Resumo expandido apresentado ao
IX Simpósio Nacional de Estudos Tectônicos / III International Symposium on Tectonics (maio 2003)
Carlos Henrique Grohmann de Carvalho ([email protected]) - IGc-USP - CNPq scholarship holder Mario da Costa Campos Neto – IGc-USP
Introduction
The Brazilian Platform was affected in all its extension by cenozoic tectonic deformations which reactivated the main precambrian brazilian lineaments, resulting in a subdivision of blocks bounded by crustal descontinuities. (3, 10).
The southern portion of the São Francisco Craton is marked by a Precambrian N50W-trending shear zone, renewed in the Quaternary (10). Erosional surfaces in the Quadrilátero Ferrífero have close relations with litostructural domains (11). Reactivations with vertical displacements, of ancient faults, would be responsible for the present-day altimetric level differences (6, 1).
Evidences of cenozoic tectonics are observed in several sedimentary deposits, and indicate three deformational events with distinct stress fields. The first one, extensive, with NNE-SSW orientation and probably related with grabens and horsts oriented ESE-WNW. A second event, more expressive, with NW- SE compression. The third event is considered the relaxing of the previous one (7).
This paper intents to present morphometric analysis made with Geographical Information System (GIS) for the eastern portion of the Quadrilátero Ferrífero (fig.1), as well as their possible relations with recent tectonic events.
Methods
The morphometric analysis were carried out for an area of about 1400 km2, correspondent to the
1:50.000 topographic sheet Catas Altas (SF-23-X-B-I-1). The main GIS used was the GRASS open source GIS (8).
The topographic contour lines and drainage network were digitized from the topo sheet and imported into the system as vector entities. Contour lines were converted to raster format and interpolated to produce a Digital Elevation Model (DEM) with 50m of spatial resolution.
Four shaded relief maps, with lighting at N90º, N45ºE, N00º, N45ºW, and with inclination of 20º above the horizon, were used to trace lineaments (9).
A lineament density map was produced as the relation of total length per area (km/km2) of cells with
500x500m. The same parameters were used in the drainage density map (5). The map of isobase (2) was elaborated manually.
The surface roughness map (4) was calculated as the ratio beetwen the real (surface) area and the flat (plan) area of cells with 1x1km. This method is useful for morphological compartimentation since it considers
Results
The surface roughness map (fig.2) shows a relatively flat area in the central region, sorrounded by areas with higher roughness. A similar schema can be seen in the map of isobase (fig.3). Inferred structures are represented in both maps, and were used to delimit five unformal morphological domains (north, south, east, west and central). Lineament and drainage densities agree with orientation of these structures.
Morphostructural lineament analysis indicate the presence of main trends oriented NE-SW and NNE- SSW through the whloe area. Secondary trends are oriented N-S, NNW-SSE and NW-SE (fig.4).
The northern morphological domain comprises an essentially plan area, with lineament frequency peak at the NE direction, and length peaks at NE, NNW and NW. The eastern and southern domains have greater roughness values and lineaments oriented mainly NE-SW and NW-SE; ENE-WSW trend occurs only in the southwest of the area.
The western domain comprises almost only the quartzite massif of the Serra do Caraça. It has the biggest roughness values, and is a strong isobase anomaly. Lineament orientation differs from all the other domains, with high counting of N-S, NW-SE and E-W trends.
The central domain encloses the Chapada de canga, a 900m-high plateaus supported by laterite. Surface is nearly flat, and lineaments show all directions mencioned above.
Slope and aspect (fig.5) maps show, in the south and north domains, NE-SW lined ridges, with steepest slope in the NW face. This orientation agrees with the regional metamorphic foliation and should be related to it. N-S and NW-SE lined ridges are less commom, best viewed in the east and west domains, and the related lineaments are probably younger.
Conclusions
Within the study of recent geologic evolution of the eastern border of the Quadrilátero Ferrífero, several morphometric methods were applied in a GIS environment. The system used was the GRASS open- source GIS, availabe at no cost for the user.
In the study area, main structures could be inferred (fig.6). NE-SW structures agree with the metamorphic foliation, and its presence in morphometric maps can be seen as reactivations, with progressive abatement to north.
Structures oriented NW-SE and NNE-SSW, agree with low-angle thrusts, and may also be reactivated as brittle faults, responsibles for the present morphological scenario, with a depressed central areal, sorrounded by uplifted hills.
101.
2 - Golts, S. & Rosenthal, E. 1993. A morphotectonic map of the northern Arava in Israel, derived from isobase lines. Amsterdam: Geomorphology, 7: 305-315.
3 - Hasuy, Y. 1990. Neotectônica e aspectos fundamentais da tectônica ressurgente no Brasil. In: Workshop Neotecto. Sedim. Cont. Cenoz. SE Brasil, 1, Belo Horizonte. Anais... Belo Horizonte, SBG-MG. Boletim 11, p.1-31.
4 - Hobson, R.D. 1972. Suface roughness in topography: quantitative approach. In: Chorley, R.J. (ed) Spatial analysis in geomorphology. Harper and Row, p.225-245.
5 - Horton, R.E. 1945. Erosional development of streams and their drainage basins: hydrophysical approach to quantitative morphology. Geol.Soc.Am.Bull. 56(3):275-370.
6 - King, L. 1956. Geomorfologia do Brasil Oriental. Rev. Bras. Geog., 18(2):147-266.
7 - Lipski, M.; Endo, I.; Castro, P.T.A.; Trzazkos-Lipski, B. 2001. Estudo do campo de tensões Cenozóicas no Quadrilátero Ferrífero, Minas Gerais. VIII Simp. Nac. Est. Tec, UFPE
8 - Neteler, M; Mitasova, H. 2002. Open Source GIS: A GRASS GIS Approach. Boston, Kluwer Academic Publishers, 464 p. (software available at http://grass.itc.it)
9 - Riccomini, C.; Crósta, A.P. 1988. Análise preliminar de lineamentos em imagens de sensores remotos aplicada à prospecção mineral na área dos granitóides Mandira, SP. Bol.Inst.Geoc. USP. Série Científica. 19:23-37
10 - Saadi, A. 1993. Neotectônica e tectônica recorrente na porção sul do Cráton do São Francisco. In: Simp.Crat. S. Franc., 2, Salvador. Anais...Salvador - SBG/BA. p.230-232
11 - Varajão, C.A.C. 1991. A questão da correlação das superfícies de erosão do Quadrilátero Ferrífero, Minas Gerais. Rev. Bras. Geoc., 21(2):138-145.
680 700 720 740 740 760 720 740 1240 1200 1800 1300 1300 860 840 820 800 760 740 780 720 680 600 940 900 600 580 680 720 780 740 680 700 780 760 800 900 720 700 660 640 700 820 800 860 780 720 760 740 5 10% 5 10% Length Frequency
Fig.2 - Surface roughness map Fig.3 - Map of isobase
Fig.4 - Interpreted lineaments Fig.5 - Aspect map
Fig.6 - Morphological scenario proposed
5 10% 5 10% 5 10% 5 10% 5 10% 510% 5 10% 5 10% 10 20% 10 20% Frequency Length Frequency Length Frequency Length Frequency Length Frequency Length
Modified from Dorr (1969), Silva,Baltazar and Brandalise (1991), Raposo (1991). 20 30'0 20 30'0 760 760 20 00'0 20 00'0 Alvinópolis Acaiaca PONTE NOVA PONTE NOVA Rio Casca Rio Casca 700 700 772020 774040 OURO PRETO OURO PRETO MARIANA Ouro Branco Ouro Branco 7720 7720 Belo Vale Belo Vale CONGONHAS CONGONHAS 660 660 668080 640 640 620 620 600 600 0 10 40 km Brumadinho Ibirité Nova Lima Nova Lima ITABIRITO 560 560 7780 7780 7760 7760 580 580 7740 7740 Bação dome Bonfim dome
Santa Rita dome Belo Horizonte
dome
Moeda syncline
Ouro Branco syncline
Gandarela syncline Serrado Curral
Minas Supergroup Rio das Velhas Supergroup SE São Francisco Craton Gneiss Neoproterozoic Belt
Dom Silvério Group Mantiqueira Gneisses Juiz de Fora Granulites
Study area N NE E SE S SW W NW Curves 1 1.05 1.1 1.15 1.2 1.25 1.3