Unconformities as mineralogical breaks in the burial metamorphism of the Andes     

Luis Aguirre  Beatriz Levi  Robin Offler 《Contributions to Mineralogy and Petrology》1978,66(4):361-366

Stratigraphical-structural units separated by regional unconformities in the Andes of Peru and Chile, display a pattern of low grade burial metamorphism. Each stratigraphical-structural unit shows a particular facies series covering part or all the range between the zeolite and the greenschist facies. These facies series were episodically generated as part of the geological evolution of each unit prior to its own folding. Mineralogical breaks are found to coincide with the regional unconformities and often cases of higher grade assemblages on top of lower grade ones occur. This pattern may be explained by a process of sealing of each unit after its particular metamorphic episode took place. Porosity and permeability conditioning P _f, as demonstrated for individual lava flows, are the significant controlling factors in the production of the metamorphic assemblages.  相似文献   

The secular variantion of longitudes and plate tectonic motion     

E. Proverbio  V. Quesada 《Journal of Geodesy》1974,48(2):187-212

Summary According to the plate tectonic theory of Le Pichon [1968] we summarized the absolute values of the angular rate of rotation of the Eurasia and America plates determined by astronomical latitude observations. The authors then tried to use the data of longitude observation so far available to emphasize the existence of similar crust movements. The analysis of longitude data has shown the minor homogeneity of these astronomical observations especially as far as the observations obtained by means of PZT are concerned. By using particularly accurate observational data [Torao & Okasahi, 1965, 1969] the data of longitude variations confirm the existence of movements in the earth’s crust, exactly equal to those deduced by the analysis of latitude observations and in agreement with the results of geophysical measurements.  相似文献   

Comparative analysis of continuous radon sensors in underground environments     

R.?Vi?as  A.?Eff-DarwichEmail author  V.?Soler  J.A.?Castro-Almazan  M.?C.?Martín-Luis  J.?Coello  M.?L.?Quesada  J.?de la?Nuez 《Environmental Geology》2004,46(8):1108-1117

Four different continuous radon (²²²Rn)-detection systems have been compared in underground environments, namely three subhorizontal tunnels excavated for groundwater exploitation. Within observational uncertainties, all sensors detected the same concentrations of radon in the air of the tunnels, regardless of the methodology used to measure this radioactive gas. In this sense, the choice of continuous long-term radon monitoring sensors in underground tunnels is constrained by factors such as robustness of the instrumentation, power supply and cost, rather than the sensitivity of the detection methodology. This is particularly important for the monitoring of radon in the context of seismo-volcanic surveillance, where the harsh environmental conditions of the monitoring sites and the absence of electrical power supply are key factors to take into account.  相似文献   

Gravity-enhanced representation of measured geoid undulations using equivalent sources     

Fernando Guspí  Antonio Introcaso  Beatriz Introcaso 《Geophysical Journal International》2004,159(1):1-8

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Generation of overtides and compound tides in Amazon estuary     

Marcos Nicolás Gallo  Susana Beatriz Vinzon 《Ocean Dynamics》2005,55(5-6):441-448

Tidal propagation in estuaries is affected by friction and fresh water discharge, besides changes in the depth and morphology of the channel. Main distortions imply variations in the mean water level and asymmetry. Tidal asymmetry can be important as a mechanism for sediment accumulation and turbidity maximum formation in estuaries, while mean water level changes can affect navigation depths. Data from several gauges stations from the Amazon estuary and the adjacent coast were analyzed and a 2DH hydrodynamic model was configured in a domain covering the continental shelf up to the last section of the river where the tidal signature is observed. Based on data, theoretical and numerical results, the various influences in the generation of estuarine harmonics are presented, including that of fresh water discharge. It is shown that the main overtide, M₄, derived from the most important astronomic component in the Amazon estuary, M₂, is responsible for the tidal wave asymmetry. This harmonic has its maximum amplitude at the mouth, where minimum depths are found, and then decreases while tide propagates inside the estuary. Also, the numerical results show that the discharge does not affect water level asymmetry; however, the Amazon river discharge plays an important role in the behavior of the horizontal tide. The main compound tide in Amazon estuary, Msf, generated from the combination of the M₂ and S₂, can be strong enough to provoke neap low waters lower than spring ones. The results show this component increasing while going upstream in the estuary, reaching a maximum and then slightly decaying.  相似文献   

Rock magnetic and geochemical proxies for iron mineral diagenesis in a tropical lake: Lago Verde, Los Tuxtlas, East–Central Mexico     

Beatriz Ortega  Margarita Caballero  Socorro Lozano  Gloria Vilaclara  Alejandro Rodríguez 《Earth and Planetary Science Letters》2006,250(3-4):444-458

Magnetic and non-magnetic mineral analyses were conducted on a lacustrine sequence from Lago Verde in the tropical coast along the Gulf of Mexico that covers the last 2000 years. The site witnessed the transformation of the environment since the early Olmec societies until forest clearance in the last century. Through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes, and compare this model with the archeological record and inferred climatic changes in the northern hemisphere of tropical America. Volcanic activity has played a major influence on sediment magnetic properties, as a purveyor of Ti-magnetites/Ti-maghemites, and as a factor of instability in the environment. Anoxic reductive conditions are evident in most of Lago Verde's sedimentary record.

Direct observations of magnetic minerals and ratios of geochemical (Fe, Ti), and ferrimagnetic (χf) and paramagnetic (χp) susceptibility (χ) data, are used as parameters for magnetite dissolution (χp/χ, Fe/χf), and precipitation (χf/Ti) of magnetic minerals. Intense volcanic activity and anoxia are recorded before A.D. 20, leading to the formation of framboidal pyrite. Increased erosion, higher evaporation rates, lower lake levels, anoxia and reductive diagenesis in non-sulphidic conditions are inferred for laminated sediments between A.D. 20–850. This deposit matches the period of historical crisis and multiyear droughts that contributed to the collapse of the Maya civilization. Dissolution of magnetite, a high organic content and framboidal pyrite point to anoxic, sulphidic conditions and higher lake levels after A.D. 850. Higher lake levels in Lago Verde broadly coincide with the increased precipitation documented during the Medieval Warm Period (A.D. 950–1350) in the northern tropical and subtropical regions of the American continent. For the Little Ice Age (A.D. 1400–1800), the relatively moist conditions inferred are in concordance with the glacial advances recorded in central Mexico. Higher erosion rates reflect destruction of the rainforest over the last 40 years.  相似文献   

A reappraisal of the structure of the Spanish segment of the Iberian Pyrite Belt     

C. Quesada 《Mineralium Deposita》1997,33(1-2):31-44

The major structural features of the Iberian Pyrite Belt are described in terms of geometry, deformation mechanisms, scale, timing, kinematics and the mutual relationships among the various architectural elements. The result of such an analysis allows this zone to be considered as a S-verging, thin-skinned, fold and thrust belt propagating southwards over a mid-crustal basal detachment. This was the response in the footwall of the suture to the major phase of Hercynian oblique collision between the South Portuguese Plate and the Ossa-Morena Zone of the Iberian Autochthon. This thin-skinned event inverted a previous extensional structure acquired during the initial stages of the collisional process and intimately linked to the formation of the ore deposits that make this region a world-class metallogenic province.  相似文献   

From platform to basin to swell: orbital control on sedimentary sequences in the Oxfordian, Spain     

André Strasser  Marc Aurell  Beatriz Bádenas  Guillermo Meléndez  Sara Tomás 《地学学报》2005,17(5):407-413

Climatic, oceanographic and ecological changes that control the formation and deposition of sediment in shallow and deep depositional environments commonly occur with periodicities of a few 10 000 years. Consequently, in order to interpret sedimentary sequences in the geological past, high time resolution is required. This is best obtained by cyclostratigraphy. Three sections have been studied in the Oxfordian of north-eastern Spain: one represents a shallow, siliciclastic-carbonate platform with repetitive subaerial exposures, one an intraplatform basin with sponge bioherms, and one a swell where iron ooids and glauconite formed. The platform section displays a well-defined stacking pattern of depositional sequences; the deeper-water sections are well dated by ammonites. The correlation between the three sections is a best-fit solution integrating biostratigraphy, sequence stratigraphy and cyclostratigraphy. It is concluded that the small-scale depositional sequences formed in tune with the 100-ka orbital eccentricity cycle. An additional factor was differential subsidence that ruled basin morphology.  相似文献   

Half-Order Stable Boundary-Layer Parametrization Without the Eddy Viscosity Approach for Use in Numerical Weather Prediction     

Richard J. Foreman  Stefan Emeis  Beatriz Canadillas 《Boundary-Layer Meteorology》2015,154(2):207-228

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The nautiloid Family Eothinoceratidae from the Floian of the Central Andean Basin (NW Argentina and South Bolivia)     

Marcela Cichowolski  Beatriz G. Waisfeld  Norberto E. Vaccari  Larisa Marengo 《Geological Journal》2015,50(6):764-782

A single confidently dated species of cephalopod is so far known in the Tremadocian of the southern Central Andean Basin (NW Argentina and southern Bolivia). This species belongs to the Eothinoceratidae and has a strong affinity mainly with Avalonia. During the Floian, a notable increase in diversity took place, with the appearance of a variety of families represented by several genera, in particular, within the Family Eothinoceratidae. In addition to the previously described species from southern Bolivia, we evaluate the other records of that family from the Central Andean Basin, and propose the following new taxa: Saloceras sikus sp. nov., Saloceras quena sp. nov., Mutveiceras gen. nov., and Mutveiceras cienagaensis sp. nov. We also describe Margaritoceras diploide, Margaritoceras sp., and Mutveiceras sp. From a palaeogeographic perspective, the cephalopod fauna shows affinities mainly with those of England, Wales, and the Montagne Noire (cold water Gondwana and peri‐Gondwana). As with other cephalopod faunas of mid to high palaeolatitudes, eothinoceratids occur along with other cephalopods forming assemblages of low morphological diversity. We interpret the forms described here as demersal with a subvertical poise, but capable of making rapid buoyancy changes, living in a wide spectrum of shallow offshore to shoreface settings. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献