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

Van der Lee (2004)

Vp 150 km crust 500 o C600 o C 1300 o C lithosphere 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 lateral resolution : km km T > 40s, long paths T > 15s, shorter paths Previous tomography studies High velocities in the Amazon craton Vs 100 km depth 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 paths for Rayleigh wave group velocities.

Rayleigh group velocity dispersion

Maps of Rayleigh group velocities: Period 20s (average 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 km depth) Low velocities in the Andes (asthenosphere) and high velocities in the cratons/shields.

Group velocity inversion: S-wave velocity in the lower crust (30km) low velocities in the Paraná basin: no underplating?

Waveform inversion Path-average 1D model IASP91, initial inverted observed inverted IASP91

~1500 paths for waveform modeling

Path-average 1D models (S + Rayleigh waveform fitting) - more resolution, less data (1550 paths) Regionalized group velocities (1 o x1 o grid) T = 20s to 150s - low resolution, more data (6100 paths) Two data sets T=100s

DISP PWI JOINT S-wave velocity at 150 km different 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 GaD 1.8 – 1.55 Ga Guyana shield Guaporé shield

Joint inversion results: Vs 100 km Generally high velocities In 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 km Vs 150 km thickest lithosphere in oldest block A TransBrasilian Lineament: generally low velocities or limits high velocity blocks.

oldest rocks Vs 200 km resolution km Ga granitoids/greenstones Carajás Iron Province

Vs 300 km

150 km Guyana shield Guaporé shield S.Francisco craton NWSE

Bouguer anomalies (Chapin, 1996) data from Leeds

Intraplate seismicity in Brazil Very low activity: max. known magnitude = 6.2 mb Non-uniform seismicity: why?

Bouguer anomalies mGal Amazon craton Suture: weakness zone ??

Paraná basin craton Seismicity: No simple correlation with surface geology suture craton

P-wave upper mantle tomography BLSP project, VanDecar et al Schimmel et al C. Escalante, 2002 M. Rocha, 2003 ~60 stations, 10,000 P+PKP readings

station craton intrusion Ma intrusions Ma, Atlantic rift

Lower Cretaceous Upper Cretaceous Assumpção et al., Geophys.J.Int.

Seismicity

Velocity anomalies ( km) beneath epicenters. Regional grid 1° x 1°, “total” magnitude within 80 km radius. Velocity beneath each grid point: a) total magnitude > 4 b) total magnitude <4 c) no events average anomaly Assumpção et al., Geophys.J.Int.

Number of earthquakes along NW-SE profile (+- 100km width) Iporá APIP SFC S.Mar/plat. mag>3,5 lithosphere/asthenosphere limit?

~1% Vp anomaly ---> ~100 O C temperature difference 400 O C in 80 Ma cools down to ~200 O C today (conduçtion!) convection accelerates cooling “thin spot” cratonfoldbelt lithosphere plume 1100 o C1500 o C km

SE Brasil: evidences for higher temperature effect in low-velocity anomalies - Good correlation between P and S anomalies, - More recent intrusions are closer to lower velocities, - Heat flow increases towards the borders of the Paraná basin: ~45 mW/m 2 in center, ~55 mW/m 2 at the borders.

mantle MPa strength of lithospheric lid (S1-S3) T crust o C 1000 o C F integrated > F average F integrada < F average DT = 100 o C Aheim dunite, strain rate s -1 proposed model: thinner lithosphere is hotter and weaker: intraplate stresses concentrate in the upper crust.

“uniform catalog”, compression tension Observed directions of the crustal stresses in Brasil

mantle crust plate lithosphere/ asthenosphere thin, hot: weak thick, cold: strong 1300 o C Assumpção et al., Geophys.J.Int. proposed model: thinner lithosphere is hotter and weaker: intraplate stresses concentrate in the upper crust.

station craton intrusion Ma intrusions Ma, Atlantic rift What caused post-rifting Igneous intrusions??

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 images are like impressionist paintings 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 filter for whole crust: width = 2 high frequency gauss filter for sedimentary layers: 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 approved For two crustal seismic lines ~700, 800 km long (USP – IRIS)

400 stations for seismic refraction lines (4 months) 40 broadband stations for upper mantle studies (2 years)

Receiver Function R Z FR(w) = R(w)/Z(w) fr(t) = response from an incident pulse

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 waves 2- receiver functions 3- isostasy

Tocantins Province deep seismic refraction line. Higher Pn velocity beneath SF craton seismic station used in tomography

Average crustal Vp/VS ratio 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