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Título

The Crustal and Mantle Structure beneath the Carajás Mineral Province and Araguaia Belt from Seismological Data

Texto do resumo

We installed a temporary network of 30 stations in the Carajás Mineral Province (CPM), located in the southeastern part of the Amazon Craton (CA). This network has an unprecedented spatial density in Brazil and was installed in the “GEODYNAMIC (4D) OF THE MINERAL PROVINCE OF CARAJÁS” project financed by the company Vale S.A. This project was developed in partnership with the Brazilian Geological Service (SGB/CPRM), with the Agency for the Development of Innovation in the Brazilian Mineral Sector (ADIMB) and with the University of Brasília, the University of São Paulo and the Federal University of Western Pará. The results have allowed us to discuss the crustal and mantle seismic structure beneath the PMC and adjacent areas.
The thickness of the crust under each station was obtained using the Receiver Function technique. It is observed that the thickness of the crust in the PMC region shows little variation, with an average value of 40 km. There is a trend of crustal thinning in the eastern portion of the CMP, close to the limit with the Araguaia Belt, with an average of around 37.8 km. The western portion has an average value of around 40.3 km, with a thickness slightly higher than average. The average thickness variation is on the order of 2.5 km, very close to the average uncertainty, which is approximately ±2 km, suggesting that there is no significant variation in crustal thickness within the PMC. Finally, it is observed that the crust is thicker in the region of the Parnaíba Basin and in the western portion of the Araguaia Belt.
Seismic lithosphere is generally defined as a high-velocity layer above the low-velocity zone (LVZ) or above a high-velocity gradient zone in the upper mantle, presumably caused by partial melting. The lithosphere-asthenosphere boundary is defined by a negative seismic velocity contrast related to the lithospheric base. In We presented 1D models of S wave velocity for the Carajás (DCJ) and Rio Maria (DRM) Domains in the CMP and for the Araguaia Belt (AB), adjacent to the CMP. These models were obtained from the inversion of average dispersion curves for surface wave phase speed of events recorded for stations installed in each of the aforementioned regions. The results indicate that there is no significant difference between DCJ and DRM, with LAB around 150 km. AB's LAB is thinner by about 25 km (125 km) than CMP's. The crustal thicknesses obtained are compatible with those obtained with the Receiver Function method.
We present results of the multi-frequency seismic tomography method with P waves for two depths in the upper mantle. It is noted that the limit between the PMC and the Araguaia Belt was well marked by a low-velocity anomaly. Another feature that draws attention is the low speed at the limit between the Bacajás and Carajás Domains. This segmentation appears further north at shallower depths (136 km slice) and migrates south at greater depths (226 km slice). This anomaly contrasts with the high velocities observed in the Bacajás and Rio Maria Domains. In general, these anomalies suggest thermal differences related to variations in the thickness of the lithosphere, however, considering the results presented of 1D Surface Wave Inversion, the low-velocity anomaly under the Carajás Dominion should not be of thermal origin, being possibly caused by compositional changes in the sublithospheric mantle.

Palavras Chave

Receiver Function; 1D Surface Wave Inversion; Multiple Frequency Seismic Tomography; Carajás Mineral Province