Zur Geologie der nordchilenischen Kordilleren

Zusammenfassung Die Arbeit enthält Ergebnisse von Geländeuntersuchungen in den nordchilenischen Kordilleren. In dem geologisch wenig bekannten Raum der Provinzen Atacama und Antofagasta wurden neue stratigraphische Tatsachen entdeckt, u. a. in der Küstenkordillere ein über 1000 m mächtiges Paläozoikum und eine Lias-Transgression, sowie in der Hochkordillere ausgedehnte Vorkommen präkambrischer Gneise und Glimmerschiefer und Lias mit reicher Fossilführung.Die seit der Trias geförderten Ergußgesteine und die Rolle des mittelkretazischen Andenplutons werden geschildert. Ferner wird auf die Beziehungen zwischen magmatischen und tektonischen Ereignissen in Nordchile eingegangen.Der Bau der Küstenkordillere seit der Trias wird vorwiegend von Bruchverformung geprägt. Faltung ist fast ausschließlich auf prämesozoische Serien beschränkt. Die Platznahme des Andenplutons in der Mittelkreide kann hier mit keiner Faltungsphase in Verbindung gebracht werden. Die Faltungsintensität in der Hochkordillere ist zwar stärker, wirklich alpinotype Verformungen werden jedoch selten erreicht.

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Resumen En este trabajo se da una visión general sobre estudios geológicos realizados en la cordillera del norte de Chile, entre los 22 y 26 grados de latidud sur. Estos estudios los realizó el autor en el año 1956, en calidad de huésped de la Universidad de Chile.En la Cordillera de la Costa entre Chañaral y Taltal, y en un perfil por la Cordillera al Este de Vallenar, pude comprobar una serie de novedades en el campo de la estratigrafía.En la Cordillera de la Costa, aflora una potente serie paleozóica (1000 m de potencia), probablemente Silúrico Superior o Devónico, la que se muestra intensamente plegada. Por otra parte pude datar como perteneciendo al Paleozóico una transgresión liásica entre Bahia Cifuncho y Las Bombas.En la Alta Cordillera, al Sur-Este de El Transita, existen espesores considerables de gneises y esquistos precámbricos. En la quebrada de Pinto y quebrada Chanchoquin, afloran sedimentos de la parte alta del Liásico inferior y Liásico medio, los que son muy fosiliferos. Dentro de los sedimentos liásicos se depositaron concordantemente coladas de porfiritas. También aparecen volcanitas postliásicas.Pudieron determinarse dos generaciones de roca plutónica: una vieja, compuesta principalmente de material diorítico que muestra ordenación de petrofabric, y una intrusión mas moderna compuesta de granito fresco no deformado. Este material está claramente intruido en la formación porfirítica en el valle del rio El Carmen entre el Alto del Carmen y San Félix. Es probable que este granito sea más jóven que el batolito mesocrétácico de la Cordillera de la Costa.Finalmente se discuten las relaciones entre procesos tectónicos y magmáticos en las Cordilleras del norte de Chile. Diversos croquis y figuras ilustran los puntos tratados.Por la ayuda prestada en la preparación de los trabajos de campo, le agradezco al Señor Jorge MuñozCrsti, Director del Instituto de Geología de la Universidad de Chile. En los trabajos de campo me asistió el Ingeniero SeñorAntonio Pena.

     

Geologisches Profil durch den mittleren Abschnitt der Anden (Chile-Bolivia)

Zusammenfassung Mit Hilfe neuer Arbeiten wird ein geologischer Schnitt durch den über 900 km breiten mittleren Abschnitt der Anden von der pazifischen Küste Chiles bis in die subandine Zone Boliviens gegeben. Die Profile zeigen folgende Tatsachen: die Anden sind ein Falten-, aber kein Deckengebirge; die Vergenzen der Strukturen sind vorwiegend nach Osten gerichtet, der Anteil von Magmatiten seit dem Beginn der Trias ist bedeutend; die Anden werden von tiefgreifenden, jungen Gräben durchzogen, mit deren Einbruch eine enorme vulkanische Tätigkeit im Zusammenhang steht.Die Küstenkordillere Chiles hat eine andere tektonische Geschichte als die Hochkordilleren.Aus dem tektonischen Bild der mittleren Anden können unmittelbar keine Beweise für eine Westdrift Südamerikas abgeleitet werden.

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By aid of recent studies a geological section is made through the central part of the Andes (900 km large), from the pacific coast of Chile up to the subandine zone of Bolivia. The profiles show the following facts: the Andes are mountains with folds and not with nappes; the overturns of the structures are mainly turned eastward; the part of the ignecus rocks since the beginning of the Triassic is considerable; the Andes are interlaced by thorough-going trenches of younger date, whose appearance an enormous volcanic activity is connected.The tectonic history of the coastal Cordillera is different from that of the High Cordillera.A western drift of South America cannot be directly proved from the tectonic picture of the central Andes.

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Résumé A l'aide de nouvelles recherches on fait une coupe géologique à travers le secteur central des Andes (large de 900 km) de la côte pacifique de Chile jusque dans la zone subandienne de la Bolivie. Les profils montrent les faits suivants: les Andes sont des montagnes à plis, non pas à nappes; les déversements en sens inverse des structures vont principalement vers l'est; la part des roches éruptives depuis le commencement du Triassique est considérable; les Andes sont sillonnées par des fossés profondes et d'un origine récent; leur apparition est en rapport avec une énorme activité volcanique.La Cordillère à la côte de Chile a une histoire tectonique différente de celle de la Haute Cordillère.On ne peut pas dériver du tableau tectonique des Andes centrales la preuve d'une drift vers l'ouest de l'Amérique du Sud.

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Calcite formation in microbial mats: modeling and quantification of inhomogeneous distribution patterns by a cellular automaton model and multifractal measures

The evolution of early diagenetic calcite cements in microbial mats of recent supratidal sediments of the southern North Sea is modeled in a two-dimensional microscale approach by a cellular automaton model (CAM). Calcite is traced out in the model by virtual calcium distribution patterns obtained from runs under different assumptions concerning sediment-intrinsic conditions. For justification of the CAM, real calcium distribution patterns, documented by scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM/EDX), are quantitatively compared with the virtual patterns on the basis of multifractal analyses. The formation of high magnesian calcite as a consequence of biogenic anaerobic decomposition of organic matter starts at certain initial calcite domains. In this stage an inhomogeneous and multifractal calcium distribution is characteristic. Nearly complete remineralization of organic matter leads to monofractal behavior of generalized fractal dimensions (D_B(q) ±1.84). The CAM results confirm that calcite formation is a self-determining morphogenetical process and diffusive transport processes of reactants within the mat affect the biogenic calcite formation.     

Upper-ocean transport mechanisms from the Gulf of Maine to Georges Bank, with implications for Calanus supply

Potential upper-ocean pathways for the supply of biota from the Gulf of Maine to Georges Bank are investigated by numerically tracking particles in realistic 3-d seasonal-mean and tidal flow fields. The flow fields, obtained from a prognostic model forced by observed M₂ tides and seasonal-mean wind stress and density fields, include the major known observational features of the circulation regime in winter, spring and summer — a wind-driven surface layer (in winter and early spring) overlying seasonally-evolving baroclinic and tidally-rectified topographic gyres. The surface layer in winter and early spring, with generally southward drift for typical northwesterly wind stress, can act as a conveyor belt for the transport of biota to Georges Bank, provided that the biota can spend a substantial fraction of time in the surface Ekman layer. The numerical experiments indicate that the upper-ocean drift pathways for biota in the southern Gulf of Maine are strongly sensitive to biological and/or physical processes affecting vertical position in relation to the surface Ekman layer and horizontal position in relation to topographic gyres. The seasonality and location of the identified pathways are generally consistent with observed distributional patterns of Calanus finmarchicus based on the 11-year MARMAP surveys.     

On the amorphisation of ZnCr2S4 spinel under high pressure: x-ray diffraction studies

Compressibility of ZnCr₂S₄ single crystals and their structure under pressure have been determined by means of x-ray methods. A pressure region of deterioration of the single crystal was observed around 10 GPa. Moreover, a plastic deformation as a result of a phase transition under pressure, an irreversible structural change, a chemical decomposition of the sample or a hysteresis effect, has been revealed. The crystals that are recovered from the pressure cell show a decrease in unit cell volume of 3% compared to the initial volume. Received: June 19, 1996 / Revised, accepted: February 1, 1997     

“Pseudosinhalite”, a new hydrous MgAl-borate: synthesis, phase characterization, crystal structure, and PT-stability

The new synthetic phase Mg₂Al₃O[BO₄]₂(OH) provisionally named “pseudosinhalite” is optically, chemically, and structurally similar to the mineral sinhalite, MgAl[BO₄], isostructural with forsterite. It grows hydrothermally from appropriate bulk compositions in the range 4–40?kbar at temperatures that increase with pressure (～650?→?900?°C), and it breaks down at higher temperatures to sinhalite?+?corundum?+?H₂O. At P?≥?20?kbar single-phase products of euhedral twinned crystals could often be obtained. Pseudosinhalite is monoclinic with a?=?7.455 (1) Å, b?=?4.330 (1) Å, c?=?9.825 (2) Å, β?=?110.68 (1)°, and space group P2₁/c. Crystal structure analysis reveals that pseudosinhalite is also based on hexagonal close packing (hcp) of oxygen atoms with Mg and Al in octahedral and B in tetrahedral coordination. In pseudosinhalite the winged octahedral chains in the plane of hcp are not straight as in sinhalite but have a zigzag, 3-repeat period (Dreierkette), and only 1/10 instead of 1/8 of all tetrahedral sites are filled by boron. Hydrogen is located at a split position between two oxygen atoms O5—O5, which are only 2.550 Å apart and thus generate strong hydrogen bonding. This may be responsible for the absence of an hydroxyl absorption band between 2800?cm^?1 and 3500?cm^?1 in the powder IR spectrum. The equilibrium breakdown curve of pseudosinhalite to form sinhalite, corundum, and water was determined by bracketing experiments to pass through 10?kbar, 745?°C and 35?kbar, 950?°C, giving a slope of about 8?°C/kbar, similar to dehydration curves of some silicates at high pressure. In nature pseudosinhalite could have been misidentified as sinhalite. A possible appearance, like sinhalite in boron-rich skarns, would require more aluminous bulk compositions than for sinhalite at relatively low temperatures. However, pseudosinhalite might also form as a hydrous alteration product of sinhalite at low temperatures, perhaps in association with szaibelyite, MgBO₂(OH).     

Variations in the chemical and isotopic composition of fluids discharged from the Taupo Volcanic Zone, New Zealand

Contents of H₂O, CO₂ and Cl in well discharges from six explored geothermal systems of the Taupo Volcanic Zone, New Zealand, point to the existence of two distinct source fluids. The fluid present in discharges from systems along the eastern boundary is characterised by high CO₂ contents, 1.6 ± 0.5 , at mole ratios of 3.9 ± 1.5. High (0.06) and (12) weight ratios in these waters suggest that all four constituents are derived from associated andesitic rock. Geothermal discharges in the western parts of the TVZ, dominated by rhyolitic magmatism, are characterised by low CO₂ contents, 0.12 ± 0.05 , and low (0.14 ± 0.1) ratios. Again, relative Cl, B, Li and Cs contents agree with those of this potential source rock. High and ratios in the east are typical of fluids affected by the addition of volatiles released from subducted marine sediments. For the western systems, these ratios resemble more closely those expected for mantle-derived volatiles. The isotopic compositions of all deep waters point to the presence of variable amounts of a magmatic component, some 14 ± 5% in the eastern and 6 ± 2% in the western systems. The observed variations are explained in terms of interaction of volatiles released from the subducted sediments with material of the mantle wedge to form a volatile-charged, high-alumina basalt. Its convective rise, in a direction opposite to that of the down-going slab, leads to high enrichment in volatiles of the magmas generated beneath the eastern parts of the TVZ and increases their ability to intrude the continental crust. Further fractional crystallisation and assimilation leads to the formation of volatile-rich andesitic melts, partly extruded to form the volcanoes of the andesitic arc, partly intruded to act as source rocks for the high-gas geothermal systems. Batches of high-alumina basalt, depleted in subducted volatiles, travel farther west to pond beneath a zone of crustal extension. Following extensive fractionation, highly siliceous melts, carrying predominantly mantle-type volatiles, rise beneath the western part of the TVZ to supply both heat and volatiles to the geothermal systems there.