling, including the Wairakei geothermal field which is used for electric power generation. Also included in the area is the Maroa Volcanic Centre, thought to have been one of the major sites of past rhyolitic eruptions (Healy, 1962). The seismic properties determined are: (1) the difference in travel-time between P-waves through the region under the Taupo Volcanic Zone and through regions to either sid.e for local mantle earthquakes; (2) re-
0377-0273/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company
254 [
Q
'PO0 k m
TAUPO VOLCANIC ZONE
+
/
/
/
~*S
d" / / / / / Fig. 1. The North Island, New Zealand. The solid circles represent large active andesite volcanoes within the Taupo Volcanic Zone; the solid triangles represent seismograph stations used to locate the mantle earthquakes indicated by crosses; the area shown in Fig.2 is indicated by the rectangle.
lative values of the P/S amplitude ratio across the Taupo Volcanic Zone for deep earthquakes to the north o f New Zealand. The first observation reflects a combination of compositional and thermal effects, the second depends largely on thermal effects, especially the degree of partial melt present (if any). Hopefully, these results will lead to a better understanding of the heat source for the near-surface hydrothermal convection. GEOLOGIC SETTING OF THE CENTRAL TAUPO VOLCANIC ZONE The volcanism in the North Island, New Zealand, forms the southern termination o f the volcanic activity of the Tonga-Kermadec island-arc, where the Pacific plate is subducted under the Indian plate. The present activity has evolved from a series o f volcanic arcs present in the North Island over the last 20 m.y., which have apparently migrated east and south during this time (Kear, 1959; Calhaem, 1973). The area of recent volcanic activity, the
255 Taupo Volcanic Zone ("TVZ" from here on) lies above mantle earthquakes of depth 100 km and coincides with the region of strong present-day shallow hydrothermal convection. Large active andesite volcanoes are present at each end of the 200 km length of the TVZ but the much more extensive central region has been characterised by intense rhyolitic activity within what is now a topographic depression 25--50 km wide (Healy, 1962). Within this central region volcanic rocks thought to represent some of the earliest activity (Te Kopia ignimbrite) have a radiometrically determined age of 0.6 m.y. (G.W. Grindley, personal communication). This is approximately twice the age of the currently active geothermal systems (Grindley, 1965; Lloyd, 1972). The youngest eruptions have deposited a thin layer of pumice ashes over a large area surrounding the TVZ itself. It is usually considered that the rhyolitic rocks of the central TVZ have been derived by melting of parts of the upper crust (Mesozoic greywacke grading downward to schist) due to intrusion of molten mafic rock at depth, a view supported by detailed petrological studies (Ewart et al., 1971). Consistent with this view, mafic intrusive material and mixtures of it with more acidic greywacke melt are present in small-scale basalt and andesite eruptions. Subsidence of the crust is thought to have followed the eruption of rhyolitic magma, resulting in a basement depression, some two to three kilometres deep on gravity evidence (Modriniak and Studt, 1959; Hochstein and Hunt, 1970), filled with a variety of volcanic rocks and debris. Drilling at Wairakei has reached a depth of 2.2 km without encountering greywacke basement although 20 km to the northeast at Broadlands, nearer the eastern boundary of the TVZ, greywacke has been found at various depths ranging from 1.0 to 2.3 km. The structural model that follows from these ideas will be referred to as the batholith model: when fully crystallised the intrusive material and associated crustal melt would form a batholith grading down in composition from granitic to mafic. This model is similar, although on a smaller scale, to that proposed by Pitcher (1978) for the coastal batholith in Peru, also a region of subduction of oceanic lithosphere under continental. Recently a different sort of model for the formation of the central TVZ has been proposed by Calhaem (1973). He considers the TVZ as the southern extension into a continental environment of the Lau-Havre Trough, a region in which Karig (19.70) has proposed that a back-arc spreading process is taking place. It is thus suggested that greywacke basement is not continuous across the central TVZ but that new crust is being created by rifting and concurrent intrusion of molten mafic rock. The geographic pattern of the ages of older volcanic rocks to the north and west of the TVZ is cited to support the contention that recently the rate of spreading has been sufficiently great to require the creation of entirely new crust rather than isolated igneous intrusions into the pre-existing crust as in the past. These ideas result in a structural model that will be referred to as the spreading model: when the intrusive rocks are fully crystallised, the result should be similar to but
256 thicker than newly created oceanic crust, with a thin veneer of rhyolitic volcanic rocks. In this model the rhyolitic rocks are accounted for b y melting of greywacke crust spalling from the edges of the intrusive region. In the central TVZ there is a very high regional heat flux o f a b o u t 0.8 W/m 2 (Studt and Thompson, 1969) which can be assumed to be representative of the flux for at least the age o f the present geothermal areas. In the case of either the batholith or spreading models, this is explained b y the upward transport of molten mafic rocks coupled with an efficient near-surface hydrothermal circulation system. The amount of intrusive material needed to maintain this heat o u t p u t is very dependent on its intrusive history and present temperature. The greater part o f the regional heat flux takes place in the areas of hydrothermal upwelling which are distributed throughout the central TVZ at a spacing o f roughly 15 km. This distribution has led to the concept of a regional " h o t plate" (e.g., Elder, 1965). It is generally thought, however, that the age o f the geothermal fields decreases eastwards. SEISMIC O B S E R V A T I O N S
IN S O M E O T H E R G E O T H E R M A L
REGIONS
Both the geologic models outlined above postulate that the crust under the central TVZ is quite different from that on either side due to the intrusion of large amounts o f magma, a process which may still be taking place and which provides the heat source to drive the near-surface geothermal activity. If substantial amounts o f this intrusive rock, or resulting crustal melt, are still molten or semi-molten (temperature > the solidus), then they should have a significant effect on the observations to be reported here. In this regard it is pertinent to examine some results obtained in other active ~eothermal regions. One such region is the Yellowstone area in the U.S.A. where substantial hydrothermal activity is associated with recent volcanism. The extent and petrology of the volcanic activity, there is similar to that in the central TVZ (Eaton et al., 1975) although the tectonic setting is different. Iyer (1979) has made a detailed study of teleseismic P-wave travel-time residuals in the Yellowstone region. He finds delays in the caldera region of up to 2 s. A three-dimensional inversion o f his data to obtain a velocity model indicates that relatively low-velocity material extends up to shallow depths in an area a b o u t 80 km in diameter under the caldera. This material has a P-wave velocity 15--20% less than the surrounding rocks in the crust b u t a lesser contrast (about 5%) at deeper levels. As to S-wave attenuation, there is some evidence that short-period S-waves from shallow local earthquakes are severely attenuated by propagation through some, b u t not all, parts o f the caldera region (Eation et al., 1975). The Geysers-Clear Lake geothermal field, California, has also been investigated seismically in a similar way (Iyer et al., 1979). Although the surface thermal and volcanic manifestations are less than at Yellowstone, P-wave delays of up to 1.0 s are observed within an area a b o u t 25 km in diameter.
257
The data require a region of low velocity (15-25% contrast) centred under the anomalous area but probably restricted to crustal depths. Iyer and Stewart (1977) have discussed the possible causes of a region of relatively low velocity in the crust. These include lateral variations in temperature, composition, fabric, and the nature of any fluid inclusions (e.g., partial melt). In the two cases discussed above the only viable explanation of the large P-wave delays appears to be presence of partial melt. This is not unexpected in such areas, of course. The data then provide very important restraints on the extent and thermal state of the heat sources responsible for the surface geothermal activity. Several other smaller geothermal areas have been investigated using the P-wave delay technique (e.g., Reasenberg et al., 1980; Robinson and Iyer, 1981). Generally, some evidence for local regions of partial melt is found, although their areal extent can be small and the resulting delays small, on the order of 0.3 s. However, given the extent of the geothermal activity in the TVZ, it would be natural to expect effects of similar magnitude to those at Yellowstone and the Geysers. The present experiment was designed with these results in mind. P-WAVE TRAVEL-TIME RESIDUALS IN THE CENTRAL TAUPO VOLCANIC ZONE
The direct application of the teleseismic method used in the Yellowstone and Geysers regions to the study.of the crust under the central TVZ would be difficult. Seismic recording conditions within the TVZ are quite noisy and the resulting low gains yield poor detection of teleseisms. Moreover, the teleseismic arrivals must pass through the whole of the subducted Pacific plate lithosphere thus introducing large travel-time perturbations (Robinson, 1976) that can not yet be estimated to the required accuracy of about + 0.2 s. In contrast to the problems that would be involved with a study of teleseismic P-wave arrivals, the mantle earthquakes occurring under and to the west of the TVZ, presumably in the top region of the subducted lithosphere, provide good sources. Events of sufficient size (M L > 3.5) are fairly frequent, produce sharp P-wave arrivals and can be accurately enough located by using observations from the permanent seismograph stations in New Zealand. Portable seismographs can then be used at temporary sites on either side and within the central TVZ to observe P-wave arrivals from these deep events. Sites used for the observation of deep earthquakes were chosen to extend to areas on or very near greywacke on either side of the TVZ (Fig.2). Site 17 was directly on greywacke, while sites 1, 15, and 16 were a short distance above it. The distance from the easternmost site, 1, to the westernmost, 17, is 90.4 km. Site 2 was near what is usually taken as the eastern boundary of the TVZ (the "Kaingaroa Fault") where resistivity data suggest that there may be about 250--300 m of volcanic rock overlying greywacke (H.M. Bibby, pets. commun.). Not all sites were occupied simultaneously (see Table 1), there being only five portable seismographs available. Site 3, however, was
258
~]/la /
12
f
~
I i
~ee
/ ~
o
arood,onds)/ '~
./-k~ ~
?'---/ (
/
Fig.2. The section o f the central Taupo Volcanic Zone investigated in this study. Solid triangles represent temporary seismograph sites used; open circles indicate sites used for the reconnaissance refraction survey. The line o f cross-section used in later figures is shown by an east-west dashed line; the boundaries o f the Taupo Volcanic Zone are indicated by the two northerly dashed lines
TABLE
1
Stationdata Station No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Relative P-wave residual
Relative P/S amplitude
number
average
std. dev.
number
average
std. dev.
6 6 17 5 5 6 3 4 2 6 3 2 2 5 4 2 1
--0.59 --0.29 0.00 0.22 0.02 0.18 0.13 0.33 0.11 0.47 0.49 0.24 0.07 0.46 0.43 0.50 0.69
0.18 0.23 0.00 0.24 0.26 0.38 0.19 0.20 0.06 0.29 0.44 0.11 0.03 0.23 0.19 0.14 --
5 6 9 3
1.5 1.9 1.0 2.1
0.6 0.8 0.0 0.4
1
3.5
2 2 1 2 2 1 3
3.1 6.7 3.5 1.0 5.9 3.7 4.6
1.2 3.2 0.2 1.6 0.8
259
occupied during all recording periods (a total of 45 days) and is used as a reference site for the calculation of relative residuals. Recordings were made with five portable smoked-paper seismographs (Kinemetrics models PS-1 and PSI-A) equipped with 1-Hz vertical seismometers {Mark Products L-4C). The recording speed was 60 mm/minute and radio time signals were recorded each day to ensure a timing accuracy of + 0.1 s. All instruments had the same frequency response, peaked at 2.5 Hz and falling by 12 db at 1 Hz and 12 Hz. Displacement gains ranged from 12,700 to 93,750. Seventeen deep earthquakes under the central North Island that were well recorded at the reference site were located using readings from the permanent stations of the New Zealand seismograph network (Fig.l). The events were located jointly using the method of Douglas (1967), which minimises relative location errors. A comprehensive study by Smith (1977) of New Zealand deep earthquakes and the means of locating them has shown that mantle inhomogeneity requires that the Jeffreys-Bullen travel-times used be scaled according to the position of the station in question. For Pwaves a percentage correction of - 9 % (higher awrage velocity) was used for stations in the east of the North Island and 0% or - 4 % for stations in the west. Corresponding S-wave corrections w e r e - 7 % and +6%. In addition, individual station corrections as determined by Smith (1977) were used. From the hypocentres of the events of interest, travel-times were calculated for those of our temporary stations which recorded the event. These were not used in the location procedure. Residuals were calculated, and the residual at site 3 was subtracted from those at the other sites for each individual event to minimise the effects of errors in origin time of the source events, and to reduce those due to spatial mislocation. The resulting relative residuals reflect the difference between paths to a station and to the reference site. Site 3 was used as the reference site because of its central position, and because of its relatively high gain which ensured that all events would be satisfactorily recorded. It has the disadvantage of being within the possibly anomalous region. If the structure under the central TVZ is so complex that paths from the different source earthquakes to site 3 encounter substantially different velocity structures, then this choice produces some complications in interpretation. For the purpose of detecting P-wave delays due to a large region of anomalous structure, however, this problem is not critical. The average relative residual for each site is given in Table 1, together with the number of observations (1--6) and, where possible, the standard deviation of the relative residual. The average standard deviation is 0.22 s. In order to estimate the effects of the near-surface, low-velocity volcanic rocks within the TVZ, a reconnaissance seismic refraction survey was carried out using the portable seismographs. This survey consisted of two explosions of 50 kg in Lakes Taupo and Ohakuri, recorded at sites between the shot points (Fig.2). Another shot was fired at the Lake Taupo shot point and recorded at sites to the northwest.
260
The results of this survey, summarised in Figs.3 and 4 can be interpreted in several ways, depending on the sort o f structural model used as a guide. Using the batholith model, the results are consistent with a uniform fiat basement with P-wave velocity o f 5.45 km/s (typical o f greywacke) overlain by 2.0 km of volcanic rocks with velocities as shown in Fig.4. Presumably this basement would be somewhat bumpy as found at Broadlands. The 2-
+/
10
F-
0
I 5
I 10
I 15
l 20
I 25
I 30
I 35
i L0
L5
DISTANCE, km
Fig.3. Travel-time data obtained in the reconnaissance refraction survey. First arrivals are shown by solid circles, strong later arrivals by crosses.
vp, km/s 0
,4.0
2.0
6.0
I
\ 1.0
[2. W
I
I
\ \
\
\ \\ \\
E
_-£
I
\
2.0
3.0
Fig.4. Velocity-depth models derived from the data shown in Fig.3. The solid line represents that m o d e l used for calculation o f corrections to the P-wave travel-time residuals. The other t w o curves represent alternative interpretations o f the first-arrival data.
261
km average depth to " b a s e m e n t " is only approximate due to the probable complexities in the velocity structure that are not well represented by a simple three-layer model. The layer o f velocity 3.2 km/s, determined primarily b y strong second arrivals, is taken as representing ignimbrite or rhyolite while the shallower material m a y be pumice, ash, etc. The velocities at depths less than 0.5 km are not well determined as there are no close-in observations to constrain them. The shallow velocities shown are similar to those measured b:~ Hochstein and H u n t (1970) at Broadlands. For the purposes o f calculating a correction to the observed relative residuals, the vertical travel-time through the upper 2 km was compared with that for a "normal" greywacke crust found near Wellington (Garrick, 1968). At sites situated on ignimbrite or rhyolite (as most were), the 3.2 km/s layer was taken as extending to the surface, giving a delay o f 0.17 s. At the few sites situated on pumice or ash the delay is 0.32 s. These corrections have been applied to the observed relative residuals at all sites within the TVZ. In another interpretation o f the refraction results, the first-arrival data can be explained by a gradual increase in velocity with depth, either linear d o w n to a final velocity o f 5.45 km/s at a depth of 2.5 km or hyperbolic as shown in Fig.4. In any case, the corrections to vertical P-wave travel-times, down to a depth of 2.5 or 3 km, respectively, are nearly the same as in the previous interpretation. The resulting relative P-wave travel-time residuals are shown in Fig.5, data from stations outside the TVZ being shown b y solid circles, from those inq )-~
0.6
\
0.~ 0
0 0.2 u~
~x O\ 0
o.o C3
0
0 \
0 0
Q.2
\Q
?1'
i
i
2O
L
i
i
4O
i
6O
~0
DISTANCE, km
Fig.5. Average P-wave travel-time residuals from local mantle earthquakes as a function of distance along the cross-section line shown in Fig.2 (site 17 equals 0.0 distance). Solid circles represent data for sites outside the Taupo Volcanic Zone, open circles represent data for sites within it. The latter have been corrected for the near-surface (< 2 kin) structure.
262
side by open circles. The major feature o f the data is the approximately linear increase in residual from east to west fo a b o u t 1.3 s, the trend being well defined b y the data from either side o f the TVZ. The configuration of o u r experiment is shown in Fig.6, drawn to scale for a typical source event. From this it is clear that a large region (on the order of 20 km in size) of relatively low velocity local to the crust under the TVZ is not indicated by the results. There are no significant positive departures (low velocity) from the regional trend in residuals at sites within the TVZ, the largest being 0.21 s which is within the expected scatter due to experimental and observational errors. The largest departure from the regional trend is negative, --0.41 s, at site 13, based on t w o consistent observations. This would imply a local region of relatively high velocity there. TVZ 17
8
1
!
I°
50 km
~v
Fig.& Simplified diagram of the configuration of our experiment drawn to scale. Triangles are seismograph sites.
Despite the apparent lack of a significant P-wave delay associated with the central TVZ itself, the substantial regional variation must reflect lateral variations in velocity, most probably in the mantle. A part of the explanation m a y be the relatively greater proportion of path length within the high-velocity subducted Pacific lithosphere for paths to the east. However, it could be argued that we are really seeing the effect of a crustal low-velocity region that extends to the west b e y o n d the boundary o f the TVZ and that if our observations were extended still further west, the relative residuals would drop again. The fact that there is little in the way of recent volcanic activity to the west, where t h e low-velocity region would have to be most well-de-
263 veloped, is then very puzzling and thus we do not think this suggestion is very likely to be true although it can not yet be ruled o u t completely. The above observations are not sufficient in themselves, however, to rule out the possibility o f a region o f partial melt in the crust under the TVZ since structural and compositional changes could offset the decrease in velocity that would result. For example, if the intrusive rocks thought to exist under the TVZ have a velocity higher, when cooled, than the surrounding crustal rocks, they could be partially molten (perhaps b y 5%) and still produce no large relative travel-time delays. The S-wave attenuation data to be discussed below are important in this regard and also in regard to the cause of the regional trend in P-wave residuals. P/S AMPLITUDE RATIOS IN THE CENTRAL TAUPO VOLCANIC ZONE The a m o u n t of inelastic attenuation of P- and S-waves in the crust seems to be quite variable. Under normal conditions, however, the value of Qp/Qs appears to be close to 2.0 with recent determinations of Qp on the order of 1000 (Hashizume, 1979; Brocher, 1979). At temperatures high enough to cause partial melting, however, theoretical (e.g., O'Connell and Budiansky, 1977) and laboratory (e.g., Murase and McBirney, 1973) results indicate that attenuation will increase substantially (low Q values). For example, taking a 4% melt fraction and a frequency of 3 Hz, the results of O'Connell and Budiansky (1977) give a Qp o f a b o u t 50, a Qs of about 10. The important point here is that the ratio Qp/Qsbecomes much larger at temperatures above the solidus than at temperatures below. Field evidence for this includes the use of S-wave "shadow zones" to map concentrations of magma beneath volcanoes (e.g., Matumoto, 1971), a zone where Qs is thought to be a b o u t 10 beneath the Mid-Atlantic Ridge (Solomon, 1973), and the attenuation of S-waves propagating through parts of the Yellowstone caldera (Eaton et al., 1975). It would seem, then, that observations of the relative attenuation of P- and S-waves for paths through and to either side of the TVZ would provide information on the presence or absence of a local region of partial melt. The values of Qp and Qs q u o t e d above for a 4% partial-melt region are sufficient to effectively cause the disappearance of 3-Hz S-waves relative to Pwaves for propagation through a region on the order of 10 km thick. Examination of the records from the deep earthquakes used above showed that strong arrivals at approximately the expected time for S-waves were sometimes recorded. Permanent stations of the New Zealand network commonly record S-arrivals of quite large amplitude, often with quite sharp onsets, from similar events. An a t t e m p t to evaluate the nature of these S-waves qualitatively produced no clear pattern of presence or absence of strong Swaves. Stations to the east of the TVZ, however, seemed to record strong Swaves more often than those within or to the west. Quantitative data on S-wave attenuation can be obtained b y study of distant events so that the probability of significant changes in radiation
P
Fig.7. Records of an earthquake in the Kermadec Islands area, north of New Zealand, at sites 3 and 15. Note the much smaller S-wave amplitude at site 15.
SITE 15
SITE 3
S
bo
265
pattern among rays to the observation points is small. However, at distances b e y o n d a b o u t 10°--15 ° short-period S-waves are effectively absent due to normal mantle inelastic attenuation, probably in the low-velocity zone o f the upper mantle. Fortunately a fair number of usable deep focus events occur to the north of New Zealand in the Kermadec region. An example is shown in Fig.7. F o r these events that were well recorded, the P/S amplitude ratios at our temporary sites have been measured and normalised in each case to the value observed at the reference site. Amplitude ratios are used rather than just amplitudes, in order to compensate for instrument and site gain factors and geometric spreading. Average ratios, based on from one to six observations, are shown in Fig.8 and Table 1; the higher the ratio, the higher the attenuation of S-waves relative to P-waves. Although there is considerable scatter, the majority o f the values define a westward increase in the ratio. Sites within the central TVZ do not exhibit abnormally high values relative to this regional trend, except perhaps at site 12. However, the two observations at site 12 have a very high variance (Table 1) so that not t o o much can be inferred from them. Mooney (1970), using observations of predominant frequency in recordings of deep earthquakes at permanent stations of the New Zealand seismograph network, has suggested that the whole o f the northwest North Island is underlain b y a relatively attenuative region in the mantle. This region is centred at a b o u t 100 km depth and is a b o u t 50 km thick. It possibly represents an intensification of the normal upper mantle low-velocity zone. It seems likely that we are observing across the eastern edge of this attenuative region. Moreover, the correspondence in trends o f the P-wave residuals and of the P/S amplitude ratios suggests a c o m m o n explanation, at least in part: that the mantle attenuative zone is also a region of relatively low P-wave velocity.
O .I)
o o L
J
20
~
~ o - ~
o i
I
~
40
I
60
I
I
t
80
DISTANCE, km
Fig.8. Average normalisecl P/S a m p l i t u d e r a t i o s as a f u n c t i o n o f d i s t a n c e along the line o f cross-section s h o w n in Fig.2. Solid circles are d a t a f r o m sites o u t s i d e t h e T a u p o Volcanic Zone, o p e n c ~ c l e s are data f r o m w i t h i n it.
266 CONCLUSION T h e c r u s t u n d e r t h e h y d r o t h e r m a l areas o f t h e c e n t r a l T V Z d o e s n o t prod u c e significant P-wave travel-tinge d e l a y s o r high S-wave a t t e n u a t i o n relative t o t h e areas i m m e d i a t e l y o n e i t h e r side. T h e s e results s e e m t o rule o u t t h e p r e s e n c e o f e x t e n s i v e regions o f s e m i - m o l t e n or m o l t e n r o c k local t o t h e crust b e n e a t h t h e region. This c o n t r a s t s to t h e s i t u a t i o n at s o m e o t h e r areas o f s t r o n g h y d r o t h e r m a l a c t i v i t y . In o t h e r words, e x c e p t f o r t h e p r o b a b i l i t y o f small localised h o t spots, t h e large v o l u m e o f intrusive r o c k s a n d associated crustal m e l t i n f e r r e d t o b e p r e s e n t o n geological g r o u n d s are n o w at t e m p e r a t u r e s b e l o w t h e solidus. It is possible t h a t this is indicative o f a m o r e active intrusive p e r i o d in t h e p a s t (as m i g h t be e x p e c t e d o n t h e b a t h o l i t h m o d e l ) so t h a t s u f f i c i e n t t i m e has elapsed f o r s u b s e q u e n t cooling t o t a k e place. O n t h e o t h e r h a n d , it c o u l d i n d i c a t e t h a t i n t r u s i o n i n t o t h e crust f r o m d e e p e r levels ( t h e m a n t l e a t t e n u a t i v e zone?) occurs~only in a q u i t e limited area, in c o n t r a s t to t h e large ar@as involved at t h e Y e l l o w s t o n e and G e y s e r s areas. This limited i n t r u s i o n m i g h t o c c u r a l o n g t h e eastern edge o f t h e m a n t l e a t t e n u a t i v e zone, w h i c h w o u l d c o r r e s p o n d t o t h e eastern T V Z , a n d a p p e a r s t o be m o r e c o m p a t i b l e w i t h t h e s p r e a d i n g m o d e l o f t h e region t h a n w i t h t h e b a t h o l i t h model.
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= 2~'~
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