Brazillan Megafaults

Abstract

Abstract. The main known Brazilian continental faults with a length of more than 100 km were summarized in the text. An attempt to describe their characteristics in the frame of the regional tectonic organization and evolution instead of just classifying them according to their geometric or genetic features was done with the intention to bind these structures to the regional geological history. Brittle and ductile megashears, sutures, rifts, extense thrusts, etc., which cut the 8,511 x 106 km2 of the Brazilian Precambrian basement and its Phanerozoic and volcano-sedimentary cover portray a typical evolution of an old tectonic platform similar to the Siberian, African, and other large and old continental areas of the world. The most striking ductile shears are generally of Upper Proterozoic age, showing transpression and transtensional features. Large thrusts, brittle and ductile, border the Middle Proterozoic terrains being also considered of Upper Proterozoic age. Inside the older cratons such as the Amazon Graton, old grabens are infilled with Middle Proterozoic volcano-sedimentary formations or apparently conjugated faults are observed. The opening of the Atlantic Ocean produced extensive activation of older lines of weakness and generation of rifts and grabens inside the continent and along the newly formed passive margino The Takutu and Jurua lineaments are probably such features where spreading was aborted. More recently, tectonic activity concentrated on some faults with compression near the Andean border and possible extension along the Atlantic border expressed, respectively, by the Iquiri and Bata faults and the Paraiba Rift System. The term "gigafault" is proposed for some more expressive faults such as welded and aborted plate margins, actual plate margins or faults with an extension of several hundreds or thousands of kilometers. Resumen. El conocimiento de las fallas continentales de Brasil, de mas de 100 km de largo, se resume en el texto. Se intenta describir sus caracteristicas en el marco de la organizacion tectonica y evolucion regionales, en vez de solo clasificarlas de acuerdo a su geometria y genesis, con la intencion de relacionarlas con la historia geologica regional. Los megacizallamientos fragiles y ductiles, suturas, rifts, sobrescurrimientos, etc., que cortan los 8.511 x 106 km2 del basamento precambrico Brasiliano y su cobertura volcano-sedimentaria fanerozoica, presentan una tipica evolucion de una plataforma antigua similar a la Siberiana, Africana y otras areas continentales antiguas en el mundo. Las zonas de cizalle ductil mas impresionantes son, generalmente, de edad proterozoica superior, y muestran rasgos transpresivos y transtensionales. Grandes corrimientos, fragiles y ductiles, rodean a los terrenos del Proterozoico medio y son consideradas, tambien, de edad proterozoica superior. En el interior de los cratones mas antiguos, como el craton Amazonico, antiguos grabens estan rellenos con formaciones vulcano-sedimentarias del Proterozoico medio. La abertura del Atlantico produjo la reactivacion extensiva de antiguas lineas de debilidad y la generacion de rifts y grabens dentro del continente y a lo largo del entonces recientemente formado margen pasivo. Los lineamientos Takutu y Jurua son, probablemente, rasgos donde hubo expansion abortada. Mas recientemente, la actividad tectonica se concentro en algunas fallas con compresion cerca de los Andes y con extension cerca del margen continental Atlantico, expresada por las fallas Iquiri y Bata Y por el Sistema de "rift" Paraiba, respectivamente. El termino "gigafalla" se propone para algunas fallas mas significativas, tales como margenes de placas, soldados y abortados, margenes actuales de placas y fallas con extension de varios centenares o miles de kilometros.