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31.
J. Fernández-Turiel C. Pérez-Miranda G. Almada M. Medina C. Riviere M. Gordillo 《Environmental Geology》2003,43(8):941-949
Spatial and temporal variations of Na, Mg, Si, K, Ca, SO42-, Cl-, HCO3-, Ag, Al, As, Au, B, Ba, Be, Bi, Br, Ce, Cd, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Ga, Ge, Gd, Hf, Hg, Ho, I, La, Li, Mn, Mo, Nb, Nd, Ni, P, Pb, Pr, Rb, Sb, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, U, V, W, Y, Yb, Zn, and Zr were monitored through 37 sampling stations to determine the main aspects influencing the surface-water quality in the Salí River watershed (Tucumán Province, NW Argentina). The influence of the regional geological setting on water chemistry allows to distinguish three sub-basins. The interaction with sedimentary rocks was found to be dominant in the northern and central-eastern sub-basin as well as in the southern sub-basin, whereas the metamorphic-granitic basement of the Pampean Ranges was noted in the central sub-basin. In addition, anthropogenic activities affect the spatial variation of K, P, Mn, Rb, and Pb as well as dissolved oxygen concentrations and Eh. Temporal water-quality variation is related to the spatial distribution of precipitation and to the seasonal character of the main local industries (sugar cane, alcohol, citrus), increasing P and K concentrations and decreasing dissolved oxygen concentration and Eh in winter. Cl-, Na, SO42-, Al, As, B, Fe, Mn, Se and U concentrations exceed the regulated drinking-water thresholds at several sampling stations. 相似文献
32.
Franco Biondi Ignacio Galindo Estrada Juan Carlos Gavilanes Ruiz Alejandro Elizalde Torres 《Quaternary Research》2003,59(3):293-299
The impact of volcanic eruptions on forest ecosystems can be investigated using dendrochronological records. While long-range effects are usually mediated by decreased air temperatures, resulting in frost rings or reduced maximum latewood density, local effects include abrupt suppression of radial growth, occasionally followed by greater than normal growth rates. Annual rings in Mexican mountain pine (Pinus hartwegii Lindl.) on Nevado de Colima, at the western end of the Mexican Neovolcanic Belt, indicate extremely low growth in 1913 and 1914, following the January 1913 Plinian eruption of Volcán de Fuego, 7.7 km to the south. That event, which is listed among the largest explosive eruptions since A.D. 1500, produced ashflow deposits up to 40 m thick and blanketed our study area on Nevado de Colima with a tephra fallout 15–30 cm deep. Radial growth reduction in 1913–14 was ≥30% in 73% of the sampled trees. We geostatistically investigated the ecological impact of the eruption by mapping the decrease in xylem increment and found no evidence of a spatial structure in growth reduction. Little information has been available to date on forest species as biological archives of past environments in the North American tropics, yet this historical case study suggests that treeline tropical sites hold valuable records of prehistoric phenomena, including volcanic eruptions. 相似文献
33.
C. Fernández M. A. Gaertner C. Gallardo M. Castro 《Meteorology and Atmospheric Physics》1997,62(3-4):179-200
Summary The numerical simulation of a long-lived, stationary mesoscale convective system (MCS) already described in a previous paper (Fernández et al., 1995) is analyzed in greater detail. The influence of various external forcings, such as sea surface temperature, local orography or terrain roughness, upon the characteristics of the system is studied. This analysis makes it possible not only to identify the most important factors, but also to deduce the importance of some other internal forcings and to propose explanations for some dynamic features of the system that were difficult to understand. Hence, the sensitivity test methodology applied seems to be a useful tool to clarify the complex dynamics of some moist convective events. In the modelled MCS, sea surface temperature and orography are identified as key factors. The results also indicate that the upstream triggering of convection provoked by an orographic blocking effect is the main cause of the development of the system, while upslope triggering plays a secondary role.With 18 Figures 相似文献
34.
Martin Connors Alan R. Hildebrand Mark Pilkington Carlos Ortiz-Aleman Rene E. Chavez Jaime Urrutia-Fucugauchi Eduardo Graniel-Castro Alfredo Camara-Zi Juan Vasquez John F. Halpenny 《Geophysical Journal International》1996,127(3):F11-F14
The buried Chicxulub impact structure is marked by a dramatic ring of sinkholes (called cenotes if containing water), and adjacent less prominent partial rings, which have been shown to coincide with maxima in horizontal gravity gradients and a topographic depression. These observations, along with the discreteness and spacing of the features, suggest a formation mechanism involving faulting in the outer slump zone of the crater, which would thus have a diameter of approximately 180 km.
An opposing view, based primarily on the interpretation of gravity data, is that (he crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.
No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels tbe shelf edge of eastern Yucatan. 相似文献
An opposing view, based primarily on the interpretation of gravity data, is that (he crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.
No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels tbe shelf edge of eastern Yucatan. 相似文献
35.
Luiz Carlos Ruiz Pessenda Adauto de Souza Ribeiro Ramon Aravena José Albertino Bendassolli 《Quaternary Research》2004,62(2):183-193
The study place is in the Barreirinhas region, Maranhão State, northeastern Brazil. A vegetation transect of 78 km was studied among four vegetation types: Restinga (coastal vegetation), Cerrado (woody savanna), Cerradão (dense woody savanna), and Forest, as well as three forested sites around Lagoa do Caçó, located approximately 10 km of the transect. Soil profiles in this transect were sampled for δ13C analysis, as well as buried charcoal fragments were used for 14C dating. The data interpretation indicated that approximately between 15,000 and ∼9000 14C yr B.P., arboreal vegetation prevailed in the whole transect, probably due to the presence of a humid climate. Approximately between ∼9000 and 4000-3000 14C yr B.P., there was the expansion of the savanna, probably related to the presence of drier climate. From ∼4000-3000 14C yr B.P. to the present, the results indicated an increase in the arboreal density in the area, due to the return to a more humid and probably similar climate to the present. The presence of buried charcoal fragments in several soil depths suggested the occurrence of palaeofires during the Holocene. The vegetation dynamic inferred in this study for northeastern Brazil is in agreement with the results obtained in areas of Amazon region, based on pollen analysis of lake sediments and carbon isotope analysis of soil organic matter (SOM), implying than similar climatic conditions have affected these areas during the late Pleistocene until the present. 相似文献
36.
The relative importance of the contribution of the lower crust and of the lithospheric mantle to the total strength of the continental lithosphere is assessed systematically for realistic ranges of layer thickness, composition, and temperature. Results are presented as relative strength maps, giving the ratio of the lower crust to upper mantle contribution in terms of crustal thickness and surface heat flow. The lithosphere shows a “jelly sandwich” rheological layering for low surface heat flow, thin to average crustal thickness, and felsic or wet mafic lower crustal compositions. On the other hand, most of the total strength resides in the seismogenic crust in regions of high surface heat flow, crust of any thickness, and dry mafic lower crustal composition. 相似文献
37.
Carlos Marquardt Alain Lavenu Luc Ortlieb Estanislao Godoy Diana Comte 《Tectonophysics》2004,394(3-4):193-219
Neotectonic observations allow a new interpretation of the recent tectonic behaviour of the outer fore arc in the Caldera area, northern Chile (27°S). Two periods of deformation are distinguished, based on large-scale Neogene to Quaternary features of the westernmost part of the Coastal Cordillera: Late Miocene to Early Pliocene deformations, characterized by a weak NE–SW to E–W extension is followed by uppermost Pliocene NW–SE to E–W compression. The Middle Pleistocene to Recent time is characterized by vertical uplift and NW–SE extension. These deformations provide clear indications of the occurrence of moderate to large earthquakes. Microseismic observations, however, indicate a lack of shallow crustal seismicity in coastal zone. We propose that both long-term brittle deformation and uplift are linked to the subduction seismic cycle. 相似文献
38.
Tortonian calcarenites of the Betic Cordillera were deposited in coastal or very shallow marine environments and represent an ideal marker for estimating vertical movements from the late Miocene to the Present. A map showing the heights at which these Tortonian marine rocks are situated has a clear correlation with the present relief, indicating that today's relief has been formed since the Tortonian. There is also a good correlation between present relief and the Bouguer anomaly distribution in the Betic Cordillera, as well as with crustal thickness. Likewise, the present relief is directly related to the geodynamic setting of a horizontal N–S to NNW–SSE compression and an almost perpendicular extension, along with isostatic readjustment, existing in the Betic Cordillera from the Tortonian. As a result of these regional stresses, faults and folds have produced notable vertical movements. The highest rates of uplift of the Betic Cordillera coincide with large antiforms, in particular those of the Sierra Nevada and the Sierra Filabres. Several subsiding sectors also exist (for example, the Granada Basin or the Guadalquivir Basin). The foreland Guadalquivir Basin has a complex history because the uplift in its eastern sector and subsidence in the western sector coexisted during the late Tortonian. Today the whole Betic Cordillera is characterized by differential regional uplift, even in the aforementioned subsiding sectors. 相似文献
39.
40.
The kinematic evolution of the Barinas–Apure Basin and the southern Mérida Andes from Lower Miocene to the Present is numerically modelled using flexural isostatic theory and geophysical and geological data. Two published regional transects are used to build up a reference section, which is then used to constrain important parameters (e.g. shortenings and sedimentary thicknesses) for the flexural modelling. To control the location of the main fault system in the flexural model earthquake information is also used. The estimated flexural elastic thickness of the South American lithosphere beneath the Barinas–Apure Basin and the Mérida Andes Range is 25 km. The value for the final total shortening is 60 km. The flexural isostatic model shows that the Andean uplift has caused the South American lithosphere subsidence and the development of the Barinas–Apure Basin.In addition, gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory. Consequently, the best-fit flexural model is used to build a gravity model across the Mérida Andes and the Barinas–Apure Basin preserving the best-controlled structural features from the flexural modelling (e.g. basin wavelength and depth) and slightly changing the main bodies density values and deep crustal structures. The final gravity model is intended to be representative of the major features affecting the gravity field in the study area. The predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the Maracaibo crust over the South American Shield. This process would use the Conrad discontinuity as a main detachment surface within an incipient NW dipping continental subduction. 相似文献