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Crustal and Upper Mantle Studies in Brazil: lithospheric thicknesses and
intraplate seismicity
USC, Geol. Sci., 2005
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recent team
Marcelo Assumpção - Univ. of SãoPaulo
Suzan Van der Lee - ETH-Zurich; Northwestern Univ.
Mei Feng - USP; Chinese Academy Geol. Sciences
Meijian An - USP; Chinese Academy Geol. Sciences
Martin Schimmel - USP; IJA-CSIC, Barcelona
Marcelo Rocha - USP, student
Marcelo Bianchi - USP, student
Thiago Costa - USP, student
Jordi Julià - USC
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Vp Vp
150 km
crust
500oC 600oC
1300oC
1300oClit
hosp
here
asthenosphere
average reference profile
Vp variation with temperature
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South America Geological Provinces
1) Surface-wave tomography, continental scale
2) P-wave regional tomography. SE Brazil
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Heintz et al. 2005
lateral resolution : 700 - 1000km 300 - 500 km T > 40s, long paths T > 15s, shorter paths
Previous tomography studiesHigh velocities in the Amazon craton
Vs 100 kmdepth
Van der Lee et al. 1999
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Global tomography (Univ. Colorado)
Few stations in South America, very long paths
-> low resolution (does not isolate Amazon craton
100 km
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P
S
P
surface
Surface
S
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BLSP02 Project
New stations in Northern Brazil
10 (ETH-Z) 5 (USP)
Cooperation and support: UnB, UFRN, IPT, UFMS, UFRA, CPRM, CVRD, DeBeers, RTDM, AngloGold.
6100 paths for Rayleigh wave group velocities.
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Rayleigh group velocity dispersion
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Maps of Rayleigh group velocities:
Period 20s
(average 0 - 30 km depth)
Low velocities in the Andes (thick crust) and sedimentary basins (Amazon, Parnaiba, Paraná).
Amazon
Parnaíba
Paraná
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Maps of Rayleigh group velocities:
Period 100s
(average 0-150 km depth)
Low velocities in the Andes (asthenosphere) and high velocities in the cratons/shields.
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Waveform inversion
Path-average 1D model
IASP91, initial
inverted
observed
invertedIASP91
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~1500 paths for waveform modeling
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Path-average 1D models (S + Rayleigh waveform fitting)- more resolution,less data (1550 paths)
Regionalized groupvelocities (1ox1o grid)T = 20s to 150s - low resolution, more data (6100 paths)
Two data sets
T=100s
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DISP PWI JOINT
S-wave velocity at 150 kmdifferent methods
Group velocity Waveform only Joint inversion
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Resolution test for joint inversion (group+waveform)
~ 300 km in mid-continent, ~600 km in northern Brazil
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Amazon geochronological provinces
Tassinari &Macambira,1999
A > 2.3 Ga
B 2.2 – 1.95 Ga
C 1.95 – 1.8 Ga
D 1.8 – 1.55 Ga
Guyana shield
Guaporé shield
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Joint inversion results:
Vs 100 km
Generally high velocitiesIn the Amazon craton.No clear separation between the Guyana and Guaporé shields.
Separate high-velocity block in Southern S.Francisco craton.
Possible cratonic block beneath the Paraná basin?
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asthenosphere
Peru
Bolívia
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resolution 300-600km
Vs 150 km
thickest lithosphere in oldest block A
TransBrasilian Lineament:generally low velocities orlimits high velocity blocks.
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oldest rocks
Vs 200 km
resolution 500-800km
2.9-3.0 Gagranitoids/greenstonesCarajás Iron Province
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150 km
Guyana shield Guaporé shield S.Francisco craton
NW SE
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Intraplate seismicity in Brazil
Very low activity:
max. known magnitude = 6.2 mb
Non-uniform seismicity: why?
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Paraná basin
craton
Seismicity:
No simple correlation with surface geology
suture
craton
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P-wave upper mantle tomography BLSP project, 1992-2004
VanDecar et al. 1995
Schimmel et al. 2003
C. Escalante, 2002
M. Rocha,
2003
~60 stations, 10,000 P+PKP
readings
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station
cratonintrusion 85-60 Ma
intrusions 130-120 Ma, Atlantic rift
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Seismicity
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600.
500.
400.
300.
200.
100.
0 .
Dep
th (
km)
A A '
Number of earthquakes along NW-SE profile (+- 100km width)
Iporá APIP SFC S.Mar/plat.
mag>3,5
lithosphere/asthenosphere limit?
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“uniform catalog”, 1955-2000
compression
tension
Observed directions of the crustal stresses in
Brasil
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mantle
crust
plate
lithosphere/ asthenosphere
thin,hot:weak
thick,cold:strong
1300oC
Assumpção et al., 2004. Geophys.J.Int.
proposed model:
thinner lithosphere is hotter and weaker:
intraplate stresses concentrate in the upper crust.
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station
cratonintrusion 85-60 Ma
intrusions 130-120 Ma, Atlantic rift
What caused post-rifting Igneousintrusions??
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Gibson et al.(1997)
Igneous Provinces : Iporá Alto Paranaíba
80 Ma
model based in Geochemistry :
plume impact beneath lithospheric “thin spots”
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Trindade Plume deviated by the root of the São Francisco craton ? (Thompson et al., 1998; Gibson et al., 1999)
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geochemistry(Gibson et al., 1997)
tomography (BLSP, 2004)
effect of ray paths
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Tomographic Tomographic images are like images are like impressionist impressionist paintingspaintings
Upper mantle tomography in Brazil
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Renoir: “Remando no Sena”
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Current collaboration with USC:structure of sedimentary basins
with high frequency receiver functions
continental flood basalt
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usual gauss filterfor whole crust:width = 2
high frequencygauss filterfor sedimentarylayers:width = 10
station trib
basalt layer
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continental flood basalts in the Paraná basin: Iguassu Falls
Thank you!
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Future collaboration with USC:crustal and upper mantle studies in NE Brazil
2-years deployment with IRIS stations (Jordi, Owens)
crustal seismic profile 500 km long (USP – USGS)
Brazilian funds just approvedFor two crustal seismic lines ~700, 800 km long (USP – IRIS)
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400 stationsfor seismicrefraction lines(4 months)
40 broadbandstationsfor upper mantlestudies(2 years)
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Teresina station
Vp/Vs = 1,71
Crustal thickness= 40 km(assuming Vp=6.4km/s)
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Crustal thicknesses from Receiver Functions
46 km
33 km
46 km
33 km
35 km
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Crustal thickness
constrained by 1- Rayleigh waves2- receiver functions3- isostasy
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TocantinsProvince
deep seismic refraction line.
Higher Pn velocity beneath SF craton
seismicstationused intomography
12
14
20
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1.74
1.76
1.74
1.711.70
Average crustal Vp/VSratio
Archean is generally more felsic, consistent wuth low Vp/Vs (<1.73).Mafic rocks, in general, have Vp/Vs >1.75, consistent with Magmatic Arc.
1.73
1.71