crustal and upper mantle studies in brazil: lithospheric thicknesses and intraplate seismicity usc,...
TRANSCRIPT
Crustal and Upper Mantle Studies in Brazil: lithospheric thicknesses and
intraplate seismicity
USC, Geol. Sci., 2005
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
Vp Vp
150 km
crust
500oC 600oC
1300oC
1300oClit
hosp
here
asthenosphere
average reference profile
Vp variation with temperature
South America Geological Provinces
1) Surface-wave tomography, continental scale
2) P-wave regional tomography. SE Brazil
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
Global tomography (Univ. Colorado)
Few stations in South America, very long paths
-> low resolution (does not isolate Amazon craton
100 km
P
S
P
surface
Surface
S
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.
Rayleigh group velocity dispersion
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
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.
Waveform inversion
Path-average 1D model
IASP91, initial
inverted
observed
invertedIASP91
~1500 paths for waveform modeling
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
DISP PWI JOINT
S-wave velocity at 150 kmdifferent methods
Group velocity Waveform only Joint inversion
Resolution test for joint inversion (group+waveform)
~ 300 km in mid-continent, ~600 km in northern Brazil
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
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?
asthenosphere
Peru
Bolívia
resolution 300-600km
Vs 150 km
thickest lithosphere in oldest block A
TransBrasilian Lineament:generally low velocities orlimits high velocity blocks.
oldest rocks
Vs 200 km
resolution 500-800km
2.9-3.0 Gagranitoids/greenstonesCarajás Iron Province
150 km
Guyana shield Guaporé shield S.Francisco craton
NW SE
Intraplate seismicity in Brazil
Very low activity:
max. known magnitude = 6.2 mb
Non-uniform seismicity: why?
Paraná basin
craton
Seismicity:
No simple correlation with surface geology
suture
craton
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
station
cratonintrusion 85-60 Ma
intrusions 130-120 Ma, Atlantic rift
Seismicity
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?
“uniform catalog”, 1955-2000
compression
tension
Observed directions of the crustal stresses in
Brasil
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.
station
cratonintrusion 85-60 Ma
intrusions 130-120 Ma, Atlantic rift
What caused post-rifting Igneousintrusions??
Gibson et al.(1997)
Igneous Provinces : Iporá Alto Paranaíba
80 Ma
model based in Geochemistry :
plume impact beneath lithospheric “thin spots”
Trindade Plume deviated by the root of the São Francisco craton ? (Thompson et al., 1998; Gibson et al., 1999)
geochemistry(Gibson et al., 1997)
tomography (BLSP, 2004)
effect of ray paths
Tomographic Tomographic images are like images are like impressionist impressionist paintingspaintings
Upper mantle tomography in Brazil
Renoir: “Remando no Sena”
Current collaboration with USC:structure of sedimentary basins
with high frequency receiver functions
continental flood basalt
usual gauss filterfor whole crust:width = 2
high frequencygauss filterfor sedimentarylayers:width = 10
station trib
basalt layer
continental flood basalts in the Paraná basin: Iguassu Falls
Thank you!
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)
400 stationsfor seismicrefraction lines(4 months)
40 broadbandstationsfor upper mantlestudies(2 years)
Teresina station
Vp/Vs = 1,71
Crustal thickness= 40 km(assuming Vp=6.4km/s)
Crustal thicknesses from Receiver Functions
46 km
33 km
46 km
33 km
35 km
Crustal thickness
constrained by 1- Rayleigh waves2- receiver functions3- isostasy
TocantinsProvince
deep seismic refraction line.
Higher Pn velocity beneath SF craton
seismicstationused intomography
12
14
20
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