On the origin of rock fragment mulches on Vertisols: A case study from the Ethiopian highlands

Many Vertisols in Tigray, Ethiopia, typically carry a discontinuous rock fragment (RF, size 0.5–> 40 · 10^− 2 m) cover with 10 to 100 RFs m^− 2. Such RF mulches are of agricultural and environmental significance because they influence the water balance in the underlying soils and the crop yield. Natural RF concentrations are mostly considered as eolian or hydraulic lag deposits, or as the result of lateral transport over the soil surface from a rock outcrop, upslope. In cultivated areas RF mulches can develop by tillage.This paper presents the case of a natural RF mulch whose lithology indicates that the RFs are up-squeezed by the local Vertisol. The study site is located in the pass of Enda Maryam, Tigray, Northern Ethiopia (39°8′ E and 13°36′ N). A circular area of 10 m diameter, about 200 m away from the water divide in the valley has been cleared annually between 01/1999 and 05/2003. During this period, 625 RFs, 17 being > 7.5 · 10^− 2 m in size, totalling a mass of nearly 62 kg, have been collected. After correction for measurement procedures, the rate of RF up-warping by the Vertisol at Enda Maryam is assessed at 5 RFs m^− 2 in 3 years. At this rate of appearance, the formation of current RF concentrations on top of active valley Vertisols is only a matter of 10^1–2 years, provided the availability of RFs below the soil surface.Although important underground displacements were measured in the Vertisol between 01/1999 and 05/2002, the supposed link between up-squeezing of RFs and plastic deformations of ‘chimney’, ‘diapir’ or ‘intrusion’-like type in the Vertisol could not be evidenced. Instead, RFs are clearly concentrated on the soil surface as well as in depth, along the existing vertical desiccation cracks, often > 1 m deep which display polygonal configurations at the soil surface. Further, bundles of slickensides containing some RFs, have been mapped at the base of the Vertisol. The slickenside configuration suggests that the RF-bearing substrate is being scraped off.While the underground displacement of RFs along active slickensides seems normal, the process of RFs ascending in ‘upright’ position in the edge of desiccation cracks needs explanation. The closure of a desiccation crack is a peristaltic-like movement, following ascent or descent of the capillary fringe. It is hypothesized that this movement gradually pushes the RF to the surface or to another place or level in the soil profile where the crack closes in last instance.The apparent young age of the valley Vertisol mulches in Ethiopia might indicate the very recent formation of yearly recurrent desiccation cracks of Vertisols in the area. Available information confirms that most valleys in the study area used to be perennially marshy. Under these conditions no movements of RFs in the soil profile are expected to occur. Gullying, leading to pronounced seasonal desiccation of the Vertisols, started in several cases not more than 50 years ago.     

Review of the neotectonics of the Eastern Turkish–Armenian Plateau by geomorphic analysis of digital elevation model imagery

Digital elevation model (DEM) images provide synoptic views of the Earth’s surface allowing the analysis of landforms of still active tectonic and volcanic structures at regional scale. A DEM at 250 m pixel size constitutes regional scale data particularly efficient to investigate the late Miocene–Quaternary deformation of the Eastern Turkish–Armenian Plateau in the Arabian–Eurasian area of convergence. Geomorphic analysis of the DEM image associated with review of fault-plane solutions of earthquakes show that faults are mostly strike-slip with small vertical component. Here we show that the orientations of the tectonic and volcanic structures fit with a tectonic regime characterized by N–S shortening and E–W lengthening, consistent with westward escape of Anatolia perpendicular to the direction of the Arabia–Eurasia shortening. The uniform uplift of the plateau, the predominance of strike-slip faulting, the lack of major thrusts and the occurrence of normal faults do not support a model of going-on crustal thickening due to intracontinental convergence. On the contrary, our observations can be better interpreted in terms of lithospheric thinning and mantle upwelling related to gravity escape of Anatolia.     

Early Estimation of Seismic Hazard for Strong Earthquakes in Taiwan

A shakemap system providing rapid estimates of strong ground shaking could be useful for emergency response providers in a damaging earthquake. A hybrid procedure, which combines site-dependent ground motion prediction models and the limited observations of the Real-Time Digital stream output system (RTD system operated by Central Weather Bureau, CWB), was set up to provide a high-resolution shakemap in a near-real-time manner after damaging earthquakes in Taiwan. One of the main factors that affect the result of ground motion prediction analysis is the existence of site effects. The purpose of this paper is to investigate the local site effects and their influence in the ground shaking and then establish an early estimation procedure of potential hazard for damaging earthquakes. Based on the attenuation law, the site effects of each TSMIP station are discussed in terms of a bias function that is site and intensity-level dependent function. The standard deviation of the site-dependent ground motion prediction model can be significantly reduced. The nonlinear behavior of ground soil is automatically taken into account in the intensity-level dependent bias function. Both the PGA and the spectral acceleration are studied in this study. Based on the RTD data, event correctors are calculated and applied to precisely estimate the shakemap of damaging earthquakes for emergency response.     

Analysis of Post-Failure Slope Movements within the Framework of Hazard and Risk Analysis

The paper first recalls briefly a methodological framework to assess landslide hazard and risk analysis in terms of predisposition, triggering andrevealing factors. This framework, that reflects the mechanisms involved in the landslide, is based on the Geotechnical Characterisation of slope movements proposed by Vaunat et al. (1994) and Leroueil et al. (1996). The Geotechnical Characterisation can be schematized by a 3-D matrix having the following axes: types of movement; types ofmaterial; and the four stages of movement: pre-failure, failure, post-failure andreactivation. For each relevant element of this 3-D matrix, there is a characterisationsheet including: the controlling laws and parameters, the predisposition factors, thetriggering or aggravating factors, the revealing factors and the consequences of the movement. The paper focuses afterwards on the post-failure stage, which generallyis the most destructive, and on the mobility index. It is shown that this laterindex can be described as the product of sub-indices associated with failure, brittlenessof the material, ability of the soil to develop pore pressures, geometry of the moving soil mass and characteristics of the terrain. It is also shown how these aspectscan be incorporated into the Geotechnical characterisation of slope movements. This seems to provide a rational basis for examining slope movements at the post-failure stage and assessing associated risks.     

An Early Cretaceous extinct spreading center in the northern Natal valley

We have identified an extinct E–W spreading center in the northern Natal valley on the basis of magnetic anomalies which was active from chron M11 (133 Ma) to 125.3 Ma, just before chron M2 (124 Ma) in the Early Cretaceous. Seafloor spreading in the northern Natal valley accounts for approximately 170 km of north–south motion between the Mozambique Ridge and Africa. This extension resolves the predicted overlap of the continental (central and southern) Mozambique Ridge and Antarctica in the chron M2 to M11 reconstructions from Mesozoic finite rotation parameters for Africa and Antarctica. In addition, the magnetic data reveal that the Mozambique Ridge was an independent microplate from at least 133 to 125 Ma. The northern Natal valley extinct spreading center connects to the spreading center separating the Mozambique Basin and the Riiser-Larsen Sea to the east. It follows that the northern Mozambique Ridge was either formed after the emplacement of the surrounding oceanic crust or it is the product of a very robust spreading center. To the west the extinct spreading center connects to the spreading center separating the southern Natal valley and Georgia Basin via a transform fault. Prior to chron M11, there is still a problem with the overlap of Mozambique Ridge if it is assumed to be fixed with respect to either the African or Antarctic plates. Some of the overlap can be accounted for by Jurassic deformation of the Mozambique Ridge, Mozambique Basin, and Dronning Maud land. It appears though that the Mozambique Ridge was an independent microplate from the breakup of Gondwana, 160 Ma, until it became part of the African plate, 125 Ma.     

Numerical Calculation of Equivalent Cell Permeability Tensors for General Quadrilateral Control Volumes

A new method for upscaling fine scale permeability fields to general quadrilateral-shaped coarse cells is presented. The procedure, referred to as the conforming scale up method, applies a triangle-based finite element technique, capable of accurately resolving both the coarse cell geometry and the subgrid heterogeneity, to the solution of the local fine scale problem. An appropriate averaging of this solution provides the equivalent permeability tensor for the coarse scale quadrilateral cell. The general level of accuracy of the technique is demonstrated through application to a number of flow problems. The real strength of the conforming scale up method is demonstrated when the method is applied in conjunction with a flow-based gridding technique. In this case, the approach is shown to provide results that are significantly more accurate than those obtained using standard techniques.     

Where the buffalo roam: Boundaries and the politics of scale in the yellowstone region

For more than twenty years, the state of Montana has pursued a controversial policy of lethal control to manage winter migrations of bison from Yellowstone National Park. In the late 1990s, as the state and a number of federal agencies attempted to cooperatively outline a more palatable scenario for bison management, local environmental groups became active participants in the debate. This paper examines the strategies pursued by these organizations in the course of their attempt to construct a new, ecosystem-level territorial space and jurisdictional scale for bison, and places their struggle within the context of existing work in geography on the politics of scale. Ultimately, the failure of their efforts illustrates some of the important constraints faced by social groups that attempt to create new territorial regimes and new scales of resistance in the interstices of existing political structures.     

Fitness-for-Purpose of Reference Material Reference Values in Relation to Traceability of Measurement, as Illustrated by USGS BCR-1, NIST SRM 610 and IAEA NBS28

As in all fields of sample analysis, reference materials play a large role in supporting measurements in the geosciences. While a rather large number of materials are in distribution (> 380), not all are equally effective or fit-for-purpose in supporting laboratory data quality and thereby assuring the desired comparability of measurements between laboratories. Equally important, reference values that are not fit-for-purpose cannot be used effectively to establish traceability links between laboratory measurements and national and international standards. The needed fitness-for-purpose is not achieved for reference values either when more than one reference value has been proposed and a consensus does not exist among users as to which should be used by all, or when reference value uncertainties are too large in comparison to those of routine laboratory measurements. The focus of this review will be, first to outline the current reality, and second to suggest ways in which certifications of RMs can be improved to provide reference values that are universally accepted and more fit-for-purpose in general laboratory use. The discussion will be illustrated largely by current uses of USGS BCR-1, NIST SRM 610 and IAEA NBS28, as these three materials are those for which the largest body of newly published data exists, according to recent bibliographies of the geoanalytical literature published annually in Geostandards Newsletter: The Journal of Geostandards and Geoanalysis.     

Eocene paleomagnetism of the Caucasus (southwest Georgia): oroclinal bending in the Arabian syntaxis

The Caucasus is very important for our understanding of tectonic evolution of the Alpine belt, but only a few reliable paleomagnetic results were reported from this region so far. We studied a collection of more than 300 samples of middle Eocene volcanics and volcano-sedimentary rocks from 10 localities in the Adjaro–Trialet tectonic zone (ATZ) in the western part of the Caucasus. Stepwise thermal demagnetization isolates a characteristic remanent magnetization (ChRM) in 19 sites out of 31 studied. ChRM reversed directions prevail, and a few vectors of normal polarity are antipodal to the reversed ones after tilt correction. The fold test is positive too, and we consider the ChRM primary. Analysis of Tertiary declinations and strikes of Alpine folds in the Adjaro–Trialet zone and the Pontides in Northern Turkey shows a large data scatter; Late Cretaceous data from the same region, however, reveal good correlation between paleomagnetic and structural data. Combining Late Cretaceous and Tertiary data indicates oroclinal bending of the Alpine structures which are locally complicated with different deformation. The overall mean Tertiary inclination is slightly shallower than the reference Eurasian inclination recalculated from one apparent polar wander path (APWP), but agrees with other. This finding is in accord with geological evidence on moderate post-Eocene shortening across the Caucasus. We did not find any indication of long-lived paleomagnetic anomalies, such as to Cenozoic anomalously shallow inclinations further to the east in Central Asia.     

Sediment yield and storage variations in the Négron River catchment (south western Parisian Basin,France) during the Holocene period

In the Négron River catchment area (162 km²), surface‐sediment stores are composed of periglacial calcareous ‘grèze’ (5 × 10⁶ t) and loess (21 × 10⁶ t), and Holocene alluvium (12·6 × 10⁶ t), peat (0·6 × 10⁶ t) and colluvium (18·5 × 10⁶ t). Seventy‐five per cent of the Holocene sediments is stored along the thalwegs. Present net sediment yield, calculated from solid discharge at the Négron outlet, is low (0·6 t km^?2 a^?1) due to the dominance of carbonate rocks in the catchment. Mean sediment yield during the Holocene period is 7·0 t km^?2 a^?1 from alluvium stores and 7·6 t km^?2 a^?1 from colluvium stores. Thus, the gross sediment yield during the Holocene period is about 18·7 t km^?2 a^?1 and the sediment delivery ratio 3 per cent. The yield considerably varies from one sub‐basin to another (3·9 to 24·5 t km^?2 a^?1) according to lithology: about 25 per cent and 50 per cent of initial stores of periglacial grèze and loess respectively were reworked during the Holocene period. Sediment yield has increased by a factor of 6 in the last 1000 years, due to the development of agriculture. The very high rate of sediment storage on the slope during that period (88 per cent of the yield) can be accounted for by the formation of cultivation steps (‘rideaux’). It is predicted that the current destruction of these steps will result in a sediment wave reaching the valley floors in the coming decades. Subboreal and Subatlantic sediments and pollen assemblages in the Taligny marsh, where one‐third of the alluvium is stored, show the predominant influence of human activity during these periods in the Négron catchment. Copyright © 2002 John Wiley & Sons, Ltd.