Yociteru Hasui

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.

Transpressional tectonics and strain partitioning during oblique collision between three plates in the Precambrian of southeast Brazil

Abstract Two orogenic belts have been recognized in south-east Brazil, which are interpreted to have been formed as a product of diachronous collisions between three continental plates. Wide crustal-scale shear belts have developed both between and inboard of the collided and amalgamated plate borders. These shear belts record frontal, oblique or lateral displacements during oblique plate converence and A-type subduction. The overall structural style of each belt depends on the angle subtended between the plate boundary and the convergence vector. The E-W branch between the São Paulo and Brasília plates, the Campo do Meio strike-slip shear belt, has undergone dominantly sinistral wrench dominated transpression along a set of folds and shear zones dipping southwards. The NE-SW branch between the São Paulo and Vitória plates, the Paraíba do Sul strike-slip shear belt, has undergone a partitioned dextral transpression, whereas the north-south branch between the Brasília and Vitória plates, the Paraíba do Sul strike-slip shear belt, has undergone a partitioned dextral transpression, whereas the north-south branch between the Brasília and Vitória plates is essentially a frontal thrust system with only a weak component of dextral strike-slip. These complex structural patterns, formed at deep to mid-crustal levels, reflect temporal and spatial partitioning at all scales between flattening and non-coaxial deformation, and down-dip and strike-slip shearing, in tangential as well as in transcurrent structural domains. Additionally, this area demonstrates that regional flower structures, lateral extrusion and other secondary deformations across the yz sections of transpressional belts are important in accommodating shortening in obliquely convergent orogens.

Tectonics and paleogeography of the Marajó Basin, northern Brazil

The Marajo Basin area presents geologic and geomorphologic features chiefly due to the Mesozoic extension and post-Miocene neotectonics. The extension event with an Early and a Late Cretaceous phases originated four sub-basins that constitutes the Marajo Basin, with a thick continental clastic sequence showing marine influence. NW and NNW normal faults and NE and ENE strike-slip faults controlled the basin geometry. The extension, related to the Equatorial Atlantic opening, propagated into the continent along crustal weakness zones of the Precambrian Tumucumaque, Amapa and Araguaia orogenic belts. The neotectonic event is a strike-slip regime which developed transtensional basins filled in by Upper Tertiary shallow marine (Pirabas Formation) and transitional sequences (Barreiras Group), followed by Quaternary fluvial deposits and transitional sequences derived from the Amazon and Tocantins rivers and the Marajoara estuary. The current landscape has a typical estuarine morphology. The coast morphology presents sea-cliffs on transitional Upper Tertiary sequences, while inwards dominate hills sustained by Mid-Pleistocene lateritic crust, with a flat erosive surface at 70 m. In the eastern Marajo Island several generations of paleochannels associated with fluvial-estuarine sequences are recognized, while a fluvial-marine plain is widespread on its western side.

The Araguaia fold belt, Brazil: A reactivated Brasiliano-Pan-African cycle (550 Ma) geosuture

Abstract The Araguaia-Tocantins geosuture, which separates the Araguaia Fold Belt (AFB) from the Archean Amazonian Craton, was active in the late Middle Proterozoic. The Baixo Araguaia Supergroup was deposited, consisting of the Estrondo Group (lower quartzites with intercalated schists), Xambioa Formation (schists), and Canto da Vazante Formation (upper feldspathic schists); and the Tocantins Group consisting of the Couto Magalhaes Formation (phyllites, quartzites, slates, limestones, and metacherts) and Pequizeiro Formation (upper chlorite schists); and associated mafic-ultramafic bodies. The deformational history includes four regional phases of deformation within this supracrustal sequence: recumbent folds with vergence to the west; refolding with a N-S trend; an intense crenulation episode; and late thrusting from east to west. Metamorphism is of intermediate or intermediate-high pressure type with garnet, biotite, chlorite, and sericite isograds succeeded by a slightly or non-metamorphosed zone, from east to west. Rocks surrounding sparse gneissic-cored domes contain isograds of staurolite, kyanite, and fibrolite. These isograds are believed to be associated with the 1100 Ma Uruacuano event. The Brasiliano Orogeny strongly affected the AFB with displacements due to transcurrent reactivation of great and old faults of the basement, slight folding in the supracrustal sequence, intrusion of small granite bodies, and development of domes with associated normal faults. The area underlain by the Estrondo Group was uplifted at this time, causing the deposition of the Rio das Barreiras polymictic conglomerate of the central area. K-Ar and Rb-Sr analyses date this thermo-tectonic event at 550 ± 100 Ma. The Archean basement is exposed in the cores of domes as a granite-gneiss association, the Colmeia complex, which shows thermo-tectonic features that may be interpreted as polycyclic imprints (Jequie, Transamazonian?, Uruacuano, and Brasiliano Events).

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.

O PRÉ-CAMBRIANO DA REGIÃO DE NATIVIDADE, GO

This paper presents the results of the geological investigation performed on the Precambrian rocks at the Natividade region (Northern Goias). Archean tonalites, mafic rocks, and supracrustals of ancient evolution not yet reconstituted were modified by strong progressive non- coaxial ductile deformation, with displacement, imbrication shearing recrytallization in high to medium amphibolite facies and some migmatization. These modified rocks grade to granite- greenstone terrain in the eastern part of the area. All these modified rocks and unaffected gneisses are assembled in the Goiano Complex . The mass displacement occurred towards SSE and the mylonitic foliation has 0N30 direction, what is interpreted as related to a not yet located oblique ramp . This important tectonic event is admited to be of Late Archean age. Afterwards, high angle ductile shear zones developed, with 10N20 direction, sinistral movement and recrystallization under green schist facies conditions. This event seems to be of Early Proterozoic age and introduced modifications on the earlier structural geometry. An ensialic basin could be developed and closed in some interval of the, 1.850-450 Ma period; its sedimentary filling, the Natividade Group, accumulated on stable shelf environment was folded with 0N20 trending axis and slightly transformed by low grade metamorphism. Transcurrent 50N60 dextral and 300N315 sinistral faults also could form, cutting this supracrustal sequence and the older Goiano Complex.

Análise de lineamentos na exploração mineral: aplicação no sudeste paulista

This paper deals with a satelite and radar imagery analysis, covering the southeastern region of the state of Sao Paulo, Brasil, aiming to select target areas, potentially favorable to host Sn and W mineralizations, spatially and genetically related to acid plutonism. These mineralizations are known to be strongly associated with fracturing (lato sensu); therefore an image analysis focusing on lineaments was done to characterize: 1) intensely fractured areas; 2) areas showing fracture patterns whith azimuths distinct from the regional one; and 3) systems of aligned fractures, of regional occurrence. The results were integrated with geological and geophysical data, allowing the selection of several target-areas, which were subject to geological and geochemical surveys. New non-foliated, discordant, intrusive granitic bodies were discovered; metassomatic alteration products (greisens) associated with them were identified along intensely fractured areas. The study suggests, for the southeastern region of the state of Sao Paulo, a model of linear discontinuities reactivated during thermo-tectonic instability periods.

Uma interpretação alternativa da origem dos turmalina leucogranitos de Mailasqui, São Paulo

Based on field, geochemical and geophysical data, an anatectic crostal source is proposed for the tourmaline leucogranites from north of the town of Mailasqui, State of Sao Paulo, Brazil. In addition, an evaluation of their metalliferous potential is presented.