Le Pico de Orizaba (Mexique): Structure et evolution d’un grand volcan andesitique complexe

Volcan Pico de Orizaba, which marks the eastern end of the Trans-Mexican Volcanic Belt, is one of the largest andesitic composite volcanoes in America. It is located above a series of crustal distensive faults making the boundary of the Coast Plains of the Gulf of Mexico from theAltiplano. For this reason, the volcano shows an asymmetry: from the west, its elevation is about 3,000 m whereas on the eastern side it reaches 4,000 to 4,500 m from its base. The Pico de Orizaba is composed of a primitive stratovolcano raised by a recent summit cone. It has been built by three very distinct volcanic and magmatic phases.

1. The first one, probably discontinuous effusive activity, lasted more than one million years. It is mainly composed of two pyroxenes-andesites with scarce associated basaltic and dacitic lava-flows. Amphibole is an accessory mineral in most differentiated lavas. On the eastern flank, numerous massive and autobrecciated lava-flows pass outward into thick conglomeratic formations. This effusive phase has built a primitive central volcano and a parasitic cone: the Sierra Negra.
2. The second phase is of short duration — about 100,000 years or less — in comparison with the first period. It seems that this period began with the formation of a caldera followed by the extrusion of amphibole dacite domes and the overflow of viscous silica-rich (andesite to dacite) lava flows on the northern flank. An intense explosive activity develops:pelean nuées ardentes are associated with extrusion of the domes; numerous plinian eruptions leading to widespread dacitic pumiceous air-falls are produced by both the central and the adventive volcanoes. This sequence of events is interpreted as the progressive emptying of a superficial chamber containing differenciated magma. A rhyolite flow erupted during this phase.
3. The age of the recent phase is better defined. It started 13,000 years B.P. with the eruption of a dacitic ash-flow containing pumice and scoria-bombs. This was such an intense event that products were found 30 km S.E. of the summit, erasing the top of the former volcano and creating a large crater (4–5 km wide). The present cone, of 1,400–1,500 m elevation, grew in this crater. During a period of 7,000 to 8,000 years, the new stratovolcano experienced various important pyroclastic eruptions with a cycle of the order of 1,000 to 1,500 years. The pyroclastic flows (ash, pumice, and bombs) associated with air-fall deposits are of Saint-Vincent type. They present an heterogeneous dacitic and andesitic magma. The dacitic component is similar to previous differenciated materials. On the other hand, the andesitic magma appears somewhat similar to lava-flows from morphologically young cones erupted outside the central vent system. This eruptive cycle can be interpreted as the result of reoccurring injections of deep basic magma within the crustal chamber. For the last 5,000 years the activity of the modern Pico de Orizaba has again been essentially effusive (andesites) with periodic plinian eruptions.

     

边坡开挖破坏过程的离心模型试验研究

边坡开挖是工程中经常遇到的问题。在清华大学土工离心机上,开发了一种离心机运转过程中的边坡开挖模拟技术。进行了土坡开挖的离心模型试验,测量了开挖过程中边坡的位移场变化。结果表明,边坡的破坏过程是从坡体下部开始的剪切带逐渐向上发展贯通的过程,并经历了滑裂面贯通前的平缓变形和贯通后变形迅速增大两个阶段。坡顶裂缝最初均是由受拉引起的,但随着剪切带的不断向上发展,裂缝的扩展是由拉伸和剪切两种作用综合引起的。     

Using Quantitative Textural Analysis to Understand the Emplacement of Shallow-Level Rhyolitic Laccoliths--a Case Study from the Halle Volcanic Complex, Germany

In qualitatively homogeneous magmatic bodies, quantitative texturalanalysis—such as crystal size distribution, modal abundance,and spatial distribution pattern analyses—allows theirinternal heterogeneity to be measured and interpreted. In thisstudy, these methods are applied to samples from a 300 m drillcore through one of the porphyritic rhyolitic laccoliths (Petersbergunit) of the     

Paleoenvironmental analysis of a middle Wisconsinan biota site, southwestern Virginia, U.S.A.

The Ratcliff Site in southwestern Virginia lies in a small second-order stream valley filled with approximately 3.5 m of organic-rich deposits that contain bones of mammoth, mastodon, deer (or antelope), logs, and plant macrofossils. Radiocarbon analyses indicate the age of the organic-rich sediment ranges from > 44,000 to 29,100 ¹⁴C yr BP, a time period with no fossil remains reported in this region of the Appalachians. Analyses of field observations, textural data, organic carbon content, and plant macrofossils indicate that the organic-rich sediments contain interbedded standing-water and debris-flow deposits. Up to 6 m of oxidized debris-flow sediments bury the organic-rich sediments. The presence of Rubus parviflorus (Thimble Berry) throughout the deposit indicates the site had a boreal environment from > 44,000 to 29,100 ¹⁴C yr BP. Plant macrofossil evidence indicates the uplands had stands of spruce/jack-pine forests while the valley contained ponds and associated wetlands. Three debris flows occurred at the site between approximately 38,000 and 29,000 ¹⁴C yr BP, suggesting a recurrence interval for major storms of approximately 3000 yr, even though the apparent stability of the boreal environment implies a climate not conducive to catastrophic rainstorms. This conflicting combination of features suggests that during the middle Wisconsinan this area experienced generally cool climates, dominated by polar air masses, but was punctuated by relatively brief warm periods marked by incursions of tropical air masses.     

Behaviour of brick infill walls strengthened with expanded steel plates

The present study was dedicated to investigate an effective, economical and convenient method for strengthening brick infill walls, i.e. the use of externally-bonded expanded steel plates. A total of 30 plastered wall assemblies were tested under monotonic diagonal loading to investigate the contribution of these plates to the load capacities, ductilities and rigidities of brick walls. The plate thickness, bolt spacing and location of the plates in the wall were the parameters of the test program. The experiments indicated that application of the plates on the outermost fine plaster layer significantly improves the diagonal load–deflection behaviour of infill walls and reducing the bolt spacing provides the strengthened walls with higher load capacity values. An analytical expression was developed for estimating the diagonal load capacities of plated brick walls. The analytical load estimates were found to be in close agreement with the experimental values.     

Geochemical Characteristics of Gold Polymetallic Deposits as Exemplified by the Laozuoshan Deposit,Heilong jiang,China

From the studies of ore deposit geologic settings,sulfur isotopes,lead isotopes,carbon isotopes and oxygen isotopes,fluid inclusions and petrochemistry in this paper,the authors have drawn a conclusion that the ore-forming hydrothermal solutions are the high-temperature magmatic hydrothermal solutions for the gold ore deposit,and at the same time,the involvemety of crustal materials can not be ruled out .It is the first time that the authors have proposed that the Laozuoshan gold-ploymetallic ore deposit in Heilongjiang Province was formed in the calc-alkaline series environment at the margin of an active continent.     

Distribution characteristics of Geo-hazards in Ganxi Valley after the Wenchuan earthquake

A devastating earthquake of Ms 8.0 struck the Wenchuan area on May 12, 2008. The quake triggered numerous landslides and avalanches in Ganxi Valley. Aerial photo interpretation indicates that the landslides and avalanches, presently in a stable state, are concentrated in 91 subregions of the valley, occupying 10.42 km², about 29% of the valley area. Some laws have been found on the gradient, aspect, elevation, distance of rupture, and formation lithology of geo-hazards in Ganxi Valley: (1) They are mainly situated on slopes between 30° and 50° and increase in number with increasing slope; (2) Failures are mainly in a southerly direction; and most occurred between 1,000 and 1,600 m, especially between 1,000 and 1,400 m; (3) Most are within 2,000 m of the Beichuan fault or within 1,000 m of other major faults; (4) most failures occurred from the T₁f + t formation in the study area. The geo-hazards status quo of Ganxi Valley provides foundation for the scheme of hazard reducing in the future.     

Natural and anthropogenic aquatic environmental changes reconstructed by paleolimnological analyses in Lake Kitaura,central Japan

In order to assess the recent anthropogenic environmental changes in Lake Kitaura, central Japan, changes during the past few centuries were reconstructed from results of radiometric and tephrochlonological age determination, magnetic susceptibility measurements, total organic carbon analyses, total nitrogen analyses and fossil diatom analyses on a sediment core from the lake. A total of six major and sub-zones are recognized according to the diatom fossil assemblages, and we discuss aquatic environmental change in Lake Kitaura mainly based on these diatom assemblage change. Zone Ia and Zone Ib (older than AD 1707) are marine to brackish. In Zone IIa (AD␣1707–AD 1836), most of the brackish diatoms disappeared, and were replaced by freshwater species indicating a decrease in salinity. We interpret the salinity decrease in Zone I–IIa as a sea-level fall during the Little Ice Age. The salinity of the lake decreased to near freshwater conditions in Zone IIb (AD 1836–AD 1970), which could arise from alteration in River Tone or development of a sandspit in the mouth of River Tone in addition to sea-level change. In Zone IIIa (AD 1970–AD 1987), the diatom assemblage indicates a freshwater environment, and sedimentation rates increase rapidly. These changes reflect sedimentary environment change and an ecosystem transition due to the construction of the tide gate. In Zone IIIb (AD 1987–AD 2002), the diatom flux (valves cm⁻² y⁻¹) increased and species composition changed. The changes in Zone IIIb show a good agreement with limnological monitoring data gathered from the lake. These paleolimnological data suggest that the recent human-induced changes of the aquatic environment of the lake after the 1970s exceed rates during the period concerned in this study.     

Assessing groundwater storage changes in the Nubian aquifer using GRACE data

Gravity Recovery and Climate Experiment (GRACE) level two (L2) data is used in estimating the groundwater storage changes (GWSC) in the Nubian Sandstone Aquifer System (NSAS). This set of data consists of spherical harmonics coefficients with specific degree and order. The GRACE data is de-correlated using a sixth degree polynomial in order to reduce the effect of the noise error resulting from the correlation between the spherical harmonics coefficients with the same degree parity. The GRACE estimates of GWSC are smoothed using Gaussian filter with half width of 1000 km. This half width is chosen in order to maximize the correlation between the GRACE estimates of GWSC and previous modeling results of the NSAS. The loss in groundwater storage occurring in each of the four countries sharing the NSAS is calculated to assess the sustainability of using the NSAS as a water resource in each country. The overarching finding in this study is that NSAS is losing its groundwater storage at a very high rate. Also, it is found that Egypt is the fastest in losing its groundwater storage from the NSAS. This loss of groundwater storage in Egypt may not necessarily be resulting from in-country extractions because of the trans-boundary nature of this aquifer. The GRACE-based estimates are found to be close to available data and previous modeling results of the NSAS.     

Linking multi-temporal analysis and community consultation to evaluate the response to the impact of Hurricane Stan in coffee areas of Chiapas,Mexico

This paper analyses the responses related to land use of coffee growers in Chiapas, Mexico to the impact of Hurricane Stan (October 2005). A multi-temporal analysis of the effect on land cover was performed through the combination of unsupervised classification of SPOT multispectral images and visual interpretation of panchromatic images (8 months previous to the hurricane, and 2, 14, and 40 months after the hurricane). The information provided by this geographic analysis was interpreted in light of information gathered though household surveys. Although the hurricane wrecked havoc across the region, the main impact in the study area was in the riparian zones where the extent of the loss experienced in terms of coffee harvest and soil was such that, even 14 months after the event, households with land in those areas were struggling to recover. Nevertheless, after 40 months, the zones that had suffered total soil loss began to support soil and vegetation, indicating the possibility of replanting coffee in those areas. Although the hurricane occurred when the coffee sector was particularly fragile as a result of the preceding several years of poor prices, the impact did not trigger extensive land use change. The surveys showed, however, that people are now more informed of the risk of living and farming on the river margins and are now performing soil conservation practices and planting trees to reduce risk.