Maurício da Silva Borges

Tectonics and paleogeography along the Amazon river

Abstract The main structural and geomorphological features along the Amazon River are closely associated with Mesozoic and Cenozoic tectonic events. The Mesozoic tectonic setting is characterised by the Amazonas and Marajo Basins, two distinct extensional segments. The Amazonas Basin is formed by NNE–SSW normal faults, which control the emplacement of dolerite dykes and deposition of the sedimentary pile. In the more intense tectonic phase (mid-Late Cretaceous), the depocentres were filled with fluvial sequences associated with axial drainage systems, which diverge from the Lower Tapajos Arch. During the next subsidence phase, probably in the Early Tertiary, and under low rate extension, much of the drainage systems reversed, directing the paleo-Amazon River to flow eastwards. The Marajo Basin encompasses NW–SE normal faults and NE–SW strike-slip faults, with the latter running almost parallel to the extensional axes. The normal faults controlled the deposition of thick rift and post-rift sequences and the emplacement of dolerite dykes. During the evolution of the basin, the shoulder (Gurupa Arch) became distinct, having been modelled by drainage systems strongly controlled by the trend of the strike-slip faults. The Arari Lineament, which marks the northwest boundary of the Marajo Basin, has been working as a linkage corridor between the paleo and modern Amazon River with the Atlantic Ocean. The neotectonic evolution since the Miocene comprises two sets of structural and geomorphological features. The older set (Miocene–Pliocene) encompasses two NE-trending transpressive domains and one NW-trending transtensive domain, which are linked to E–W and NE–SW right-lateral strike-slip systems. The transpressive domains display aligned hills controlled by reverse faults and folds, and are separated by large plains associated with pull-apart basins along clockwise strike-slip systems (e.g. Tupinambarana Lineament). Many changes were introduced in the landscape by the transpressive and transtensive structures, such as the blockage of major rivers, which evolved to river-lakes, transgression of the sea over a large area in the Marajo region, and uplift of long and narrow blocks that are oblique to the trend of the main channel. The younger set (Pliocene–Holocene) refers to two triple-arm systems of rift/rift/strike-slip and strike-slip/strike-slip/rift types, and two large transtensive segments, which have controlled the orientation of the modern drainage patterns.

Tectonics and sedimentation in the Curitiba Basin, south of Brazil

Abstract The Curitiba Basin, Parana, lies parallel to the west side of the Serra do Mar range and is part of a continental rift near the Atlantic coast of southeastern Brazil. It bears unconsolidated and poorly consolidated sediments divided in two formations: the lower Guabirotuba Formation and the overlying Tinguis Formation, both developed over Precambrian basement. Field observations, water well drill cores, and interpretations of satellite images lead to the inference that regional tectonic processes were responsible for the origin of the Basin in the continental rift context and for morphotecatonic evolution through block tilting, dissection, and erosion. The structural framework of the sediments and the basement is characterized by NE–SW-trending normal faults (extensional tectonic D 1 event) reactivated by NE–SW-trending strike–slip and reverse oblique faults (younger transtensional tectonic D 2′ to transpressional tectonic D 2″ event). This tectonic event, which started in the Paleogene and controlled the basin geometry, began as a halfgraben and was later reactivated as a pull-apart basin. D 2 is a neotectonic event that controls the current morphostructures. The Basin is connected to the structural rearrangement of the South American platform, which underwent a generalized extensional or trantensional process and, in late Oligocene, changed to a compressional to transpressional regime.