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Nicolae Bălcescu, Romania

Rossel P.,University of Concepcion | Oliveros V.,University of Concepcion | Ducea M.N.,University of Arizona | Ducea M.N.,Universitatea Bucharest | And 5 more authors.
Lithos | Year: 2013

The Upper Jurassic volcanic rocks of the Pre-Cordillera and High Andes of northern Chile (26-31°S) represent a back-arc magmatic chain formed during an earlier stage of Andean subduction. After the Callovian, the back-arc basin gradually changed from marine to continental conditions and was characterized by basaltic to rhyolitic rocks erupted along two belts, parallel to the coeval arc. The western belt comprises the Picudo and Algarrobal formations, whereas the eastern belt comprises the Lagunillas Formation and the Quebrada Vicuñita Beds. New major and trace element data, along with whole rock Sr, Nd and Pb isotopes are presented for these volcanic belts and compared to the geochemical features of the Jurassic and Early Cretaceous arc magmatism. Ar-Ar and U-Pb ages constrain the back arc volcanism to have evolved between 163.9. ±. 1.4 and 148.9. ±. 1.2. Ma.Rocks belonging to the western belt have steep multi-element patterns and low concentrations of HREE, suggesting the presence of garnet in the source, and a more radiogenic isotopic composition than the arc magmatism. Parental magmas of these back-arc lavas would have been generated through melting of a depleted mantle, although less depleted than the sub-arc mantle, and interacted with minor amounts of Paleozoic crust. The geochemical composition of the rocks belonging to the eastern belt is more heterogeneous and suggests involvement of different magmatic sources, including depleted mantle as well as an OIB-type mantle within the wedge. In spite the fact that the Jurassic Andean arc was built over a continental plate, the architecture of the volcanic chains and geochemical variations observed among the arc and back-arc rocks in northern Chile resemble those in modern island arcs, and thus support the hypothesis that early Andean subduction developed under extensional tectonic conditions. © 2013 Elsevier B.V.

Tanase A.M.,Universitatea Bucharest
Bacteriologia, virusologia, parazitologia, epidemiologia (Bucharest, Romania : 1990) | Year: 2010

Prokaryotes are the most abundant living organisms and also most diverse from genetically and metabolically point of view, being responsible for the majority of biogeochemical processes playing the most important role in life cycle on the planet. Considering this, there is a general agreement among taxonomists, that there is a very small number of bacterial species recognized and described today, mostly because of controversial issues concerning bacterial species concept. One of the most accepted approaches, even today, is the polyphasic taxonomy because it is based on diverse information, obtained from classic taxonomy but also from molecular level. The development of new molecular techniques, especially sequencing rRNA genes conducted to an improved concept, that we intended to evaluate in this review, and even more, to reconstruction of group specific phylogenetic tree.

Pearson D.M.,University of Arizona | Pearson D.M.,Idaho State University | Kapp P.,University of Arizona | DeCelles P.G.,University of Arizona | And 5 more authors.
Geosphere | Year: 2013

The retroarc fold-and-thrust belt of the Central Andes exhibits major along-strike variations in its pre-Cenozoic tectonic configuration. These variations have been proposed to explain the considerable southward decrease in the observed magnitude of Cenozoic shortening. Regional mapping, a cross section, and U-Pb and (U-Th)/He age dating of apatite and zircon presented here build upon the preexisting geological framework for the region. At the latitude of the regional transect (24-25°S), results demonstrate that the thrust belt propagated in an overall eastward direction in three distinct pulses during Cenozoic time. Each eastward jump in the deformation front was apparently followed by local westward deformation migration, likely refl ecting a subcritically tapered orogenic wedge. The first eastward jump was at ca. 40 Ma, when deformation and exhumation were restricted to the western margin of the Eastern Cordillera and eastern margin of the Puna Plateau. At 12-10 Ma, the thrust front jumped ~75 km toward the east to bypass the central portion of a horst block of the Cretaceous Salta rift system, followed by initiation of new faults in a subsystem that propagated toward the west into this preexisting structural high. During Pliocene time, deformation again migrated >100 km eastward to a Cretaceous synrift depocenter in the Santa Bárbara Ranges. The sporadic foreland-ward propagation documented here may be common in basement-involved thrust systems where inherited weaknesses due to previous crustal deformation are preferentially reactivated during later shortening. The minimum estimate for the magnitude of shortening at this latitude is ~142 km, which is moderate in magnitude compared to the 250-350 km of shortening accommodated in the retroarc thrust belt of southern Bolivia to the north. This work supports previous hypotheses that the magnitude of shortening decreases significantly along strike away from a maximum in southern Bolivia, largely as a result of the distribution of pre-Cenozoic basins that are able to accommodate a large magnitude of thin-skinned shortening. A major implication is that variations in the pre-orogenic upper-crustal architecture can infl uence the behavior of the continental lithosphere during later orogenesis, a result that challenges geodynamic models that neglect upper-plate heterogeneities. © 2013 Geological Society of America.

Murray K.E.,University of Arizona | Ducea M.N.,University of Arizona | Ducea M.N.,Universitatea Bucharest | Schoenbohm L.,University of Toronto
Memoir of the Geological Society of America | Year: 2015

Investigations of lithospheric foundering and related magmatism have long focused on the central Andes, where there are postulated links between the eruption of mantle-derived lavas and periodic loss of the lower lithosphere. Whole-rock elemental and Nd-Sr-Pb isotopic results from a suite of late Miocene-Quaternary mafic lavas erupted onto the Puna Plateau clarify the relationship between this hypothesized process and lava composition. Zinc and Fe provide a critical perspective because they are partitioned differently during the melting of asthenospheric and lithospheric mantle. All Puna lavas have Zn/FeT (×104) values >13, which requires clinopyroxene and perhaps garnet to be the dominant phase(s) in the melt source; this precludes a melt source of typical mantle asthenosphere. This result is contrary to classic models of delamination magmatism that suggest asthenospheric peridotite melts to generate these lavas. Pyroxenite (±garnet)-bearing lithospheric materials in the central Andes are likely common and heterogeneous in age, volatile content, and mineralogical composition, and if they are the melt source, this can explain the diversity in the elemental (La/Yb = 11-45; La/Ta = 22-40) and isotopic (87Sr/86Sr = 0.7055-0.7080;εNd = -1 to -7) compositions of these mafic magmas (MgO > 8%, Mg number > 60). We propose that compositionally diverse, gravitationally unstable pyroxenites both drive "dripping" of the lower lithosphere and are the source of the resulting melt. We also postulate that mantle-derived lavas erupted on the Puna Plateau were generated during localized foundering and melting of these materials. The cumulative effect of these drip events is a modern Puna Plateau with geodynamic anomalies including thin lithosphere and anomalously high surface elevation. © 2014 The Geological Society of America. All rights reserved.

Currie C.A.,University of Alberta | Ducea M.N.,University of Arizona | Ducea M.N.,Universitatea Bucharest | DeCelles P.G.,University of Arizona | Beaumont C.,Dalhousie University
Memoir of the Geological Society of America | Year: 2015

Cordilleran orogens, such as the central Andes, form above subduction zones, and their evolution depends on both continental shortening and oceanic plate subduction processes, including arc magmatism and granitoid batholith formation. Arc and batholith magma compositions are consistent with partial melting of continental lithosphere and magmatic differentiation, whereby felsic melts rise upward through the crust, leaving a high-density pyroxenite root in the deep lithosphere. We study gravitational removal of this root using two-dimensional thermal-mechanical numerical models of subduction below a continent. The volcanic arc position is determined dynamically based on thermal structure, and formation of a batholith-root complex is simulated by changing the density of the arc lithosphere over time. For the model lithosphere structure, magmatic roots with even a small density increase are readily removed for a wide range of root strengths and subduction rates. The dynamics of removal depend on the relative rates of downward gravitational growth and lateral shearing by subduction-induced mantle flow. Gravitational growth dominates for high root densification rates, high root viscosities, and low subduction rates, resulting in drip-like removal as a single downwelling over 1-2.5 m.y. At lower growth rates, the root is removed over >3 m.y. through shear entrainment as it is carried sideways by mantle flow and then subducted. In all models, >80% of the root is removed, making this an effective way to thin orogenic mantle lithosphere. This can help resolve the mass problem in the central Andes, where observations indicate a thin mantle lithosphere, despite significant crustal shortening and thickening. © 2014 The Geological Society of America. All rights reserved.

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