Kenji Freire Motoki

The pyroclastic dyke and welded crystal tuff of the Morro dos Gatos alkaline intrusive complex, State of Rio de Janeiro, Brazil

The present article reports the geologic, lithologic, and petrographic characteristics of the pyroclastic dyke and phenocryst-rich welded crystal tuff of the Morro dos Gatos alkaline intrusive body, State of Rio de Janeiro, Brazil. The intrusive complex has an extension of 1.2 x 0.8 km occupying an area of 0.5 km2. It is constituted mainly by monzonite and trachyte. At the south flank of the massif, there exits a pyroclastic dyke and at the northwest flank, an intrusive welded crystal tuff. The pyroclastic dyke is 40 to 80 cm wide and intrudes into the basement porphyritic granite. This rock is constituted by welded tuff breccia with large amount of lapilli-size angular lithic fragments of trachyte. Along the contact plane of this dyke, there is an intercalation of the trachyte. The crystal tuff is characterised by abundant alkaline feldspar phenocrysts and mineral fragments of alkaline feldspar, quartz, biotite, and amphibole, showing a crystal matrix-supported texture. Large phenocrysts are aggregations of idiomorphic alkaline feldspar crystals with a general size of 2 mm and a maximum one of 6 mm. The matrix is composed mainly of mineral fragments of less than 1 mm in general size. The size-frequency diagram for the mineral fragments suggests that they have been originated from an explosive eruption. The matrix is strongly welded and completely devitrified showing a cryptocrystalline texture. The crystals were concentrated by sorting with the help of the volcanic gas ascending in the volcanic conduit during a continuous and effusive phase of the eruption. The limited special distribution of the pyroclastic rocks and small outcrop size indicate that they are not constituent of pyroclastic flow deposits, but of subvolcanic conduit and fissure.

Terremoto fóssil evidenciado pela brecha tectônica silicificada da área de Cabo Frio, RJ, e sua relação com a gênese do movimento intermitente da falha e o hidrotermalismo amagmático associado

This paper shows present the present state of the geologic and petrographic studies of the silicified tectonic breccia in the Cabo Frio area, State of Rio de Janeiro, Brazil. Based on the results, the authors propose a model for the stick-slip fault displacement. The tectonic breccia zones are 50 cm to 20 m wide and of NE-SW general strike. They are cut by early Cretaceous mafic dykes. The clasts are angular and characterized by auto-brecciation texture. The matrix is firmly consolidated by the silicification with the dissemination of hematite, carbonates, and sericite. The breccia zones are distributed randomly in a wade area of the State of Rio de Janeiro. This observation affirms that the silicification cannot be related to the local magmatism of Cretaceous to early Cenozoic alkaline intrusions. An alternative idea is the hydrothermalism originated form geothermal gradient. According to the normal continental geothermal gradient, of 25 to 30oC/km, the temperature attributed to the depth from 6 to 8 km is 180 to 220oC. In this condition, the H2O present there behaves as hydrothermal liquid. This type of hydrothermalism, without participation of magmas, is called amagmatic hydrothermalism. When a earthquake occurs, a cataclastic zone is formed. The surface water migrated down through it and arrived at the depth of 6 to 8 km transforms into amagmatic hydrothermal liquid. According to the stress adjustment and consequent accommodation of the crustal rock blocks, the hydrothermal liquid is compressed and forced to rise up along the cataclastic zone. During the ascension and cooling of the hydrothermal liquid, the cataclastic zone is silicified and consolidated. The silicified tectonic breccia observed in the studied area can represent this phenomenon which took place in the geologic time. By means of the silicification and consolidation, the fault plane transforms form a weak zone into a mechanically firm zone. This transformation enables the accumulation of tectonic energy in form of elastic deformation. That is, during and just after the earthquake, the fault plane is a weak zone. However, in the period between earthquakes it is a firm zone. The clasts of the fault breccia are composed of silicified cataclastic rock. This observation indicates that the fault plane rupture and its hydrothermal consolidation took place repeatedly, which implies repeated earthquakes along the same fault. The authors propose this process as an additional mechanism for stick-slip movement of the faults. The amagmatic hydrothermalism can be a geologic representation of asperity. It is a seismologic conception that corresponds to the specific locality on the fault plane in which the fault movement is stagnated and tectonic energy is accumulated in form of elastic deformation. A new earthquake occurs by means of asperity rupture.

Cristalização fracionada e assimilação da crosta continental pelos magmas de rochas alcalinas félsicas do estado do Rio de Janeiro

This paper presents a general model for the magmatic evolution for the felsic alkaline rocks of the State of Rio de Janeiro, Brazil. The rocks are meta-alkaline nepheline syenite, alkaline syenite, phonolite, and trachyte of potassic and ultrapotassic series. The compositional variation diagrams indicate the fractionation crystallisation of clinopyroxene, amphibole, titanite, ilmenite, and apatite. The magmatic fractionation for the mafic and intermediate alkaline rocks is characterised by the crystallisation of leucita and alkaline feldspar, and for the felsic alkaline rocks, of leucita and alkaline feldspar. By means of fractionation crystallisation, the felsic alkaline magma changes its composition from meta-alkaline to peralkaline and from potassic to sodic. The SSI diagram (silica saturation index) for the felsic alkaline rocks presents a linear trend crossing over the thermal divide, pointing out a strong effect of continental crust assimilation. The magmatic evolution occurs in three stages: 1) Leucite crystallisation; 2) Alkaline feldspar and nepheline crystallisation; 3) Continental crust assimilation and consequent transformation of the silica-undersaturated composition into oversaturated one. The assimilation rate is high, reaching up to 54%. The assimilation events took place during the Stage 2, which suggests magma super-reheating and consequent wall rock partial melting, proposing the origin of alkaline syenite and trachyte magmas of thermodynamically unstable composition.

Subvolcanic neck of Cabugi Peak, state of Rio Grande do Norte, Brazil, and origin of its landform

This article reports geologic, petrographic, and geomorphological observations of the mafic alkaline subvolcanic neck of the Cabugi Peak, located in the State of Rio Grande do Norte, Brazil. The massif is 370 m high and has 0.4 km3 of total volume. It is constituted mainly by Caico orthogneiss. The neck is exposed on the top of the massif forming a conical morphologic protrusion with relative height of 160 m and diameter of 500 m. The volume of the mafic alkaline rock is 0.056 km3 occupying 14% of the whole massif. The general form of the massif is strongly convex with the MCI (Macro Concavity Index) of -2.3. The neck is constituted by olivine-rich alkaline micro-gabbro in the centre and alkaline dolerite at the contact zone. There are well-developed cooling columnar joints with typical diameter of 60 cm. They are steep at the centre of the neck and sub-horizontal at the contact zone. On the foothill surface, called Sertaneja surface, no outcrops of mafic alkaline lava, other eruptive deposits, or volcanic rock debris have been observed. These observations allow a conclusion that the volcanic edifice and eruptive deposits of the late Oligocene were completely removed by later uplift and consequent regional denudation and that the present-day surface is significantly lower than that of the eruption time. The outcrops of the Cabugi Peak exhibit the underground geologic structure of the late Oligocene volcano. The original volcano form is not preserved anymore and the present morphologic elevation is attributed to differential erosion of the subvolcanic neck. According to the volcanological definition, the present-day morphology of the Cabugi Peak is not classified as an extinct volcano.

Exumação das Rochas Mantélicas no Arquipélago de São Pedro e São Paulo, Oceano Atlântico Equatorial, e sua Implicação na Possível Geração de Hidrocarbonetos Abiogenéticos por Serpentinização

This paper presents the mantle rock exhumation at Saint Peter and Saint Paul Islets, Equatorial Atlantic Ocean, and its implication to possible generation of abiogenic hydrocarbons by serpentinization. In Saint Paul transform fault zone, there are two contrasting tectonic processes for mantle exhumation: Extension at amagmatic spreading centre and compression along a pressure ridge. On the ocean bottom, the ultramafic rocks of exhumed mantle react chemically with sea-water generating heat and abiogenic hydrocarbons, the phenomenon called serpentinisation. The amagmatic spreading with megamullion formation occurs along inter-transform ridge segments. The ultramafic rocks are originated from uppermost mantle and have advanced serpentinisation and little expressive plastic deformation. The olivine porphyroblasts show brittle fracturing. On the other hand, the pressure-ridge occurs along the transform fault span where the fault directions and relative plate motion are oblique. Due to the direction discordance, the strike-slip movement of the transform fault generates compression stress perpendicular to the fault. This stress squeezes out the underlying deep mantle from the ductile deformation depth up to the Earths surface. The pressure ridge at Saint Peter and Saint Paulo Islet, namely Brachiosaurus Ridge, is the only confirmed example of mantle exhumation above sea level in Atlantic Ocean. The mantle rocks have little expressive serpentinisation and extremely developed plastic deformation, showing mylonitic texture. The porphyroclasts and matrix show either brittle fracturing or plastic deformation.

SATELLITE-DERIVED GRAVIMETRY FOR ABROLHOS CONTINENTAL SHELF, STATES OF ESPÍRITO SANTO AND BAHIA, BRAZIL, AND ITS RELATION TO TECTONIC GENESIS OF SEDIMENTARY BASINS

ABSTRACT. This paper presents the results of satellite-derived gravimetry for Abrolhos Continental Shelf and the adjacent areas, States of Esp´irito Santo and Bahia,Brazil, and discusses tectonic genesis of the sedimentary basins. The free-air and Bouguer anomalies on the continental shelf show a gradual increase from coast tocontinental shelf break. The total difference of 135 mGal indicates crust thinning of 4.5 km. The Bouguer anomaly increases beyond the shelf break and continues upto the abyssal plane. The total difference of 260 mGal suggests crustal thinning of 9.0 km. The studied area has two types of passive continental margin: 1) ES-typewith high continental slope, low and narrow continental rise, and high free-air and Bouguer anomalies; 2) SP-type with low continental slope, high and wide continentalrise, and low gravity anomalies. There are three types of sedimentary basins with different geologic structure and tectonic genesis: 1) Basins on the continental shelf,CSH-type; 2) Basin on the continental rise, CRS-type; 3) Continental rift basin, CRF-type. The CSH-type, as Esp´irito Santo basin, has coherent free-air and Bougueranomalies 40 to 50 mGal lower than the adjacent areas. The sedimentary deposits are 2.0 to 2.5 km thick. The basement depression is sustained by mechanical strengthwithout isostatic compensation. The normal faults of basin border are limited within upper crust. The CRS-type, as Pedro Can´ario basin, has low local residual Bougueranomaly with difference of 40 mGal. The sedimentary deposits are more than 2.7 km thick. The continental side basin border is under isostatic compensation due todeep normal fault between continental slope and continental rise. The oceanic side border is sustained by mechanical strength of the basement. The CRF-type, BesnardPassage, is a continental rift basin originated from mantle pull-apart tectonism. The rift bottom is under complete isostatic compensation. The normal faults of the grabenborder continue to the mantle. The Moho depth along the rift is at least 6.5 km shallower than the adjacent areas. Keywords: satellite-derived gravimetry, Abrolhos Continental Shelf, sedimentary basin, crustal thinning, isostatic compensation. RESUMO. Este artigo apresenta os resultados da gravimetria por satelite para a Plataforma Continental de Abrolhos e as areas adjacentes, ES-BA, e discute as geneses tectonicas das bacias sedimentares. As anomalias ar-livre e Bouguer na plataforma continental mostram um aumento gradativo a partir da costa ate a quebra da plataforma continental. A diferenca total de 135mGal indica afinamento da crosta continental de 4,5 km. A anomalia Bouguer se apresenta alem da quebra da plataformae continua ate o assoalho abissal. A diferenca total de 260 mGal sugere afinamento crustal de 9,0 km. Na area estudada, observam-se dois tipos de margem continental passiva: 1) Tipo ES com talude continental alto, elevacao continental baixa e curta e anomalias ar-livre e Bouguer altos; 2) Tipo SP com talude continental baixo, elevacao continental alta e extensa e anomalias gravimetricas baixas. Existem tres tipos de bacias sedimentares com estrutura geologica e genesis tectonicas diferentes: 1) Bacias na plataforma continental, Tipo CSH; 2) Bacias na elevacao continental, Tipo CRS; 3) Bacia de rifte continental, Tipo CRF. O Tipo CSH, como a bacia do Espirito Santo, possui anomalias ar-livre e Bouguer coerentes 40 a 50 mGal mais baixas do que as areas adjacentes. Os depositos sedimentares sao de 2,0 a 2,5 km de espessura. A depressao do embasamento e sustentada pela rigidez mecânica sem compensacao isostatica. As falhas normais das bordas de bacias sao limitadas dentro da crosta superior. O Tipo CRS, como a bacia Pedro Canario, tem baixa anomalia Bouguer local com diferenca de 40 mGal. Os depositos sedimentares tem mais de 2,7 km de espessura. A borda da bacia do lado continental esta sob compensacao isostatica devido a falha normal profunda entre talude continental e elevacao continental. Aborda do lado oceânico e sustentada pela rigidez mecânica do embasamento. O Tipo CRF, Passagem Besnard, e uma bacia do rifte continental originada do tectonis mode distensao do manto. O fundo da bacia esta sob compensacao isostatica completa. As falhas normais da borda do graben continuam ate o manto. A profundidade da Moho ao longo do rifte e pelo menos 6,5 km mais rasa do que as areas adjacentes. Palavras-chave: gravimetria por satelite, Plataforma Continental de Abrolhos, bacia sedimentar, afinamento crustal, compensacao isostatica.

Análises geomorfológicas do Platô de Teresópolis e da Serra do Mar, RJ, com o auxílio de seppômen e ASTER GDEM e sua relação aos tectonismos cenozoicos

This paper presents geomorphological analyses of the Teresopolis Plateau, the Serra do Mar Cliff, and the adjacent areas, State of Rio de Janeiro, Brazil, with the help of summit level technique using ASTER GDEM satellite topographic data, and its relation to the Cenozoic tectonism. According to the summit level map of the grid of 4 km, the studied area is divided into three geomorphologic domains, Rio de Janeiro Lowland (RJL), Serra do Mar Scarp (SMS), and Teresopolis Highland (TRH). The Rio de Janeiro Lowland, which corresponds to the Guanabara Graben area, is constituted by alluvial deposit planes and the areas of low hills that expose the metamorphic basement. Their limits are sharp and linear suggesting existence of small faults and grabens. The tops of the low hills are almost at the same altitude, constituting a summit level surface with approximate altitude of 100 m. The summit level maps of the grid of 4 km and 2 km show that the top of the Serra do Mar Scarp is of approximate altitude of 1300 m, and therefore, this normal fault has displacement of about 1200 m. The scarp strikes N75oW and divided into the segments of 15 km to 30 km in length. At some localities on the halfway of the scarp, mega-landslide morphology is observed. The Teresopolis highland is tilted to N30oW with mean dip of 0.9o. The drainage systems on the plateau shows three orientations, N60oE, N10oE e N75oW. Along the Paraiba do Sul River, there is a morphologic depression of 20 km wide and 300 m deep, suggesting presence of a shallow graben. On the Teresopolis Highland, there are three alignments of morphologic protrusions oriented to N50oE, with relative height up to 900 m. The height distribution diagrams for the summit level surfaces suggest that they should be remnant inselbergs before the fault movement of the Serra do Mar Scarp. The Cenozoic normal faults and the fracture systems of this area are often discordant with the basement structure. This phenomenon is explained by the model of the reactivation of the structures of the lithospheric mantle, and not of the continental crust.