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