Influence of variable uncertainties in seismic tomography models on constraining mantle viscosity from geoid observations

The radial viscosity structure of the Earth is explored on the basis of the geoid observations. The variations of uncertainty in seismic tomography models are accounted for when finding the radial viscosity structure. The new methodology we propose attempts to fit more closely those features of the geoid that are better constrained by tomography models and avoids to fit those features that are poorly constrained. This approach is particularly important because the error of geoid predictions caused by uncertainties in seismic tomography models is overwhelmingly larger than the noise in the geoid measurements. The synthetic tests indicate that the viscosity structures obtained by disregarding the uncertainty variations in seismic tomography models can be biased depending on the geoid spectral band and on the ‘input’ seismic tomography model. When the uncertainty variations in seismic models are considered in the inversion process, results do not indicate a viscosity in the transition zone lower than in the upper mantle. A robust feature found with the new method is a viscosity in the upper mantle two orders of magnitude smaller than in the lower mantle. The error covariance of seismic tomography models is critical for the method we suggest. A covariance matrix rigorously derived by seismologists should help to even more reliably infer the viscosity structure and relation between anomalies in density and seismic velocities from surface observations such as the geoid, and thus lead to a better knowledge of the Earth interior.     

Progressive development of s-type flanking folds in simple shear

Flanking structures are deflections of planar or linear fabric elements in a rock alongside a crosscutting element (CE), e.g. a vein or fault. This study provides new results from analogue experiments, which test and extend recent numerical models of flanking structures. A linear viscous matrix material (PDMS) was deformed in a ring shear rig that allows continuous observation to large values of shear strain. Rotational behaviour, offset and deflection of marker lines around a predefined, lubricated CE were monitored for different initial orientations of the fault with respect to the shear zone boundary, and the results were compared with numerical results and natural examples. At high initial angles to the shear zone boundary (>135°), a structure previously described as an ‘s-type flanking fold’ develops. During progressive deformation, an initially straight marker line passing through the centre of the CE is offset in a sense synthetic with the bulk sense of shear and shows a shortening displacement across the CE. Simultaneously, this central marker line is deflected and forms symmetrical folds, which are convex in the direction of shear along the CE (i.e. normal drag). Both offset and deflection of the marker lines decrease towards the tips of the fault. Natural examples of s-type flanking folds, directly comparable with the model results, are more common than is generally appreciated.     

Zur Stabilität von Margarit im System CaO-Al2O3-SiO2-H2O

An increasing number of occurrences of margarite have been reported in the last years. However, previous experimental investigations in the system CaO-Al₂O₃-SiO₂-H₂O are limited to the synthesis of margarite and to the upper stability limit according to the reaction (1) 1 margarite?1 anorthite +1 corundum +1 H₂O (Chatterjee, 1971; Velde, 1971). Since margarite often occurs together with quartz, the upper stability limit of margarite in the presence of quartz is of special interest. Therefore, the reactions (5) 1 margarite +1 quartz ?1anorthite +1 kyanite/andalusite +1 H₂O and (6) 4 margarite+3 quartz ? 2 zoisite+5 kyanite+3 H₂O were investigated experimentally using mixtures of natural margarite (from Chester, Mass., USA), quartz, kyanite, andalusite, zoisite, and synthetic anorthite. The indicated equilibrium temperatures at water pressures equal to total pressure are: 515± 25°C at 4 kb, 545 ±15°C at 5 kb, 590±10°C at 7 kb, and 650±10°C at 9 kb for reaction (5), and 651±11°C at 10 kb, 648 ± 8°C at 12.5kb, and 643±13°C at 15kb for reaction (6), respectively. Besides this, additional brackets for equilibrium temperatures were determined for the above cited reaction (1): 520±10°C at 3 kb, 580±10°C at 5 kb, and 640± 20°C at 7 kb. On the basis of these experimentally determined reactions (1), (5), and (6) and of the reactions (3) 2 zoisite +1 kyanite? 4 anorthite +1 corundum +1 H₂O (7) 2 zoisite +1 kyanite +1 quartz ? 4 anorthite +1 H₂O and (10) 1 pyrophyllite ? 1 andalusite/kyanite+3 quartz+1 H₂O for which experimental or, in the case of reaction (3), calculated data were already available, a pressure-temperature diagram with 3 invariant points and 11 univariant reactions was developed using the method of Schreinemakers. This diagram, summarizing both experimental and phase relation studies, allows conclusions about the conditions under which margarite has been formed in nature. Margarite is limited to low grade metamorphism at water pressures up to approximately 3.5 kb; in the presence of quartz, margarite is even limited to low grade metamorphism at water pressures up to 5.5 kb. Only at water pressures higher than the values stated before margarite, and margarite+quartz, respectively, can occur in medium grade metamorphism (as defined by Winkler, 1970 and 1973). For the combined occurrence of margarite+quartz and staurolite as reported by Harder (1956) and Frey (personal communication, 1973) it may be estimated that water pressure has been greater than approximately 5.5 kb, wheras temperature has been in the range from 550 to 650°C. Furthermore, the present study shows that the assemblage zoisite+kyanite (+ H₂O) is an indicator of both pressure [P _{H 2 O}> approximately 9kb]and temperature [T> approximately 640 to 650° Cat water Pressures up to 15 kb].     

Holocene ice dynamics and bottom-water formation associated with Cape Darnley polynya activity recorded in Burton Basin,East Antarctica

A multi-proxy study including sedimentological, mineralogical, biogeochemical and micropaleontological methods was conducted on sediment core PS69/849-2 retrieved from Burton Basin, MacRobertson Shelf, East Antarctica. The goal of this study was to depict the deglacial and Holocene environmental history of the MacRobertson Land–Prydz Bay region. A special focus was put on the timing of ice-sheet retreat and the variability of bottom-water formation due to sea ice formation through the Holocene. Results from site PS69/849-2 provide the first paleo-environmental record of Holocene variations in bottom-water production probably associated to the Cape Darnley polynya, which is the second largest polynya in the Antarctic. Methods included end-member modeling of laser-derived high-resolution grain size data to reconstruct the depositional regimes and bottom-water activity. The provenance of current-derived and ice-transported material was reconstructed using clay-mineral and heavy-mineral analysis. Conclusions on biogenic production were drawn by determination of biogenic opal and total organic carbon. It was found that the ice shelf front started to retreat from the site around 12.8 ka BP. This coincides with results from other records in Prydz Bay and suggests warming during the early Holocene optimum next to global sea level rise as the main trigger. Ice-rafted debris was then supplied to the site until 5.5 cal. ka BP, when Holocene global sea level rise stabilized and glacial isostatic rebound on MacRobertson Land commenced. Throughout the Holocene, three episodes of enhanced bottom-water activity probably due to elevated brine rejection in Cape Darnley polynya occured between 11.5 and 9 cal. ka BP, 5.6 and 4.5 cal. ka BP and since 1.5 cal. ka BP. These periods are related to shifts from warmer to cooler conditions at the end of Holocene warm periods, in particular the early Holocene optimum, the mid-Holocene warm period and at the beginning of the neoglacial. In contrast, between 7.7 and 6.7 cal. ka BP, brine rejection shut down, maybe owed to warm conditions and pronounced open-water intervals.