135 research outputs found
Rio grande cone tectono-stratigraphic model – brazil: seismic sequences
Análisis sísmicos integrados con técnicas de interpretación, procesamiento, información de velocidades y datos geofísicos permiten modelar diferentes rasgos de tipo tectónico, estructural o geomorfológico. Este trabajo se constituye en una primera aproximación a un modelo tridimensional de un rasgo geomorfológico ubicado en el fondo oceánico. El modelo inicial comprende una configuración bidimensional de una grilla interpretada a partir de secciones sísmicas llevadas a un volumen tridimensional, esto mediante la concepción de una aproximación dos y medio dimensional. Además de un cambio de configuración, se hizo un análisis de atributos sobre las imágenes de las secciones y se determinaron las características presentes desde un área de la plataforma, hasta el offshore del sudeste Brasilero. El rasgo geomorfológico a tratar es conocido como el Conode Rio Grande, el cual pudo ser caracterizando con sus principales estructuras tectónicas y estratigráficas, mediante la cartografía y modelamiento tridimensional del subsuelo, con la integración geológica, geofísica y geomorfológica. La geomorfología del cono, se ve influenciada por procesos gravitacionales, corrientes defondo, estructuras de tipo tectónico y composición litológica, como sus principales controladores que están presentes a partir del Neógeno. Así, a partir de la interpretación sísmica pueden ser extractados diferentes estructuras tectónicas, sedimentarias y geoformas, por ejemplo presencia de fallas, pliegues, canales, levee naturales, contornitos, etc
Whole-rock and isotope geochemistry of Ordovician to Silurian units of the Cuyania terrane, Argentina: insights for the evolution of SW Gondwana margin
The Cuyania terrane in central Argentina (Fig. 1) is characterized by a Mesoproterozoic (Grenvillianage) basement with depleted Pb isotopic signatures and Mesoproterozoic Nd model ages resembling basement rocks of the same age from Laurentia (Ramos, 2004; Sato et al., 2004 and references therein). Several authors have proposed para-autochthonous (Aceñolaza et al., 2002; Finney et al., 2005) versus allochthonous (e. g. Ramos et al., 1986; Dalziel et al., 1994; Astini et al., 1995; Thomas and Astini, 1996) geotectonic models for the early Palaeozoic evolution of the Cuyania terrane. The tectonic evolution of the Cuyania terrane is a substantial part of the understanding of the evolution of the western border of southwest Gondwana. Several morphostructural units form the Cuyania composite terrane (Fig. 1; Ramos et al., 1996): The Precordillera s.s., the Western Pampeanas Ranges and the San Rafael and Las Matras blocks. However, the boundaries of the terrane are still not well-constrained (Astini and Dávila, 2004; Porcher et al., 2004; Casquet et al., 2006). A combination of several methodologies including geochemistry, Sm-Nd, Pb-Pb and U-Pb detrital zircon dating was applied to several clastic Ordovician (Los Sombreros, Gualcamayo, Los Azules, La Cantera, Yerba Loca, Empozada, Trapiche, Sierra de la Invernada, Portezuelo del Tontal, Las Vacas, Las Plantas and Alcaparrosa Formations) and Ordovician to Silurian (Don Braulio and La Chilca Formations) units of the Cuyania terrane (Fig. 2). The combination of these different approaches can give accurate information in order to constrain the probable sources that provided detritus to the Cuyania terrane and ultimately to constrain the existing models about its origin.Centro de Investigaciones Geológica
Provenance of Ordovician clastic sequences of the San Rafael Block (Central Argentina), with emphasis on the Ponón Trehué Formation
The Ordovician Ponón Trehué Formation is the only early Palaeozoic sedimentary sequence known to record a primary contact with the Grenvillian-age basement of the Argentinean Cuyania terrane, in its southwards extension named the San Rafael block. Petrographic and geochemical data indicate contributions from a dominantly upper continental crustal component and a subordinated depleted component. Nd isotopes indicate εNd of −4.6, ƒSm/Nd −0.36 and TDM 1.47 Ga in average. Pb-isotope ratios display average values for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of 19.15, 15.69 and 38.94 respectively. U–Pb detrital zircon ages from the Ponón Trehué Formation cluster around values of 1.2 Ga, indicating a main derivation from a local basement source (Cerro La Ventana Formation). The Upper Ordovician Pavón Formation records a younger episode of clastic sedimentation within the San Rafael block, and it shows a more complex detrital zircon age population (peaks at 1.1 and 1.4 Ga as well as Palaeoproterozoic and Neoproterozoic detrital grains). Detailed comparison between the two Ordovician clastic units indicates a shift with time in provenance from localized basement to more regional sources. Middle to early Upper Ordovician age is inferred for accretion of the Cuyania terrane to the proto-Andean margin of Gondwana.Centro de Investigaciones Geológica
Nd isotopes from Yerba Loca Formation (Upper Ordovician), Cuyania terrane, Argentina
Several arguments as well as geochemical data from the basement rocks support the allochthonous models of the Precordillera (Cuyania) Terrane as derived from Laurentia (e.g. Ramos et al., 1986; Dalla Salda et al., 1992; Astini et al., 1995). Less certain are the time and type of collision with Gondwana. However, other authors have proposed a parautochthonous evolution based on biostratigraphical and structural data, displaced during Ordovician–Devonian times (e.g., Aceñolaza et al., 2002; Finney et al., 2005). To contribute to the discussion about tectonic models for the Precordillera, preliminary Nd isotope data from an ongoing provenance study on the Caradocian rocks from the Yerba Loca Formation are here presented.Centro de Investigaciones Geológica
U-Pb detrital zircon data from the paleozoic Sierra Grande Formation, North Patagonian Massif, Argentina
The Sierra Grande Formation is a siliciclastic sedimentary unit with oolithic iron levels of economical importance which crops out in the Argentinean North Patagonian Massif. It was deposited within an open marine environment, with littoral to sub-littoral conditions. A Silurian-Lower Devonian age is assigned based on fossil content. It is mainly composed of medium to fine quartz arenites, wackes, silts, shales and conglomerate beds. In order to establish the source rocks for the Silurian-Devonian basin and also to constrain the sedimentation age, different U-Pb geochronological studies have been carried out on detrital zircons. The zircons have been studied with comparative morphology analysis using SEM images to asses the different population types. For dating using U-Pb isotopes in zircon detrital grains, the high resolution SHRIMP and LA-ICP-MS methodologies were used. The predominant obtained age groups are: Neoproterozoic (547–991 Ma), Cambrian (497-546 Ma) and Ordovician (443-495 Ma). All the analyzed samples also comprise Mesoproterozoic ages of about 1009 to 1382 Ma and smaller proportion Paleoproterozic (1641 to 2248 Ma) and Neo-Archean (2649-2657 Ma) ages. It is important to note that Silurian ages (428 and 440 Ma) were obtained from a group of zircon crystals, constraining the sedimentation age of the Sierra Grande basin to be not older than Middle Silurian. With these isotopic data it is possible to discuss the provenance of the Silurian-Devonian siliciclastic sedimentary cover of the North Patagonian Massif.Centro de Investigaciones Geológica
Wind-pattern circulation as a palaeogeographic indicator: case study of the 1.5-1.6 Ga Mangabeira Formation, São Francisco Craton, Northeast Brazil
The preserved deposits of dune-scale aeolian bedforms provide valuable palaeoenvironmental indicators of atmospheric circulation patterns and the latitudinal position and distribution of land masses. However, no attempts to use palaeowind directions and palaeogeographic reconstructions of ancient land mass distribution have been published to model Precambrian atmospheric circulation. The Mangabeira Formation is a large Mesoproterozoic aeolian erg succession (1.6 to 1.5 Ga) composed of two aeolian units that accumulated in the São Francisco Craton, Brazil. The Lower Unit records multiple drying-upward depositional cycles, each of which represents an episode of erg expansion and contraction driven by climate changes. The Upper Unit is composed dominantly of stacked aeolian dune strata that lack intervening interdune deposits and which record extreme aridity. Palaeowind directions recorded from cross-strata of transverse, crescentic aeolian dunes of the Lower and Upper Units record dune migration under the influence of two dominant winds that blew to the southeast and northwest. Analysis of these palaeowind data in relation to assessment of regional palaeogeographic reconstructions for the period 1.6 to 1.5 Ga reveals a correlation between atmospheric circulation and land mass distribution. At this time the São Francisco Craton was located between the mid-latitudes and the equatorial zone. The wind regime determined from analysis of dip azimuths of cross-strata of the Lower Unit (1.6 to 1.54 Ga) are consistent with a palaeogeographic position between 25° to 35° S. Analysis of cross-strata dip azimuths of the Upper Unit indicate northwest-directed palaeowinds and a dominant monsoonal wind pattern from 1.54 to 1.5 Ga. During this time the large land mass of the São-Francisco-Congo and Siberian cratons drifted northwards through the equatorial zone from palaeolatitude 30° S to 30°N
Drainage evolution and exhumation history of the eastern Himalaya: Insights from the Nicobar Fan, northeastern Indian Ocean
The eastern Himalayan syntaxis, where the Yarlung Tsangpo sharply bends, is one of the areas experiencing most rapid exhumation on Earth. The rapid exhumation is often regarded as the result of capture of the Yarlung Tsangpo by the Brahmaputra River. However, both the timing of integration of the Yarlung Tsangpo-Brahmaputra River and initiation of the rapid syntaxial exhumation are debated. As the ultimate sedimentary trap of the Yarlung Tsangpo-Brahmaputra River, the Nicobar Fan is a window to look into the drainage evolution and exhumation history of the eastern Himalaya. International Ocean Discovery Program Expedition 362 drilled the Nicobar Fan for the first time, recovering fan sediments dating back to the Early Miocene (∼19Ma). We apply trace elements and Sr-Nd isotopes to investigate the provenance of the sediments in the Nicobar Fan with the aim of constraining the timing of integration of the Yarlung Tsangpo-Brahmaputra River and initiation of the rapid syntaxial exhumation. The geochemical and Sr-Nd isotope compositions indicate an eastern Himalayan source dominated by the Greater Himalaya, with significant Gangdese arc contribution and primarily carried by the Brahmaputra River. Flux of Gangdese arc material appears to have been continuous from the base of the Nicobar Fan, suggesting that the Yarlung Tsangpo-Brahmaputra River has been established at least since ∼19Ma. Synchronously with the sharp rise in sedimentation rate, the abrupt change of geochemical and isotope compositions at ∼9.2Ma indicates an increase in erosion of the Greater Himalaya as the result of initiation of rapid exhumation in the broad syntaxial region. The proportion of Greater Himalayan material increased again at 3.5–1.7Ma, consistent with a younger pulse of rapid exhumation focused in the core of the syntaxis since ∼3.5Ma. Our results show that initiation of the rapid syntaxial exhumation postdated integration of the Yarlung Tsangpo-Brahmaputra River by at least ∼10 m.y. Therefore, tectonic uplift rather than river capture could be responsible for the initiation of the rapid syntaxial exhumation
Anatase crystals of unusual habit in quartz veins from the Diamantina region (Espinhaço Range, Minas Gerais)
- …