The magnitude, symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites

Summary. Seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals. The symmetry and magnitude of anisotropy depend upon: (1) the volume percentages of the minerals constituting the upper mantle, (2) the degree and symmetry of preferred orientation of each mineral and (3) the alignment of the minerals' crystallographic axes relative to one another. The nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle. Petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the Samail ophiolite, Oman, the Troodos ophiolite, Cyprus, the Bay of Islands ophiolife, Newfoundland, the Twin Sisters ultramafic, Washington, USA, the Dun Mountain ophiolite, New Zealand, the Red Hills ophiolite, New Zealand and the Red Mountain ophiolite, New Zealand. The compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent, in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle. The symmetry varies from orthorhombic to axial, with the axial symmetry axis corresponding to the olivine a-axes maxima and subparallel to spreading directions in oceanic upper mantle. Pyroxene a -, b - and c -axes maxima generally parallel olivine b -, c - and a -axes, respectively, and anisotropy decreases with increasing pyroxene content. Shear-wave splitting is predicted for all propagation directions within the upper mantle. Symmetry is also orthorhombic or axial, with the minimum difference in velocity between the two shear-waves parallel to the maximum compressional wave velocity.     

Generalized Shifted-Factor Analysis Method for Multivariate Geo-Referenced Data

Multivariate data with spatial dependencies arise in many areas of application, including geology, precision agriculture, and ecology. For analysis of such data, a methodology based on a generalized shifted-factor model is developed. The model incorporates potential lagged dependencies between factors and observed variables, representing asymmetric spatial dependencies observed in practice. Identification and estimation issues are discussed. A prediction procedure that exploits both the multivariate and spatial dependence in the data is proposed and illustrated.     

Correlation between ground failure and soil conditions in Adapazari, Turkey

Ground failure in Adapazari, Turkey during the 1999 Kocaeli earthquake (M_w=7.4) was severe. In four central downtown districts, where more than 1200 buildings collapsed or were heavily damaged, hundreds of structures tilted and penetrated into the ground due in part to liquefaction and ground softening. Based on a multi-institutional subsurface investigation program, soil conditions along four lines in which ground failure was surveyed after the earthquake are classified into four generalized subsurface site categories. This classification is primarily based on the presence or absence of shallow and intermediate depth liquefiable soils. Observations of ground failure are found to correlate well with site categories that are susceptible to liquefaction according to current state-of-the-art methods without strict adherence to the Chinese criteria. Soils that liquefied were found to meet the liquid limit and liquidity index conditions of the Chinese criteria. However, soils that liquefied did not typically meet the clay-size condition for liquefiable soils by the Chinese criteria.     

Factors controlling stratal pattern and facies distribution of fluvio‐lacustrine sedimentation in the Sivas mini‐basins,Oligocene (Turkey)

The Sivas Basin, located in the Central Anatolian Plateau of Turkey, is a foreland basin that records a complex interaction between sedimentation, salt tectonics and regional shortening during the Oligo‐Miocene leading to the formation of numerous mini‐basins. The Oligocene sedimentary infill of the mini‐basins consists of a thick continental succession, the Karayün Formation, comprising a vertical succession of three main sub‐environments: (i) playa‐lake, (ii) fluvial braided, and (iii) saline lacustrine. These sub‐environments are seen as forming a large Distributive Fluvial System (DFS) modified through time as a function of sediment supply and accommodation related to regional changes in climate and tectonic regime. Within neighbouring mini‐basins and despite a similar vertical stratigraphic succession, subtle variations in facies assemblages and thickness are observed in stratigraphic units of equivalent age, thus demonstrating the local control exerted by halokinesis. Stratigraphic and stratal patterns reveal in great detail the complex interaction between salt tectonics and sedimentation including different types of halokinetic structures such as hooks, wedges and halokinetic folds. The regional variations of accommodation/sediment supply led to coeval changes in the architectural patterns recorded in the mini‐basins. The type of accommodation regime produces several changes in the sedimentary record: (i) a regime dominated by regional accommodation limits the impact of halokinesis, which is recorded as very small variations in stratigraphic thickness and facies distribution within and between mini‐basins; (ii) a regime dominated by localized salt‐induced accommodation linked to the subsidence of each individual mini‐basin enhances the facies heterogeneity within the DFS, causing sharp changes in stratigraphic thickness and facies assemblages within and between mini‐basins.     

Potential and actual trace gas fluxes in Arctic terrestrial ecosystems

This paper provides an overview of results obtained through a number of studies of actual and potential trace gas exchanges in Eurasian and Greenlandic tundra ecosystems. The chief findings include:
i) Long-term accumulation rates of carbon in organic tundra soils, i.e. net uptake of atmospheric CO₂, are strongly controlled by simple climatic parameters (mean July temperature, annual precipitation). Warmer and wetter conditions stimulate carbon sequestration rates in Arctic terrestrial ecosystems.
ii) The release of carbon through ecosystem respiration is also heavily influenced by climate. However, the release of dead organic soil carbon as CO₂ is constraind by the lability of the stored organic compounds. This lability decreases significantly with depth (i.e. age) of the soils; moreover, this in turn decreases the temperature sensitivity of the decomposition process.
iii) Methane emissions from typical tundra habitats in northern Eurasia are slightly lower than from seemingly similar habitats in North America although this difference probably can be attributed to the colder climatic setting of the studied sites compared with the general climatic conditions at the North American sites. There is a strong linkage between CO₂ exchange, CH₄ formation and emission rates in some wet tundra ecosystems.
iv) Atmospheric uptake of CH₄ occurs in some dry and mesic tundra habitats and there are indications that these uptake rates could be affected negatively by atmospheric nitrogen deposition. Emissions of N₂O are rarely seen fromArctic soils but there appear to be a strong potential for denitrification and, hence, N₂O release. This might be due to high rates of denitrification during the spring thaw and possibly associated significant releases of N₂O in this period.