MORPHOTECTONICS OF THE CURITIBA SEDIMENTARY BASIN The morphostructural framework differentiation in the Precambrian/Eo-Paleozoic basement and Cenozoic sedimentary cover of the Metropolitan Area of Curitiba is due to extensional and transtensional neotectonic movements, with the concurrence of climatic changes. Drainage morphometric analysis indicates tectonic influence in the morphogenetic processes. Structural alignments control the sub-rectangular and sub-parallel patterns of the drainage system, the asymmetry of the second-order drainage sub-basins of the Iguaçu basin river, the main local base level, and the geometry of the Cenozoic sediments, including the alluvial deposits, which present asymmetric spatial distribution and narrowing segments in areas of crossed-alignments. The main second order tributaries of the Iguaçu river, like the Pequeno and Barigüi rivers, present tectonically controlled channels, as indicated by the coincidence of anomalies with older structures reactivated since the Lower Tertiary, as well as scarps that may be clearly observed in digital elevation models, even not so accentuated. The higher northnorthwestern margin inclination of the Iguaçu River in relation to its south-southeastern margin is a salient morphostructural characteristic related with the Upper Iguaçu Fault, which deformed the Curitiba sedimentary basin and to which part of the upper Iguaçu valley is adjusted. The fault was active during and after the Pleistocene, reflecting the more recent neotectonic activity, and may be recognized through the faulted colluvial-alluvial deposits and the weak seismic activities in the Serra do Mar.

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The Precambrian Rio Paraiba do Sul Shear Belt comprises a 200-km-wide anastomosing network of NE-SW trending ductile shear zones extending over 1000 km of the southeastern coast of Brazil. Granulitic, gneissic-migmatitic, and granitoid terrains as well as low-to medium-grade metavolcanosedimentary sequences are included within it. These rocks were affected by strong contractional, tangential tectonics, due to west-northwestward oblique convergence of continental blocks. Subsequent transpressional tectonics accomodated large dextral, orogen-parallel movements and shortening. The plutonic Socorro Complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ductile shear zones. The tectonic and magmatic structural features of the Serra do Lopo Granite provide a model of emplacement by sheeting along shear zones during coeval strike-slip and cross shortening of country rocks. Geochronological data indicate that the main igneous activity of Socorro Complex spanned at least 55 million years, from the late stage of the northwestward ductile thrusting (650 Ma), through right-lateral strike slip (595 Ma) deformation. The country rocks yield discordant age data, which reflect a strong imprint of the Transamazonian tectono-metamorphic event (1.9 to 2.0 Ma). We propose a model for the origin of calcalkaline granites of the Ribeira Belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have tbcused granite migration from deeper into higher Crustal levels along these shear zones. M A.S. Basel, Nb and Pb Isotope studies bearing on the crustal evolution of southeastern Brazil, Rev. , Approximation of terrestrial lead isotope evolution by two-stage model, Earth Planet. Sci. Lett., 26, 207-221, 1975. Steiger, R.H., and E. Jager, Subcommission on geochronology: Convention on the use of decay constant in geo-and costoo-chronology, Earth Planet. Sci. Lett., 36, 359-362, 1977. Tommasi, A, A Vauchez, L.A.D. Fernandes, and C C. Potschef, Magma-assisted strain localization in an orogen-parallel transcurrent shear zone in southern Brazil, Tectonics, 13, 421-437, 1994. UIbrich, H H G.J, S.R F. Vlach, V.A. Janasi, and A R. Alineida, Ascens•1o e coloca$,no de granit6ides proteroz6icos em ,'ireas do embasamento do Estado de S•1o Paulo, Brasil Meridional: paradoxos da amtlise geom•trica, Bol. lnstituto de Geoci•ncias-Universidade de Sc•o Paulo, Publ. Esp., 12, 143-144, 1992. Vlach, S.R.F., V.A. Janasi, and C.A.C.B. Vasconc•los, The Itu-belt: associated calcalkaline a•'d aluminous A-type late Brasiliano granitoids 2n the states of S•Io Paulo and Paramt SE-B•'azil, paper presented at the 36. Congresso Bra,zileiro de Geologia, Sociedade Br...

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Palavras-chave: convergência pré-colisional, Neoproterozóico, Província Mantiqueira. RESUMOIdades U-Pb em zircão com valores entre 660 e 640 Ma, obtidas em uma série de ortognaisses e plútons de afinidades cálcio-alcalinas, no sudeste da Província Mantiqueira Central, sugerem que um importante período de magmatismo cordilherano ocorreu nesta região antes da etapa colisional da aglutinação do Gondwana Ocidental (até então reportado ao período entre 625 e 580 Ma). A natureza isotópica deste magmatismo ainda não foi bem caracterizada, no entanto, alguns dados isotópicos Sm-Nd preliminares sugerem que estes magmas não foram derivados exclusivamente de litosfera paleoproterozóica, mas constituem produtos híbridos de fontes paleoproterozóicas e neoproterozóicas. Tais misturas são comumente observadas em ambiente de arcos continentais. É possível que as rochas com idades entre 660 e 640 Ma representem um magmatismo de arco tipo cordilherano, resultado da subducção de crosta oceânica Neoproterozóica durante a convergência pré-colisional e fechamento de oceano (nesta região de ligação entre Adamastor e Goianides).Keywords: precollisional convergence, Neoproterozoic, Mantiqueira Province. ABSTRACTUranium-lead zircon ages between 660 and 640 Ma, obtained from a series of calc-alkaline orthogneisses and plutons in southeast Brazil's Central Mantiqueira Province, suggest that a significant period of magmatism occurred in this region prior to the collisional assembly of West Gondwana (presently constrained in the region between ca. 625 and 580 Ma). While the nature of this earlier magmatism is presently unclear, some preliminary Sm-Nd data suggest that these magmas were not solely derived from the Paleoproterozoic lithosphere, but appear to represent hybrid products of Paleoproterozoic and Neoproterozoic sources. As such hybrid mixtures have been most commonly observed in continental arc settings, it is possible that the 660 to 640 Ma magmatism represents arc magmatism that resulted from subduction of Neoproterozoic oceanic crust during early precollisional convergence and closure of a branch of either the Adamastor or Goianides oceans.

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