Estrutura crustal do setor central da província Tocantins utilizando ondas P, S e fases refletidas com dados de refração sísmica profunda

Abstract

This research is based on an execution of two lines of deep seismic refraction of 300 km of extension (L1-Porangatu and L2-Cavalcante), crossing over central sector of Tocantins Province from west to east by using, in each line, 120 digital seismographs and explosions with controlled time and explosive charges between 500 and 1,000 kg in each 50 km; GPS receivers were employed in order to control the time and geographical coordinates from recording and shot points. This kind of experiment under these explained conditions is pioneer in Brazil. Initially experiment data, which have been considered of good quality, allowed the elaboration of 1D models, using TTInvers program. Successive models were related to represent layers with similar characteristics in a preliminary model aiming of modelling in 2D, accomplished with MacRay program. Obtained 2D models represent the final result of seismic velocity distribution from crust beneath L1 and L2 lines. Results show crust under central section of Tocantins Province with thickness varying from 36 to 43 km, and whose parameters are correlated to main geological structures existents in surface. VP as well as VP/VS ratio mean values vary about 6.5 km/s and 1.74, respectively, with the exception of fold-and-thrust belt, whose values are 6.3 km/s and 1.73. Those values reach 6.8 km/s and 1.74 beneath São Francisco craton. There are indicia of double subduction occurred in the eastern portion of Tocantins Province with São Francisco Cráton subducting to west (in ∼ 760 Ma), as well as in the western portion, withAmazon Cráton subducting to east (in∼620 Ma). The gravimetric model, obtained in this work in terms of seismic model, adequately adjusts with observed gravimetric data by using theoretical densities slightly modified, within limits allowed by the function employed to calculating the densities based on VP values achieved from this work. Adopted mantle densities to modelling took in consideration Paleoproterozoic age, beneath São Francisco Cráton, less dense (3.31 g/cm3), and with higher VP (8.26 km/s), as well as Neoproterozoic one, beneath Tocantins Province, denser (3.34 g/cm3), and with lower VP (8.07 km/s).