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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B06302, doi:10.1029/2003JB002790, 2004
Continental crust under compression: A seismic refraction study
of South Island Geophysical Transect I, South Island, New Zealand
Harm J. A. Van Avendonk,1,2 W. Steven Holbrook,1 David Okaya,3
Jeffrey K. Austin,1,4 Fred Davey,5 and Tim Stern6
Received 9 September 2003; revised 27 February 2004; accepted 13 April 2004; published 15 June 2004.
[1] The 1996 South Island Geophysical Transect (SIGHT) active source seismic survey was
designed to show the style of lithospheric thickening due to late Cenozoic oblique
convergence across the Australian-Pacific plate boundary in New Zealand. As part of this
study, two seismic refraction lines were shot across central South Island and offshore
extensions of the continental crust in the Tasman Sea and Pacific Ocean. We present the data
and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of
these seismic transects. A tomographic inversion of 62,563 travel times from crustal and
upper mantle refractions and wide-angle reflections resulted in a model with a two-layer
crust. Upper crustal velocities were between 5.9 and 6.3 km/s, and lower crustal velocities
were between 6.5 and 7.0 km/s. Continental compression has locally reduced the seismic
velocities in the Pacific plate crust by 0.2–0.3 km/s, a possible effect of high strain and fluids
in the crust. The thickening of the crust from 28 km at the east coast of South Island to 37 km
beneath the Southern Alps can account for about 25% of the 80–110 km shortening of
Pacific plate crust, while the rest must be accounted for by rapid erosion of Mesozoic
sedimentary rocks on the west side of the orogen. In our model the lower crust forms a
continuous 2–6 km thick layer beneath central South Island. The asymmetric topography of
the Southern Alps is reflected in the crustal root which has a steeper flank at the west coast.
This observation is consistent with westward underthrusting of Pacific lower lithosphere
INDEX TERMS: 3025 Marine
beneath South Island that has been suggested in earlier studies.
Geology and Geophysics: Marine seismics (0935); 8102 Tectonophysics: Continental contractional orogenic
belts; 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 8110 Tectonophysics:
Continental tectonics—general (0905); KEYWORDS: crustal structure, continental collision, refraction
seismology
Citation: Van Avendonk, H. J. A., W. S. Holbrook, D. Okaya, J. K. Austin, F. Davey, and T. Stern (2004), Continental crust under
compression: A seismic refraction study of South Island Geophysical Transect I, South Island, New Zealand, J. Geophys. Res., 109,
B06302, doi:10.1029/2003JB002790.
1. Introduction
[2] Owing to its buoyancy, the continental crust has been
deformed but rarely subducted during various collision
events in the Earth’s history. Orogens from Proterozoic to
recent times are well preserved [e.g., Lucas et al., 1993;
Meissner and Tanner, 1993], providing ample opportunities
to investigate the role of deep-seated geological processes
1
Department of Geology and Geophysics, University of Wyoming,
Laramie, Wyoming, USA.
2
Now at University of Texas Institute for Geophysics, Austin, Texas,
USA.
3
Department of Geological Sciences, University of Southern California,
Los Angeles, California, USA.
4
Now at ChevronTexaco, Houston, Texas, USA.
5
Institute of Geological and Nuclear Sciences, Lower Hutt, New
Zealand.
6
School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand.
Copyright 2004 by the American Geophysical Union.
0148-0227/04/2003JB002790$09.00
during the creation of these mountain belts [Arndt and
Goldstein, 1989; Nelson, 1992]. Yet it is difficult to infer
the tectonic stresses and lithospheric rheology at the onset
of continental compression. The yield strength of continental lithosphere, the distribution of strain in the deep crust,
and the mechanical coupling with the lower lithosphere may
be obscured as the orogen matures and erodes. To gain more
insight in the behavior of continental crust under compression we must look at young convergent zones, such as the
Southern Alps of New Zealand.
[3] The Southern Alps of South Island, New Zealand, are
the surface expression of a relatively small orogen that
formed over the last 5 –10 Myr in this submerged continent
by transpression across the Pacific-Australian plate boundary. The relative plate motion was strike slip in the early
Miocene [Kamp, 1987; Sutherland, 1995], but a change in
the Euler rotation pole of the Pacific plate resulted in 80–
100 km of shortening [Stock and Molnar, 1982; Cande and
Kent, 1995; Sutherland, 1995; Walcott, 1998]. Most of the
shearing and compressional deformation has taken place
near the Alpine Fault [e.g., Norris et al., 1990], a major
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Figure 1. Two SIGHT transects crossed the island where
it is 150 km wide, and two transects investigated the
structure that ran parallel to the plate boundary on the
Australian and Pacific side. The R/V Maurice Ewing air gun
shots (in black) were recorded by ocean bottom instruments
(circles) and dense arrays of land seismometers on the two
transects. On-land explosions (stars) were also recorded by
the land arrays.
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debate [e.g., Pysklywec et al., 2002] because the seismic
constraints on the deep crustal structure were insufficient
prior to the 1996 South Island Geophysical Transect
(SIGHT) study.
[5] During the SIGHT experiment, seismic refraction and
reflection data were collected along four seismic transects
by a consortium of U.S. and New Zealand scientists. The
width of South Island, 150 km from the Tasman Sea to the
Pacific Ocean, made it a suitable target for a controlledsource, two-sided onshore-offshore seismic experiment
[Okaya et al., 2002]. Two of these transects are parallel to
the east and west coast of South Island. The two other
transects extend from the Tasman Sea across South Island
into the Pacific Ocean (Figure 1). Linear arrays of land
seismometers across the island recorded air gun shots that
were fired offshore from both the east and west coast, such
that both flanks of the crustal root could be imaged [Davey
et al., 1998]. This data set, combined with land explosion
and ocean bottom refraction data, provides an unprecedented
opportunity to image the deeper structure of an orogen and
undeformed portions of the flanking continental plates
[Okaya et al., 2002]. In this paper we present a twodimensional (2-D) seismic velocity model obtained by a
tomographic inversion of seismic refraction data that were
recorded along the northern of the two transects across the
waist of South Island, referred to here and elsewhere as
Transect 1 (Figure 1). Our seismic velocity model consists
of layers with progressively increasing seismic velocity
bounded by continuous, reflecting interfaces.
2. Geologic Setting
lithological discontinuity that runs straight along the west
flank of the Southern Alps. Significant strain has also been
accommodated in Pacific plate crust to the east [Walcott,
1984; Langdale and Stern, 1998]. The geologic observations are generally consistent with the current strain distribution known from Global Positioning System (GPS)
surveys [Beavan et al., 1999; Beavan and Haines, 2001]
across South Island. These detailed kinematic constraints
make the Southern Alps an attractive place to study the deep
structure of a continental collision.
[4] Over the years a number of studies have shown that
the Alpine Fault dips 40– 50 southeast from its surface
trace, forming an oblique thrust along which sedimentary
rocks of the Pacific plate are transported from lower crustal
levels to the surface [Wellman, 1979; Sibson et al., 1979;
Rattenbury, 1986; Cooper and Norris, 1994; Kleffmann et
al., 1998]. The localization of convergence and uplift in the
Southern Alps is reflected in the thermal history of rocks
exposed in South Island [Adams, 1980; Tippett and Kamp,
1993; Batt et al., 2000], by shoaling of the brittle-ductile
transition [Leitner et al., 2001; Wannamaker et al., 2002],
and in low seismic velocities in the Pacific plate crust
[Kleffmann et al., 1998; Stern et al., 2001; EberhartPhillips and Bannister, 2002]. In contrast with the shortening in the upper crust, the strain in the mantle lithosphere is
inferred to be distributed over a few hundred kilometers
[Klosko et al., 1999; Molnar et al., 1999; Scherwath et al.,
2002; Kohler and Eberhart-Phillips, 2002]. The coupling
between these two modes of deformation is an issue of
[6] We describe some of the New Zealand geology with a
focus on the area where the South Island Geophysical
Transect (SIGHT) active seismic data were collected. The
distribution in age and origin of basement rocks of South
Island reveals two very distinct metamorphic belts, the
Western and Eastern Province, which are joined by a suture
called the Median Tectonic Zone (MTZ, Figure 2) [Landis
and Coombs, 1967; Bradshaw, 1993; Kimbrough et al.,
1993]. The 450 km dextral slip of the Alpine Fault has
offset the MTZ over much of South Island in the Cenozoic,
such that in central South Island the Western and Eastern
Provinces lie on the Australian and Pacific plates, respectively (Figure 2).
[ 7 ] The Western Province, comprising the NelsonWestland block in the north and Fiordland block in the
south, is considered an autochthonous part of Gondwana
[e.g., Spörli, 1987]. It consists of lower Paleozoic metasediments that were cratonized during the middle Paleozoic and
intruded by large volumes of granitoids [Cooper, 1979;
Tulloch, 1983; Mattinson et al., 1986]. During the early
Cretaceous, accretion of the Eastern Province onto the
Gondwana margin was accompanied by continental arc
volcanism. Some granulites are found in the Nelson block
[Tulloch, 1983], and these rocks may constitute a significant
portion of the lower crust as in much of the aged continental
crust worldwide [Fountain and Salisbury, 1981; Rudnick
and Fountain, 1995]. SIGHT Transect 1 intersects only a
25 km wide subaerial strip of the Westland, but Paleozoic
basement has also been dredged offshore on the Challenger
Plateau [Tulloch et al., 1991]. Therefore the western portion
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side of the Southern Alps. The divide between these watersheds, 50 km from the west coast locally reaches 1700 m
elevation. Transect 2 lies just 60– 70 km southward along
the strike of the Alpine Fault [Scherwath et al., 2003], and
crosses a higher section of the divide just south of the
Mount Cook area, where the band of Haast Schist narrows
to 12 km, and the Bouguer gravity anomaly deepens toward
the south [Woodward, 1979]. The variation in thickness of
the Haast Schist terrane may be the result of more pervasive
compression at Transect 2 compared with Transect 1.
3. Wide-Angle Data
[10] An overview of the SIGHT active source seismic
data was given by Davey et al. [1998] and Okaya et al.
[2002]. We summarize the source and receiver geometry
along Transect 1 and give an inventory of the travel time
data that were used in the tomographic inversion. Before we
can use the data in a tomographic inversion we must
interpret and label travel time arrivals. We present a travel
time interpretation based on a two-layer crust that applies to
all the subbasement seismic refraction data collected along
SIGHT Transect 1.
Figure 2. Map of South Island shows the location of
terranes from the Eastern and West Provinces relative to the
Alpine Fault in central South Island. Pearson et al. [1995]
measured 38 mm/yr convergence in central South Island.
MTZ, Median Tectonic Zone.
of Transect 1 covers a large portion of old Gondwana
foreland that is characterized mostly by metagraywackes
and granitic rocks in the upper crust.
[8] The Eastern Province includes several terranes that
appear to have been derived from an island arc system [e.g.,
Coombs et al., 1976], but most of the eastern and central
South Island is covered by the Torlesse graywackes. These
quartzofeldspathic sediments, mainly Permian to Jurassic in
age in central South Island, may have been deposited on
Pacific oceanic crust [Smith et al., 1995] near a trench
system [Adams and Kelley, 1998]. In the Early Cetaceous,
sedimentary rocks of the Eastern Province were exhumed
from mid to lower crustal depths. Because of the more
recent deeper exhumation near the Alpine Fault, the metamorphic grade of basement ranges from zeolite and
prehnite-pumpellyite facies in the Torlesse terrane in the
east [MacKinnon, 1983] to greenschist in most of the Haast
Schist terrane [Wood, 1978]. The dextral shear between the
Australian and Pacific plates in the Cenozoic has bent the
Eastern Province in a reversed ‘S’ shape [Molnar et al.,
1975; Walcott, 1978], and the concurrent offset across the
Alpine Fault has placed the Paleozoic crust of the Western
Province against the Haast Schists where SIGHT Transect 1
crosses central South Island. The Torlesse and Haast terranes probably extend offshore east of South Island on
Transect 1, since Mesozoic sandstones and schists are found
on Chatham Rise and on the Chatham Island [Cullen, 1965;
Andrews et al., 1978; MacKinnon, 1983].
[9] The seismic Transects 1 and 2 both make use of
river valleys that provide access into the Southern Alps.
Transect 1 follows the Whataroa river through Westland,
and then it follows the Rangitata river system at the other
3.1. Marine Seismic Data
[11] Marine seismic data were acquired using an array of
20 air guns on board the R/V Maurice Ewing with a total
capacity of 139 liter. The air guns were fired every 50 m on
the offshore portions of the seismic transects (Figure 1). The
two-way travel time to the acoustic basement is constrained
using stacked multichannel seismic (MCS) data on both the
east and west coast of South Island. Eight ocean bottom
instruments were deployed in both the Tasman Sea and the
Pacific Ocean in order to provide data coverage at large
distances of the plate boundary. All instruments recorded
with a 10 ms sample interval. After suppression of previous
shot noise (PSN) [Nakamura et al., 1987], predictive
deconvolution and band-pass filtering, the ocean bottom
records show coherent arrivals to 80 km source-receiver
offset. Crustal refractions (labeled Pg) have apparent velocities somewhat less than 6 km/s in receiver gathers, and
midcrustal and Moho reflections are visible at offsets larger
than 40 km with apparent velocities larger than 6 km/s
(Figure 3). A midcrustal reflection, referred to as PiP in this
paper, was observed and correlated between receiver gathers
on both the east and west coast of South Island. Mantle
refractions (Pn) are observed intermittently in the ocean
bottom data.
3.2. Onshore-Offshore Data
[12] The R/V Maurice Ewing air gun shots were also
recorded by an array of 126 land seismometers along
Transect 1. The instruments were spread from the east to
the west coast with an average spacing of 1.5 km,
although the gaps between instruments in the Southern
Alps were larger. The sample interval of all instruments
was 10 ms. Noise levels varied strongly among instrument
sites, but some of these receiver gathers are of very high
quality (Figure 4). In comparison with the ocean bottom
data the onshore-offshore data have a better signal/noise
ratio and the signal does not degrade as fast with distance,
presumably because the recording is not affected by PSN.
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Ocean onshore-offshore records. Offshore to the west, the
PmP arrivals are weaker than PiP, but in the Pacific
Ocean records PmP are usually the strongest phase. Weak
mantle refractions (Pn) are observed in most onshoreoffshore records followed by relatively strong reflections
that appear to arrive from the uppermost mantle.
Figure 3. An example ocean bottom seismometer (OBS)
gather shows seismic refractions and reflections from
instrument C1 at 145 km on the Australian plate west of
South Island. The horizontal distances are measured along
the 603 km long Transect 1 from northwest to southeast.
The vertical time axis is reduced by 6 km/s. Inset shows the
true shot spacing of 50 m.
[13] A crustal seismic refraction (Pg) with apparent
velocity of approximately 6 km/s is observed in the
offshore data east and west of South Island to sourcereceiver offsets of 150 km. In most onshore-offshore
receiver gathers the lower crust is characterized by a few
wide-angle reflections. Some of these lower crustal reflections are spatially incoherent, but the first arriving of
these reflections is always a clear waveform that correlates well with the midcrustal PiP observed in the ocean
bottom seismograph data. Offshore to the west, PiP is
sometimes observed to source-receiver distances larger
than 200 km, but at the east coast most PiP are recorded
within 150 km. The Moho reflection (PmP) is also a
prominent arrival on both the Tasman Sea and Pacific
3.3. Explosion Data
[14] The onshore-offshore seismic data provide excellent
coverage of both flanks of the Southern Alps, but instruments positioned near the east and west coast did not record
many useful offshore data from across the 150 km wide land
section of Transect 1. Seismic coverage of the crust beneath
central South Island was provided mostly by 16 large
chemical explosions along Transect 1 in South Island,
recorded by 417 instruments spaced at 400 m (Figure 1).
Turning waves and postcritical reflections with a dominant
frequency of 10 Hz constitute most of the observed
seismic phases in the ocean bottom and onshore-offshore
air gun data. In contrast the explosion data, recorded with
4 ms sample interval, contain seismic energy to at least
35 Hz. High-frequency precritical seismic reflections from
the deep crust are commonly observed. The turning wave
in the crust (Pg) is generally the first and strongest arrival in
the explosion data, and it is often coherent to offsets over
100 km (Figure 5). On some explosion shot records, distinct
PiP and PmP arrivals are consistent with the onshoreoffshore data recorded at larger source-receiver offsets.
Other explosion records show a train of seismic energy that
appears to arrive from the deep crust. We interpret the onset
of this package as PiP and the tail as PmP.
3.4. Travel Time Picks
[15] The four seismic travel time phases Pg, PiP, PmP,
and Pn can be used to constrain a two-layer crustal model
provided that these arrivals are consistent among the different seismic gathers. Reversed coverage on ocean bottom
and explosion records allowed us to verify source-receiver
Figure 4. Onshore-offshore data shot from the Tasman Sea recorded by an instrument located at shot
point 7 (SP07) located at 301 km. The horizontal and vertical axes are as in Figure 2. An upper crustal
refraction (Pg) is observed to 130 km source-receiver offset. Very strong and continuous reflections PiP
and PmP come in with higher apparent velocity and are recorded to 180 km source-receiver offset.
Another strong mantle reflection is visible after PmP. A weak mantle arrival (Pn) appears at 150 km
offset. Inset shows the coherence of wide-angle reflection PiP.
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Figure 5. Explosion refraction data, from shot point SP07
at 301 km, recorded across South Island show a strong firstarrival Pg. The time and distance axes are as in Figure 2.
The instruments were spaced every 400 m along much of
the 150 km wide traverse across South Island, except for the
high portions of the Southern Alps between 253 and 265 km
where helicopters were used to deploy instruments.
reciprocity. The onshore-offshore data are unreversed, but
the dense receiver spacing results in consistent seismic
arrival times at constant offset between neighboring land
instruments.
[16] In section 4 we describe a least squares inversion of
all wide-angle seismic refraction and reflection travel times.
Appropriate weighting of the travel time data is necessary to
ensure that the modeling procedure finds a good balance
between fitting explosion and air gun data, and to account
for different noise levels. Although the shot spacing in the
air gun data is 50 m, we picked the arrivals in these receiver
gathers with intervals of at least 400 m, or 8 seismic traces,
to achieve a data density comparable to the explosion data.
Data errors were assigned visually to all travel time picks.
They varied between 70 and 150 ms in the ocean bottom
data, between 40 and 120 ms in the onshore-offshore data,
and between 40 and 160 ms in the explosion data. The
62,563 travel time picks are separated by data type and
seismic phase in Table 1.
4. Tomography
[17] Tomographic inversions aim to invert seismic refraction data for a minimum-structure seismic velocity model
[Zelt, 1999; Zelt et al., 2003]. The smooth seismic velocity
models that result from this approach usually give a
conservative but robust representation of the true earth
structure [Zelt et al., 2003]. However, the multichannel
seismic data collected during SIGHT show reflections from
the basement offshore New Zealand [e.g., Davey et al.,
1998]. These reflections are caused by a contrast in seismic
properties at this boundary where probably Mesozoic rocks
are overlain by postcollision Tertiary rocks. Sharp seismic
boundaries can also be inferred from the consistent occurrence of wide-angle reflections PiP and PmP in the wideangle data. We use the method of Van Avendonk et al.
[2001a] to construct a model with smooth spatial variations
in seismic velocity that is intersected by four velocity
discontinuities: The seafloor, acoustic basement, lower crust
and Moho.
4.1. Ray Tracing
[18] The calculation of ray paths and travel times in a
reference seismic velocity model is an essential step in a
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tomographic inversion. When applied to seismic refraction data, a ray-tracing scheme must handle dense source
or receiver gathers efficiently. Korenaga et al. [2000] and
Van Avendonk et al. [2001b] showed that a combination
of a grid-based global travel time solver such as the
shortest path method [Moser, 1991] or eikonal equation
solver [Vidale, 1988] with a local optimization scheme
[i.e., Um and Thurber, 1987; Moser et al., 1992] is
computationally efficient. Using a shortest path grid
spacing of 500 by 150 m in the method of Moser
[1991] and the ray-bending method of Moser et al.
[1992] we calculate travel times with an accuracy of
approximately 10 ms.
[19] An apparent problem with travel time solvers is their
inability to find secondary travel time arrivals, with the
exception of reflections from first-order seismic velocity
discontinuities [Moser, 1991]. We model the seismic reflections PiP and PmP as reflections from a midcrustal and
crust-mantle seismic velocity step. Seismic reflections and
refractions originating from a velocity discontinuity in the
model can be found by first calculating the travel times from
a source to the interface, and by propagating the travel times
back to the Earth’s surface using the interface as a timevarying source.
4.2. Inversion
[20] The travel time data carry information on both the
seismic velocities and reflecting boundaries. We therefore
seek to solve for these parameters simultaneously. However, the structure of the acoustic basement appears to be
more complicated than the deeper structure. We therefore
carry out a ‘‘layer-stripping’’ step to determine the seismic
structure of the shallow sediments and basement before we
image the deep crustal structure. By forward modeling of
shallow seismic refractions from within the sediments and
basement reflections from multichannel seismic data
[Davey et al., 1998] we obtained a basement profile along
Transect 1. The PiP and PmP arrival times of the
explosion data recorded in the Southern Alps are consistent with a crustal root that extends deeper than the crust
outside the collision zone [e.g., Smith et al., 1995]. We
generated a starting model with the lower crust and Moho
deepened over a wide region beneath South Island
(Figure 6). The seismic velocity of the model layers and
the geometry of the lower crust and Moho tomographic
inversion
[21] The tomographic inversion solves for a model vector
m that represents both seismic slowness, which is the
reciprocal of seismic velocity, and the depth of the velocity
discontinuities [Van Avendonk et al., 2001a]. The inversion
procedure seeks a model perturbation dm that minimizes
Table 1. Number of Picks for Each Data Type and Phase
Phase
Pg
PiP
PmP
Pn
Total
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Ocean Bottom
Onshore-Offshore
Land Explosion
2362
796
1838
69
5065
11,043
16,709
15,541
4,707
48,000
3712
3231
2564
0
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VAN AVENDONK ET AL.: CONTINENTAL COMPRESSION SOUTH ISLAND
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Figure 6. Starting model for tomographic inversion of deep crustal arrivals contains detailed basement
topography and a first guess of the geometry of the crustal root.
both the data misfit and the roughness of the new velocity
model using least squares:
F ðdmÞ ¼ ðd A dmÞT C1
d ðA dm dÞ
þ lðm þ dmÞT DT Dðm þ dmÞ:
ð1Þ
The matrix A is the Fréchet derivative matrix that maps dm
into the data residual vector d. Assuming that travel time
pick errors are uncorrelated, the a priori data covariance
matrix Cd reduces to a diagonal matrix with elements Cii =
s2i , where s are the errors assigned to the travel time data.
The second term of equation (1) minimizes both horizontal
and vertical gradients in seismic velocity and the slopes of
layer boundaries in the model. The step in seismic velocity
across these boundaries is not restricted in the inversion
[Van Avendonk et al., 2001a]. The first-derivative roughness
matrix D applies to the newly constructed model m + dm.
This strategy has been named the ‘‘jumping’’ approach
[Shaw and Orcutt, 1985] because it attempts to jump from
the starting solution to the final solution without restraining
the size of dm. The relative strength of this smoothing term
is varied by the scalar l. The trade-off between data misfit
and model roughness is tuned to achieve a normalized data
misfit that approximately averages unity:
c2 ¼
2
1
12
Cd ðA dm dÞ 1:
N
ð2Þ
[22] We carried out regularized inversions on a rectangular
grid with horizontal node spacing of 1 km and the vertical
cell spacing varied between 200 m near the surface and
500 m at 50 km depth. The resultant model vector m of
3.3 104 parameters was determined by 1.5 105 equations.
The fraction of nonzero elements in the inversion matrix
was 1.3 103, sufficiently sparse to invert the system of
equations using LSQR [Paige and Saunders, 1982].
4.3. Data Fit
[23] We carried out 22 iterations of ray tracing and
inversion of all the wide-angle travel time picks. The
procedure yielded a stable solution after eight steps. Although a c2 data misfit of 1.0 is preferred, the trade-off
between model roughness and data misfit is subjective, and
we found that a c2 misfit of 1.3 produced a more conservative model that is reproduced between different tomography iterations (Figure 7). The inversion procedure
guarantees that the summed data misfit compares well with
the assigned errors, but the spatial distribution of the data
misfit may still be important where details of the seismic
velocity model are concerned.
[24] In Figure 8 we show the ray paths and travel time fits
for three ocean bottom seismometers (OBSs) on Transect 1.
The data density on the distal portions of Transect 1 was not
high because the instruments were widely spaced and
observed deep refractions and reflections were intermittent.
However, the good data fit of these ocean bottom records
validates the two-layer model of the extended continental
crust on both the east and west side of the collision zone.
The ray paths of the onshore-offshore data (Figure 9)
sample the east and west flank of the orogen, and many
deep reflections in the explosion data return from the keel of
Figure 7. Seismic velocity structure along Transect 1 stretches from submerged and undeformed
Australian continental crust to the northwest across the west coast of South Island at 213 km, the Alpine
Fault at 228 km, the east coast at 271 km, and into the submerged Pacific plate. Red triangles show the
location of ocean bottom and land instruments that recorded the wide-angle seismic data. See color
version of this figure at back of this issue.
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Figure 8. Ocean bottom data from the (a and b) Tasman Sea and (c) Pacific Ocean show a good data fit
of seismic refractions that turn in the upper crust and wide-angle reflections from the top of the lower
crust (PiP) and Moho (PmP). Picked travel times are shown as solid lines, and travel times calculated in
our new seismic velocity model are shown by dashed curves.
the crustal root. The Moho and lower crust beneath the plate
boundary (Figure 7) apparently dip too steeply to the east to
be illuminated by wide-angle reflections.
4.4. Seismic Velocity Model
[ 25 ] The 603 km long seismic velocity model of
Transect 1 covers both undeformed and compressed portions of continental crust (Figure 7). At large distance from
the collision zone, both the Australian and Pacific plate
crust is characterized by a 15 km thick upper crust with
seismic velocities of 5.7– 6.2 km/s overlying a thinner lower
crust with seismic velocities ranging from 6.5 to 7.1 km/s
(Figure 7). This distinct bimodal velocity distribution fits
the travel time data, and it explains the prominence of
midcrustal reflection PiP on both sides of the plate boundary. However, the contrast between these two crustal layers
may be accentuated by the additional constraints in the
inversion. The roughness operator D was applied to seismic
velocity gradients within layers such that locally the flattest
possible seismic velocity structure was found. The increase
in seismic velocity between the upper and lower crust was
not penalized. The Australian lower crust is substantially
thicker and laterally more heterogeneous than the Pacific
lower crust. At 200 km northwest of the plate boundary the
lower crustal velocity averages 7.1 km/s, but it decreases to
6.4 km/s near the Alpine Fault.
[26] The thick crustal root beneath the Southern Alps
lies entirely on the Pacific side of the plate boundary. The
maximum Moho depth of 37 km below sea level is
reached at 275 km in the profile, 45 km southeast of
the surface expression of the Alpine Fault. The upper
crust makes up most of the crustal root beneath the
Southern Alps and its seismic velocity is only 5.8 km/s.
The lower crust is only 3 – 5 km thick beneath the crustal
root and its seismic velocity of 6.5 km/s is also less than
the 7.0 km/s in the lower crust eastward in the Pacific
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Figure 9. Seismic records of six explosion records are shown along with onshore-offshore refraction
data that were recorded from the same location as these large explosions. The Figures 9a to 9f are records
from shot point 14 (233.1 km), shot point 11 (265.7 km), shot point 10 (272.7 km), shot point 7
(301.3 km), shot point 4 (333.6 km) and shot point 3 (347.8 km), respectively. The shallow waters
offshore the east and west coast resulted in gaps in these data records.
plate. The upper mantle beneath South Island also has
reduced seismic velocities. The minimum seismic velocity
in the upper mantle (7.8 km/s) is found near the west
coast and the Alpine Fault. The crustal thickness
decreases rapidly from the plate boundary, but the east
flank of the crustal root beneath South Island is more
gradual, extending over at least 100 km. The seismic
velocity beneath the Canterbury plains is significantly
higher than the velocity further offshore to the east: The
upper crustal seismic velocities of 6.2– 6.3 km/s there are
higher than elsewhere in the Mesozoic upper crust. The
lower crust is variable in thickness (2 – 6 km), and the
mantle seismic velocity beneath the east coast is very
high at 8.6 km/s.
4.5. Model Resolution
[27] Before we discuss implications of the new seismic
velocity model for Transect 1, we must assess the non-
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Figure 9. (continued)
uniqueness in the model. The tomographic inversion was
designed to find a simple seismic velocity model that fits
the travel time data. A first impression of reliability of
our solution in Figure 7 is given by the derivative weight
sum (DWS) [Scales, 1989; Toomey and Foulger, 1989].
By summing up the elements for each column of derivate
matrix A and scaling by data errors we obtain a measure
of ray density (Figure 10). The SIGHT experiment
succeeded in covering the crust beneath the Southern
Alps with ray paths without leaving large gaps. However,
model resolution also depends on the rank of A. Crossing
ray paths are essential to independently resolving model
parameters. The solution dm that minimizes the penalty
function (1) can be expressed in the generalized inverse
of d:
1 T
T
^ ¼ AT C1
dm
A d:
d A þ lD D
ð3Þ
The resolution matrix can be found by substituting A dm for
d in equation (3) [e.g., Tarantola, 1987]. The resolution
matrix R, a blurring filter between the ‘‘true’’ model dm and
^ can be expressed as
the minimum norm solution dm,
1 T
T
R ¼ AT C1
A A:
d A þ lD D
ð4Þ
We calculate R using an eigenvalue decomposition of
T
ATC1
d A + l D D by Lanczos iteration [Vasco et al., 1999].
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Figure 9. (continued)
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Figure 10. Derivative weight sum (DWS) measures the ray density scaled by the errors in the travel
time data. Thickness of the reflecting boundaries indicates the density of coverage of these model
boundaries.
The diagonal of the resolution matrix gives a direct
indication of the quality of the solution (Figure 11). The
resolution is very good at depths less than 10 km in the
collision zone. Adequate resolution is also obtained in
the lower crust of the Australian and Pacific plates where
paths of PiP and PmP cross. Deeper in the crustal root and
farther offshore South Island the resolution becomes poor.
Compared to the ray density (Figure 10) the resolution
decreases faster with depth, presumably because the long
ray paths of PiP, PmP, and Pn arrivals cover many model
parameters in the deep crust that cannot be determined
independently.
5. Discussion
[28] We have obtained a detailed crustal seismic velocity
profile across the Southern Alps of New Zealand in central
South Island. A similar seismic profile was obtained across
the Mount Cook area [Scherwath et al., 2003], with similar
results. These two transects currently provide the most
detailed information on the deep crustal structure of the
collision zone where 20– 25 km thick Pacific and Australian plates are juxtaposed. The 3-D tomographic inversion of
first-arrival times in South Island of Eberhart-Phillips and
Bannister [2002] shows that the crustal structure does not
vary much along strike in central South Island.
5.1. Structure of Continental Crust Offshore
Central South Island
[29] The distinct two-layer crust in the Australian and
Pacific plates is consistent with the geological history of
the Western and Eastern Provinces of New Zealand. In
the Australian plate, the Westland crust has experienced
both convergent margin and extensional processes [e.g.,
Bradshaw et al., 1981; Bishop, 1992] that have resulted
in high-grade metamorphic rocks in the lower crust. A
thin lower crust with high seismic velocity beneath the
Pacific plate was previously observed by Smith et al.
[1995]. They interpreted this layer as the old oceanic
crust on which the Caples and Torlesse sedimentary rocks
were deposited [e.g., Wood, 1978]. The thickness of 3 –
6 km and the seismic velocity of 7.0 km/s in the lower
crust in our model of Transect I are consistent with this
interpretation.
[30] The seismic velocity structure along Transect 1
exhibits large variations in crustal thickness. The crustal
thickness of the Australian plate averages 23 km in our
profile. The Pacific plate crust in the Bounty Trough is just
18 km thick (Figure 12). These values are low in
comparison with global compilations of continental crustal
thickness [e.g., Mooney et al., 1998], but they are not
unusual for extended and submerged continental lithosphere. The continental lithosphere of New Zealand was
stretched mostly during and after the separation from
eastern Gondwana in the late Cretaceous [Carter et al.,
1974; Kamp, 1986], and smaller rift basins can also be seen
at 75 km and 475 km in our new crustal profile (Figure 7).
The gradual westward thickening of Pacific crust from
18 km in the head of the Bounty Trough to 28 km beneath
the east coast (Figure 12) is unlikely caused by the
Cenozoic convergence in New Zealand because GPS and
geochronology studies indicate that the recent and current
Figure 11. Diagonal of the resolution matrix shows very good resolution near the surface in South
Island. Resolution is much poorer at larger depth in the model. Trade-offs between various model
parameters at large depth result in low resolution. See color version of this figure at back of this issue.
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parallel to the plane of foliation [Godfrey et al., 2002].
The seismic velocities of 6.3 km/s imaged in the middle
crust between 350 and 400 km (Figure 7) may be due to
anisotropy, but we do not have sufficient ray coverage at
depths below 10 km to distinguish anisotropic effects from
spatial variations in isotropic seismic velocity structure.
Figure 12. Seismic velocity as a function of depth in our
model at 65 km, 275 km, 370 km, and 500 km shows that
seismic velocities in the upper crust are very consistent but
the depth of the top of lower crust and Moho varies laterally.
deformation is largely confined to South Island [e.g., Kamp
and Tippett, 1993; Beavan and Haines, 2001]. Besides the
mentioned Cretaceous thinning in the Bounty Trough, the
South Island crust may have been thicker toward the plate
boundary. The Torlesse terrane that largely makes up the
Pacific upper crust in central South Island may have been
laterally compressed when it docked in the late Triassic or
early Jurassic [e.g., Landis and Bishop, 1972; MacKinnon,
1983].
[31] The seismic velocity of 6.0– 6.2 km/s in the upper
crust in the Australian and Pacific plates is not unusual
for rocks of granitic and quartzofeldspathic composition
[Holbrook et al., 1992; Christensen and Mooney, 1995].
Although the seismic velocity in the Australian and Pacific
plates falls well within the range of global observations, the
vertical gradient is remarkably low (Figure 12). This modest
increase in seismic velocity with depth may indicate that
these upper crustal layers are homogeneous in composition
[Christensen and Mooney, 1995].
[32] Petrophysical studies of Torlesse greywacke and
Haast schists of South Island show an average compressional velocity of just over 6.0 km/s [Okaya et al., 1995]
which is in agreement with our velocity model. However,
Haast schist samples exhibit up to 20% anisotropy with fast
velocities in the plane of foliation [Okaya et al., 1995;
Godfrey et al., 2000]. A small seismic refraction study
offshore Otago showed compressional seismic velocities
parallel and perpendicular to the inferred foliation differed
only 6% on a macroscopic scale [Smith, 1999]. The presence of Haast schist could therefore result in higher seismic
velocities if seismic refraction paths run predominantly
5.2. Shortening of South Island
[33] Walcott [1998] reviewed conceptual models for deformation of the South Island crust based on data collected
prior to the SIGHT experiment. The role of two basic modes
of crustal shortening, exhumation and thickening of the
crust, cannot be quantified without knowledge of the
geometry of the deeper crust. The crustal root beneath
South Island, with a crustal thickness of up to 37 km along
Transect 1, and the Cenozoic sediments offshore provide
new constraints on the mass transfers through this young
orogen. The crustal structure along SIGHT Transect 2 is
mostly similar to our seismic velocity model, although
Scherwath et al. [2003] found a maximum crustal thickness
in the root of 44 km. This difference can be explained as an
along-strike variation in the amount of compression along
the plate boundary, which has also raised the Mount Cook
area, where Transect 2 crosses the Main Divide, to the
highest elevations of South Island. It should also be noted,
however, that our tomographic inversion was designed to
find the smoothest model that fits the data. The keel of the
crustal root in our image may be biased toward smaller
depths. The minimum-parameter model [Zelt and Smith,
1992] of Scherwath et al. [2003] is a model with minimum
features, but does not apply smoothing of the model layer
boundaries.
[34] To explain the westward increase in uplift rate and
metamorphic grade in the Mesozoic basement, Wellman
[1979] suggested that the Pacific crust is sheared from the
subducting upper mantle in South Island and upturned along
the Alpine Fault toward the plate boundary. If the rock
volume eroded at the plate boundary equals the amount of
crustal shortening, the orogeny may be in a steady state.
However, the band of highly metamorphosed schists that
are currently exhumed in the Southern Alps only represent
15 km thickness of incoming Pacific crust [Grapes and
Watanabe, 1992]. Consequently, these schists are probably
bounded at the base by décollement within the Pacific crust
[Norris et al., 1990]. The section of Pacific crust beneath
the décollement is transferred into a crustal root, and thus
causes the root to grow over time (Figure 13).
[35] Until recently the contributions of thickening and
erosion to crustal shortening have been difficult to evaluate
because of limited constraints on deep crustal structure in
South Island. If the mean elevation was near sea level at the
onset of convergence [Kamp and Tippett, 1993], crustal
thickening must have dominated at least until the Southern
Alps rose high enough to increase precipitation. Comparing
river-suspended sediment load with rock uplift, Adams
[1980] concluded that shortening and erosion are now
approximately balanced in South Island, so that further
growth of the crustal root beneath the Southern Alps must
be small. Using older constraints on the deep structure of
South Island [e.g., Stern, 1995], Walcott [1998] estimated
that about one third of the total crustal rocks that entered the
orogen from the east now reside in the crustal root. The
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Figure 13. Schematic picture shows our seismic velocity model without vertical exaggeration. Location
of the Alpine Fault is projected on the profile.
other two thirds of the mass budget are then dispersed as
sediments, perhaps as far as the Taranaki basin and Tasman
Sea. Our estimate of 28 km for undeformed Pacific crust is
slightly thicker than Walcott’s assumed 25 km thickness.
Our thickness of the crustal root is a little less than Walcott’s
assumed 10 to 15 km thickening of the original crust, but it
is in better agreement with the results of Transect 2
[Scherwath et al., 2003].
[36] It remains difficult to refine the 3-D crustal mass
balance with 2-D constraints on the crustal structure because
of the along-strike variation in structure [Eberhart-Phillips
and Bannister, 2002; Scherwath et al., 2003] and the large
component of strike-slip motion (38 mm/yr) in the orogen
[Walcott, 1998]. However, the new seismic constraints
confirm that 75% of the incoming crust must have eroded
off the Southern Alps. The first-order comparison shows
that surface erosion has been the dominant factor in the
mountain building process and may lead to a flux steady
state where all crustal rocks are exhumed [Willet and
Brandon, 2002]. Alternatively, if erosion cannot keep pace
with convergence, the crustal root of this young mountain
belt could over time reach depths where phase changes
make it dense enough to allow delamination, as probably
happened in the European Alps [e.g., Chopin, 1987;
Laubscher, 1988; Kissling, 1993; Marchant and Stampfi,
1997].
[37] The Pacific lower crust forms a wide synform
beneath the crustal root of the Southern Alps, apparently
without substantial thickening. Both its thickness and seismic velocity vary along Transect 1, but its unique reflective
character is evident in almost every seismic refraction
record in the explosion refraction and east coast onshoreoffshore data (Figure 8). The downward deflection of the
lower crustal layer (Figure 7) can account for half of its 80–
110 km convergence. Given the low model resolution at
large depth, we cannot rule out that pieces of the missing
lower crustal material have been mixed into the crustal root.
In our modeling we have parameterized the lower crust as a
layer that is distinct from the upper crust. The two-layer
model seems justified because of the widespread observation of the PiP reflection. However, there is no evidence that
the lower crust remains intact in the west flank of the root
where it may have been intensely deformed during the
oblique convergence. The image of the lower crustal keel
along transect 2 [Scherwath et al., 2003] shows that the
Pacific lower crust is deformed in the Mount Cook area.
[38] Seismic studies of the mantle beneath South Island
[Stern et al., 2000; Kohler and Eberhart-Phillips, 2002]
show that the crustal root of the Southern Alps overlies a 2 –
4% high-velocity and high-density mantle root. The area
experiences earthquakes between 30 and 70 km depth
[Kohler and Eberhart-Phillips, 2003]. The anomaly indicates lower upper mantle temperatures that may be caused
by a cold downwelling beneath the collision zone.
5.3. Crustal Deformation
[39] The oblique compression in South Island evolved
from a strike-slip setting but crustal deformation now occurs
almost entirely on the Pacific side, leaving the Australian
plate undisturbed. Numerical modeling shows that greater
initial crustal thickness and weaker rheology of the Pacific
plate may have caused it to deform more easily, effectively
making the Australian plate an indentor [e.g., Beaumont et
al., 1996]. The maximum depth of upper mantle earthquake
hypocenters beneath South Island increases from 40 km
near the east coast of South Island to 70 km beneath the
Alpine Fault [Reyners, 1987; Kohler and Eberhart-Phillips,
2003]. Reyners [1987] interpreted this trend as due to
westward underthrusting of Pacific mantle lithosphere.
However, the large shear strain beneath the plate boundary
may also contribute to the occurrence of these deep earthquakes [Kohler and Eberhart-Phillips, 2003].
[40] The new seismic constraints from the SIGHT experiment indicate that the crustal structure of the Pacific and
Australian role may also have played a role in the asymmetric
development of the Southern Alps. If we can assume that the
seismic velocity structure at the east coast is the best
approximation of the Pacific crustal structure before convergence started, quartzofeldspathic rocks of the Torlesse or
Haast terranes may have been juxtaposed against more
competent rocks of the Westland lower crust at 20– 25 km
depth at the start of convergence. The upper crust of the
cratonic blocks of the western Province may also be stronger
than the Torlesse graywacke (Figure 13). Indentation of the
more competent Australian plate subsequently resulted in
thickening, deformation, and strain softening in the Pacific
plate crust [e.g., Koons, 1987; Gerbault et al., 2002, 2003],
probably greatly aided by the expulsion of fluids from the
crustal root [Koons et al., 1998; Wannamaker et al., 2002].
The low-resistivity anomaly imaged by Wannamaker et al.
[2002] roughly coincides with a low seismic velocity anomaly of 0.2– 0.3 km/s beneath the Southern Alps in our study
of SIGHT Transect 1. In other seismic studies of central
South Island [Kleffmann et al., 1998; Stern et al., 2001;
Eberhart-Phillips and Bannister, 2002] this anomaly is found
to be larger in amplitude and confined to a smaller region in
the Pacific crust. The wider low-velocity anomaly in our
study (Figure 7) may result from our use smoothness con-
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straints (equation (1)) instead of damping. The damped
inversion of Eberhart-Phillips and Bannister [2002] did
not attempt to minimize the seismic velocity gradients in
the model. Low seismic velocities in the crust of the Alpine
Fault zone may be evidence for elevated pore pressures
[Christensen, 1984; Stern et al., 2001] which further facilitates strain localization in the Pacific plate crust.
[41] Two other positive feedback mechanisms that may
have localized deformation are mechanical unloading of the
crust by erosion and the advection of heat. The interaction
between rock uplift and surface erosion has been studied
extensively in New Zealand and other mountain belts
[Koons, 1989; Beaumont and Quinlan, 1994; Batt and
Braun, 1999]. The prevailing west wind in South Island
may have accelerated erosion near the west coast, and it
may be largely responsible for the 40 km lateral offset
between the Alpine Fault and the root of the Southern Alps
[Willett, 1999]. The upward advection of relatively hot
lower crust locally elevates the depth of the brittle-ductile
transition and thus influenced the mechanical strength and
strain distribution near the plate boundary [Adams, 1980;
Shi et al., 1996; Batt and Braun, 1999; Leitner et al., 2001].
[42] In our interpretation of the continental compression
we have ignored the large amount of strike slip at the plate
boundary. Three-dimensional models [e.g., Koons, 1994;
Braun and Beaumont, 1995] can better predict the failure
and deformation of crustal rocks in an obliquely convergent
setting. Earthquake studies have provided evidence for
partitioning of strike-slip and convergence along different
fault systems in northern South Island [Anderson et al.,
1993; Doser et al., 1999; Abercrombie et al., 2000; Gledhill
et al., 2000; Leitner et al., 2001]. In this area the crustal
stresses are influenced by the Hikurangi subduction zone,
which lies just north of the continent-continent collision
[Anderson et al., 1993; Reyners and Cowan, 1993; Leitner
et al., 2001]. In contrast, in the crust of central South Island
the Alpine Fault appears to accommodate both strike slip
and convergence [Sibson et al., 1979; Norris et al., 1990;
Cooper and Norris, 1994]. The lack of strain partitioning in
the crust may be characteristic for continental collision
zones, depending on the degree obliquity of convergence
[Braun and Beaumont, 1995]. While convergence deeper in
the crust is thought to be accommodated by a listric
detachment further east [e.g., Norris et al., 1990], the
strike-slip deformation probably extends more vertically
down into the zone of reduced upper mantle seismic
velocity between 200 and 260 km in our profile
(Figure 7). This low-velocity anomaly, with seismic velocities of 7.8 km/s, can be interpreted as a zone of distributed strain in the uppermost mantle that gives rise to
azimuthal anisotropy with the fast velocities parallel to the
plate boundary [Klosko et al., 1999; Scherwath et al., 2002].
Seismic velocities measured in the same location on
Transect 1 in the direction parallel to the Alpine Fault are
as high as 8.2 km/s (A. Melhuish et al., Crustal and upper
mantle seismic structure of the Australian plate, South
Island, New Zealand, submitted to Tectonophysics, 2004).
6. Conclusions
[43] Tomographic inversion of seismic refraction travel
times of the SIGHT data has provided unprecedented
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constraints on the deep structure of South Island, New
Zealand, that give us more insight in the style of crustal
shortening in this young continent-continent collision.
[44] Our seismic velocity model with a two-layer crust
covers crust from the Pacific and Australian plates both
inside and outside the orogen. The 80– 110 km of convergence has locally increased the crustal thickness from
28 km to 37 km beneath the Southern Alps. The deepest
part of the crustal root lies 40 km southeast of the surface
expression of the Alpine Fault. Extensive crustal thickening
has taken place over a 80 km wide zone in the Pacific
plate crust, and it can account for about half the crustal
shortening in central South Island. In contrast, the Australian plate crust is largely undeformed. We attribute the onset
of deformation in the Pacific plate crust to the larger
thickness of weaker upper crust at the start of transpression.
The upper crust of the Pacific plate shows seismic velocities
that are 0.2– 0.3 km/s lower than elsewhere in the quartzofeldspathic rocks of the Pacific plate crust. Overpressured
fluids released from the crustal root may be responsible for
lowering the seismic velocity in the continental collision
zone. The exhumation of hot lower crustal rocks and
coupling between uplift and erosion in the Southern Alps
has also contributed to the localization of strain in a narrow
zone southeast of the Alpine Fault. The results from the
SIGHT experiment can provide much improved constraints
for geodynamic models of this plate boundary.
[45] Acknowledgments. We thank the land crew in South Island and
officers and crew of the R/V Maurice Ewing that made the SIGHT study a
success. Discussions with M. Scherwath regarding SIGHT transect 2 are
much appreciated. The comments made by the Associate Editor and two
anonymous reviewers improved our manuscript. All figures were created
using GMT software [Wessel and Smith, 1998]. Funding was provided by
the New Zealand Science Foundation and U.S. National Science Foundation under grant EAR-9418530. Publication support was provided by the
University of Texas Institute for Geophysics.
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J. K. Austin, ChevronTexaco, North America Upstream, 11111 S.
Wilcrest - N2018, Houston, TX 77099, USA. ([email protected])
F. Davey, Institute of Geological and Nuclear Sciences, P.O. Box 30368,
Lower Hutt, New Zealand. ([email protected])
W. S. Holbrook, Department of Geology and Geophysics, University of
Wyoming, Laramie, WY 82071-3006, USA. ([email protected])
D. Okaya, Department of Geological Sciences, University of Southern
California, University Park, Los Angeles, CA 90089-0740, USA.
([email protected])
T. Stern, School of Earth Sciences, Victoria University of Wellington,
P.O. Box 600, Wellington, New Zealand. ([email protected])
H. J. A. Van Avendonk, University of Texas Institute for Geophysics, 4412
Spicewood Springs Road, Austin, TX 78759, USA. ([email protected])
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Figure 7. Seismic velocity structure along Transect 1 stretches from submerged and undeformed
Australian continental crust to the northwest across the west coast of South Island at 213 km, the Alpine
Fault at 228 km, the east coast at 271 km, and into the submerged Pacific plate. Red triangles show the
location of ocean bottom and land instruments that recorded the wide-angle seismic data.
Figure 11. Diagonal of the resolution matrix shows very good resolution near the surface in South
Island. Resolution is much poorer at larger depth in the model. Trade-offs between various model
parameters at large depth result in low resolution.
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