The Under-side of the Andes: Using Receiver Functions to Map the North Central Andean Subsurface
Jamie Ryan , Susan Beck , George Zandt , Lara Wagner , Estela Minaya and Hernando Tavera
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1
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Abstract: T21E-2626
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University of Arizona 2University of North Carolina, Chapel Hill 3El Observatorio San Calixto, La Paz, Bolivia 4Instituto Geofísico del Perú, Lima, Peru
Depth to Moho map:
Receiver Function Cross Sections and Plots:
−13
-2
Moho
−14
0
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Peru
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Cusco
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CAUGHT Station
GSN Station
City
Active Volcanoes
F
E
CP18
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Ý
CP16
CP15
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CP10
CP11
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Arequipa
LPAZ
LaPaz
CB31
CB24 CB26
CB22
CP06
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CP03
CP02
CP01
CB2E
CB2D
CB11
A
CB28
CB27
Ý
CB14
CB25
SICA
CB23
CB13
CB21
CB20
CB12
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B
CB2C
CB2B
CB2A
CB29
CB41
CP07
FA
CB2F
CB33
CB32
WC
CP04
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AP
CP08
CB2G
CHUQ
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CB04
300
Horizontal Offset (km)
600
Figure 4 (above): Cross-section B, gridded and CCP stacked wtih 15km bins between the stations and 80km bins off the line, with a gaussian value of
1.0, crustal velocity of 6.1km/s mantle velocity of 8.0km/s and a Vp/Vs of 1.75. Note the consistent positive Moho arrival under the section except the
western portion of the Andes, and the apparent lack of significant crustal thickness under the high elevation of the Eastern Cordillera. The black line
represents the slab1.0 model (Hayes et al., 2012).
P data CB25 54 rad traces
-4
A
-0
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180
B
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60
P data CB25 54 tan traces
P data CB28 65 rad traces
P data LPAZ 159 rad traces
P data CB28 65 tan traces
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40CP09 0
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EC
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CB23
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CB11
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Longitude(degrees)
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SA
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FA
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CB2C
LPAZ
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P data LPAZ 159 tan traces
350
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−16
AP
WC
Figure 5 (below, left): Cross-sections A-F, gridded and CCP stacked with 50km square bins at a 42 degree angle to best align with the dense line of
stations, and the same velocities and gaussian as the above cross-section. Note the Moho arrivals, and the apparent lack of adequate crustal material
underneath the high topography of the Eastern Cordillera. There is also evidence to support a large low velocity body underneath the Western
Cordillera and, in sections D and E, some evidence for mid-crustal arrivals.
-0
LAJO
450
50
40
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CP09
CP05
C
CB42
CP12
Ý
CB34
CB43
EC
CP14
Bolivia
D
CB44
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CB41
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CP13
SA
CP17
m
0
-.075
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CB54
Abancay
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BANJO/SEDA Station
CP20
CP19
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50
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backazimtuh
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−15
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Latitude( degrees)
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140
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Bolivia
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CAUGHT array of broadband seismic stations
B
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Peru
.075
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The seismology component of the Central Andean Uplift and Geodynamics of High
Topography (CAUGHT) experiment was a deployment of 50 broadband seismometers in
northwestern Bolivia and southern Peru to investigate the interplay between crustal
shortening, lithospheric removal and surface uplift.
There is a total of 275 km of documented upper crustal shortening in northwest Bolivia (15°
to 17° S) (McQuarrie et al, 2008). Associated with such shortening is crustal thickening and
the potential for lithospheric removal as the thickened lithospheric root becomes unstable.
The seismology data collection has now been completed and two years of data cataloged.
With that data receiver functions images were created to constrain Moho depth underneath
the Central Andes, and to search for intracrustal anomalies.
Depth (km)
The CAUGHT Experiment:
−67
−66
Geomorphic provinces modified from Tassara 2005
Figure 9: CAUGHT array, topography and geomorphic provinces overlain with an interpolated grid of Moho contours based on the 50km by 50km
binned receiver functions shown in the previous section. Note the overall good correlation of contours with the geomoprhic provinces, and the
deeper Moho arrivals present just south and northwest of Lake Titicaca. Example stations from previous plots are darkened for reference.
seconds
100 km
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180
-4
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Receiver functions utilize P-S conversions generated at sharp
impedance boundaries in the Earth, i.e. velocity transitions. A
comparison of the arrival times of the primary phase to the
converted phases yields the depth to the interface causing the
conversion, while the amplitude of the arrival is dependent on the
size of the velocity contrast. Generally the largest amplitude
arrivals in receiver functions are the primary arrival from the direct
P wave, and the high velocity contrast conversion arrival
Figure 2: Basic receiver function conversions.
associated with the Moho.
PpPhs PpShs Pp
The raw waveforms are cut, checked for
appropriate signal-to-noise ratios and then
rotated into their radial and transverse
components. We then deconvolved the rotated
components using the iterdecon method to create
the receiver functions (Liggoria and Ammon,
1999). Common conversion-point (CCP) stacking
was later utilized to take large numbers of
quality-controlled receiver functions and collapse
them around the points where phase conversions
occurred.
Ps
Moho
60
Depth (km)
Receiver Function Methodology:
C
-0
Figure 1: CAUGHT seismometer array with cross-sections and geopmorphic provinces marked;
forearc (FA), Western Cordillera (WC), Altiplano (AP), Eastern Cordillera (EC) and Subandean (SA) zones.
50
seconds after P-arrival
Ý
Figure 6 (above): Backazimuth plots for 3 sample stations in the CAUGHT array,
plotting the receiver functions against event backazimuth.
180
-4
D
-0
CB28
CB41
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LPAZ
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.085 .05
ray parameter (s/km)
.085 .05
.085
Figure 7 (above): Ray-parameter plots for 3 sample stations in the array,
demonstrating the existence of multiple arrivals off the Moho.
180
-4
E
-0
Figure 8 (below): HK plots for 6 sample stations, comparing various depth and
Vp/Vs combinations to better constrain the Vp/Vs and Moho depth underneath
each station in the CAUGHT array. Contours are found by stacking the Moho
arrivals into the first arrival amplitude and finding the highest likelihood (Zhu
and Kanamori, 2000).
60
2.0
CB11
CB2C
CB23
1.9
Figure 3: Example calculated receiver
function prior to CCP stacking.
180
-4
1.8
F
-0
1.7
Vp/Vs ratio
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Ý
relative amplitude
1
1
60
1.6
2.0
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70 40
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LPAZ
CB41
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CP09
1.9
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seconds
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Horizontal Offset (km)
600
1.7
1.6
-We identified arrivals for the Moho and generated a 3D map of crustal thickness underneath the array that reveals distinct
characteristics for the 4 major tectonomorphic provinces.
-The Subandean crust thickens westward from 40 km adjacent to the foreland to 50 km adjacent to the Eastern Cordillera.
-The crust beneath the Eastern Cordillera varies between 50-65 km, generally thickening westward.
-The crustal thickness beneath the Altiplano is variable along strike between 60-65 km, although it appears to thin to the
south.
-The crust under the Western Cordillera is equally variable between 60-75 km, with numerous intracrustal interfaces.
These crustal thicknesses will be an important part of developing better subsurface images for the Central Andes, providing
constraints for other seismological techniques to build upon. These results can also help constrain shortening estimates that
use crustal thickness as a constraint.
Future work on this project will involve incorporating these results into other CAUGHT studies focusing on the lower crust and
upper mantle.
References
Beck, Susan, and George Zandt. "The nature of orogenic crust in the central Andes." Journal of Geophysical Research 107, no. B10 (2002): 1-16.
Hayes, G. P., D. J. Wald, and R. L. Johnson (2012), Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302,
doi:10.1029/2011JB008524.
Ligorria, J. and Ammon, C. (1999) Iterative deconvolution and receiver-function estimation. Bulletin of the Seismological Society of America (October 1999),
89(5):1395-1400.
McQuarrie, N., Barnes, J.B. & Ehlers, T.A. (2008) Geometric, kinematic, and erosional history of the central Andean Plateau, Bolivia (15–17°S). Tectonics, TC3007, doi:
DOI: 0.1029/2006TC00205.
Tassara, A. "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 399-357 (2005): 39-57.
Zhu, Lupei, and Hiroo Kanamori. "Moho depth variation in southern California from teleseismic receiver functions." Journal of Geophysical Research 105, no. B2
(2000): 2969-2980.
Acknowledgments
1.8
180
Interpretations:
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70 40
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Depth (km)
70 40
50
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We’d like to acknowledge the following groups: NSF Continental Dynamics for funding the CAUGHT project (#0907880), our collaborators and the staff members
of both El Observatorio San Calixto in Bolivia and the Instituto Geofísico del Perú in Peru, IRIS-PASSCAL for providing the equipment, technical support and
field assistance, the numerous individuals responsible for committing to field work on the project, and also the generous people and families in Bolivia and Peru
who graciously allowed us the use of their property for this experiment. J. Ryan also received support from Chevron to work on this project.