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Lower crustal and upper mantle electrical conductivity contrasts in the central North Island of New Zealand
304
Physics of the Earth and Planetary Interiors, 49 (1987) 304—3 13 Elsevier Science Publishers BY., Amsterdam — Printed in The Netherlands
Lower crustal and upper mantle electrical conductivity contrasts in the central North Island of New Zealand Malcolm R. Ingham Research School of Earth Sciences, Victoria University of Wellington, Private Ba& Wellington (New Zealand) (Received October 3, 1986; revision accepted April 1, 1987)
Inghain, M.R., 1987. Lower crustal and upper mantle electrical conductivity contrasts in the central North Island of New Zealand. Phys. Earth Planet. Inter., 49: 304—313. Magnetotelluric soundings made in the central North Island of New Zealand over the last 20 years have been reanalysed, together with more recent soundings, in an attempt to detect any electrical conductivity contrast associated with a known boundary in other geophysical properties. ‘Invariant’ apparent resistivity curves from 11 sites in all have been compared and curves which appear to be representative of the different regions have been selected. One-dimensional modelling has led to the identification of a good conductor at lower crustal or upper mantle depths in the northwest of the North Island. No such conductor is identified in the south and east. Modelling of a single site in the Taupo Volcanic Zone has led to the first indication yet obtained of a deeper conductor beneath the geothermal region.
1. Introduction £ Active volcano
Mooney (1970) presented evidence that the upper mantle beneath the North Island of New Zealand could be divided into two regions. The boundary between the two regions was proposed as running from northeast to southwest down the centre of the island to a point just south of the central volcanic plateau and then due west to the coast (Fig. 1). To the northwest of the line Mooney reported the attenuation in the mantle of seismic frequencies of 3 Hz and greater. To the south and east of the boundary such frequencies are transmitted. Hatherton (1970), in a companion paper to Mooney’s, correlated the proposed boundary with various other geological and geophysical features. He pointed out that in the North Island of New Zealand calc-alkaline andesites occur only over the attenuating region. Furthermore all the recently active andesite volcanoes lie along the boundary between the attenuating and transmitting regions (Fig. 1). A good correlation also exists 0031-9201/87/$03.50
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between the measured heat flow and the supposed upper mantle inhomogeneity. To the south and east of the boundary, heat flow is below normal whilst it is near or above normal in the northwest. To the northeast of the central volcanic plateau the Taupo Volcanic Zone (TVZ in Fig. 1) lies along the boundary between the two regions and marks the main area of geothermal activity in the North Island. All the hot springs and other surface manifestations of geothermal activity occur in the Volcanic Zone or, to a lesser extent, above the attenuating region. Hatherton also noticed that the trend of Bouger gravity anomalies follows the boundary quite closely. Positive anomalies occur to the northwest whilst predominantly negative anomalies occur to the south and east. All the above features are themselves linked to the active subduction zone off the east coast of the North Island. The Hikurangi Trench (Fig. 1) is the southern extension of the Tonga—Kermadec Trench and marks the line along which the Pacific plate, to the east, is subducted beneath the Indo—Australian plate, to the west. As can be seen from Fig. 1 the bend in the attenuating—transmitting boundary follows the bend in the Trench axis. Hence the low heat flow in the south and east can be understood as being the result of the presence of the cold subducted plate beneath the region. To the northwest frictional heating as the downgoing slab interacts with the overlying lithosphere leads to higher heat flow, geothermal manifestations and the line of active volcanoes. The higher temperature and presence of magma results in the seismic attenuation. The aim of this paper is to compare magnetotelluric sounding results from the attenuating and transmitting regions, as well as the Taupo Volcanic Zone, to see if there is any evidence for upper mantle inhomogeneity in electrical conductivity. Such inhomogeneity might be expected if vastly different thermal regimes exist beneath the different regions. The results presented are to a large extent drawn from various masters and doctoral theses completed at Victoria Umversity of Wellington, New Zealand and themselves represent a
soundings carried out by the author are also presented. The results also give an indication of the variations in electrical conductivity structure likely to be found during a proposed major study along a traverse across the Taupo Volcanic Zone.
review of previously unpublished magnetotelluric studies carried out in the North Island of New Zealand during the last 20 years. More recent
Fig. 2. The locations of magnetotelluric sites in the central North Island in relationship to the attenuating—transmitting boundary and the Taupo Volcanic Zone.
2. MT results 1969—1979 Before 1980 magnetotelluric soundings in the central North Island of New Zealand had been carried out by Marriott (1969), Hurst (1974) and Midha (1979). The locations of the sites occupied by these authors in relationship to the attenuating—transmitting boundary and the Taupo Volcamc Zone are shown in Fig. 2. Also shown in Fig. 2 are the locations of four recent magnetotelluric soundings by the present author, the results of which are presented in the next section. The data quality and methods of analysis and presentation of results used by Marriott, Hurst and Midha are very variable and are discussed individually below. For the purposes of this paper the results have all been reduced to a common form. As the main interest is to attempt to compare gross one-dimensional electrical conductivity structures for the attenuating and transmitting regions, and also the Taupo Volcanic Zone, the method which has been chosen is to present curves of ‘invariant’ apparent resistivity against period.
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The ‘invariant’ apparent resistivity used, p1, is that calculated from the impedance Z = 0 5(Z z ‘~
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The magnetotelluric results of Marriott, Hurst and Midha are therefore presented as curves of p. against period, T (Fig. 3). The lengths of the bars giving the apparent resistivity at each point represent the range of values between the bounds expressed by eqs. 3a, b. Marriott (1969) made recordings of the tellunc and magnetic fields at a single site PUK. The location of the site (Fig. 2) was southwest of Lake Taupo some 20 km north of the central North Island volcanoes. Apparent resistivity curves were obtained in the co-ordinate system of the measurement axes (magnetic NS and EW) both by visual inspection of the chart records and by spectral analysis. Results were calculated in the period range 2—300 s. Marriott reported the p~,and ~ curves to be similar at short periods but to be slightly anisotropic at periods above 40 s with ~ being the larger of the two apparent resistivities. Marriott used curve matching techniques to model the ~ curve in terms of a three layer model which had a resistivity of 37.5 ~m at the surface, a layer
of resistivity of the order of 10 000 ~2mextending from 4.5 to 90 km depth and a basement layer with a resistivity of tens of ohm-metres. A check on the applicability of one-dimensional curve —
matching was made by calculating the ratio of vertical to horizontal magnetic field variations. This ratio was reported as being <0.1 for periods <50 s and between 0.15 and 0.2 at longer periods. Rotation of the impedance tensor axes was carried out by Marriott only for the period of 25 s. The result showed the principle impedance axis at this period to be oriented N60°W magnetic—approximately perpendicular to the attenuating— transmitting boundary shown in Fig. 2. The p, curve for PUK is shown in Fig. 3a. The multiple apparent resistivity estimates of Mamott smoother useestimates of eq. 3 above. have beenvariation averaged before to give the fewer with a This has led to the identification of a possible inflection in the curve at around 50 s period which was not visible in the original data. Alternatively the possible inflection may be due purely to the scatter in the original data. As part of a study centered just to the north of the central volcanic plateau Hurst (1974) recorded magnetotelluric data at three sites. At one of these, Ngakonui, problems with man-made noise prevented the calculation of apparent resistivity curves. The locations of the other two sites, TER and TAH, are shown in Fig. 2 and are important because both lie within the Taupo Volcanic Zone. At TER Hurst obtained apparent resistivity curves in the period range 10—500 5 with a single estimate at micropulsation periods around 2 s. Taking uncertainties into account the curves were only slightly amsotropic with ~ larger than p~,. There was very little variation in apparent resistivity with period apart from a gradual increase in ~ at periods above 40 s. Hurst, perhaps rather ambitiously, attempted to model his results from TER using two-dimensional numerical calculations. His final model placed TER at the eastern edge of an outcropping block of resistivity 6.7 ~2m, approximate width 25 km and thickness 5.4 km, striking along the Taupo Volcanic Zone and embedded in an Earth of resistivity 35 fm. The p 1 curve for TER shown in Fig. 3a has been calculated directly from the original p~, and
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308
curves without any averaging of the data. The resultant scatter in values is apparent. For the second site, TAH, good quality ~ and curves were obtained by Hurst in the period range 2—200 s. The apparent resistivities were generally low, of the order of 10 ~m, with some anisotropy at the shortest periods. Rotation of the impedance tensor led to the determination of the principle axis as being oriented N90°E (— N70°E magnetic). A one-dimensional model fitted to the apparent resistivity curves had a 100 ~lm surface layer to a depth of 3.2 km. This was underlain by an 11 ~lm layer the thickness of which was unresolvable but suggested to be a minimum of 10 km. Hurst proposed that the surface layer might be anisotropic to account for the anisotropy in the apparent resistivity curves at short period. A site very close, if not identical, to TAH was occupied by Midha (1979) as part of the largest magnetotelluric and magnetovariational study yet conducted in the region. Apparent resistivity estimates were obtained by Midha in the period range 600—6000 s using autoregressive spectral techniques. Although it is of some concern that very little data were used by Midha in obtaining the apparent resistivity curves they do smoothly extend the results of Hurst to longer periods. Hence the p. curve for TAH in Fig. 3a has been derived from the original results of both authors. The two separate recording bands are clearly distinguishable. Midha also carried out magnetotelluric soundings at four other sites the locations of which are shown in Fig. 2. Of these MMK and MKN lie above the attenuating region of Mooney (1970) whilst GAL and RTK are above the eastern part of the transmitting region. In all cases maximum and minimum apparent resistivity curves were derived from relatively small amounts of data using autoregressive techniques. At all four sites there are unrealistically steep increases in the Pm~ curve at the shortest periods and at MMK, MKN and GAL considerable scatter in the minimum apparent resistivity curve. The effects of these factors can be seen in the p, curves shown in Fig. 3b. No attempt was made by Midha to model these magnetotelluric results. However, the principle imPYX
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pedance directions were determined. At all four sites MMK, MKN, GAL and RTK, at all periods, the principle directions were approximately perpendicular to the Taupo Volcanic Zone. 3. Recent MT results During February 1985 four additional magnetotelluric sites were deployed in an attempt to detect any inhomogeneity in electrical conductivity structure across the attenuating—transmitting boundary. The locations of the sites (Fig. 2) were chosen to avoid any anomalous effects due to the Taupo Volcanic Zone. However, this criterion did lead to one site, TEA, being within 10 km or so of a coastline. In Fig. 4a, b maximum and minimum apparent resistivities are shown for each of the sites along with the phase curve for the maximum impedance. At all four sites the skew was low (<0.2) and the azimuth of the impedance tensor variable. The most unsatisfactory results were obtained at TEA where noise problems were encountered. These, together with the relative proximity to both the coast and the deep sediments of the Waganui Basin to the east, result in the rather scattered, anisotropic apparent resistivity curves for TEA shown in Fig. 4a. Noise problems are also the cause of some scatter in the curves for NGM. Nevertheless both the Pm~ and p,,...11~ curves are reasonably well defined at NGM. The best data were obtained at MAK and GOW, both sites above Mooney’s attenuating region. Indeed the results from these two sites are most striking for their similarity. In particular, both maximum apparent resistivity curves exhibit a well defined inflection at a period of 100 s. It seems reasonably apparent from the results for these four sites that if any boundary in electrical conductivity structure does occur it lies to the south of MAK. The maximum apparent resistivity curve at NGM is about half a decade higher in value than at MAK and GOW. However, the extraneous effects at TEA make it difficult to judge whether, if a boundary exists, it lies between MAK and NGM or NGM and TEA. —
309
The two ‘invariant’ apparent resistivity curves from the Taupo Volcanic Zone, TER and TAH,
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oow. 4. Comparison and modelling of MT results Smoothed p. curves for all the sites discussed above are shown in Fig. 5. The curves are distinguished both by the site name and according to the site location in the attenuating region (labelled N/W), the Taupo Volcanic Zone (TVZ) or the transmitting region (S/E).
cal setting it seems reasonable that higher resistivities might be expected at the former three sites. Inspection of Fig. 5 shows that the curves from PUK and MMK effectively parallel those of MAK and 00W but are displaced by half a decade. Indeed as mentioned above, just as at MAK and .
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apparent resistivity to be much less marked.
The p, curves for sites on the south and east, i.e. the transmitting region, are much more disparate. As indicated above, TEA is not considered to be typical because of the noise problems encountered and its location. The other three sites NGM, RTK and GAL are all similar at the longest periods (>1500 s). At shorter periods the curves for RTK and GAL show a rapid fall in apparent resistivity. This can be attributed to the exceptionally and unrealistically steep portions of the Pmax
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curves mentioned above and it seems likely that the curve for NGM is better representative of the transmitting region. RTK and GAL are close to boundaries between ignimbrite, bordering the Taupo Volcanic Zone, and greywacke further east. NGM has the same geological setting as MAK and GOW. In light of the above discussion it seems reasonable to accept MAK and 00W as representative of sites in the attenuating region. More tentatively, because of uncertainty in the exact location of the boundary, NGM has been taken as representative of those in the transmitting region. For the purposes of comparing the regional structures one-dimensional modelling of the response for NGM and the average response for MAK and 00W has therefore been carried out. In addition the curve for TAH has been modelled to give a resistivity structure for the Taupo Volcanic Zone. It must be remembered, however, that each of the regions covers a considerable area and that no single resistivity structure is necessarily valid for the whole region. In the case of the Taupo Volcanic Zone in particular the variation in near surface electrical conductivity associated with the geothermal areas means that no single site is likely to be
typical. Because of this the relative variations in resistivity with depth are of greater importance in comparing the regions than are the actual values of resistivity and depth themselves. Figure 6a shows a one-dimensional model fit to the average ‘invariant’ curve for MAK and GOW. The five layer model shown provides a reasonable fit to both the apparent resistivity and phase curves. The resistivity and thickness of the surface conductive layer are well defined only in that the integrated conductance of the layer is 450 S. The depth and thickness of the deeper conductive layer are not particularly well resolved. Such a layer, of resistivity <50 ~m, is required, in either the lower crust or upper mantle, to give the inflection in the apparent resistivity curve at around 100 s. The phase curve for NGM is too scattered to be of any use in modelling. A typical model fitting the apparent resistivity curve at the site is shown in Fig. 6b. The lower conductive layer of the three layer model is necessary to give the flattening of the curve at longer periods. No lower crustal or upper mantle conductor is evident indicating a major difference in the conductivity structures for the attenuating and transmitting regions. As was —
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suggested above, the cause of greater lower crustal—upper mantle conductivity beneath the attenuating region is likely to be the existence of higher temperatures and possibly partial melt aris-
ing from frictional heating of the subducted plate. Also shown in Fig. 6b is a model giving a fit to the p1 data from TAH. A four layer model is required to give a satisfactory fit. The surface
312
conductive layer can probably be identified with the surface geothermal manifestations and possibly gives an estimate of the depth to which the system extends. The deeper conductor at midcrustal to upper mantle depths is perhaps the first indication of the deeper source of the geothermal features of the Taupo Volcanic Zone. An important point to note is that a model without the intermediate resistive layer between the two conductors does not fit the data nearly as well.
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The magnetotelluric results seem to suggest the existence of a lower crustal—upper mantle electrical conductivity contrast between the attenuating and transmitting regions. In addition considerably lower resistivities exist throughout the crust and possibly into the upper mantle beneath the Taupo Volcanic Zone. One area in which it is possible to look for support for this hypothesis is in the results of magnetovariational studies in the region. Marriott (1969) did not calculate induction arrows or their equivalent for his site PUK. Parkinson vectors were calculated for both TAH and TER by Hurst (1974). He reported that at TAH for periods between 40 and 120 s the vector had a magnitude of about 0.14 and was within 5° of magnetic north (— N20 E). At TER, for penods <60 s, the vector had magnitude 0.28 and a direction close to N70°W. At longer periods the vector was oriented magnetic north. By far the most extensive magnetovariational results were obtained by Midha (1979) who calculated single-station transfer functions at 16 sites. Reversed real and imaginary induction arrows for these sites and for 00W, MAK, NGM and TEA as well as one additional site originally reported by Ingham (1985) are shown for two periods of variation in Fig. 7. The results for a period of 1500 s, Fig. 7a, make clear why little can be learnt about any conductivity contrast associated with the attenuating—transmitting boundary from the results of Midha. All of the real arrows from Midha’s sites point northeast and have very urnform magnitude. This behaviour is characteristic of the arrows at all periods above 1000 s and was interpreted by Midha as evidence of a major con-
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ductive region trending northwest to southeast to the north of New Zealand. Indeed at periods 3000 s the real induction arrows at GOW, MAK, NGM and TEA also swing to point in this northeastern direction. The existence of such a conductive structure is currently under investigation using ocean bottom based measurements (D.A. Christoffel, personal communication, 1986). At the shorter period of 375 s, Fig. 7b, Midha’s arrows have generally small amplitude and considerable scatter in direction. There is a hint of q reversal in real arrows associated with the western edge of the Taupo Volcanic Zone but again no obvious feature is apparent. The more recent sites further south yield more promising results. The real arrow at TEA is unam>
313
biguously effected by the coastline to the south and the Waganui Basin to the east. However, a persistent reversal in real arrows occurs between NGM and the site reported by Ingham (1985). This reversal is in fact south of the proposed attenuating—transmitting boundary and it is possible that NGM is a site intermediate between the two regions rather than truly representative of the transmitting region. At the shorter period the real arrows at MAK and 00W point towards Mt. Taranaki possibly indicating the existence of shallow high conductivity beneath the volcano. The imaginary arrows at all of these recent sites also point towards Mt. Taranaki at 1500 s period. At the longer period the real arrows at MAK and 00W are very small in magnitude, an observation which is compatible with the sites being on relatively uniform layered structure.
proposed major magnetotelluric traverse across the Taupo Volcanic Zone should not only elucidate the electrical conductivity contrast and the relationship to it of the Taupo Volcanic Zone itself, but also enable interpretation of the contrast in terms of the tectonic structure of the North Island of New Zealand.
6. Summary and conclusion
Geophysics, University of Edinburgh and thanks go to Prof. K.M. Creer for the facilities made available. Finally, this work was funded in part by
This paper has reported on a comparison of magnetotelluric sounding results obtained over the last 20 years in and around the Taupo Volcanic Zone of the North Island of New Zealand. In particular an attempt has been made to investigate if any electrical conductivity contrast exists coincident with a major geophysical boundary reported by Mooney (1970) and Hatherton (1970). Arguments have been put forward for selecting magnetotelluric curves typical of the attenuating and transmitting regions of Mooney and of the Taupo Volcanic Zone. The ‘invariant’ apparent resistivity curves selected have been modelled using layered Earth models. The results obtained are that: (1) there is evidence for a lower crustal or upper mantle conductor beneath Mooney’s attenuating region; (2) no such conductor appears to exist beneath the transmitting region; and (3) one-dimensional modelling of a single site in the Taupo Volcanic Zone suggests the existence of a deep conductor beneath the geothermal area. In conclusion it seems likely that a conductivity inhomogeneity does exist and some support for this comes from magnetovariational results. A
Acknowledgements Prof. D.A. Christoffel supervised most of the students whose research is reported. D. Jepsen and E. Broughton provided valuable aid in the field and laboratory, respectively. The helpfulness and kindness of all the landowners on whose property measurements were made is gratefully acknowledged. This paper was written whilst the author was on study leave at the Department of
New Zealand University Grants Committee grant No. 83/15 3 and the Victoria University Internal Research Fund.
References Hatherton, T., 1970. Upper mantle inhomogeneity beneath New Zealand: surface manifestations. J. Geophys. Res., 75:
269-284. Hurst, A.W., 1974. Magnetic Effects in Volcanic Regions.
Ph.D. 369 Thesis, land, pp. Victoria University of Wellington, New ZeaIngham, M.R., 1985. Magnetovariational measurements in the Cook Strait region of New Zealand. Phys. Earth Planet. Inter., 39: 182—193. In&oano, M.R., 1987. The use of invariant impedances in magnetotelluric interpretation. Geophys. J. R. Astron. Soc., in press. Marriott, W.G., 1969. A Magnetotelluric Investigation of the North Island Volcanic Plateau. M.Sc. Thesis, Victoria Urnversity of Wellington, 81 pp. Midha, R.K., 1979. Geoelectromagnetic induction studies in the North Island VolcanicofRegion, New 342 Zealand. Ph.D. Thesis, Victoria University Wellington, pp. Mooney, H.M., 1970. Upper mantle inhomogeneity beneath New Zealand: seismic evidence. J. Geophys. Res., 75: 285-309.
Physics of the Earth and Planetary Interiors, 49 (1987) 304—3 13 Elsevier Science Publishers BY., Amsterdam — Printed in The Netherlands
Lower crustal and upper mantle electrical conductivity contrasts in the central North Island of New Zealand Malcolm R. Ingham Research School of Earth Sciences, Victoria University of Wellington, Private Ba& Wellington (New Zealand) (Received October 3, 1986; revision accepted April 1, 1987)
Inghain, M.R., 1987. Lower crustal and upper mantle electrical conductivity contrasts in the central North Island of New Zealand. Phys. Earth Planet. Inter., 49: 304—313. Magnetotelluric soundings made in the central North Island of New Zealand over the last 20 years have been reanalysed, together with more recent soundings, in an attempt to detect any electrical conductivity contrast associated with a known boundary in other geophysical properties. ‘Invariant’ apparent resistivity curves from 11 sites in all have been compared and curves which appear to be representative of the different regions have been selected. One-dimensional modelling has led to the identification of a good conductor at lower crustal or upper mantle depths in the northwest of the North Island. No such conductor is identified in the south and east. Modelling of a single site in the Taupo Volcanic Zone has led to the first indication yet obtained of a deeper conductor beneath the geothermal region.
1. Introduction £ Active volcano
Mooney (1970) presented evidence that the upper mantle beneath the North Island of New Zealand could be divided into two regions. The boundary between the two regions was proposed as running from northeast to southwest down the centre of the island to a point just south of the central volcanic plateau and then due west to the coast (Fig. 1). To the northwest of the line Mooney reported the attenuation in the mantle of seismic frequencies of 3 Hz and greater. To the south and east of the boundary such frequencies are transmitted. Hatherton (1970), in a companion paper to Mooney’s, correlated the proposed boundary with various other geological and geophysical features. He pointed out that in the North Island of New Zealand calc-alkaline andesites occur only over the attenuating region. Furthermore all the recently active andesite volcanoes lie along the boundary between the attenuating and transmitting regions (Fig. 1). A good correlation also exists 0031-9201/87/$03.50
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Fig. 1. The North Island of New Zealand showing the division into attenuating and transmitting regions proposed by Mooney (1970). The Taupo Volcanic Zone is also shown.
305
between the measured heat flow and the supposed upper mantle inhomogeneity. To the south and east of the boundary, heat flow is below normal whilst it is near or above normal in the northwest. To the northeast of the central volcanic plateau the Taupo Volcanic Zone (TVZ in Fig. 1) lies along the boundary between the two regions and marks the main area of geothermal activity in the North Island. All the hot springs and other surface manifestations of geothermal activity occur in the Volcanic Zone or, to a lesser extent, above the attenuating region. Hatherton also noticed that the trend of Bouger gravity anomalies follows the boundary quite closely. Positive anomalies occur to the northwest whilst predominantly negative anomalies occur to the south and east. All the above features are themselves linked to the active subduction zone off the east coast of the North Island. The Hikurangi Trench (Fig. 1) is the southern extension of the Tonga—Kermadec Trench and marks the line along which the Pacific plate, to the east, is subducted beneath the Indo—Australian plate, to the west. As can be seen from Fig. 1 the bend in the attenuating—transmitting boundary follows the bend in the Trench axis. Hence the low heat flow in the south and east can be understood as being the result of the presence of the cold subducted plate beneath the region. To the northwest frictional heating as the downgoing slab interacts with the overlying lithosphere leads to higher heat flow, geothermal manifestations and the line of active volcanoes. The higher temperature and presence of magma results in the seismic attenuation. The aim of this paper is to compare magnetotelluric sounding results from the attenuating and transmitting regions, as well as the Taupo Volcanic Zone, to see if there is any evidence for upper mantle inhomogeneity in electrical conductivity. Such inhomogeneity might be expected if vastly different thermal regimes exist beneath the different regions. The results presented are to a large extent drawn from various masters and doctoral theses completed at Victoria Umversity of Wellington, New Zealand and themselves represent a
soundings carried out by the author are also presented. The results also give an indication of the variations in electrical conductivity structure likely to be found during a proposed major study along a traverse across the Taupo Volcanic Zone.
review of previously unpublished magnetotelluric studies carried out in the North Island of New Zealand during the last 20 years. More recent
Fig. 2. The locations of magnetotelluric sites in the central North Island in relationship to the attenuating—transmitting boundary and the Taupo Volcanic Zone.
2. MT results 1969—1979 Before 1980 magnetotelluric soundings in the central North Island of New Zealand had been carried out by Marriott (1969), Hurst (1974) and Midha (1979). The locations of the sites occupied by these authors in relationship to the attenuating—transmitting boundary and the Taupo Volcamc Zone are shown in Fig. 2. Also shown in Fig. 2 are the locations of four recent magnetotelluric soundings by the present author, the results of which are presented in the next section. The data quality and methods of analysis and presentation of results used by Marriott, Hurst and Midha are very variable and are discussed individually below. For the purposes of this paper the results have all been reduced to a common form. As the main interest is to attempt to compare gross one-dimensional electrical conductivity structures for the attenuating and transmitting regions, and also the Taupo Volcanic Zone, the method which has been chosen is to present curves of ‘invariant’ apparent resistivity against period.
) J /
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~ PRESENT PAPER
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306
The ‘invariant’ apparent resistivity used, p1, is that calculated from the impedance Z = 0 5(Z z ‘~
\
XY
YXI
‘ ‘
which is invariant with respect to rotation of the axes of the impedance tensor. Even in the vicinity of three-dimensional structure a one-dimensional interpretation of Z~gives a reasonable approximation to the resistivity—depth structure beneath the measurement site (Ingham, 1987). As only the gross one-dimensional structure of each region is of interest the use of p is thus appropriate. Unfortunately none of the above authors calculated the variation of phase with period. However, p, can be expressed in terms of ~ 4~.,and 4~(where the phases he between 0 and 90 ) as +~ cos( ci~ I~ + p, = (2) Hence, in the absence of phase data p~can still be said to lie between the bounds ~
0.25k
—
(max’l = 0 251 ~PXY
/
and p.(mm) I
=
+
+ PYX
O.25(pxy + pYX )
2 I VP~~YPYX /
~3a~ “
/
(3b)
The magnetotelluric results of Marriott, Hurst and Midha are therefore presented as curves of p. against period, T (Fig. 3). The lengths of the bars giving the apparent resistivity at each point represent the range of values between the bounds expressed by eqs. 3a, b. Marriott (1969) made recordings of the tellunc and magnetic fields at a single site PUK. The location of the site (Fig. 2) was southwest of Lake Taupo some 20 km north of the central North Island volcanoes. Apparent resistivity curves were obtained in the co-ordinate system of the measurement axes (magnetic NS and EW) both by visual inspection of the chart records and by spectral analysis. Results were calculated in the period range 2—300 s. Marriott reported the p~,and ~ curves to be similar at short periods but to be slightly anisotropic at periods above 40 s with ~ being the larger of the two apparent resistivities. Marriott used curve matching techniques to model the ~ curve in terms of a three layer model which had a resistivity of 37.5 ~m at the surface, a layer
of resistivity of the order of 10 000 ~2mextending from 4.5 to 90 km depth and a basement layer with a resistivity of tens of ohm-metres. A check on the applicability of one-dimensional curve —
matching was made by calculating the ratio of vertical to horizontal magnetic field variations. This ratio was reported as being <0.1 for periods <50 s and between 0.15 and 0.2 at longer periods. Rotation of the impedance tensor axes was carried out by Marriott only for the period of 25 s. The result showed the principle impedance axis at this period to be oriented N60°W magnetic—approximately perpendicular to the attenuating— transmitting boundary shown in Fig. 2. The p, curve for PUK is shown in Fig. 3a. The multiple apparent resistivity estimates of Mamott smoother useestimates of eq. 3 above. have beenvariation averaged before to give the fewer with a This has led to the identification of a possible inflection in the curve at around 50 s period which was not visible in the original data. Alternatively the possible inflection may be due purely to the scatter in the original data. As part of a study centered just to the north of the central volcanic plateau Hurst (1974) recorded magnetotelluric data at three sites. At one of these, Ngakonui, problems with man-made noise prevented the calculation of apparent resistivity curves. The locations of the other two sites, TER and TAH, are shown in Fig. 2 and are important because both lie within the Taupo Volcanic Zone. At TER Hurst obtained apparent resistivity curves in the period range 10—500 5 with a single estimate at micropulsation periods around 2 s. Taking uncertainties into account the curves were only slightly amsotropic with ~ larger than p~,. There was very little variation in apparent resistivity with period apart from a gradual increase in ~ at periods above 40 s. Hurst, perhaps rather ambitiously, attempted to model his results from TER using two-dimensional numerical calculations. His final model placed TER at the eastern edge of an outcropping block of resistivity 6.7 ~2m, approximate width 25 km and thickness 5.4 km, striking along the Taupo Volcanic Zone and embedded in an Earth of resistivity 35 fm. The p 1 curve for TER shown in Fig. 3a has been calculated directly from the original p~, and
307 ,~Iflm)
PUK
1
1
100
TI’
I ii 1 (Am)
10
100
0001
i(s)
~
i(s)
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~
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tim) 100
10
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101
10
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III III
100
100 1000
10000
T(s)
1000
I(s)
10000
I(s)
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MKN
1000
RIK
II
0
TI
100
10000
i
ii T
P
I
1 ~‘
1000
100
10000
11
I(s) 1000
Fig. 3. (a) ‘Invariant’ apparent resistivity estimates for the sites PUK, TER and TAH. The length of the bars correspond to the limits given by eq. (3). (b) As for Fig. 3(a) for the sites MMK, MKN, GAL and RTK.
308
curves without any averaging of the data. The resultant scatter in values is apparent. For the second site, TAH, good quality ~ and curves were obtained by Hurst in the period range 2—200 s. The apparent resistivities were generally low, of the order of 10 ~m, with some anisotropy at the shortest periods. Rotation of the impedance tensor led to the determination of the principle axis as being oriented N90°E (— N70°E magnetic). A one-dimensional model fitted to the apparent resistivity curves had a 100 ~lm surface layer to a depth of 3.2 km. This was underlain by an 11 ~lm layer the thickness of which was unresolvable but suggested to be a minimum of 10 km. Hurst proposed that the surface layer might be anisotropic to account for the anisotropy in the apparent resistivity curves at short period. A site very close, if not identical, to TAH was occupied by Midha (1979) as part of the largest magnetotelluric and magnetovariational study yet conducted in the region. Apparent resistivity estimates were obtained by Midha in the period range 600—6000 s using autoregressive spectral techniques. Although it is of some concern that very little data were used by Midha in obtaining the apparent resistivity curves they do smoothly extend the results of Hurst to longer periods. Hence the p. curve for TAH in Fig. 3a has been derived from the original results of both authors. The two separate recording bands are clearly distinguishable. Midha also carried out magnetotelluric soundings at four other sites the locations of which are shown in Fig. 2. Of these MMK and MKN lie above the attenuating region of Mooney (1970) whilst GAL and RTK are above the eastern part of the transmitting region. In all cases maximum and minimum apparent resistivity curves were derived from relatively small amounts of data using autoregressive techniques. At all four sites there are unrealistically steep increases in the Pm~ curve at the shortest periods and at MMK, MKN and GAL considerable scatter in the minimum apparent resistivity curve. The effects of these factors can be seen in the p, curves shown in Fig. 3b. No attempt was made by Midha to model these magnetotelluric results. However, the principle imPYX
—
pedance directions were determined. At all four sites MMK, MKN, GAL and RTK, at all periods, the principle directions were approximately perpendicular to the Taupo Volcanic Zone. 3. Recent MT results During February 1985 four additional magnetotelluric sites were deployed in an attempt to detect any inhomogeneity in electrical conductivity structure across the attenuating—transmitting boundary. The locations of the sites (Fig. 2) were chosen to avoid any anomalous effects due to the Taupo Volcanic Zone. However, this criterion did lead to one site, TEA, being within 10 km or so of a coastline. In Fig. 4a, b maximum and minimum apparent resistivities are shown for each of the sites along with the phase curve for the maximum impedance. At all four sites the skew was low (<0.2) and the azimuth of the impedance tensor variable. The most unsatisfactory results were obtained at TEA where noise problems were encountered. These, together with the relative proximity to both the coast and the deep sediments of the Waganui Basin to the east, result in the rather scattered, anisotropic apparent resistivity curves for TEA shown in Fig. 4a. Noise problems are also the cause of some scatter in the curves for NGM. Nevertheless both the Pm~ and p,,...11~ curves are reasonably well defined at NGM. The best data were obtained at MAK and GOW, both sites above Mooney’s attenuating region. Indeed the results from these two sites are most striking for their similarity. In particular, both maximum apparent resistivity curves exhibit a well defined inflection at a period of 100 s. It seems reasonably apparent from the results for these four sites that if any boundary in electrical conductivity structure does occur it lies to the south of MAK. The maximum apparent resistivity curve at NGM is about half a decade higher in value than at MAK and GOW. However, the extraneous effects at TEA make it difficult to judge whether, if a boundary exists, it lies between MAK and NGM or NGM and TEA. —
309
The two ‘invariant’ apparent resistivity curves from the Taupo Volcanic Zone, TER and TAH,
Panm)
• 100
s’S
S
•++
•
are comparable and lower than those for any of the other sites. In the case of the most extensive curve, TAH, the two flatter portions of the curve suggest a possible interpretation in terms of a conductor—resistor—conductor—resistor layered
Pa~
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. ~• •,
too 100
10
I
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attenuating and transmitting regions is less clear. The curves for MAK and GOW are practically identical and, interestingly, the inflection in the
4~
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model. Any distinction between the p1 curves from the
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GOW as being broadly representative of the northwestern or attenuating region. Taking into
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account the geology of the region the curves for
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classification. MKN MMK and PUK are all situated on the
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ignimbrite plateau to the west of the Taupo Volcanic Zone. MAK and GOW are on young sediments to the southwest. Given such a geologi-
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maximum resistivity curves Fig. 4b also occurs apparent in the ‘invariant’ curves. Theofsimilarity of the curves supports the inflection as a real feature and it is tempting to classify MAK and
10000 i(s)
10
100
1000 —~
10000 I(s)
00W
Fig. 4. (a) Maximum and minimum apparent resistivities and maximum phase for the sites TEA and NGM. Uncertainty bars, where large enough to be shown, correspond to one standard error. (b) As for Fig. 4(a) for the sites MAK and
oow. 4. Comparison and modelling of MT results Smoothed p. curves for all the sites discussed above are shown in Fig. 5. The curves are distinguished both by the site name and according to the site location in the attenuating region (labelled N/W), the Taupo Volcanic Zone (TVZ) or the transmitting region (S/E).
cal setting it seems reasonable that higher resistivities might be expected at the former three sites. Inspection of Fig. 5 shows that the curves from PUK and MMK effectively parallel those of MAK and 00W but are displaced by half a decade. Indeed as mentioned above, just as at MAK and .
.
GOW, an mflection is evident m the curve for PUK. The p, curve for MKN is much more siniilar to those for MAK and 00W except that it does not flatten at periods over 1500 s~MKN is close to outcropping Mesozoic greywacke and it is likely that local effects cause the raising of the
apparent resistivity to be much less marked.
The p, curves for sites on the south and east, i.e. the transmitting region, are much more disparate. As indicated above, TEA is not considered to be typical because of the noise problems encountered and its location. The other three sites NGM, RTK and GAL are all similar at the longest periods (>1500 s). At shorter periods the curves for RTK and GAL show a rapid fall in apparent resistivity. This can be attributed to the exceptionally and unrealistically steep portions of the Pmax
310 p
1Ulm)
1000
JTN~ —
MKN
— —
i0
1
10
100
1000 —*
10000 I(s)
Fig. 5. Smoothed ‘invariant’ apparent resistivity curves for all sites. N/W refers to the attenuating region, TVZ to the Taupo Volcanic Zone and S/E to the transmitting region.
curves mentioned above and it seems likely that the curve for NGM is better representative of the transmitting region. RTK and GAL are close to boundaries between ignimbrite, bordering the Taupo Volcanic Zone, and greywacke further east. NGM has the same geological setting as MAK and GOW. In light of the above discussion it seems reasonable to accept MAK and 00W as representative of sites in the attenuating region. More tentatively, because of uncertainty in the exact location of the boundary, NGM has been taken as representative of those in the transmitting region. For the purposes of comparing the regional structures one-dimensional modelling of the response for NGM and the average response for MAK and 00W has therefore been carried out. In addition the curve for TAH has been modelled to give a resistivity structure for the Taupo Volcanic Zone. It must be remembered, however, that each of the regions covers a considerable area and that no single resistivity structure is necessarily valid for the whole region. In the case of the Taupo Volcanic Zone in particular the variation in near surface electrical conductivity associated with the geothermal areas means that no single site is likely to be
typical. Because of this the relative variations in resistivity with depth are of greater importance in comparing the regions than are the actual values of resistivity and depth themselves. Figure 6a shows a one-dimensional model fit to the average ‘invariant’ curve for MAK and GOW. The five layer model shown provides a reasonable fit to both the apparent resistivity and phase curves. The resistivity and thickness of the surface conductive layer are well defined only in that the integrated conductance of the layer is 450 S. The depth and thickness of the deeper conductive layer are not particularly well resolved. Such a layer, of resistivity <50 ~m, is required, in either the lower crust or upper mantle, to give the inflection in the apparent resistivity curve at around 100 s. The phase curve for NGM is too scattered to be of any use in modelling. A typical model fitting the apparent resistivity curve at the site is shown in Fig. 6b. The lower conductive layer of the three layer model is necessary to give the flattening of the curve at longer periods. No lower crustal or upper mantle conductor is evident indicating a major difference in the conductivity structures for the attenuating and transmitting regions. As was —
311 MAK-GOW
lO0)i (flm)
75 Am
4o::onm
100
I
0epth)~)
20000fLm
I
1
175
100
100
10000 ~ I(s)
NOM TOOC
T,0,
—
~
~
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i~o
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ioooo i(s)
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100
1000
10000 —~
i(s)
•
Fig. 6. (a) One-dimensional model and its fit to the averaged P~and ~, results for MAK and GOW. Uncertainty bars represent the differences between the two sites. (b) One-dimensional models and their fits to the smoothed p, results for NGM and TAH. Uncertainty bars for NGM are as for Fig. 4, those for TAB are as for Fig. 3.
suggested above, the cause of greater lower crustal—upper mantle conductivity beneath the attenuating region is likely to be the existence of higher temperatures and possibly partial melt aris-
ing from frictional heating of the subducted plate. Also shown in Fig. 6b is a model giving a fit to the p1 data from TAH. A four layer model is required to give a satisfactory fit. The surface
312
conductive layer can probably be identified with the surface geothermal manifestations and possibly gives an estimate of the depth to which the system extends. The deeper conductor at midcrustal to upper mantle depths is perhaps the first indication of the deeper source of the geothermal features of the Taupo Volcanic Zone. An important point to note is that a model without the intermediate resistive layer between the two conductors does not fit the data nearly as well.
/
/
)
I
( /
,~
/ /
~
/
~.
~
//
~
-
(
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5.
0
Magnetovanational results
The magnetotelluric results seem to suggest the existence of a lower crustal—upper mantle electrical conductivity contrast between the attenuating and transmitting regions. In addition considerably lower resistivities exist throughout the crust and possibly into the upper mantle beneath the Taupo Volcanic Zone. One area in which it is possible to look for support for this hypothesis is in the results of magnetovariational studies in the region. Marriott (1969) did not calculate induction arrows or their equivalent for his site PUK. Parkinson vectors were calculated for both TAH and TER by Hurst (1974). He reported that at TAH for periods between 40 and 120 s the vector had a magnitude of about 0.14 and was within 5° of magnetic north (— N20 E). At TER, for penods <60 s, the vector had magnitude 0.28 and a direction close to N70°W. At longer periods the vector was oriented magnetic north. By far the most extensive magnetovariational results were obtained by Midha (1979) who calculated single-station transfer functions at 16 sites. Reversed real and imaginary induction arrows for these sites and for 00W, MAK, NGM and TEA as well as one additional site originally reported by Ingham (1985) are shown for two periods of variation in Fig. 7. The results for a period of 1500 s, Fig. 7a, make clear why little can be learnt about any conductivity contrast associated with the attenuating—transmitting boundary from the results of Midha. All of the real arrows from Midha’s sites point northeast and have very urnform magnitude. This behaviour is characteristic of the arrows at all periods above 1000 s and was interpreted by Midha as evidence of a major con-
0.2 0.4
REAL — (MAO —--a 90000 isoo~
)
/ ~
~/
~_-_-~
~“
/
.‘~‘
/
) (
~
~
~
/
~
,
-
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Fig. 7. (a) Reversed real and imaginary induction arrows for a penod of 1500 s. The boundary between the attenuatmg and transmitting regions and the Taupo Volcanic Zone is also shown. (b) As for Fig. 7(a) for a period of 375 s.
ductive region trending northwest to southeast to the north of New Zealand. Indeed at periods 3000 s the real induction arrows at GOW, MAK, NGM and TEA also swing to point in this northeastern direction. The existence of such a conductive structure is currently under investigation using ocean bottom based measurements (D.A. Christoffel, personal communication, 1986). At the shorter period of 375 s, Fig. 7b, Midha’s arrows have generally small amplitude and considerable scatter in direction. There is a hint of q reversal in real arrows associated with the western edge of the Taupo Volcanic Zone but again no obvious feature is apparent. The more recent sites further south yield more promising results. The real arrow at TEA is unam>
313
biguously effected by the coastline to the south and the Waganui Basin to the east. However, a persistent reversal in real arrows occurs between NGM and the site reported by Ingham (1985). This reversal is in fact south of the proposed attenuating—transmitting boundary and it is possible that NGM is a site intermediate between the two regions rather than truly representative of the transmitting region. At the shorter period the real arrows at MAK and 00W point towards Mt. Taranaki possibly indicating the existence of shallow high conductivity beneath the volcano. The imaginary arrows at all of these recent sites also point towards Mt. Taranaki at 1500 s period. At the longer period the real arrows at MAK and 00W are very small in magnitude, an observation which is compatible with the sites being on relatively uniform layered structure.
proposed major magnetotelluric traverse across the Taupo Volcanic Zone should not only elucidate the electrical conductivity contrast and the relationship to it of the Taupo Volcanic Zone itself, but also enable interpretation of the contrast in terms of the tectonic structure of the North Island of New Zealand.
6. Summary and conclusion
Geophysics, University of Edinburgh and thanks go to Prof. K.M. Creer for the facilities made available. Finally, this work was funded in part by
This paper has reported on a comparison of magnetotelluric sounding results obtained over the last 20 years in and around the Taupo Volcanic Zone of the North Island of New Zealand. In particular an attempt has been made to investigate if any electrical conductivity contrast exists coincident with a major geophysical boundary reported by Mooney (1970) and Hatherton (1970). Arguments have been put forward for selecting magnetotelluric curves typical of the attenuating and transmitting regions of Mooney and of the Taupo Volcanic Zone. The ‘invariant’ apparent resistivity curves selected have been modelled using layered Earth models. The results obtained are that: (1) there is evidence for a lower crustal or upper mantle conductor beneath Mooney’s attenuating region; (2) no such conductor appears to exist beneath the transmitting region; and (3) one-dimensional modelling of a single site in the Taupo Volcanic Zone suggests the existence of a deep conductor beneath the geothermal area. In conclusion it seems likely that a conductivity inhomogeneity does exist and some support for this comes from magnetovariational results. A
Acknowledgements Prof. D.A. Christoffel supervised most of the students whose research is reported. D. Jepsen and E. Broughton provided valuable aid in the field and laboratory, respectively. The helpfulness and kindness of all the landowners on whose property measurements were made is gratefully acknowledged. This paper was written whilst the author was on study leave at the Department of
New Zealand University Grants Committee grant No. 83/15 3 and the Victoria University Internal Research Fund.
References Hatherton, T., 1970. Upper mantle inhomogeneity beneath New Zealand: surface manifestations. J. Geophys. Res., 75:
269-284. Hurst, A.W., 1974. Magnetic Effects in Volcanic Regions.
Ph.D. 369 Thesis, land, pp. Victoria University of Wellington, New ZeaIngham, M.R., 1985. Magnetovariational measurements in the Cook Strait region of New Zealand. Phys. Earth Planet. Inter., 39: 182—193. In&oano, M.R., 1987. The use of invariant impedances in magnetotelluric interpretation. Geophys. J. R. Astron. Soc., in press. Marriott, W.G., 1969. A Magnetotelluric Investigation of the North Island Volcanic Plateau. M.Sc. Thesis, Victoria Urnversity of Wellington, 81 pp. Midha, R.K., 1979. Geoelectromagnetic induction studies in the North Island VolcanicofRegion, New 342 Zealand. Ph.D. Thesis, Victoria University Wellington, pp. Mooney, H.M., 1970. Upper mantle inhomogeneity beneath New Zealand: seismic evidence. J. Geophys. Res., 75: 285-309.