Vertical-axis rotations determined from paleomagnetism of Mesozoic and Cenozoic strata within the Bolivian Andes

Thermal demagnetization and principal component analysis allowed determination of characteristic remanent magnetization (ChRM) directions from 256 sites at 22 localities in Mesozoic and Cenozoic sedimentary strata of the Bolivian Altiplano and Eastern Cordillera. An inclination-only fold test of site-mean ChRM directions from Cenozoic units (principally the Santa Lucía Formation) indicates optimum unfolding at 97.1% unfolding, consistent with a primary origin for the ChRM. For Mesozoic strata, optimum unfolding occurred at 89.2%, perhaps indicating secondary remagnetization at some locations. For Cenozoic units, comparison of locality-mean directions with expected paleomagnetic directions indicates vertical-axis rotations from 33° counterclockwise to 24° clockwise. Euler pole analysis of along-strike variation in crustal shortening within the Subandean and Interandean zones indicates 18° clockwise rotation south of the axis of curvature of the Bolivian Andes and 6° counterclockwise rotation northwest of the axis during the past 10 m.y. Along-strike variation of shortening within the Eastern Cordillera indicates 8° clockwise rotation south of the axis and 8° counterclockwise rotation northwest of the axis from 35 to 10 Ma. These vertical-axis rotations produced by along-strike variations in crustal shortening during development of the Bolivian fold-thrust belt agree well with observed rotations determined from paleomagnetism of Cenozoic rocks in the Eastern Cordillera and in the Subandean and Interandean zones. However, local rotations are required to account for complex rotations in the Cochabamba Basin and within the Altiplano. The curvature of the Bolivian Andes has been progressively enhanced during Cenozoic fold-thrust belt deformation

The structure and segmentation of the Southeast Indian Ridge

The Southeast Indian Ridge (SEIR) spreads at a relatively narrow range of intermediate rates (59–75 km/Ma) but exhibits the full range of slow to fast spreading morphology and segmentation. Satellite gravity data reveal transitions in the structure of the spreading center where it is influenced by the Amsterdam and Kerguelen hotspots and at the Australian– Antarctic Discordance (AAD). Although the spreading rate between the hotspots and the AAD is nearly constant, the ridge exhibits a variety of distinct styles of morphology and segmentation not observed at fast or slow spreading centers. Recently, collected multibeam bathymetry data reveal a transition from East Pacific Rise style overlapping axial highs near 92°E to Mid-Atlantic Ridge style axial valleys with non-transform offsets near 116°E. The intervening segmentation is characterized by propagating offsets coexisting with stationary transforms, which exhibit different degrees of temporal stability. Currently, there are 10 transform offsets between the hotspots and the AAD but only five of these have persisted since seafloor spreading stabilized at 35 Ma. The other five appear to have formed since 35 Ma and several more have disappeared by transform shortening or coalesced by along-axis propagation. There is a transition from monotonic offset propagation near the hotspots to oscillatory propagation approaching the AAD. This change in offset stability corresponds to transitions in depth, axial morphology and offset structure. Through much of the transitional region, higher order segmentation is characterized by en-echelon offsets of a diffuse spreading axis that generally lacks a well-defined neovolcanic zone. Since the spreading rate is nearly constant, the regional variation in axial morphology and segmentation appears to be controlled by an upper mantle thermal gradient - possibly a result of flux of asthenosphere from the hotspots to the AAD. This is consistent with the gradual increase in average ridge flank depths along this part of the plate boundary but segment scale changes in axial depth reveal spatio-temporal variability in the dynamic topography that are not preserved on older lithosphere. Intrasegment transitions in axial morphology and en-echelon offsets within first order segments suggest that local variations in mantle thermal structure introduce short-lived instabilities in higher order segmentation and dominate the short term evolution of the plate boundary

Tertiary remagnetization of Paleozoic rocks from the Eastern Cordillera and sub-Andean Belt of Bolivia

Paleomagnetic samples were collected from 98 sedimentary horizons in eight different Devonian to Permian sedimentary units at eight localities in the Eastern Cordillera and the sub-Andean Belt of Bolivia. For 77 sites, thermal demagnetization allowed determination of a characteristic magnetization (ChRM) with site-mean 95% confidence limit, α95, ≤15°. The ChRM is carried predominantly or entirely by hematite. Fold and reversal tests from two of the sampled localities indicate that the characteristic magnetization is synfolding, likely acquired during the earliest stages of deformation. Additionally, a modified conglomerate test at one locality and the nearly uniform direction of ChRM across the Devonian to Permian age units clearly reveals the secondary nature of the characteristic magnetization. Finally, the ChRM directions are discordant from any expected Paleozoic directions. Paleomagnetic poles calculated from the ChRM directions fall near the Cenozoic portion of the apparent polar wander path for South America. We interpret these observations to indicate widespread chemical remagnetization of these Paleozoic strata during, but prior to completion of, Cenozoic Andean folding

Geostatistical radar-raingauge combination with nonparametric correlograms: methodological considerations and application in Switzerland

Modelling spatial covariance is an essential part of all geostatistical methods. Traditionally, parametric semivariogram models are fit from available data. More recently, it has been suggested to use nonparametric correlograms obtained from spatially complete data fields. Here, both estimation techniques are compared. Nonparametric correlograms are shown to have a substantial negative bias. Nonetheless, when combined with the sample variance of the spatial field under consideration, they yield an estimate of the semivariogram that is unbiased for small lag distances. This justifies the use of this estimation technique in geostatistical applications. Various formulations of geostatistical combination (Kriging) methods are used here for the construction of hourly precipitation grids for Switzerland based on data from a sparse realtime network of raingauges and from a spatially complete radar composite. Two variants of Ordinary Kriging (OK) are used to interpolate the sparse gauge observations. In both OK variants, the radar data are only used to determine the semivariogram model. One variant relies on a traditional parametric semivariogram estimate, whereas the other variant uses the nonparametric correlogram. The variants are tested for three cases and the impact of the semivariogram model on the Kriging prediction is illustrated. For the three test cases, the method using nonparametric correlograms performs equally well or better than the traditional method, and at the same time offers great practical advantages. Furthermore, two variants of Kriging with external drift (KED) are tested, both of which use the radar data to estimate nonparametric correlograms, and as the external drift variable. The first KED variant has been used previously for geostatistical radar-raingauge merging in Catalonia (Spain). The second variant is newly proposed here and is an extension of the first. Both variants are evaluated for the three test cases as well as an extended evaluation period. It is found that both methods yield merged fields of better quality than the original radar field or fields obtained by OK of gauge data. The newly suggested KED formulation is shown to be beneficial, in particular in mountainous regions where the quality of the Swiss radar composite is comparatively low. An analysis of the Kriging variances shows that none of the methods tested here provides a satisfactory uncertainty estimate. A suitable variable transformation is expected to improve this

A Dust-Penetrated Classification Scheme for Bars as Inferred from their Gravitational Force Fields

The division of galaxies into ``barred'' (SB) and ``normal'' (S) spirals is a fundamental aspect of the Hubble galaxy classification system. This ``tuning fork'' view was revised by de Vaucouleurs, whose classification volume recognized apparent ``bar strength'' (SA, SAB, SB) as a continuous property of galaxies called the ``family''. However, the SA, SAB, and SB families are purely visual judgments that can have little bearing on the actual bar strength in a given galaxy. Until very recently, published bar judgments were based exclusively on blue light images, where internal extinction or star formation can either mask a bar completely or give the false impression of a bar in a nonbarred galaxy. Near-infrared camera arrays, which principally trace the old stellar populations in both normal and barred galaxies, now facilitate a quantification of bar strength in terms of their gravitational potentials and force fields. In this paper, we show that the maximum value, Qb, of the ratio of the tangential force to the mean radial force is a quantitative measure of the strength of a bar. Qb does not measure bar ellipticity or bar shape, but rather depends on the actual forcing due to the bar embedded in its disk. We show that a wide range of true bar strengths characterizes the category ``SB'', while de Vaucouleurs category ``SAB'' corresponds to a much narrower range of bar strengths. We present Qb values for 36 galaxies, and we incorporate our bar classes into a dust-penetrated classification system for spiral galaxies.Comment: Accepted for publication in the Astrophysical Journal (LaTex, 30 pages + 3 figures); Figs. 1 and 3 are in color and are also available at http://bama.ua.edu/~rbuta/bars

Criteri topologici per l'ottimizzazione sicura di reazioni potenzialmente fuggitive

In this work, optimization procedures, particurarly suitable for potentially runaway reactions carried out in indirectly cooled semibatch reactors operated in isoperibolic temperature control mode, have been developed. Such optimization procedures based on a particular criterion, referred to as "topological", in order tio select a set of operating conditions which is able of ensuring both process thermochemical stability and high productivity and selectivity with respect the desired product. This topological approach has been validated both experimentally and theoretically by studing kinetic schemes characterized by different complexity degrees. As an example, the relevant case study of the free radical emulsion polymerization of vinyl acetate has been analyzed and reported

Dark matter within high surface brightness spiral galaxies

We present results from a detailed dynamical analysis of five high surface brightness, late type spirals, studied with the aim to quantify the luminous-to-dark matter ratio inside their optical radii. The galaxies' stellar light distribution and gas kinematics have been observed and compared to hydrodynamic gas simulations, which predict the 2D gas dynamics arising in response to empirical gravitational potentials, which are combinations of differing stellar disk and dark halo contributions. The gravitational potential of the stellar disk was derived from near-infrared photometry, color-corrected to constant (M/L); the dark halo was modelled by an isothermal sphere with a core. Hydrodynamic gas simulations were performed for each galaxy for a sequence of five different mass fractions of the stellar disk and for a wide range of spiral pattern speeds. These two parameters mainly determine the modelled gas distribution and kinematics. The agreement between the non-axisymmetric part of the simulated and observed gas kinematics permitted us to conclude that the galaxies with the highest rotation velocities tend to possess near-maximal stellar disks. In less massive galaxies, with v_max<200 km/s, the mass of the dark halo at least equals the stellar mass within 2-3 R_disk. The simulated gas morphology provides a powerful tool to determine the dominant spiral pattern speed. The corotation radius for all galaxies was found to be constant at R_corotation ~ 3 R_disk and encloses the strong part of the stellar spiral in all cases.Comment: 28 pages, 7 figures; to appear in the Astrophysical Journal, Vol. 586, March 200