K-Ar ages of basic rocks in the Patia Valley, southwest Colombia

The Patia Valley situated between the Western and Central Cordilleras of the southwest Colombian Andes contains two areas in which Mesozoic basic and ultrabasic rocks crop out in abundance. Late Cretaceous Diabase Group pillow basalts which make up much of the Western Cordillera are at least 81 ± 5 Ma in the E1 Tambo-E1 Peñol area. 105-97 Ma hornblende and whole-rock dates from the ophiolitic Los Azules complex indicate an Albian age of formation, although many dates are lower (65-62 Ma) owing the low-grade ocean-floor metamorphism. The metamorphic age distribution here supports an end of Cretaceous emplacement for the complex rather than the Early-Mid Cretaceous emplacement suggested for North Colombian ophiolites similarly aligned along the Romeral fault system. Tertiary dacites intruding the Low Azules complex are 36-15 Ma.     

Redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite

Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total ²³²Th was gradually removed during weathering. Although the whole-rock ${}^{228}\text{Th}{}^{232}\text{Th}$ activity ratios were in equilibrium, there were large excesses of ²²⁸Th in the leachable fraction of both parent rock (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 2.06}$) and partially weathered saprolite (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 3–6.5}$), due to alpha recoil and release of daughter ²²⁸Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and ${}^{228}\text{Th}{}^{232}\text{Th}\text{= 1}$, indicating that even the more resistant minerals were attacked.The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and ${}^{234}\text{U}{}^{238}\text{U}$ were in equilibrium in both the whole-rock and leachable fractions. ²³⁰Th was deficient relative to ²³⁴U and ²²⁶Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction.     

The pre-Cretaceous deep-seated tectonics of the Abukuma massif and its place in the structural framework of Japan

The structures and microstructures of the Takanuki and Hitachi areas in the Abukuma massif, Northeast Japan are described. In the Takanuki area, the basic Gosaisho series thrusts the pelitic Takanuki ones in a HP metamorphic context. The nappe structure is afterwards refolded by a migmatitic dome: the Samegawa dome, in a HT metamorphic context. Microtectonic analysis shows that the nappe was transported from south to north along the stretching lineation. Geometric features suggest that the Samegawa dome was emplaced by diapirism. The role of the thrust surface as an instable interface promoting the doming is emphasized. The Hitachi metamorphic rocks composed of basic schist, limestone and sandstone shist thrust the pelitic rocks of the western Hitachi gneisses. As for the Takanuki area, the thrusting occurred in ductile synmetamorphic conditions with a north or northeastward displacement. Owing to lithologic, petrologic, structural similitudes, the nappe of the Hitachi metamorphic rocks and that of the Gosaisho series are unified into a unique nappe with a northward motion. The emplacement occurred between late Permian and late Cretaceous likely in late Jurassic. The allochthonous units of the Abukuma massif are correlated with the Green Schist nappe described in Southwest Japan, since they are surrounded by the same zones, namely the Tanba zone and the Kurosegawa-Kitakami one. Moreover both in Southwest and Northeast Japan, the emplacement of the Green Schist nappes is due to a shear deformation inducing rotational structures along the stretching lineation indicating the same sense of transport, that is eastward in Southwest Japan and northward in Northeast Japan, owing to the late bending of the Japanese Islands. The late Jurassic nappe structure is obliquely overprinted by a HT metamorphism, Ryoke in Southwest Japan, Abukuma in Northeast Japan, and afterwards cut by late faults as the Median Tectonic Line or the Tanakura fault, giving rise to the present complexity.     

An energy approach to sliding of single-span simply supported slab-on-girder steel highway bridges with damaged bearings

This paper investigates the non-linear inelastic seismic response of existing single-span simply supported bridges having bearings which can remain stable and slide after their anchor bolts are ruptured. A simplified equivalent model is developed for the inelastic analysis of these single-span simply supported bridges. Non-linear inelastic time-history analyses are conducted for various acceleration inputs. It is found that narrower bridges with longer spans may have considerable sliding displacements and fall off their supports if adequate seat width is not provided. It is also found that for the same ratio of friction coefficient to peak ground acceleration, the sliding displacement of a structural system is linearly proportional to the amplitude of the peak ground acceleration beyond a certain threshold value. This is also demonstrated analytically from an energy approach point of view. The distribution of the energy content of an earthquake, which is related to its velocity time history, can be an indication of the propensity of an earthquake to cause high sliding displacements. Ground motions with high frequency content or high A_p/V_p ratio may produce smaller sliding displacements than ground motions with relatively lower A_p/V_p ratios.     

Direct and Indirect pCO2 Measurements in a Wide Range of pCO2 and Salinity Values (The Scheldt Estuary)

Recent improvements in both Infra-red spectroscopy and equilibrator techniqueshave allowed to determine, for the first time, pCO₂using simultaneously and continuously both the direct and indirect methods in an estuary where pCO₂ values range from 500 to 8500 atm and salinity from 0 to 30. Our results show that both methods are in excellent agreement in the wholeestuary (r² = 0.999, n = 1075, p < 0.0001). Thus, the NBS (US National Bureau of Standards) scale, although inadequate for seawater samples, is appropriate for estuarine waters and can be applied with confidence to calculate pCO₂.     

Coseismic and post-seismic signatures of the Sumatra 2004 December and 2005 March earthquakes in GRACE satellite gravity

The GRACE satellite mission has been measuring the Earth's gravity field and its temporal variations since 2002 April. Although these variations are mainly due to mass transfer within the geofluid envelops, they also result from mass displacements associated with phenomena including glacial isostatic adjustment and earthquakes. However, these last contributions are difficult to isolate because of the presence of noise and of geofluid signals, and because of GRACE's coarse spatial resolution (>400 km half-wavelength). In this paper, we show that a wavelet analysis on the sphere helps to retrieve earthquake signatures from GRACE geoid products. Using a wavelet analysis of GRACE geoids products, we show that the geoid variations caused by the 2004 December ( M _w= 9.2) and 2005 March ( M _w= 8.7) Sumatra earthquakes can be detected. At GRACE resolution, the 2004 December earthquake produced a strong coseismic decrease of the gravity field in the Andaman Sea, followed by relaxation in the area affected by both the Andaman 2004 and the Nias 2005 earthquakes. We find two characteristic timescales for the relaxation, with a fast variation occurring in the vicinity of the Central Andaman ridge. We discuss our coseismic observations in terms of density changes of crustal and upper-mantle rocks, and of the vertical displacements in the Andaman Sea. We interpret the post-seismic signal in terms of the viscoelastic response of the Earth's mantle. The transient component of the relaxation may indicate the presence of hot, viscous material beneath the active Central Andaman Basin.     

Reconstructing spatio-temporal patterns of debris-flow activity using dendrogeomorphological methods

Debris flows are a major threat in many parts of the Alps, where they repeatedly cause severe damage to infrastructure and transportation corridors or even loss of life. Nonetheless, the spatial behavior of past debris-flow activity and the analysis of areas affected during particular events have been widely neglected in reconstructions so far. It was therefore the purpose of this study to reconstruct spatio-temporal patterns of past debris flows on a forested cone in the Swiss Alps (Bruchji torrent, Blatten, Valais). The analysis of past events was based on a detailed geomorphic map (1:1000) of all forms related to debris flows as well as on tree-ring series from 401 heavily affected trees (Larix decidua Mill. and Picea abies (L.) Karst.) growing in or next to deposits. The samples were analyzed and growth disturbances related to debris-flow activity assessed, such as tangential rows of traumatic resin ducts, the onset of reaction wood or abrupt growth suppression or release.In total, 960 growth disturbances were identified in the samples, belonging to 40 different event years between A.D. 1867 and 2005. In addition, the coupling of tree-ring data with the geomorphic map allowed reconstruction of eleven formerly active channels and spatial representation of individual events. Based on our results we believe that before 1935, debris flows preferentially used those channels located in the western part of the cone, whereas the eastern part of the cone remained widely unaffected. The spatial representation of the 40 events also allowed identification of five different spatial patterns for debris flows at the study site.     

Response of littoral chironomid communities and organic matter to late glacial lake—level,vegetation and climate changes at Lago dell’Accesa (Tuscany,Italy)

This study focuses on the response of lacustrine littoral chironomid communities to late glacial changes in limnological, environmental and climate conditions in the Mediterranean context. Late glacial chironomid (Diptera: Chironomidae) assemblages, organic petrography and geochemistry were analysed in a sediment core from the littoral zone of Lago dell’Accesa (Tuscany, Italy), where the lake-level fluctuations and the vegetation history have been previously reconstructed. Comparison of the chironomid stratigraphy to other proxies (pollen assemblages, organic petrography and geochemistry, lake-level) and regional climate reconstruction suggested the predominant influence of lake-level changes on the littoral chironomid fauna. The main lowering events that occurred during the Oldest and the Younger Dryas were followed by higher proportions of taxa typical of littoral habitats. A complementary study of organic matter suggested the indirect impact of lake-level on the chironomids through changes in humic status and habitat characteristics, such as the type of substrate and aquatic macrophyte development. Several chironomid taxa, such as Glyptotendipes, Microtendipes and Cricotopus type patens, were identified as possible indicators of low lake-level in the late glacial records. Nevertheless, this study suggested that parallel analyses of organic matter and chironomid assemblages may be needed to circumvent misinterpretation of littoral chironomid assemblage stratigraphy. There was a weak response of the chironomid assemblages to small lake-level lowerings that corresponded to the Older Dryas and Preboreal oscillations. A higher level of determination, e.g. to the species group level, may be necessary to increase the sensibility of the indicators to lake-level changes.