Shallow Miocene basaltic magma reservoirs in the Bahia de Los Angeles basin, Baja California, Mexico

The basement in the Bahía de Los Angeles basin consists of Paleozoic metamorphic rocks and Cretaceous granitoids. The Neogene stratigraphy overlying the basement is formed, from the base to the top, by andesitic lava flows and plugs, sandstone and conglomeratic horizons, and Miocene pyroclastic flow units and basaltic flows. Basaltic dikes also intrude the whole section. To further define its structure, a detailed gravimetric survey was conducted across the basin about 1 km north of the Sierra Las Flores. In spite of the rough and lineal topography along the foothills of the Sierra La Libertad, we found no evidence for large-scale faulting. Gravity data indicates that the basin has a maximum depth of 120 m in the Valle Las Tinajas and averages 75 m along the gravimetric profile. High density bodies below the northern part of the Sierra Las Flores and Valle Las Tinajas are interpreted to be part of basaltic dikes. The intrusive body located north of the Sierra Las Flores is 2.5 km wide and its top is about 500 m deep. The lava flows of the top of the Sierra Las Flores, together with the distribution of basaltic activity north of this sierra, suggests that this intrusive body continues for 20 km along a NNW-trending strike. Between the sierras Las Flores and Las Animas, a 0.5-km-wide, 300-m-thick intrusive body is interpreted at a depth of about 100 m. This dike could be part of the basaltic activity of the Cerro Las Tinajas and the small mounds along the foothills of western Sierra Las Animas. The observed local normal faulting in the basin is inferred to be mostly associated with the emplacement of the shallow magma reservoirs below Las Flores and Las Tinajas.     

深海热液喷口金属硫化物中甲烷菌的多样性研究

对两个来自太平洋洋脊胡安·德富卡的两个热液烟囱的金属硫化物4136-2和4148-B1进行甲烷菌甲基辅酶 M 还原酶的编码基因 mcrA 的序列扩增,构建克隆文库并进行分子进化分析.结果表明在这个甲烷富集的热液喷口周围含有丰富的甲烷产生菌,没有任何甲烷氧化菌存在.两个硫化物样品的甲烷产生菌种类完全不同.在4136-2硫化物中的甲烷产生菌都与热液口的高温环境有关系,主要属于甲烷球菌目的甲烷暖球菌(Methanocaldococcus),少部分属于甲烷火菌目甲烷嗜高热菌属的坎氏甲烷嗜高热菌(Methanopyrus kandleri).与这两个属的可分离菌株的氨基酸同源性为89%~97%,核苷酸同源性高达92%~100%.4148-B1硫化物中发现的一类疑似甲基辅酶 M 还原酶的编码序列,它们与已知的甲烷菌 mcrA 序列核苷酸同源性为69%~72%,氨基酸同源性仅为43%~47%.这可能是由于4148-B1来自于正在喷发的超高温热液喷口相关.由于与已发表的甲烷菌克隆子或菌株同源性较低,有可能是热液口特有的以前未发现的甲烷菌     

Comparative study of top soil magnetic susceptibility variation based on some human activities

An investigation of the effect of some human activities on the magnetic susceptibility and frequency dependent susceptibility was conducted on top soil samples from, a commercial area, a motor park and a school environment in Jalingo, Taraba State, N-E Nigeria. The purpose was to assess the variation of magnetic susceptibility with different land use, detect pollution hotspots using magnetic proxy parameters and evaluate the contribution of superparamagnetic (SP) grain size contribution to the magnetic susceptibility from calculation of the frequency dependence of magnetic susceptibility (MS). The results of the mass specific low frequency magnetic susceptibility measurements showed significant enhancement with values ranging from 67.8 − 495.3 × 10⁻⁸ m³kg⁻¹ with a mean value of 191.61 × 10⁻⁸ × m³kg⁻¹ for the Jalingo College of Education (JCOE) data; 520.1 − 1612.8 × 10⁻⁸ m³kg⁻¹ with a mean value of 901.34 × 10⁻⁸ m³kg⁻¹ for the Jalingo main Market (JMM) and 188.5 − 1203.6 × 10⁻⁸m³kg⁻¹ with an average value of 574 92 × 10⁻⁶ m³kg⁻¹ for the Jalingo Motor Park (JMP). The significant magnetic enhancement indicates high concentration of ferrimagnetic minerals in the soil and hence increased pollution. The magnetic susceptibility of the different land use studied decreased in the order commercial area (market) > motor park > school premises. The results of the percentage frequency dependence susceptibility showed that most of the samples had a mixture of SP and coarse multi domain grains or SP grains < 0.05 µm. The value of χ_fd% range from 2.68 to 13.80% with an average value of 8.67% in the JCOE samples, 0.49 to 10.04% with an a-verage of 5.05% in the JMM samples and 0.56 to 13.04% with an average value of 5.86% in the JMP samples.     

Ca. 13 Ma strike-slip deformation in coastal Sonora from a large-scale,en-echelon,brittle-ductile,dextral shear indicator: implications for the evolution of the California rift

The Rancho Nuevo semi-circular structure is a geomorphological structure defined by drainage patterns in coastal Sonora, about 160 km NW of Hermosillo. The structure is about 15 by 30 km, and it is cored by felsic to intermediate plutons (granodiorite, monzogranite, quartz-porphyry) covered by Miocene volcanic rocks. This work is focused on the deformation of the intrusives which cover most of study area.The plutons are a co-magmatic suite dated between 71 ± 1.1 and 67.9 ± 1.0 Ma (U-Pb zircon, LA-ICPMS). The most voluminous unit is a granodiorite characterized by conspicuous sigmoidal fractures at the scale of high resolution satellite images, along which rhyolite dikes were emplaced about 13.2 Ma. Magnetic fabric (AMS) and paleomagnetic data were collected from 27 sites in the granodiorite. Magnetic fabrics are weak but well developed, and are characterized by steep foliation planes with strikes that follow the sigmoidal fracture pattern and suggest NE-SW to NW-SE flattening after emplacement. The characteristic magnetization is of dual polarity, but it is dominantly reverse consistent with emplacement during chron C31r. The prevalent magnetization is southwest and moderately steep negative (ten sites), a discordant direction rotated clockwise about 41 ° ± 11 with respect to the expected Late Cretaceous reference direction, also indicating gentle southward tilt. There is, however, paleomagnetic evidence suggesting that the structure did not rotate as a rigid body, but it deformed internally instead. These data are interpreted to indicate that the Rancho Nuevo semicircular structure is a large-scale, dextral, brittle-ductile shear indicator. The age of the dikes and the fact that they are covered discordantly by rocks assigned to the tuff of San Felipe indicate that northwest, srike-slip, motion of Baja California peninsula (and thus the Pacific plate relative to North America) was accommodated by faults in coastal Sonora about 13 Ma ago.     

Neural estimation of strong ground motion duration

This paper presents and discusses the use of neural networks to determine strong ground motion duration. Accelerometric data recorded in the Mexican cities of Puebla and Oaxaca are used to develop a neural model that predicts this duration in terms of the magnitude, epicenter distance, focal depth, soil characterization and azimuth. According to the above the neural model considers the effect of the seismogenic zone and the contribution of soil type to the duration of strong ground motion. The final scheme permits a direct estimation of the duration since it requires easy-to-obtain variables and does not have restrictive hypothesis. The results presented in this paper indicate that the soft computing alternative, via the neural model, is a reliable recording-based approach to explore and to quantify the effect of seismic and site conditions on duration estimation. An essential and significant aspect of this new model is that, while being extremely simple, it also provides estimates of strong ground motions duration with remarkable accuracy. Additional but important side benefits arising from the model’s simplicity are the natural separation of source, path, and site effects and the accompanying computational efficiency.     

Morphologic analysis of the temporal change of forest cover

A morphologic analysis of forest cover in a time span of twelve years is presented in this work. Two multispectral LANDSAT TM-4 images of the years 1989 and 2001 were used. These images cover a forest area where significant changes have occurred in such years. These images were expanded in terms of canonical-expansion variables that describe the spatial-spectral response of the forest masses. The images were modeled as a vector field of as many dimensions as the number of bands employed in the analysis. A vector field was constructed using canonical-variable bands. The set of canonical variables for each year was used to quantify a vector change of the forest masses. This vector change quantifies the degree of alteration of the forest masses. A region-growth algorithm was used to segment the areas occupied by the forest. This algorithm uses as input the canonical variables. The result of such segmentation is a binary image named the bitmap. From this bitmap, a morphologic analysis of the area occupied by the forest was undertaken. A DEM generated from a radar interferometric pair of the RADARSAT 1 satellite, was used to perform a cross-reference with the bitmaps. This cross-reference leads to the derivation of the elevation of occurrence of changes in the forest masses.     

The role of GPR techniques in determining ice cave properties: Peña Castil ice cave,Picos de Europa

The structure and ice content of ice caves are poorly understood. Ground penetrating radar (GPR) can provide useful insights but has only rarely been applied to ice caves. This paper interprets GPR images (radargrams) in terms of internal structure, stratification, compaction, thickness and volume of the ice block in the Peña Castil ice cave (Central Massif of Picos de Europa, northern Spain), providing the endokarst geometry of the ice cave in GPR data reflections. Eight radargrams were obtained by applying a shielded ground‐coupled antenna with a nominal frequency of 400 MHz. Although the radargrams do not depict the ice–basal bedrock interface, they suggest that the ice block is at least 54 m deep and similarly thick. Some curved reflection signatures suggest a potential vertical displacement in the block of ice, and thus certain dynamics in the ice body. Other images show numerous interbedded clasts and thin sediment layers imaged as banded reflections. In this particular cave a direct visual inspection of the ice stratigraphy is a difficult task but GPR provides clear reflectivity patterns of some of its internal features, making GPR a suitable instrument for this and future studies to achieve a better and broader understanding of the internal behavior of ice caves.     

Concordance degrees in macrozoobenthic monitoring programmes using different sampling methods and taxonomic resolution levels

In summer 1997, an intensive survey on the hard bottoms of the 'Abra de Bilbao' (N. Iberian Peninsula) was carried out in the context of the macrozoobenthic monitoring programmes developed to assess the biological recovery of the area. Three types of measurements (abundance, biomass and cover) were used to describe and compare the structure and composition of the communities at three littoral zones: subtidal, lower intertidal and upper intertidal. In addition, several taxonomic aggregation levels of data were successfully applied. The main objective of this paper is to explore the relative effect caused on the results of such programmes by earlier decisions concerning the type of measurement and the taxonomic resolution level to be applied. A 'second stage' multivariate procedure of analysis has been perforrmed based on the previously obtained sampling site ordinations. The measurement type chosen has been found to have a greater effect on the results than the taxonomic resolution used. Moreover, it is suggested that analyses based on abundance data usually lose more information when taxonomic resolution decreases than those based on biomass or cover estimates. The highest concordance among the different analyses has been found in the subtidal zone, which is considered the most appropriate habitat for the development of benthic monitoring programmes.     

Modelling Thermohaline Properties in an Estuarine Upwelling Ecosystem (R?&#x0301;a de Vigo: NW Spain) Using Box-Jenkins Transfer Function Models

Since 1987, twice weekly, hydrological variables have been monitored at a fixed station in the R?&#x0301;a de Vigo (NW Spain), aiming to examine the time scales of variability and the relationships to meteorological conditions. The present paper analyses: (1) the advantage of Box-Jenkins transfer function (TF) models (single output–multiple input), a type of linear stochastic model, to describe the dynamic behaviour of the system; and (2) the coupling between the R?&#x0301;a and meteorological events at the time scale of autonomy of this coastal inlet affected by the Iberian coastal upwelling, approximately a fortnightly period. In order to achieve these objectives, thermohaline properties have been used to characterize the estuarine ecosystem (output variables), while wind regime, runoff in the drainage basin and incoming solar radiation have been considered as the main forcing variables (input variables). The use of the amplitude time series, derived from principal component analysis (PCA) applied to the deseasonalized meteorological variables, is also explored as a different set of input variables.When compared with standard regression models, all TF models built to describe thermohaline behaviour had reduced residual variance. Similar TF models, as well as percentage of explained variance, were also obtained when meteorological variables or the amplitude time series were used as input variables. The fitted TF models provided an insight into the ‘ inertial ’ behaviour of the system and the time scales of coupling of the system with the forcing variables. The plausible physical mechanisms which link the response of the system with the observed meteorological variability are also discussed. As could be expected, bottom thermohaline properties show a stronger inertial behaviour than the surface ones, which is particularly marked for bottom temperature. Besides, the shelf domain, by means of upwelling-downwelling events, strongly influences surface and bottom temperature, as well as bottom salinity; by contrast, surface salinity is mainly influenced by the effect of wind along the main axis of the R?&#x0301;a and runoff. In relation to the time scales of coupling between the system and the forcing variables, thermohaline properties show a dependance with the meteorological conditions in, at least, the immediately preceding fortnight period. It was concluded that: (1) TF models that incorporate meteorological information described the dynamic behaviour of the system adequately; and (2) this type of model can be useful as a first approximation to develop more sophisticated (deterministic) models, since, with the purpose of modelling any state variable of the system, both the coupling between different domains and the time scales of the interactions must be taken into account.     

Fluage du sel gemme sous très faibles charges

Long-term creep tests have been performed under very small uniaxial loadings ($\sigma =0.02$ to 0.1 MPa). Testing devices were set in an underground gallery, where temperature fluctuations are in the order of one hundredth of a Celsius degree. The mechanical loading was provided by dead weights. The displacements were measured through special sensors whose resolution is $\mathrm{\Delta }?={10}^{\mathrm{?8}}$. Strain rates as small as $\dot{?}={10}^{\mathrm{?12}}{\text{s}}^{\mathrm{?1}}$ were measured. The tests prove that constitutive laws deduced from tests performed on rock salt samples under standard mechanical loadings cannot be extrapolated to very small loadings. To cite this article: P. Bérest et al., C. R. Geoscience 336 (2004).