Geophys. J. Int. (2001) 147, 319–329
The thermal influence of the subducting slab beneath South
America from 410 and 660 km discontinuity observations
J. D. Collier* and G. R. Helffrich
Department of Earth Sciences, University of Bristol, Queen’s Road, Bristol, BS8 1RJ, UK
Accepted 2001 June 6. Received 2001 January 3; in original form 2000 April 28
SUMMARY
Regional seismic network data from deep South American earthquakes to western United
States and western European seismic arrays is slant stacked to detect weak near-source
interactions with upper mantle discontinuities. These observations are complemented by
an analysis of earlier work by Sacks & Snoke (1977) who observed S to P conversions
from deep events to stations in South America, and similar observations from 1994–95
events using the BANJO and SEDA networks. Observations of the depth of the 410 km
discontinuity are made beneath central South America in the vicinity of the aseismic
region of the subducting Nazca Plate. These results image the 410 km discontinuity over
a lateral extent of up to 850 km perpendicular to the slab and over a distance of 2700 km
along the length of the slab. Away from the subducting slab the discontinuity is mainly
seen near its global average depth, whilst inside the slab there is evidence for its elevation
by up to around 60 km but with significant scatter in the data. These results are consistent with the presence of a continuous slab through the aseismic region with a thermal
anomaly of 900 uC at 350 km depth. This value is in good agreement with simple
thermal models though our data are too sparse to accurately constrain them. Sparse
observations of the 660 km discontinuity agree with tomographic models suggesting
penetration of the lower mantle by the slab in the north but stagnation at the base of the
transition zone in the south. The geographical distribution of the data, however, does
not allow us to rule out the possibility of slab stagnation at the base of the transition
zone in the north. These observations, together with the presence of deep earthquakes,
require more complicated thermal models than previously used to explain them, possibly
including changes in slab dip and age with depth.
Key words: 410 discontinuity, 660 discontinuity, subduction zone.
1
INTRODUCTION
At the south-eastern shore of the Pacific Ocean the Nazca and
South American Plates converge at a rate of around 8 cm yrx1
and trench rollback is closing this margin of the Pacific basin
at 3.5 cm yrx1 (Fig. 1). The pattern of seismicity in this
region includes an aseismic gap between around 200–300 and
500–550 km depth. This enigmatic feature has been attributed
to either the absence of subducted material or the high slab
temperatures associated with the subduction of relatively young
material. Both seismic tomography (van der Hilst et al. 1997;
Bijwaard et al. 1998) and observations of seismic signals interpreted to be reflections from the upper slab/mantle interface
(James & Snoke 1990) seem to indicate that the slab is in fact
* Now at: QinetiQ, St. Andrews Road, Malvern, Worcestershire,
WR14 3PS, UK. E-mail: [email protected]
# 2001
RAS
continuous through this region. At the base of the transition
zone tomographic studies (van der Hilst et al. 1997; Grand
1994; Bijwaard et al. 1998) suggest a change in the behaviour
of the slab reaching the 660 km discontinuity. Beneath Peru
penetration of the 660 is favoured by the presence of high
velocity material in the lower mantle beneath the deepest
seismicity. Further south, beneath Argentina, a shallowing
of the dip in the region of the deepest seismicity seems to be
apparent with high velocity material being evident in a
horizontal layer at the base of the transition zone.
Another way to analyse the mantle tectonics of the region is
through the thermal effects of a slab on the seismic discontinuities at depths of 410 and 660 km. The discontinuities mark
phase changes in the olivine component of the mantle (Bernal
1936; Ringwood 1969) which, through their Clapeyron slopes,
respond to temperature changes by moving shallower or
deeper. By searching for the thermal effect of the slab on the
319
320
J. D. Collier and G. R. Helffrich
Figure 1. Map of South America showing the location of the discontinuity data relative to the subduction zone. Diamonds show data from this study
and circles those of Sacks & Snoke (1977). The thick solid line shows the NUVEL-1 plate boundary model (DeMets et al. 1990) whilst the thin solid
lines show slab surface contours from Gudmundsson & Sambridge (1998) from 0 to 650 km every 50 km. Azimuths to the western United States and
northwest Europe are shown by the dashed lines. The boxes designate the areas in each of the cross sections (a) to (d) in Fig. 6. Absolute motion
vectors and rates are based on Gripp & Gordon (1990).
410 km discontinuity we can look for further evidence either
for or against slab continuity, and if topography on the discontinuity is present these results would help to constrain the
thermal anomaly that is associated with it.
The 410 km discontinuity is caused by the olivine to wadsleyite
phase change. This reaction has a positive seismic dP/dT slope
of 2.04 MPa uCx1 (Helffrich & Bina 1994) so the discontinuity
will move to shallow depths in cooler regions such as near to
a subducted slab. Numerical thermal modelling (e.g. Helffrich
et al. 1989) can be used to estimate the topography of the discontinuity in a subduction zone assuming an equilibrium phase
change for comparison with observed data. The 660 km discontinuity is widely thought to be caused by the breakdown of
ringwoodite to perovskite plus magnesiowustite, this transition
has a negative Clapeyron slope and so will occur at higher
pressures in the vicinity of cold regions of the mantle. Depression
of this discontinuity may therefore be used to infer the presence
of cold material in subduction zones. Thus it may be possible to
discover the fate of subducted material at the base of the upper
mantle by imaging the depth of this discontinuity.
2
DATA AND METHODS
In the main part of this study we use near source underside
P-wave reflections and S to P conversions (Fig. 2) observed
in teleseismic records of deep South American earthquakes
recorded by distant seismic networks to image the 410 km
discontinuity and downgoing S to P conversions to image the
660 km discontinuity. The large number of stations in regional
networks allows us to stack all the records from a particular
network/earthquake combination to enhance weak but coherent
signals above the background noise level and thus to image the
410 and 660 km discontinuities. Our choice of networks is
constrained by the requirement of a source to receiver distance
less than about 97u when our reference phase, P, starts to
diffract around the core-mantle boundary. We therefore use the
University of Washington, University of Utah and Southern
Californian networks in the western United States and the
United Kingdom and French networks in western Europe. We
retrieved archived data for the UK network from tapes held
by the British Geological Survey in Edinburgh. Data from the
#
2001 RAS, GJI 147, 319–329
South American subducting slab
321
Table 1. List of earthquakes with locations of Engdahl et al. (1998) for
events from 1964 and onwards.
Figure 2. Schematic showing the phases and notation used in this
study for both teleseismic and local receivers and for earthquakes at
depths of 200 and 590 km shown by the stars. Initially upgoing phases
(from source or discontinuity) are designated by a lowercase letter and
initially downgoing ones by a capital.
other networks was obtained mostly via Automatic Data
Request Managers (AutoDRM). The number of earthquakes
that we are able to use is constrained by a number of practical
considerations. For the AutoDRM data the earthquake must
trigger the networks, originally setup to monitor regional rather
than teleseismic events, and at least 60 seconds of data after
the P arrival must be available. Earthquake magnitudes are
important since those with a magnitude less than 5.3 mb are
generally too noisy to use and large magnitude events (mbi6.1)
are often clipped and therefore unusable. After the removal
of noisy and clipped records a minimum of around 30 seismograms are required for stacking in order to successfully suppress
incoherent noise so that the discontinuity phases are visible.
Seventeen deep earthquakes beneath South America provide
the observational base (Tables 1 and 2). We manually align the
records for each event/network combination on the P-wave
arrival and slant stack them to enhance weak coherent arrivals
above the background noise using standard nth root methods
(Muirhead 1968; McFadden et al. 1986; Collier & Helffrich
1997). Examples of the data for both an earthquake above both
discontinuities and one between them are shown in Fig. 3, some
examples of stacked data are shown in Fig. 4.
Both p410P, s410P (see Fig. 2 for notation) can sometimes
be observed in the stacked teleseismic records given favourable
source mechanisms and geometry. Near-source 410 km discontinuity interactions are often difficult to observe in stacked
data (Collier & Helffrich 1997; Collier et al. 2001; Castle 1998).
The properties of the discontinuity are not as favourable to
its observation in subduction zones as they are in the normal
mantle. Topography on the discontinuity may cause defocusing
(or focusing) of waves and the thermodynamic properties of
the discontinuity may cause smaller reflections/conversions in
subduction zones than in the normal mantle. For instance, low
slab temperatures (e.g. Katsura & Ito 1989) and the presence of
water (Wood 1995) both broaden the depth interval over which
the olivine to wadsleyite phase change occurs making it more
difficult to observe seismically. The kinetics of the phase change
#
2001 RAS, GJI 147, 319–329
Date
Latitude
(u)
Logitude
(u)
Depth
(km)
Magnitude
mb
1959/07/06
1959/07/06
1959/12/27
1962/09/29
1964/12/09
1965/03/05
1965/07/30
1966/12/20
1967/01/17
1967/09/09
1968/08/23
1969/07/25
1986/03/26
1987/11/06
1989/05/05
1990/02/27
1990/10/17
1993/05/06
1994/01/10
1994/04/29
1994/08/08
1994/08/19
1994/11/04
1995/03/14
1995/05/12
1996/04/24
1997/03/25
1997/04/01
1997/07/20
1997/11/28
1999/03/02
1999/09/15
x26.50
x26.50
x28.00
x27.00
x27.55
x27.10
x22.86
x26.20
x27.40
x27.68
x22.03
x25.63
x7.13
x22.88
x8.34
x17.27
x11.00
x8.50
x13.42
x28.33
x13.83
x26.68
x9.43
x15.19
x19.41
x8.162
x9.07
x18.33
x22.91
x13.81
x22.90
x20.82
x61.00
x61.50
x63.00
x63.60
x63.32
x63.38
x63.83
x63.18
x63.30
x63.18
x63.67
x63.35
x71.62
x63.65
x71.38
x64.14
x70.71
x71.45
x69.44
x63.21
x68.34
x63.37
x71.30
x64.84
x63.91
x74.28
x71.24
x69.29
x66.24
x68.81
x68.51
x67.23
600
600
650
575
585
569
531
585
592
579
535
591
608
538
605
614
590
580
608
571
605
565
599
591
596
150
609
106
247
606
107
213
6.8
6.9
x
x
5.7
5.6
4.3
5.8
5.6
5.9
5.6
5.4
5.8
5.8
6.2
5.3
6.5
5.8
6.4
6.3
5.4
6.4
5.8
5.0
5.2
5.5
5.3
5.8
5.7
6.3
5.8
6.0
may also be important since lower temperatures may cause
the transformation to proceed more sluggishly, thus influencing
both its depth and sharpness. These effects, however, have yet
to be quantified experimentally. Observations of the 410 km
discontinuity in the northwest Pacific (Collier et al. 2001) provide evidence for a combination of some of these effects since
the scatter in observed p410P reflection coefficients is much
larger near subducting slabs than in the mantle. Similar effects
beneath South America make it equally difficult to observe
the 410 km discontinuity there. In this study we have obtained
teleseismic data for 36 earthquake/network combinations where
more than about 30 records are suitable for stacking and the
discontinuity interactions are not obscured by larger arrivals
in the same time window such as pP and sP. In these data
we make five out of a possible eight S410P observations, 13
out of 28 s410P, 13 out of 28 p410P and 16 out of 36 S660P
observations. Observations of downgoing S to P conversions
at the 660 km discontinuity (S660P) are often made difficult by
the close proximity of the deepest earthquakes to the discontinuity in this study, making this phase difficult to distinguish
from the P-coda. These figures show that just over 50 per cent
of both s410P and p410P interactions are at or below the noise
level even after the data are stacked, clearly demonstrating the
difficulty in making such observations. Sources of error in the
vertical location of the 410 km discontinuity obtained by this
322
J. D. Collier and G. R. Helffrich
Table 2. Discontinuity interaction points for the earthquakes studied using the teleseismic data from the University of Washington (UW),
United Kingdom (UK), University of Utah (UU), Southern Californian (SC) and French (FR) networks.
Date
Net.
No. of
stations
Az.
(u)
Back
az. (u)
D
(u)
Time
(s)
Phase
Conv.
Lat. (u)
Conv.
Long. (u)
1986/03/26
1987/11/06
UK
UW
34
61
34.16
324.55
247.91
129.03
82.74
86.35
1989/05/05
1990/02/27
UW
UW
55
58
325.95
324.20
126.28
125.69
70.71
81.80
1990/10/17
SC
FR
66
25
315.75
42.61
126.35
252.43
72.10
88.34
UW
FR
SC
UU
54
15
79
30
326.07
42.72
317.20
325.40
127.19
256.01
125.47
132.80
73.29
88.38
61.19
61.30
FR
14
42.55
251.31
90.64
UK
69
33.09
243.34
87.45
UW
SC
UW
UW
UW
SC
66
67
66
49
82
53
323.07
317.46
323.75
325.29
323.83
317.18
122.65
127.29
131.61
126.78
130.51
132.35
79.01
66.11
90.73
76.81
89.64
79.83
1996/04/24
1997/03/25
UK
UW
33
61
32.76
326.10
249.17
126.82
85.65
71.21
1997/04/01
1997/07/20
1997/11/28
UK
UW
UW
SC
35
59
62
81
32.40
325.50
325.12
317.28
239.989
130.84
127.13
127.05
91.32
85.02
76.19
66.98
FR
29
41.71
249.50
89.65
UW
UK
46
56
326.53
32.18
132.56
236.73
83.86
91.89
FR
UW
SC
47
66
44
41.90
325.68
318.69
243.10
130.28
131.32
92.95
82.82
73.03
39.00
25.60
38.50
11.05
46.25
31.95
47.00
5.25
5.65
35.70
51.00
10.35
55.30
35.45
8.20
26.85
42.35
35.00
56.90
47.35
64.35
35.40
5.25
30.65
9.65
23.98
6.85
39.80
59.70
23.30
43.65
67.70
53.30
39.45
6.45
7.80
52.00
5.75
41.50
65.89
57.80
17.35
45.50
18.05
15.30
18.75
45.70
p394P
p410P
s401P
S663P
s431P
p424P
s429P
S659P
S673P
p398P
s406P
S710P
s385P
p391P
S660P
p408P
s410P
p419P
s403P
p355P
s377P
p409P
S652P
s414P
S714P
p431P
S641P
p347P
s350P
S376P
p357P
s358P
S656P
S661P
S678P
S690P
s408P
S672P
p384P
s370P
S704P
S383P
S694P
S390P
S361P
S392P
S679P
x6.16
x22.33
x22.61
x22.61
x7.87
x16.35
x16.78
x17.02
x17.09
x10.32
x10.68
x10.77
x10.49
x7.83
x8.27
x7.45
x8.05
x12.75
x13.06
x12.38
x12.94
x12.48
x13.26
x28.03
x13.54
x26.17
x26.46
x25.73
x26.20
x7.78
x7.66
x8.42
x17.33
x22.08
x13.62
x13.58
x13.32
x13.68
x13.03
x13.39
x21.71
x20.54
x19.90
x20.55
x20.55
x20.48
x19.81
x70.96
x64.07
x63.86
x63.86
x71.10
x64.74
x64.43
x64.24
x64.30
x70.08
x70.41
x70.49
x71.06
x70.83
x71.67
x72.19
x71.77
x68.81
x69.10
x68.75
x69.12
x70.16
x69.54
x63.46
x68.55
x63.79
x63.53
x64.28
x63.80
x74.02
x72.19
x71.68
x68.63
x66.85
x68.94
x69.02
x69.27
x68.68
x68.09
x68.42
x69.35
x67.04
x66.61
x66.98
x67.42
x67.54
x68.17
1993/05/06
1994/01/10
1994/04/29
1994/08/08
1994/08/19
1999/03/02
1999/09/15
method come mainly from earthquake depth errors, arrival
time picking errors and slab velocity anomalies (Collier &
Helffrich 1997) and are around t14 km (Collier 1999) for the
South American subduction zone geometry.
To complement these results we search for upgoing S to P
and P to S conversions at the 410 km discontinuity (Fig. 2)
in the individual records of local South American stations.
Sacks & Snoke (1977) studied 15 deep earthquakes in this way
(Table 3), and observed arrivals a few tens of seconds after P
that they attributed to interactions with a seismic discontinuity
near 400 km depth. At that time they interpreted these arrivals
as interactions with the lithosphere/asthenosphere boundary.
Here we favour an interpretation associating them with interactions with the globally observed 410 km discontinuity (Table 3)
since there is little evidence for lithosphere thicker than 150
to 200 km even below continents (e.g. Anderson 1995). We
re-analysed their time picks where possible with the earthquake
locations of Engdahl et al. (1998) to improve the accuracy of
both the vertical and lateral locations of the discontinuity
observations. Additionally we use the records of temporary
stations of the Broadband Andean Joint Experiment (BANJO)
(Beck et al. 1996) and the Seismic Exploration of the Deep
#
2001 RAS, GJI 147, 319–329
South American subducting slab
323
Table 3. Discontinuity interaction points observed from the data of Sacks & Snoke (1977) and the BANJO and SEDA stations used in this study.
Date
Station
Elev.
(m)
Az.
(u)
D
(u)
Channel
Time
(sec)
Phase
Conv.
lat. (u)
Conv.
long. (u)
1959/07/06
1959/07/06
LQA
JUJ
LQA
3464
1270
3464
315.26
302.49
318.64
6.06
4.08
5.76
ZON
LPA
ZON
ZON
ZON
TCN
730
14
730
730
730
3300
229.64
149.12
223.18
228.22
224.94
277.31
8.05
8.14
6.34
6.14
6.41
4.05
ZON
LPA
ARE
LPA
ZON
LPA
730
14
2452
14
730
14
220.86
153.74
319.78
149.77
229.82
159.82
7.19
9.79
12.4
8.79
6.15
13.78
LPA
HIZO
SCHO
HIZO
DOOR
TACA
POOP
UYUN
DOOR
CHUQ
UYUN
CRUZ
14
3688
2591
3688
3749
3810
3721
3936
3749
4178
3936
4267
154.44
179.87
146.66
326.07
158.65
155.15
155.45
158.68
208.45
225.74
198.82
198.53
10.37
5.75
6.37
8.39
10.62
10.34
9.82
11.82
4.73
3.96
5.55
4.11
NS&EW
NS&EW
NS
EW
NS
NS
EW
EW
EW
EW
NS
EW
V&NS
NS
V
EW
V
NS
V
BHZ
BHZ
BHZ
BHZ
BHZ
BHZ
BHZ
BHZ
BHZ
BHZ
BHZ
SICA
HIZO
4065
3688
232.76
266.57
3.49
4.17
24
22
25
22
21.5
30
20
48
19
15
14.5
41
26
30
24
24
20
21
29
49.4
22.7
28.3
29.0
28.0
26.42
32.64
22.7
15.04
28.48
22.4
46.7
38.74
26.5
s398p
s389p
s386p
s410p
s437p
s417p
s412p
p402s
s415p
s393p
s397p
p339s
s410p
s403p
s420p
s382p
s420p
s415p
s402p
p421s
s415p
s367p
s409p
s412p
s418p
s397p
s384p
s443p
s349p
s378p
p415s
p352s
s347p
x25.64
x27.06
x25.55
x25.67
x27.19
x29.35
x27.73
x30.4
x27.76
x22.77
x22.77
x30.51
x27.26
x25.27
x28.40
x28.45
x22.73
x22.76
x26.78
x17.85
x14.80
x25.55
x10.63
x10.58
x10.55
x10.70
x16.14
x15.65
x16.53
x16.14
x17.62
x16.62
x19.48
x61.94
x60.57
x62.42
x62.31
x62.42
x62.07
x64.37
x67.17
x64.13
x64.54
x64.52
x67.57
x62.59
x64.05
x62.64
x64.22
x63.39
x63.38
x62.73
x68.33
x67.68
x64.21
x70.82
x70.76
x70.78
x70.80
x65.38
x65.33
x65.32
x65.17
x65.70
x66.83
x65.20
1959/12/27
1962/09/29
1964/12/09
1965/03/05
1965/07/30
1966/12/20
1967/01/17
1967/09/09
1968/08/23
1969/07/25
1994/08/08
1994/08/19
1994/11/04
1995/03/14
1995/05/12
Altiplano (SEDA) (Beck et al. 1994) for five deep South
American events. For this data travel time differences between
direct P and the phase of interest are calculated by crosscorrelating manually picked time windows of between 4 and
8 s duration around each phase. Fig. 5 illustrates examples of
the arrivals seen and all the measurements are summarized
in Table 2. Care has been taken to discard arrivals in both
datasets with characteristics that could mean they are caused
by crustal reverberations. The number of observations is small
compared to the number of records analysed. For instance
there were 16 stations in the BANJO deployment and seven in
SEDA, yet even for the best event, with the clearest P wave
arrival and the least noise, discontinuity interactions are only
observed in five records from both the networks (Fig. 5).
Errors in the locations of the 410 km discontinuity interaction points observed by the local stations are harder to
quantify than for the teleseismic data given the high variability
of the source/receiver geometry. Relative travel time errors
are estimated by measuring the half-width of the envelope of
the cross-correlation. For the data in our study this yields a
mean value of 1.4 s equating to at most 10 km in the depths
calculated. Other major errors come from near source velocity
anomalies with respect to the velocity model used, here IASP91
(Kennett & Engdahl 1991). The crust beneath the Andes may
be as thick as 70 km in places (e.g. Beck et al. 1996) but will
#
2001 RAS, GJI 147, 319–329
BHZ
BHZ
have only a limited effect for s410p since both the direct P
and s410p phase travel through fairly similar paths in the crust
as P waves (Fig. 2). For the p410s phase, however, it may
be larger and contribute up to around a second to the travel
time difference using crustal velocities of VP=6.25 km sx1 and
VS=4.33 km sx1 (Beck et al. 1994). The thick crust would have
the effect of increasing the time separation between P and
s410p, leading to the discontinuity inferred to be shallower than
in reality. A potentially much larger effect are long slab paths
for either P or the discontinuity phase and is especially difficult
to quantify here since the slab is undefined by seismicity.
Assuming the worst case where one leg, either direct P or the
discontinuity interaction phase, spends all its time in material
with a velocity 5 per cent faster than the ambient mantle whilst
the other travels through normal mantle, relative travel times
may be affected by up to 5 s. Though it is unlikely that the
effect is as large as this such a travel time anomaly would
yield a depth error of up to 40 km in the interpreted 410 km
discontinuity depths and also a significant lateral error.
3
DISCUSSION
The subduction zone structure beneath South America changes
significantly along the length of the region studied here (Fig. 1).
In the north the aseismic gap covers a larger depth range than
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J. D. Collier and G. R. Helffrich
Figure 3. Example record sections for the 1987 November 6 (a) and the 1999 September 15 earthquakes (b) recorded by the University of Washington
network. The seismograms have been lowpass filtered with a corner frequency of 1.0 Hz and manually aligned on the P-wave arrival. Theoretical
arrival times for 410 and 660 km discontinuity phases as well as for PcP and pP are marked.
farther south and tomographic studies (e.g., van der Hilst et al.
1997; Bijwaard et al. 1998) indicate that the slab is steeper at the
base of the transition zone there. To relate the discontinuity
topography to slab structure we plot four carefully chosen
cross-sections (Fig. 6) so that the abrupt changes in slab dip
and strike minimally interfere with our interpretation. In the
far north (Fig. 6a) there is evidence for near normal 410 depths
at distances greater than around 100 km away from the slab
and the data is suggestive of some elevation near the slab that
could be associated with a thermal anomaly, though we only
have dense data east of the slab’s probable position. Further
south in Fig. 6(b) the picture is less clear with one data point
in particular suggesting a large elevation far from the slab. It
should be noted, however, that interpretation of this crosssection is severely hampered by rapid changes in the slab dip
and strike both laterally and with depth (Fig. 1), highlighted by
the large scatter in the shallow seismicity in this figure. With
this caveat, mislocation of the interaction points by unaccounted
for velocity anomalies is the most likely cause of the unexpected
uplift in Fig. 6(b). Alternatively the elevated 410 interactions
may represent a break in the slab and an eastward jump in its
position with respect to the Gudmundsson & Sambridge (1998)
slab contours, with the consequence of the thermal anomaly
appearing far from the location of the expected continuous
slab.
Where slab dip and strike change more slowly (Fig. 6c and d)
interpretation is easier. Both Fig. 6(c) and Fig. 6(d) show
near-normal 410 depths away from the slab with evidence for
perturbation in the 410 depth near to it. Fig. 6(c) shows clear
discontinuity uplift in a region where the slab might be expected,
though slightly farther east than suggested by the slab contours
of Gudmundsson & Sambridge (1998). These data from individual
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2001 RAS, GJI 147, 319–329
South American subducting slab
p408P
p401P
325
s410P
s401P
P
P
S663P
P
P
S383P
S383P
S693P
S693P
Figure 4. Examples of Nth root slant stacked data with N=3. The P wave and the 410 and 660 km discontinuity interactions are labelled.
Earthquakes (see Table 1 for locations) on (a) 1987 November 6 recorded by the University of Washington network, (b) 1993 May 6 recorded by the
University of Utah network, and 1999 September 15 recorded by (c) the United Kingdom and (d) Southern Californian networks. Data to the right of
the vertical line are magnified by the amount shown.
observations are clear evidence for discontinuity uplift of
60t40 km and indicate the presence of a thermal anomaly.
In Fig. 6(d) the 410 is near its global average depth far from
the slab. Nearby there is some suggestion of discontinuity
uplift but with significant scatter in the data. Data at the same
horizontal location in this cross-section seem to suggest either
zero or 60 km elevation, the values at the lower end of this
range coming from the data of Sacks & Snoke (1977). The error
bars from these two groups of data overlap so they do not
directly contradict each other, but the scatter obscures the true
nature of the discontinuity topography there.
A limited number of identified interactions with the 660 km
discontinuity show a trend of decreasing depths beneath the
deepest seismicity from north to south. The data in Figs 6(a)
and (b) show the 660 to be depressed by around 50 km consistent with the depression expected from slab penetration of
this discontinuity and in agreement with tomographic results.
However, slab stagnation on top of the 660 km discontinuity
can cause it to be depressed by a similar amount, as has been
observed in the northwest Pacific (Castle & Creager 1998; Collier
et al. 2001), so the data do not conclusively prove penetration
of the discontinuity by the slab. Further data eastward of the
current cluster are necessary to prove this argument one way or
another, but this is difficult to achieve with downgoing S to P
660 km discontinuity interactions due to the locations of the
source earthquakes. Further south the observations in Fig. 6(d)
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2001 RAS, GJI 147, 319–329
show near normal depths beneath the seismicity and suggest
no slab penetration of the 660 but a shallowing of dip angle at
the base of the transition zone due to trench rollback (Olson &
Kincaid 1991; van der Hilst & Seno 1994).
4
THERMAL MODELLING
The deflection of the transition zone discontinuities in the
presence of a subducted slab yields information about its
thermal structure. This in turn has implications for both the
cause of the gap in seismicity between around 200 and 500 km
beneath South America and also for proposed mechanisms of
the deep earthquakes in that region. If the slab is continuous
410 km discontinuity observations inside it act as a thermometer to measure its temperature, and hence give us insight into
the absence of seismicity there.
We use a simple thermal model (Minear & Toksöz 1970) to
estimate the thermal anomaly and 410 and 660 km discontinuity topography expected to be associated with the subducting slab beneath South America (Fig. 7a). In this model
the subducting material at the trench has a constant age of
45 Myr. Using a Clapeyron slope of 2.04 MPa uCx1 (Helffrich
& Bina 1994) the 410 km discontinuity would be elevated by
57 km in the coldest part of the slab at a temperature of 670 uC.
This represents a thermal anomaly of 900t250 uC relative to
the surrounding mantle. The 410 km discontinuity topography
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J. D. Collier and G. R. Helffrich
P
P
s443p
S
P
s378p
P
s384p
P
S
p352s
p352s
S
S
s349p
S
Figure 5. Examples of four s410p and two p410s conversions observed in the records of BANJO and SEDA stations SICA, CHUQ, CRUZ, DOOR
and UYUN for an earthquake on 1995 March 14 (Table 1). Theoretical arrival times for both the discontinuity interaction phases as well as P and S
are marked using the source locations in Table 2 and the IASP91 velocity model. Relative traveltimes between the discontinuity phases and P are
calculated by cross-correlating the two waveforms.
would be consistent with the observations in Figs 6(a), (c) and
(d), though other models might also explain the seismic data,
and there are possible systematic uncertainties stemming from
the experimental constraints associated with the Clapeyron
slope under these conditions (Bina & Helffrich 1994). Also, the
possible effects of lower temperatures on the kinetics of the
olivine to wadsleyite transformation means that this is a lower
limit to the thermal anomaly. Coupled with other evidence such
as seismic tomography (van der Hilst et al. 1997; Bijwaard et al.
1998) and observations of the slab/mantle interface (James &
Snoke 1990) the data indicates a continuous slab through the
aseismic zone with a temperature of around 700t250 uC in its
core.
An important question to our understanding of deep
earthquake mechanisms is whether the absence of earthquakes
here is related to the temperature of the slab. It has been
suggested (e.g. Furukawa 1994) that the 600 uC isotherm may
define the cut-off depth for deep seismicity if it is linked to
the phase change. Fig. 7(a) shows that in this simple model the
depth extent of this isotherm matches the maximum depth of
the shallow seismicity. However the high slab temperatures
in this model at depths between 500 and 660 km, due to the
young lithosphere age at the trench (45 Myr in this model), are
incompatible with the occurrence of the deep earthquakes there
if cold slab temperatures are required (Kirby et al. 1996) since
in this model the temperature in the centre of the slab at
660 km depth is 925 uC.
Two possibilities present themselves as a solution to this
problem. Firstly, the deep earthquakes might not be related to
the kinetically hindered phase change but may be due to some
other mechanism as suggested by Furukawa (1994). Alternatively
an abrupt increase in the age of the subducted slab at a depth
between around 300 and 500 km is plausible (Engebretson &
Kirby 1992; Kirby et al. 1995, 1996) to allow the presence of a
remnant of metastable olivine at the base of the transition zone.
We model this (Fig. 7b) with a jump in the age of the subducting slab at the trench from 45 to 140 Myr at a time 15 Myr
before present so that at the end of the model it lies at a depth
of just over 400 km. The top part of this model is very similar to
that in Fig. 7(a) with topography differences on the 410 being
undetectable seismically. Now, however, at depths below 400 km
the thermal anomaly is much broader and colder with a temperature of around 680 uC at 660 km depth. This model may
be consistent with the transformation of a remnant metastable
olivine wedge as the mechanism for the deep earthquakes.
However the presence of cold and dense material at the base of
the transition zone seems to favour penetration of the 660 by
the subducting slab as indicated by 660 depths and tomography
(Bijwaard et al. 1998) in the north of the region (Figs 6a and b).
It may therefore be difficult to reconcile this model with the
normal 660 depths and shallowing velocity anomaly in the
south as indicated by our near normal 660 km discontinuity
depths and also seismic tomography (Bijwaard et al. 1998).
Observations of transition zone discontinuity topography in
subduction zones such as those in this study offer a unique
opportunity to measure slab temperatures at depths of between
around 350 and 410 km and 660 and 710 km. Whilst the data
beneath South America are not yet of sufficient quality to draw
firm conclusions about the slab thermal structure there is evidence
for topography on both the 410 and 660 km discontinuities
caused by thermal anomalies associated with the presence of a
subducting slab. The aim of future work should be to increase
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2001 RAS, GJI 147, 319–329
South American subducting slab
327
Figure 6. Cross-sections of the South American subduction zone corresponding to the boxes marked on Fig. 1. Squares show discontinuity
observations made with the teleseismic records, circles the re-interpreted data of Sacks & Snoke (1977) and triangles observations made from recent
earthquakes with the BANJO and SEDA networks and station CPUP. Earthquake locations are from Engdahl et al. (1998). The nominal 410 and
660 km discontinuity depths are shown by the dashed lines. The thick solid line indicates the positions of the contour lines of Gudmundsson &
Sandbridge (1998) and helps to illustrate the possible location of the slab if it is continuous through the aseismic region. The dotted line shows an
alternative interpretation of where the centre of the slab lies further to the east in (c) and (d). The 410 and 660 km discontinuity depths using Clapeyron
slopes of 2.04 and x1.9 MPa uCx1 respectively are shown by the thin solid lines.
the data coverage thus enabling more accurate slab temperature measurements thereby placing tighter constraints on the
slab position.
5
CONCLUSIONS
The 410 km discontinuity beneath South America has been
imaged over a large lateral extent using both local and teleseismic records. The data provide some evidence for elevation
of the discontinuity above its global average depth near the
aseismic region of the subducting slab. This is evidence for
the thermal effect of a subducted slab and indicates that it
may be continuous through this region. A thermal anomaly
of 900t250 uC relative to the mantle is implied at a depth of
350 km in the slab by the shallowest discontinuity interactions
observed, but scatter in the data suggests complications arising
from velocity anomalies associated with the subducting slab
and/or thick crust beneath South America. Variations in the
660 km discontinuity depths observed across the region are
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2001 RAS, GJI 147, 319–329
compatible with slab penetration of the discontinuity in the
north but shallowing of the slab dip at the base of the transition
zone in the south. Stagnation of the slab at the base of the
transition zone cannot be ruled out, however, with the data
coverage available.
ACKNOWLEDGMENTS
We thank the British Geological Survey for access to archived
and AutoDRM UK network data, RENASS for AutoDRM
French network data, and IRIS for access to BANJO/SEDA,
University of Washington and University of Utah network data.
We also thank SCEC for access to Southern Californian network data. JC thanks Luisa Braña for providing UK data for
two events, Bob Engdahl for providing his earthquake catalogue
for events up to the end of 1999, and Harmen Bijwaard for
cross-sections of his tomographic model beneath South America.
Two constructive reviews by anonymous referees helped to vastly
improve the manuscript. Some figures were created with GMT
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J. D. Collier and G. R. Helffrich
Figure 7. Numerical models for the thermal structure of a slab subducting with a velocity of 8.3 cm yrx1 with a dip of 35u. (a) A constant slab age at
the trench of 45 Myr. (b) An age of 140 Myr when at the trench for the deep subducted slab with a jump in age at the trench to 45 Myr introduced
15 Myr before the end of the model with the boundary finishing at a depth of just over 400 km. The thick solid lines across the slab and the doubleended arrow show the location of the aseismic zone. Isotherms are shown every 200 uC by thin solid lines with the 600 and 1400 uC isotherms shown by
thicker solid lines. The largest source of errors in the absolute temperatures come from the basal lithosphere temperature used and are around
t250 uC. The 410 and 660 km discontinuity depths using Clapeyron slopes of 2.04 and x1.9 MPa uCx1 respectively are shown by the dashed lines.
software Wessel & Smith (1998). JDC was supported by two
NERC awards during this work. Correspondence to JDC.
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