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GDP-01 Elevated, passive continental margins: Timing and mechanisms of uplift
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The evolution of the elevated, passive continental margin of southeastern Brazil: Magmatism, changes of stress field, uplift, and river captures
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Claudio Riccomini, Institute of Geosciences, University of São Paulo (Brazil)
Silvio Takashi Hiruma, Geological Institute, State Secretariat of Environment (Brazil)
May Christine Modenesi-Gauttieri, Geological Institute, State Secretariat of Environment (Brazil)
Peter Christian Hackspacher, Institute of Geosciences and Exact Sciences, São Paulo State University (Brazil)
Julio Cesar Hadler Neto, Institute of Physics Gleb Wataghin, State University of Campinas (Brazil)
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The stress field in the passive continental margin of southeastern Brazil has changed many times since the Gondwana break up. Early Cretaceous tholeiitic dike swarms of Santos-Rio de Janeiro (trending ENE-WSW), Ponta Grossa (NW-SE) and Florianópolis (N-S) were the feeders to extensive lava flows and constituted a probable triple junction. Magmatism in the Ponta Grossa swarm started under NE-SW-oriented extension and vertical shortening between 132 and 131 Ma, followed by NE-SW extension with a NW-SE compression around 130 Ma. Along the Santos-Rio de Janeiro and Florianópolis dike swarms the magmatism occurred ca. 127 Ma under the action of a NW-SE extension and NE-SW compression. Late Cretaceous alkaline massifs (80-65 Ma) and coeval dikes indicate a NW-SE-oriented extension during their emplacement. Tertiary stress field changed from a NNW-SSE-oriented extension during the Eocene-Oligocene to a NW-SE extension and local NE-SW compression during early Miocene. Quaternary deformations comprise a late Pleistocene-Holocene stress field with NW-SE compression and NE-SW extension, followed by two phases of Holocene deformation, first an E-W to WNW-ESE extension and later an E-W compression. The Bocaina tableland, one of the highest portions of the Serra do Mar reaching more than 2000 meters in altitude, presents a succession of NW-SE topographically decreasing low-relief isolated plateaus separated by deep valleys. Corrected apatite fission track ages in different isolated plateaus range from 326±30 to 54±5 Ma, including a significant set greater than 200 Ma, the oldest such ages yet registered in the Serra do Mar. Ages decrease NW-SE from the continent coastwards, perpendicularly to the main structural grain of the Precambrian basement, indicating a structural control, as evidenced by the correlation between brittle structures, morphotectonic features and the distribution of main alluvial plains and drainage pattern. Contrasting ages within a single plateau and similar ages at different altitudes indicate complex regional tectono-thermal evolution, which cannot be explained by the simple scarp-retreat model. Thermal histories show important cooling events related to continental break-up, Cretaceous alkaline magmatism and Cenozoic tectonism. Changes of the stress field and uplifts played a major role in the development of the drainage network of this region. Major rivers are controlled by the E-W to NE-SW-oriented basement structures. Pulses of tectonic activity along NW-SE-oriented faults, mainly during Neogene and Quaternary, have promoted numerous river captures. One of the most spectacular example is the Miocene, fault-controlled elbow of capture at Guararema, along which the Paraíba do Sul river captured the headwaters of the Tiet river. Such alterations in drainage basins have important implications for biodiversity, notably of fishes. (Financial support by FAPESP, grant 03/08031-0; CR is a research-fellow of CNPq, grant 304649/2005-8).
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