Razão de Poisson da crosta superior da região de Porangatu, Goiás, Província Tocantins: um estudo por refração sísmica

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

This work presents a refined seismic model of compressional waves (Vp) showing the distribution of Poisson ratio along the upper crust of central Tocantins Province, central Brazil, based on two dimensional modelling of the Porangatu seismic refraction data. The Porangatu seismic line begins in the Araguaia Belt (FXA) and continues through the western portion of the Brasília Belt (FXB), encompassing the Goiás Magmatic Arc (AMG) and the Goiás Massif (GM). The transect is almost E-W and is 320 km long with vertical seismic receptors at every 2.5 km and explosive sources at every 50 km. The western Tocantins Province presents lateral discontinuities marked by Sm-Nd TDM model ages (Neoproterozoic in the magmatic arc, Paleoproterozoic in the Araguaia Belt and Goiás Massif), and by anomalies identified in airborne magnetic and ground gravimetric surveys. Prominent surface contacts are in Serra Azul, marking the Transbrasiliano Lineament, and the Rio dos Bois fault, which separates the Goiás Magmatic Arc from the Goiás Massif. Poisson ratio was directly calculated from the ratio of compressional and shear wave velocities (Vp/Vs). It is an elastic constant and its values are function of rock composition, existence of fractures and porosity, allowing to recognize lateral discontinuities in the crust. The upper crust in the study area is typically felsic with one exception in the uppermost layer. The seismic model presents one surface with two steps: a gentle one at the contact between the Araguaia Belt and the Goiás Magmatic Arc and a steep one over the contact of the Goiás Magmatic Arc and the Goiás Massif. The upper layer in the Araguaia Belt is 3.5 km thick increasing gradually to 4.2 km under the Goiás Magmatic Arc. In the easternmost part of the model, the upper layer displays a 2.3 km step, coinciding at the surface with the Rio dos Bois fault. Considering Vp and σ values, the upper crustal layer is divided in to eight sections from west to east. The sections are described as: i) Araguaia Belt with Vp=5,95 km/s and σ=0,26; ii) Transbrasiliano Lineament terrains have Vp=6,02 km/s and σ=0,24; iii) eastern Goiás magmatic terrains with Vp=6,11 and σ=0,23; iv) mafic body with Vp=7,05 km/s and σ=0,28; v) Rio dos Bois fault terrains and metasedimentary sequence of Serra da Mesa Group with Vp=4,59 km/s and σ=0,12; vi) Serra Dourada batholit with Vp=5,77 km/s and σ=0,22; vii) Cana Brava layered complex and Palmeirópolis volcanosedimentary sequence with Vp=6,42 km/s and σ=0,27; viii) fold-and-thrust belt of the Brasília Belt with Vp=5,82 km/s and σ=0,21. xiv The second layer is more homogeneous presenting Vp=6,14-6,18 km/s and σ=0,24, which indicates felsic basement, probably of the granite-granodiorite type.

ASSUNTO(S)

geofísica aplicada poisson ratio tocantins province seismic refraction razão de poisson goiás magmatic arc geofisica aplicada arco magmático de goiás província tocantins applied geophysics refração sísmica

ACESSO AO ARTIGO

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