Observations of vertical ground accelerations exceeding gravity during the 1997 Umbria-Marche (central Italy) earthquakes

We found extensive evidence that the vertical ground accelerations produced during the largest shock (M = 6.0) of the 1997 Umbria-Marche earthquake sequence exceeded 1g in two areas close to the heavily-damaged villages of Annifo and Colle Croce. This evidence comes from the striking observation of thousands of freshly fractured and broken rocks and stones in these areas. Some of the broken stones lie isolated on soft detritic soil while others had been previously piled up, probably a long time agoto clear the fields for farming. The freshness of the cuts and fractures and the consistency of the observations for thousands of rocks and stones in these areas indicate that these rocks were thrown upwards during the earthquake, with breakage occurring at the time of impact. Ground motion calculations consistent with the static deformation inferred from GPS and interferometry data, show that the broken stones and rocks are found in the zone where the strongest shaking took place during the earthquake and that most of the shaking there was vertical.     

Water relation characteristics ofAlhagi sparsifolia and consequences for a sustainable management

Water relation characteristics of the desert legumeAlhagi sparsifolia were investigated during the vegetation period from April to September 1999 in the foreland of Qira oasis at the southern fringe of the Taklamakan Desert, Xinjiang Uygur Autonomous Region of China. The seasonal variation of predawn water potentials and of diurnal water potential indicated thatAlhagi plants were well water supplied over the entire vegetation period. Decreasing values in the summer months were probably attributed to increasing temperatures and irradiation and therefore a higher evapotranspirative demand. Data from pressure-volume analysis confirmed thatAlhagi plants were not drought stressed and xylem sap flow measurements indicated thatAlhagi plants used large amounts of water during the summer months. Flood irrigation had no influence on water relations inAlhagi probably becauseAlhagi plants produced only few fine roots in the upper soil layers. The data indicate thatAlhagi sparsifolia is a drought-avoiding species that utilizes ground water by a deep roots system, which is the key characteristic to adjust the hyper-arid environment. Because growth and survival ofAlhagi depends on ground water supply, it is important that variations of ground water depth are kept to a minimum. The study will provide a theoretical basis for the restoration and management of natural vegetation around oasis in arid regions.

     

Linking Tarim Basin sea retreat (west China) and Asian aridification in the late Eocene

The Tarim Basin in western China formed the easternmost margin of a shallow epicontinental sea that extended across Eurasia and was well connected to the western Tethys during the Paleogene. Climate modelling studies suggest that the westward retreat of this sea from Central Asia may have been as important as the Tibetan Plateau uplift in forcing aridification and monsoon intensification in the Asian continental interior due to the redistribution of the land‐sea thermal contrast. However, testing of this hypothesis is hindered by poor constraints on the timing and precise palaeogeographic dynamics of the retreat. Here, we present an improved integrated bio‐ and magnetostratigraphic chronological framework of the previously studied marine to continental transition in the southwest Tarim Basin along the Pamir and West Kunlun Shan, allowing us to better constrain its timing, cause and palaeoenvironmental impact. The sea retreat is assigned a latest Lutetian–earliest Bartonian age (ca. 41 Ma; correlation of the last marine sediments to calcareous nannofossil Zone CP14 and correlation of the first continental red beds to the base of magnetochron C18r). Higher up in the continental deposits, a major hiatus includes the Eocene–Oligocene transition (ca. 34 Ma). This suggests the Tarim Basin was hydrologically connected to the Tethyan marine Realm until at least the earliest Oligocene and had not yet been closed by uplift of the Pamir–Kunlun orogenic system. The westward sea retreat at ca. 41 Ma and the disconformity at the Eocene–Oligocene transition are both time‐equivalent with reported Asian aridification steps, suggesting that, consistent with climate modelling results, the sea acted as an important moisture source for the Asian continental interior.     

An evaluation of CM patterns for grain-size studies of fine grained sediments

Grain-size data of Boom clay (Belgium) are represented on CM, AM, LM diagrams as suggested by Passega. They show a typical ‘uniform suspension’ pattern. It is argued that a ‘uniform suspension’ pattern can be due simply to the clay-rich nature of the sediments. As these sediments are not deposited by a single transport mechanism, the ‘uniform suspension’pattern does not necessarily indicate a specific transport mechanism.     

Kenntnis der Randstrukturen des Witwatersrand-Beckens im nördlichen Oranje-Freistaat-Goldfeld (Südafrika)

Zusammenfassung Eine Analyse der Randstrukturen des Witwatersrand-Beckens im nördlichen Oranje-Freistaat-Goldfeld zeigt, daß das tektonische Bild der Beckenrandregion nacheinander durch verschiedene Deformationsakte von unterschiedlichem Charakter geprägt worden ist. Auf eine Einengungsphase von spät-Witwatersrand-Alter, die zur Bildung einer randlichen Aufrichtungszone und zum Aufreißen eines Systems von 'normalen und antivergenten Aufschiebungen führte, folgte während des Unteren Ventersdorp eine jüngere Zerrungsphase, bei der es zu einem staffeiförmlgen Zerbrechen der ursprünglich eingeengten Scholle kam. Die Zerrung hängt sehr wahrscheinlich mit dem Zerbrechen der Beckenrandregion während oder unmittelbar nach der Extrusion der Unteren Ventersdorp-Laven zusammen. Dabei wurde ein schmaler randparalleler Streifen der Beckenfüllung in einem Graben versenkt ('Odendaalsrus-Graben). Die abgesunkenen Schollenteile sind in sich noch stark zerbrochen und gewöhnlich schiefgestellt. Da der versenkte Streifen durch ein primäres Einmuldungsstadium hindurchgegangen ist, deutet die Grabenbildung offenbar eine Fortsetzung der Beckenbildungsvorgänge 'mit anderen Mitteln an. — Die Bewegungen haben ein Alter von etwa 2,1 Mrd. Jahren; sie fallen somit ins frühe Präkambrium und sind keinesfalls 'intraalgonkisch, wie es noch bis in die jüngste Vergangenheit angenommen wurde.Tektonische Experimente haben ergeben, daß sich die wichtigsten Störungssysteme der realen Beckenrandstrukturen bei entsprechend gewählter Versuchsanordnung auch künstlich erzeugen lassen.

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The Orange Free State goldfield represents the southernmost part of the Witwatersrand Basin (Fig. 1) which is completely buried under a cover of Ventersdorp and Karroo rocks. The pre-Karroo geology of this area is characterized by a major rift system trending SSW-NNE ('Odendaalsrus graben), followed to the east by a V-shaped horst and two minor rift blocks (Fig. 2). The main graben is bounded by two principal faults (Border Fault, De Bron Fault, Fig. 4) which are roughly parallel to the western rim of the basin.A tectonic analysis of the basin-edge structures in the northern part of the graben reveals that the basin rim has been subjected to a sequence of deformational acts working over a considerable period of time. A primary compression of the basin-edge region in Upper Witwatersrand times resulted in the formation of a marginal fold; since sedimentation continued, the different stages of the folding process were sometimes preserved by a set of minor unconformities within the youngest sediments (Elsburg A Reefs, Fig. 5). With increasing lateral pressure a couple of reverse faults developed to support the folding (faults No. 10 and 11, Figs. 6 and 7); the minor thrust faults No. 19 and 21 represent the second component of this fault system. A reconstruction of the original depositional plain of the Lower Agglomerate shows that the main thrust faults (No. 10 and 11) are definitely older than the Elsburg A 1 Reef and have been obviously revived after the younger sediments had been laid down (Fig. 8 a).In a later stage compression gave way to tensional forces bringing about a fracturing of the overturned limb of the marginal fold along normal faults (faults No. 4 and 7, Figs. 6 and 7). There is reason to believe that the tensional phase was associated with the incipient rifting during early Ventersdorp times and that these faults were more or less contemporaneous with the general tilting of the basin-edge as displayed to-day (cf. Figs. 3 and 4). Because of the westerly tilt the Boulder Beds dip towards the west, although they must have been originally deposited on a ± horizontal or slightly basinward dipping plain. Faulting was mostly accomplished before deposition of the Ventersdorp sediments and the Upper Ventersdorp Lavas took place; the latter are seldom or only to a small degree affected by the fractures (Figs. 3 and 4). — The age of the tectonics is about 2,1×10⁹ years, i. e. early Precambrian and not Algonkian as formerly supposed.Experimental work aiming at an imitation of the observed basin-edge structures has shown that the principal tectonic features can be produced artificially (Figs. 8 a and b). This refers in particular to the main system of thrust-faults as well as to the younger step-faults caused by tension.

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Résumé Les structures tectoniques au bord du Bassin de Witwatersrand dans le district de mines d'or septentrional de la République d'Orange (Afrique du Sud) ont été analysées. Il s'est montré qu'elles sont déterminées par l'action de plusieurs phases de déformation successives de caractères différents. Une phase de compression d'âge Witwatersrand supérieur a d'abord amené un redressement des couches dans une zone marginale du bassin, avec chevauchements à vergences «normales» et inverses. Elle était suivie pendant le Ventersdorp inférieur d'une phase de traction qui produisit des cassures en gradins dans le secteur primitivement comprimé. La traction est très vraisemblablement en relation avec l'effondrement de la zone marginale du bassin pendant ou peu après l'extrusion des laves inférieures de Ventersdorp. Une bande étroite de sédiments, parallèle au bord du bassin, fut alors affectée d'un affaissement (formation du «graben d'Odendaalsrus») où de nombreuses cassures à l'intérieur des compartiments affaissés ont résulté dans la formation de blocs plus ou moins inclinés. Puisque cette zone a d'abord passé par un stade synclinal, on a l'impression que l'effondrement du graben ne représente que la reprise de l'affaissement général du bassin à l'aide d'une «technique nouvelle». - Les mouvements ont un âge de 2.1 milliards d'années, ils datent par conséquent du Précambrien inférieur et ne sont point «intra-algonkiques» comme il fut encore admis tout récemment.Il a été possible de reproduire artificiellement les systèmes de failles les plus importants de la zone marginale du bassin, si les conditions de l'expériment étaient favorables.

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Kornerupine in Mg-Al-rich gneisses from Namaqualand,South Africa: mineralogy and evidence for late-metamorphic fluid activity

Three kornerupine occurrences are reported in distinctive SiO₂-poor, MgO- and Al₂O₃-rich paragneisses from the Namaqualand Metamorphic Complex in South Africa. Kornerupine coexists stably with phlogopite, cordierite, orthopyroxene, gedrite, sapphirine, sillimanite and plagioclase and, in sapphirine-free rocks, with spinel and corundum. Tourmaline of a texturally older generation than kornerupine is commonly present in the same samples.Ten analysed kornerupines show a variation in total Fe as FeO from 1.8 to 10.9 weight per cent. B₂O₃ contents are estimated from x-ray data and a few spectrochemical analyses to range from 0.9 to 3.5 weight per cent. There is a strong inverse correlation between B³⁺ and Al³⁺. Total iron content has a strong and systematic effect on refractive index, colour and dispersion. Fe and Mg are systematically partitioned with the other minerals, and Mg/(Mg+Fe) ratios increase as follows: spinel 相似文献   

Reaching critical mass? Theory, politics, and the culture of debate in children's geographies

A small but growing number of voices have begun to raise questions about the current direction of children's geographies as a subfield and its status within the wider discipline. This article intervenes in these emerging discussions to examine the status of debate itself within children's geographies. I argue that children's geographies over the past decade has operated primarily in a consensus-based mode, with a number of potential tensions and differences between practitioners masked as a result. I develop the example of notions of children's competent social agency, a core theoretical assumption that is rarely interrogated in much depth. In closing, I pose questions regarding several contemporary political issues concerning children's agency about which geographers have had surprisingly little to say. I suggest that explicitly addressing some of these vexing issues would contribute to a richer state of debate within children's geographies.