THE STRUCTURE OF THE CRUST AND UPPERMOST MANTLE BENEATH THE CENTRAL ANDES FROM AMBIENT NOISE TOMOGRAPHY
Kevin M. Ward , Ryan C. Porter , George Zandt , Susan L. Beck , Estela Minaya , Hernando Tavera , Lara S. Wagner , Maureen D. Long
1
20˚S
21˚S
D
24˚S
25˚S
Figure 1. Study area map with the location of broadband seismometers used shown as
diamonds. The locations of four shear-wave velocity cross-sections (Figure 9) are
shown as solid black lines.
10
20
resolution (km)
Depth (km)
Depth (km)
69˚W
67˚W
68˚W
14˚S
30
40
50
15˚S
60
70
70˚W
71˚W
2 2.5 3 3.5 4 4.5 5
Vs (km/s)
2 2.5 3 3.5 4 4.5 5
Vs (km/s)
66˚W
Bolivia
Peru
ter
n
Co
rd
ille
ra
19˚S
3.3
3.2
20˚S
3.1
Subandean Zone
Bolivian EC
Peruivan EC
APVC
Central Altiplano
Northern Arc
Peruivan Arc
Bolivian Arc
Peruvian Forearc
Chilean Forearc
2.9
2.8
8
10
12 14 16
20
25
30
Period (s)
35
18˚S
2 2.5 3 3.5 4 4.5 5
Vs (km/s)
Chilean Forearc
0
10
20
30
40
50
60
70
2 2.5 3 3.5 4 4.5 5
Vs (km/s)
APVC
0
Chile
70˚W
50
20˚S
22˚S
71˚W
40
60
19˚S
21˚S
72˚W
30
16˚S
22˚S
23˚S
73˚W
20
70
21˚S
40
10
17˚S
Altiplano
Bolivian Arc
0
15˚S
We
s
16˚S
Vs (km/s)
65˚W
13˚S
14˚S
2 2.5 3 3.5 4 4.5 5
68˚W
67˚W
20
30
40
50
60
Puna
69˚W
10
23˚S
65˚W
66˚W
70
2 2.5 3 3.5 4 4.5 5
Vs (km/s)
Figure 4 (right). Map showing the locations and 1-D shear-wave velocity profiles of 10 representative points (black/white stars), Holocene volcanic activity (black triangles), simplified geology [modified
from Barnes and Ehlers, 2009], and major morphotectonic provinces [modified from Tassara, 2005]. Each 1-D shear-wave velocity profile is assigned an uncertainty envelope (gray area) by inverting for every
possible permutation of reasonable values adjustable in the inversion with the results from our preferred values shown as dashed black lines. Profiles are corrected for elevation and zero refers to sea-level.
100
50
22˚S
2.8
2.7
2.6
24˚S
2.5
2.4
26˚S
Phase Velocity (km/s)
200
400
600
800
path density
1000
Figure 2 (above). Phase velocity results for 8 seconds. All results shown have at least fair
resolution (< 800 km) and the black line contours regions with at least good resolution
(< 150 km). Note the first-order correlation of velocity anomalies with the major
morphotectonic provinces shown in Figure 4.
From the 2-D phase velocity maps at 8, 10, 12, 14, 16, 20,
25, 30, 35, and 40 seconds, we construct phase velocity
profiles at every grid point (0.1° by 0.1°) in our array. At each
grid point where all ten periods used to construct the phase
velocity profiles have fair resolution (< 800 km), we
iteratively invert for the 1-D shear-wave velocity structure
[Herrmann, 1987]. The inversion method requires a 1-D
shear-wave velocity starting profile and we use a constant
value starting model (dz = 1 km, Vs = 4.6 km/s, Vp/Vs =
1.75, ρ = 3100 kg/m3, Q = 200). Although we have customized
these values for our study area and data set, we define the
uncertainty of our results by inverting for every possible
permutation of reasonable values (Figure 4).
Figure 3 (above). Phase velocity results for 20 seconds. All results shown have at least fair resolution (< 800 km) and the black line contours regions with at least
good resolution (< 150 km). Note the low-velocity-zone (LVZ) centered near the common border of Bolivia, Chile, and Argentina in the 20 second map agrees with
previous work that has identified a LVZ associated with the upper-crustal Altiplano-Puna Magma body (APMB).
REFERENCES AND ACKNOWLEDGMENTS
This work is supported by NSF award #0907880, #0943991, and #0909254. The instruments used in the CAUGHT field program were provided by the PASSCAL facility of
the Incorporated Research Institutions for Seismology (IRIS) through the PASSCAL Instrument Center at New Mexico Tech. Data collected during this experiment will be
available through the IRIS Data Management Center. The facilities of the IRIS Consortium are supported by the National Science Foundation under Cooperative Agreement
EAR-0552316 and by the Department of Energy National Nuclear Security Administration. We acknowledge the GEOFON Program of GFZ Potsdam as an additional source of
waveform data. We further acknowledge all of those who helped to deploy, maintain, and service the seismic stations from the 13 different international networks. Finally, we
thank all the PIs from the PLUTONS and the PeruSE experiments for sharing some of their restricted data prior to their release date. Additional support was provided to K. M.
Ward by Chevron-Texaco and Conoco-Phillips.
Barmin, M. P., M. H. Ritzwoller, and A. L. Levshin (2001), A fast and reliable method for surface wave tomography, Pure Applied Geophys., 158(8), 1351-1375, doi: 10.1007/PL00001225.
Barnes, J.B. and Ehlers, T.A. (2009), End member models for Andean plateau uplift, Earth Sci. Rev., 97, pp. 105–132
Bensen, G. D., M. H. Ritzwoller, M. P. Barmin, A. L. Levshin, F. Lin, M. P. Moschetti, N. M. Shapiro, and Y. Yang (2007), Processing seismic ambient noise data to obtain reliable broad band surface wave dispersion measurements, GJI., 169(3), 1239-1260, doi:
10.1111/j.1365-246X.2007.03374.x.
de Silva, S.L., and Gosnold, W.A., (2007), Episodic construction of batholiths: insights from the spatiotemporal development of an ignimbrite flare-up. Journal of Volcanology and Geothermal Research, v. 167, p.320-335. doi:10.1016/j.jvolgeores.2007.07.015
Götze H.-J. and S. Krause, (2002), The Central Andean gravity high, a relic of an old subduction complex? Journal of South American Earth Sciences, 14, pp. 799–811
Herrmann, R. B. (1987), in Computer Programs in Seismology, edited, St. Louis University, St. Louis, Mo.
Salisbury MJ, Jicha BR, de Silva SL, Singer BS, Jimenez NC, Ort MH (2011) 40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province. Geol Soc Am Bull doi:10.1130/B30280.1.
Tassara, A., (2005), Interaction between the Nazca and South American plates and formation of the Altiplano-Puna Plateau: Review of a flexural analysis along the Andean margin (15°–34°S): Tectonophysics, v. 399, p. 39–57, doi: 10.1016/j.tecto.2004.12.014.
Whitman, D., (1999), The isostatic residual gravity anomaly of the Central Andes, 12° to 29°S: A guide to interpreting crustal structure and deeper lithospheric processes, Int. Geol. Rev., 41, 457–475.
Zandt, G., Leidig, M., Chmielowski, J., Baumont, D., Yuan, X., (2003), Seismic detection and characterization of the Altiplano–Puna magma body, central Andes. Pure and Applied Geophysics 160, 789–807.
73˚W
EC
3.5
4
72˚W
71˚W
70˚W
13˚S
14˚S
14˚S
14˚S
14˚S
We
ste
rn
3.
Co
rd
5
ille
Bolivia
ra
17˚S
15˚S
16˚S
17˚S
Altiplano
18˚S
19˚S
4.5
20˚S
4.0
21˚S
22˚S
3.5
23˚S
3.0
24˚S
25˚S
Bolivia
15˚S
3.5
3.5
16˚S
17˚S
3
APVC
18˚S
18˚S
19˚S
19˚S
20˚S
20˚S
21˚S
21˚S
22˚S
22˚S
23˚S
24˚S
2.5
Argentina
Chile
25˚S
26˚S
26˚S
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W
Figure 6 (above). 7 km depth slice results shown with 0.25 km/s contour intervals
and the same major morphotectonic provinces [modified from Tassara, 2005]. The
Altiplano-Puna Volcanic Complex (APVC) is delineated with a dashed blue line
[Zandt et al., 2003]. Many of the same first order correlations seen in the phase
velocity results are seen in the 7 km depth slice. Note the correlations of velocity
anomalies with the morphotectonic provinces. Maps are corrected for elevation and
depth refers to sea-level.
Figure 9 (below). Cross-section results shown with 0.1 km/s contour intervals and with the same color
palates used in the depth slices. The same major morphotectonic provinces [modified from Tassara, 2005] are
projected onto the topographic profiles. Top of Nazca subducting slab shown as thick red line. Cross-sections
are corrected for elevation and zero refers to sea-level. Below sea-level, the cross section vertical exaggeration
(VE) is approximately 1:1 and above sea-level the VE is 10:1. Cross-section locations are shown on Figure 1.
WC
13˚S
16˚S
We have selected our color palate to saturate to white at velocities typically
associated with mantle rocks (≥ 4.5 km/s), which allows us to form a first order
estimate of crustal thickness. Visible in our cross-sections is a thick (~ 70 km)
crust under the Andean Plateau that thins to the north and east under the
Subandes (Figure 9). The top of the subducting Nazca slab (shown as a thick red
line) agrees well with the 4.5 km/s contour in the western part of our study
area. The Altiplano-Puna Magma Body (APMB) is clearly visible in the D-D’
cross section. Also visible in the D-D’ cross section is a distinctly separate
low-velocity zone west of the active volcanic arc.
A’
13˚S
15˚S
The 50 km depth slice results show localized zones of high velocity that
correlate with the locations of Neogene ignimbrite eruptive centers (Figure 8).
These zones of high velocity lower crust may be pockets of melt residuum
associated with magmatic differentiation related to Neogene volcanism.
A
13˚S
14˚S
Of particular interest is the location of a distinctly separate low-velocity zone
west of the Altiplano-Puna Magma Body (APMB). A low-velocity body (2.8
km/s) centered on 22.2˚S, 68.7˚W west of the active volcanic arc (Figure 7)
suggests the presence of melts or fluids in the upper crust. Independent active
source seismic results observe a “bright spot” in the same area. We interpret this
low-velocity zone as a satellite magna body, distinctly separate from the APMB
and call it the “Calama Magma Body” after the nearby city in northern Chile.
7
6
5
4
3
2
1
0
10
20
30
40
50
60
70
13˚S
15 km
Peru
15˚S
15˚S
16˚S
16˚S
17˚S
17˚S
5
7
6
5
4
3
2
1
0
10
20
30
40
50
60
70
B
WC
AP
EC
3.5
4
70˚W
69˚W
68˚W
“Calama Magma Body”
4.5
3.5
B’
23˚S
3.0
24˚S
25˚S
APMB
2.5
CAGH
2.5
18˚S
18˚S
19˚S
19˚S
20˚S
20˚S
21˚S
21˚S
22˚S
22˚S
23˚S
23˚S
24˚S
24˚S
25˚S
25˚S
Argentina
Chile
Figure 7 (above). 15 km depth slice results shown with 0.25 km/s contour intervals.
The 3.25 km/s contour has been thickened to emphasize a low-velocity body.
Known Neogene ignimbrite eruptive centers are enclosed by white outlines
[Salisbury et al., 2011]. The Altiplano-Puna Magma Body (APMB), imaged by
previous seismic studies is delineated with a dashed yellow line [Zandt et al., 2003].
The Central Andean Gravity High (CAGH), a positive anomaly in isostatic residual
gravity is enclosed by a dashed blue line [Götze and Krause, 2002].
C
WC
16˚S
17˚S
AP
68˚W
EC
67˚W
18˚S
19˚S
4.5
4.0
3.5
3.0
20˚S
21˚S
22˚S
23˚S
24˚S
2.5
Chile
25˚S
Argentina
“Calama Magma Body”
C’
3.5
69˚W
4
Figure 8 (above). 50 km depth slice results shown with 0.25 km/s contour intervals.
Known Neogene ignimbrite eruptive centers are enclosed by white outlines
[Salisbury et al., 2011]. Note the correlation of high velocity anomalies in the lower
crust with the locations Neogene ignimbrite eruptive centers.
3
70˚W
15˚S
26˚S
26˚S
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W
26˚S
26˚S
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W
7
6
5
4
3
2
1
0
10
20
30
40
50
60
70
13˚S
14˚S
Bolivia
4
3.
3
4.0
50 km
Peru
4
2.9
The 7 km depth slice results show excellent agreement with the major
morphotectonic provinces (Figure 6). A high velocity body tracks the Eastern
Cordillera with localized pockets of higher velocity that correlate well with
mapped Mesozoic and Cenozoic plutonic bodies (Figure 4). The 15 km depth
slice results show excellent correlation between zones of low velocity and
locations of known Neogene ignimbrite eruptive centers, for example, the
Altiplano-Puna Volcanic Center (APVC) (Figure 7).
7 km
Peru
Shear Velocity (km/s)
3.0
0
66˚W
Depth (km)
3.1
SHEAR WAVE RESULTS: 2-D
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W
12˚S
12˚S
3
3.2
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W
12˚S
12˚S
4
20˚S
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W
12˚S
12˚S
Shear Velocity (km/s)
3.3
Depth (km)
3.4
3
3.5
3.5
18˚S
3.5
1000
0
Vs (km/s)
Vs (km/s)
72˚W
73˚W
13˚S
2 2.5 3 3.5 4 4.5 5
Shear Velocity (km/s)
path density
Vs (km/s)
2 2.5 3 3.5 4 4.5 5
4
800
Peruvian Forearc
2 2.5 3 3.5 4 4.5 5
5
600
2 2.5 3 3.5 4 4.5 5
4.
400
70
3
26˚S
Phase Velocity (km/s)
200
70
3.5
24˚S
0
70
3
22˚S
50
70
5
20˚S
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3.0
2.9
2.8
2.7
2.6
2.5
2.4
70
3.
18˚S
70
ra
Cordille
100
60
Depth (km)
150
16˚S
60
Eastern
250
60
4
14˚S
60
3
400
60
3.4
3.0
50
60
3.5
2.4
50
40
4
resolution (km)
50
40
30
5
800
50
40
30
4
64˚W
50
40
30
5
3.
66˚W
50
40
30
Our first order results confirm that the high Andes
13˚S
13˚S
are supported by a thick (~70 km) low-velocity (and
Peru
presumably low-density) crust with localized but
14˚S
14˚S
regionally extensive mid-crustal low-velocity zones.
Working under the hypothesis that voluminous
15˚S
15˚S
Bolivia
ignimbrites are the surface expression of batholith
formation at depth as exemplified by the APVC [de Silva
16˚S
16˚S
and Gosnold, 2007], we combine our results with the
locations of known Neogene ignimbrite eruptive centers
17˚S
17˚S
and isostatic residual gravity and conclude the 3.25
km/s contour at 15 km’s depth generally outlines the
18˚S
18˚S
extent of a Neogene batholith with isolated pockets of
partial melt (Figure 10). This newly outlined batholith
100
generally correlates with the Western Cordillera, but
19˚S
19˚S
80
locally extends into the western Altiplano, Eastern
Cordillera, and Puna, and more surprisingly, locally
60
20˚S
20˚S
extends into the forearc in the form of the newly
40
discovered Calama Magma Body. The modern volcanic
21˚S
21˚S
20
arc is located predominantly on the western margin of
the batholith, suggesting some density control on the
0
22˚S
22˚S
localization of melt migration paths. The batholith is
-20
relatively narrow (~100 km) between 16° and 19°S, but
23˚S
23˚S
splits into two branches farther north in Peru and in
-40
southern Bolivia it occupies a significantly greater width
-60
Chile
of the orogen. The young batholith mostly occupies
24˚S
24˚S
regions of high elevation, suggesting the generation of
-80
low-density granitoids and the prevailing high
Argentina
-100
25˚S
25˚S
temperatures must counteract the densifying effects of
the formation of mafic residues at depth. Although the
26˚S
26˚S
prevailing consensus is that the central Andes are built
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W
predominantly by crustal thickening due to
Figure 10. Summary figure combining the results of our 15 km depth slice with the
compressional shortening, the identification of a large
locations of known Neogene ignimbrite eruptive centers [Salisbury et al., 2011] and
volume Neogene batholith recalls the possible important
isostatic residual gravity [Whitman, 1999]. The 3.25 km/s velocity contour (thick black
role of magmatic addition, especially in Peru and the
line from the 15 km depth slice) correlates well with areas of low isostatic residual
Puna where shortening estimates are relatively smaller
gravity and the locations of known Neogene ignimbrite eruptive centers. We interpret
this correlation as compelling evidence for the 3.25 km/s contour outlining the
than in the central Bolivian segment. Additionally, recent
general extent of a Neogene batholith with isolated pockets of partial melt
multi-disciplinary studies documenting late Cenozoic
uplift on the western margin of the central Andes, with relatively little documented shortening, also suggest magmatic addition
as a possible important causative uplift mechanism. Future interdisciplinary studies should reexamine the potential role of
magmatic addition and its interactions with crustal scale structures in the uplift history of the Andes.
5
68˚W
40
30
rporter@dtm.ciw.edu
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W
12˚S
12˚S
3.
70˚W
30
3.
74˚W
72˚W
20 Seconds
20
3.6
2.5
800
150
16˚S
20
18˚S
2.6
64˚W
250
20
3.7
400
14˚S
20
es
nd
12˚S
66˚W
20
ba
Su
The fundamental basis for Ambient Noise Tomography
(ANT) is that the cross-correlation of ambient seismic noise as
recorded by two contemporaneously operating seismometers
can be used to extract surface wave empirical Green’s functions
(EGFs) for the path between the two stations. We follow the
method outlined by Benson et al., [2007] to obtain and quality
control interstation surface wave dispersion measurements.
Although the cross-correlation step requires stations to be
operating simultaneously, all calculated interstation dispersion
curves are combined before inverting for 2-D phase velocity
maps (Figures 2 and 3) following the method outlined by
Barmin et al., [2001]. A distinct advantage of utilizing this
approach is that dispersion curves from multiple networks,
deployed during non-overlapping time periods can be combined
to invert for high quality 2-D phase velocity maps with
increased horizontal coverage.
68˚W
20
3.8
Depth (km)
AMBIENT NOISE METHODOLOGY
12˚S
70˚W
10
17˚S
2.7
74˚W
72˚W
8 Seconds
10
Forearc
26˚S
26˚S
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W 64˚W
10
ra
Cordille
Argentina
10
3.9
23˚S
Chile
10
4.0
22˚S
D’
10
Vs (km/s)
Figure 5 (below). “Observed” phase velocity profiles shown as
dashed lines with different colors corresponding to different
morphotectonic provinces. Solid black lines represent the phase
velocity profiles calculated from the final shear-wave velocity model.
0
Eastern
25˚S
Bolivia
C
CAUGHT Station
PULSE Station
PeruSE Station
PLUTON Station
IPOC Station
REFUCA Station
Temp GEOFON Station
APVC Station
BANJO/SEDA Station
GT/IU Station
Holocene Volcano
La Paz, Bolivia
6
CONCLUSIONS
Central Altiplano
0
es
24˚S
~7 cm/yr
19˚S
0
Subandean Zone
nd
ba
23˚S
C’
0
Bolivian EC
Su
22˚S
18˚S
B
Pacific
Ocean
5
Gravity Anomaly (mGal)
17˚S
0
Peruivan Arc
Depth (km)
A
19˚S
21˚S
16˚S
Peruivan EC
Depth (km)
16˚S
20˚S
15˚S
B’
18˚S
4
Depth (km)
15˚S
17˚S
3
Depth (km)
14˚S
Both the phase velocity plots and the 1-D
shear-wave results indicate a large range of velocity
structures, however, we observe a good correlation
between profiles selected from the same major
morphotectonic provinces (Figure 4). With the
exception of the forearc and the Subandes, we
observe generally very slow shear velocities (<3.6
km/s) in the upper 30 to 40 km of the crust. Locally,
in regions of young volcanic activity, we observe
significant crustal low-velocity zones. Crustal
anisotropy may be responsible for some but not all of
the observed anomalously low-velocity zones.
Northern Arc
Depth (km)
Peru
14˚S
1
Depth (km)
13˚S
Phase Velocity (km/s)
13˚S
0
Depth (km)
A’
SHEAR WAVE RESULTS: 1-D
Depth (km)
75˚W 74˚W 73˚W 72˚W 71˚W 70˚W 69˚W 68˚W 67˚W 66˚W 65˚W 64˚W
12˚S
12˚S
The Central Andes of southern Peru, Bolivia, and
northern Chile (Figure 1) form a wide orogenic
plateau with an average elevation in excess of 3 km
punctuated by volcanic peaks and pluton-cored
ridges exceeding 6 km. The uplift history of the
Andean plateau remains a lingering question in the
geosciences and two competing end-member
geodynamic models have emerged as possible
solutions: (1) slow-and-steady uplift associated with
crustal shortening, verses (2) rapid uplift associated
with isostatic rebound following a delamination
event. Each model makes specific predictions that
are recorded in the history of the plateaus climate,
elevation, lithospheric thickness, composition, and
magnitude of deformation. In part, the controversy
between models persists because of a lack in
fundamental data sets that overlap in time and
space. As part of the Central Andes Uplift and the
Geodynamics of High Topography (CAUGHT)
project, the main objective of the seismological
component is to map the crustal and uppermost
mantle structure of the Central Andes using multiple
techniques including Ambient Noise Tomography
(ANT). In this study, we use data from 153
broadband seismic stations (Figure 1) from 13
different international seismic networks, deployed
incrementally in the Central Andes from May 1994
through March 2012, to image the vertically
polarized shear-wave velocity structure of the
Central Andes.
1
1. The University of Arizona, Tucson, Arizona, United States. 2. Carnegie Institution of Washington, Washington DC, United States. 3. El Observatorio San Calixto, La Paz, La Paz, Bolivia.
4. El Instituto Geofísico del Perú, Lima, Lima, Peru. 5. The University of North Carolina, Chapel Hill, North Carolina, United States. 6. Yale University, New Haven, Connecticut, United States.
wardk@email.arizona.edu
INTRODUCTION
1,2
7
6
5
4
3
2
1
0
10
20
30
40
50
60
70
D
WC
3
3.5
PN
D’
APMB2.5
4
70˚W
69˚W
68˚W
67˚W