Dispersion Of Directions Research Articles

Global Analysis of Topographic and Climatic Controls on Drainage Basin Shapes

AbstractDrainage basins are fundamental units of Earth's surface, describing how flows accumulate across landscapes. They are direct expressions of how tectonics and climatic forces alter Earth's surface morphology. Here, we measure the width‐to‐length ratios (WLRs) of 386,931 drainage basins (average area ∼157 km2), covering all continents except Antarctica and Greenland. Global variations in WLRs are correlated with climatic aridity, whole‐basin slope, and local topographic roughness. Basins in arid landscapes tend to be narrower, potentially reflecting a higher prevalence of surface runoff and therefore a stronger slope‐parallel component of the transporting flow. Local topographic roughness is associated with wider basins, potentially reflecting greater dispersion of flow directions. Conversely, whole‐basin topographic gradients, potentially reflecting gradients in uplift, are associated with narrower basins. However, steeper basins are also often rougher, so revealing the effects of whole‐basin slope requires correcting for the confounding effects of roughness variations.

RETRO-mapping: A novel algorithm automating wavefront categorization using activation mapping during persistent atrial fibrillation demonstrates a reduction in wavefront collisions following pulmonary vein isolation.

RETRO-mapping was developed to automate activation mapping of atrial fibrillation (AF). We used the algorithm to study the effect of pulmonary vein isolation (PVI) on the frequency of focal, planar, and colliding wavefronts in persistent AF. An AFocusII catheter was placed on the left atrial endocardium to record 3 s of AF at sixsites pre and post-PVI in patients undergoing wide circumferential PVI for persistent AF. RETRO-mapping analyzed each segment in 2 ms time windows for evidence of focal, planar, and colliding waveforms and the automated categorizations manually validated. Ten patients were recruited. A total of 360 s of data in 120 segments of 3 s from 60 left atrial locations were analyzed. RETRO-map was highly effective at identifying focal waves and collisions during AF. PVI significantly reduced collision frequency but not focal and planar activation frequency. However, there was a significant reduction in the dispersion of activation directions. Larger studies may help determine factors associated with successful clinical outcome.

Magnetic Properties of Star-forming Dense Cores

Abstract Magnetic and energetic properties are presented for 17 dense cores within a few hundred parsecs of the Sun. Their plane-of-sky field strengths B pos are estimated from the dispersion of polarization directions, following Davis, Chandrasekhar, and Fermi (DCF). Their ratio of mass to magnetic critical mass is 0.5 ≲ M / M B ≲ 3 , indicating nearly critical field strengths. The field strength B pos is correlated with column density N as B pos ∝ N p , where p = 1.05 ± 0.08 , and with density n as B pos ∝ n q , where q = 0.66 ± 0.05 . These magnetic properties are consistent with those derived from Zeeman studies, with less scatter. Relations between virial mass M V , magnetic critical mass M B , and Alfvén amplitude σ B / B match the observed range of M/M B for cores observed to be nearly virial, 0.5 ≲ M / M V ≲ 2 , with moderate Alfvén amplitudes, 0.1 ≲ σ B / B ≲ 0.4 . The B − N and B − n correlations in the DCF and Zeeman samples can be explained when such bound, Alfvénic, and nearly critical cores have central concentration and spheroidal shape. For these properties, B ∝ N because M/M B is nearly constant compared to the range of N, and B ∝ n 2 / 3 because M 1/3 is nearly constant compared to the range of n 2/3. The observed core fields that follow B ∝ n 2 / 3 need not be much weaker than gravity, in contrast to core fields that follow B ∝ n 2 / 3 owing to spherical contraction at constant mass. Instead, the nearly critical values of M/M B suggest that the observed core fields are nearly as strong as possible, among values that allow gravitational contraction.

Assessment of the effect of long-range transport on air pollution trends for different Russian regions

The features of the dynamics of long-range atmospheric pollution of the territories of the Russian Federation under current conditions, in the immediate and distant prospects (from 1980 to 2050) taking into account two scenarios of the changing climate in the 21st century were studied. To assess contribution and trends of long-range atmospheric pollution in Russian regions, a two-dimensional numerical model is constructed, which implements the solution of the coupled problem of transport and diffusion of impurities in spherical coordinates, as well as an improved method for identifying sources of transboundary and local air pollution. The results indicate general trends in the directions of the flow of pollutants to the Russian regions in 1980-2050 from zonal (western) to meridional (southern). Such dynamics is markedly expressed in the regions of Siberia and the Far East and slightly in the European territory of Russia and the Urals. In the climatic scenario RCP8.5, despite the general tendency towards the southern transport of impurities, a certain dispersion of directions from where the regions are polluted becomes noticeable. It indicates an increase in atmospheric pollution from the west in certain regions, mainly in the central part and in the south of the ETR. The greatest danger to long-range atmospheric pollution of the regions of Siberia and the Far East in the coming decades will be sources located south of these territories; of the regions of the ETR and the Urals – sources located to the west and south. These results are important for the development of proposals for ensuring the environmental safety of Russian regions and for planning economic development of the country’s territories that affect the pollution of the environment of ecologically significant zones.

Social Investments as the Highest Manifestation of Implementation of Social Responsibility of the Companies of Agribusiness

The purpose of article is determination of features of social investments, sources and directions of their implementation in a format of social responsibility and their justification as its highest manifestation. It was for this purpose established their forms, endogenous and exogenous factors of implementation, manifestation as a part of innovation investment projects, borders of washing out and integration are established. Problems are revealed and financing sources of social investments on the principles of social responsibility are developed. Are defined their rating, assessment of their level and efficiency in comparison with the European and world indicators is performed. It is revealed bias of owners and management of the enterprises of domestic agribusiness for social investments and social responsibility. Instruments of its overcoming are called. Productive mechanisms and the priority directions of introduction of social investments on prospect are developed. Methodological bases of a research are based on use of modern provisions of the economic theory, business economics of agribusiness, concepts of innovation investment activities, social responsibility. In article traditional general scientific and ad latest economic and sociological hoc methods of researches - historical, monographic, system structural analysis, statistical and economic, economic-mathematical, sociological polls are used. Content, features, forms, factors, problems and the priority directions of implementation of social investments as highest manifestation of social responsibility of the enterprises of agribusiness are determined. Calculation of their rating, level, efficiency in the European and world comparisons is performed. Reserves, tools, improvement mechanisms are reasonable. Results of article can be used in activity of the enterprises of agribusiness, non-governmental organizations, agricultural communities, local government authorities and state bodies of the power at implementation of social investments, social responsibility. It is expedient to use them in educational and scientific and educational process of the highest and other educational institutions. As innovations in financial and investment ensuring social responsibility dispersion of the charitable directions, functioning durations is offered to refuse the endowment mechanism in case of its availability. But it is expedient to concentrate financial and investment resources on the solution of one or several priority social problems.

Nonlinear higher-order shell theory for incompressible biological hyperelastic materials

In the present study, a geometrically nonlinear theory for circular cylindrical shells made of incompressible hyperelastic materials is developed. The 9-parameter theory is higher-order in both shear and thickness deformations. In particular, the four parameters describing the thickness deformation are obtained directly from the incompressibility condition. The hyperelastic law selected is a state-of-the-art material model in biomechanics of soft tissues and takes into account the dispersion of collagen fiber directions. Special cases, obtained from this hyperelastic law setting to zero one or some material coefficients, are the Neo-Hookean material and a soft biological material with two families of collagen fibers perfectly aligned. The proposed model is validated through comparison with the exact solution for axisymmetric cylindrical deformation of a thick cylinder. In particular, the shell theory developed herein is capable to describe, with extreme accuracy, even the post-stability problem of a pre-stretched and inflated Neo-Hookean cylinder until the thickness vanishes. Comparison to the solution of higher-order shear deformation theory, which neglects the thickness deformation and recovers the normal strain from the incompressibility condition, is also presented.

Ice‐marginal forced regressive deltas in glacial lake basins: geomorphology, facies variability and large‐scale depositional architecture

This study presents a synthesis of the geomorphology, facies variability and depositional architecture of ice‐marginal deltas affected by rapid lake‐level change. The integration of digital elevation models, outcrop, borehole, ground‐penetrating radar and high‐resolution shear‐wave seismic data allows for a comprehensive analysis of these delta systems and provides information about the distinct types of deltaic facies and geometries generated under different lake‐level trends. The exposed delta sediments record mainly the phase of maximum lake level and subsequent lake drainage. The stair‐stepped profiles of the delta systems reflect the progressive basinward lobe deposition during forced regression when the lakes successively drained. Depending on the rate and magnitude of lake‐level fall, fan‐shaped, lobate or more digitate tongue‐like delta morphologies developed. Deposits of the stair‐stepped transgressive delta bodies are buried, downlapped and onlapped by the younger forced regressive deposits. The delta styles comprise both Gilbert‐type deltas and shoal‐water deltas. The sedimentary facies of the steep Gilbert‐type delta foresets include a wide range of gravity‐flow deposits. Delta deposits of the forced‐regressive phase are commonly dominated by coarse‐grained debrisflow deposits, indicating strong upslope erosion and cannibalization of older delta deposits. Deposits of supercritical turbidity currents are particularly common in sand‐rich Gilbert‐type deltas that formed during slow rises in lake level and during highstands. Foreset beds consist typically of laterally and vertically stacked deposits of antidunes and cyclic steps. The trigger mechanisms for these supercritical turbidity currents were both hyperpycnal meltwater flows and slope‐failure events. Shoal‐water deltas formed at low water depths during both low rates of lake‐level rise and forced regression. Deposition occurred from tractional flows. Transgressive mouthbars form laterally extensive sand‐rich delta bodies with a digitate, multi‐tongue morphology. In contrast, forced regressive gravelly shoal‐water deltas show a high dispersion of flow directions and form laterally overlapping delta lobes. Deformation structures in the forced‐regressive ice‐marginal deltas are mainly extensional features, including normal faults, small graben or half‐graben structures and shear‐deformation bands, which are related to gravitational delta tectonics, postglacial faulting during glacial‐isostatic adjustment, and crestal collapse above salt domes. A neotectonic component cannot be ruled out in some cases.

A generalized micromorphic approach accounting for variation and dispersion of preferred material directions

Materials exhibiting a heterogeneous and non-uniform composition in terms of elastic and anisotropic properties such as biological tissues require special efforts to accurately describe their constitutive behavior. In contrast to classical models, micromorphic formulations can predict the macroscopically observable material response as originated from distinct scale-dependent micro-structural deformation mechanisms. This is facilitated by additional independent degrees of freedom and associated additional strain and stress quantities. Here, a generalized continuum is mathematically constructed from a macro-continuum and a micro-continuum which are both adequately coupled on kinematics and constitutive levels as well as by micro-boundary conditions. In view of biomechanical modeling, the potential of the formulation is studied for a number of academic examples characterized by an anisotropic material composition to elucidate the micromorphic material response as compared with the one obtained using a classical continuum mechanics approach. The results demonstrate the ability of the generalized continuum approach to address non-affine elastic reorientation of the preferred material direction in the macro-space and its dispersion in the micro-space as affecting deformation, strain and stress on the macroscopic level. In particular, if the anisotropy in the micromorphic formulation is solely linked to the extra degrees of freedom and associated strain and stress measures, the deformation for small and large strains is shown to be distinctly different to the classical response. Together with the ability to implicitly account for scale-dependent higher-order deformation effects in the constitutive law the proposed generalized micromorphic formulation provides an advanced description, especially for fibrous biological materials.

Abstract T P112: The Effect of Monoaminergic Drugs on Motor Cortex Excitability, Motor Performance and Psychophysics

INTRODUCTION: The benefit of pharmacologic augmentation of stroke recovery remains unclear. As post-stroke motor recovery depends on training-dependent plasticity, we wanted to test the effect of these drugs on the excitability of primary motor cortex (M1), kinematic and psychophysical measures. METHODS: Nine able-bodied individuals (4 males) participated in 4 test sessions at least 1 week apart during which they were administered one oral dose of 4 different medications: placebo (P), amphetamine 10mg (A), methylphenidate 20mg (M), carbidopa/levodopa 25/100mg (D). We used transcranial magnetic stimulation (TMS) to measure the effects of the different drugs on M1 excitability. Motor evoked potentials (MEP) were elicited using TMS at intensities of 35-80% of maximum stimulator output. The Boltzmann function was calculated for MEP amplitudes and compared across the 4 drug conditions. The effects of the drugs on psychophysics and movement kinematics were captured by a 2d accelerometer mounted on the dorsum of the hand. We measured the reaction time and the peak acceleration and dispersion of directions of auditory paced ballistic wrist extension movements. RESULTS: Comparison of the 4 different equations for the Boltzmann sigmoid curve where R max represents the maximum MEP amplitude; k, the slope of the curve; and S50, the intensity that produced half of the maximum MEP amplitude revealed significant differences between drug conditions. Compared to P, A led to a significant decrease in the maximum MEP amplitude (p= 0.00015), while D increased it (p=0.013). The other two parameters were similar to placebo. M had no effect on any of the three parameters. The different drugs had no statistically significant effect on the kinematic measures or reaction time. CONCLUSIONS: These results indicate that the neuromodulatory effect of drugs on M1 excitability differs, which may point to different mechanisms mediating the beneficial effects of these drugs on functional recovery after stroke. It would also support the notion that the beneficial effect of the drugs on recovery is likely due to improved kinematics or psychophysics of executions during training.

Paleomagnetic and 40Ar/39Ar geochronological results from the Linzizong Group, Linzhou Basin, Lhasa Terrane, Tibet: Implications to Paleogene paleolatitude and onset of the India–Asia collision

The Linzizong Group (64–44Ma) of the Lhasa Terrane in Tibet is critically positioned for establishing the paleoposition of the southern leading edge of the Asian continent during Paleogene times and constraining onset of the India–Asia collision. Here we report paleomagnetic results from a collection comprising 384 drill-core samples from 34 sites embracing all three formations of this group. Comprehensive demagnetization and field tests isolate characteristic remanent magnetizations (ChRM) summarized by overall tilt-corrected formation-mean directions of D=183.6°, I=−12.4° (α95=8.1°) for the Dianzhong (64–60Ma), D=1.0°, I=18.1° (α95=8.1°) for the Nianbo (60–50Ma), and D=12.4°, I=23.2° (α95=7.3°) for the Pana (50–44Ma). Fold tests are positive in each formation suggesting a pre-folding origin and we interpret the magnetizations as quasi-primary and acquired at, or slightly later than, formation of the Linzizong Group. Revised Paleogene paleopoles with Ar–Ar age constraints for the Lhasa Terrane indicate that onset of the India–Asia collision occurred no later than ∼60.5±1.5Ma at a low paleolatitude of ∼10°N. Analysis of 60 site-mean observations from a range of studies of the Pana Formation in the higher part of the succession highlight a large dispersion of ChRM directions; a number of possible causes are suggested but further study of this formation over a wider area is required to resolve this issue.

The development of the pinnacles (Lena pillars) along Middle Lena (Sakha Republic, Siberia, Russia)

The study deals with the Lena Pillars (Sakha Republic, Russia, Siberia). The Lena Pillars are pinnacle features which are characteristic on karsts. The importance of these features is that they are extremely large and occur on very large areas. Their occurrence is interesting because they do not appear on tundra karst. We investigated the veneers of cliffs, the fracture density of the building rock, the distribution and dispersion of the fracture directions, direction distribution and dispersion of the giant grikes between the pillars, the layer thickness of the building rock, the size and dispersion of debris. We distinguished pillar types, and then we classified the features on the area of the pillars from a morphogenetical point of view. According to investigations and morphological analyses, the pillars developed from palaeokarsts. During the former karstification grikes and caverns developed close to the surface. These features coalesced into each other. The giant grikes of great size became filled and the pillars were covered. After the cut of the Lena River these features were revealed and became exhumed. Now the pillars are destroyed and continue developing by frost weathering.

All-space existence and dispersion of athermal directions in monoclinic KY(WO4)2

The analytical expressions for thermo-optic coefficients, dn/dT, for “fast” and “slow” light waves propagating along the arbitrary direction in a biaxial crystal are derived. On the basis of these expressions, the all-space analysis of existence of athermal directions is performed for monoclinic and biaxial KY(WO4)2 at 1.03μm. The calculations are performed for an arbitrary light propagation direction and polarization (not restricted to the principal planes). The appearance of directions that can be athermal for both “fast” and “slow” waves is predicted. The dispersion of athermal directions is analyzed for visible and near-IR. The existence of upper and lower dispersion limits for athermal behavior of KY(WO4)2 is shown. It is shown that the optical indicatrix axes can be athermal itself at some light wavelengths. Wavelength- and temperature-dependent position of the optical axes of KY(WO4)2 is also determined.

Quantifying paleosecular variation: Insights from numerical dynamo simulations

Thanks to advances in dynamo modelling, it is now possible to produce numerical sequences of magnetic polarity reversals over periods of time equivalent to several tens of millions of years. Using a set of five numerical models integrated over the equivalent of 40–50 Myr, we generated synthetic data analogous to paleomagnetic data derived from volcanic flows. We find that the distributions of directions are remarkably similar to those observed in the paleomagnetic database. Paleosecular variation among the five models is best discriminated by the relative variability in paleointensity (εF) and the relative dispersion of directions or poles as defined by the precision parameter (k). Whether the geodynamo operated in different regimes in its past can be best tested with these parameters in combination. Roughly one million years of time with 200 time-independent samples is required to achieve convergence of εF and k. The quantities εF and k correlate well with the average chron duration (μchr), which suggests that excursions and reversals are an integral part of paleosecular variation. If applicable to the geodynamo, the linear dependence of k on μchr could help to predict μchr for the Earth during geologic times with no available reversal frequency data; it also predicts much higher average k for directions during superchrons (k≈2500 for the Cretaceous normal superchron) than during actively reversing times (k≈35 for the last 80 Myr). As such high k values are not observed, either this family of dynamo models is not applicable to the geodynamo, or the geodynamo regime acting during superchrons is independent of that acting during times with frequent reversals.

Propriedades Sísmicas Anisotrópicas Derivadas da Orientação Cristalográfica Preferencial de Muscovita-Quartzo Milonitos

Seismic wave propagation in organized matter usually results in azimuthal variations of longitudinal waves (Pwaves), as well as the effect of birefringence in transversal waves (S-waves), which results in two orthogonal shear waves with contrasting velocities. In this paper we present the results of the anisotropic seismic properties of five samples of muscovitequartz mylonites collected in different parts of a fold in the Saas Fee region, Western Internal Alps. The P-wave velocities in these rocks varies from 5.73 to 6.32 km/s, whereas the high-velocity shear wave (S1) varies from 3.82 to 4.22 km/s and the low velocity (S2) from 3.73 to 4.09 km/s. The anisotropy in these rocks is relatively high and reaches values from 9.5% for P-waves, and almost 11% for shear wave splitting. Both anisotropy and propagation directions seem to be related to from the strong preferred orientation of quartz and muscovite but also depend of muscovite modal content within the different specimens. Development of preferred orientation of minerals destroys and disperses the single crystal seismic properties, which causes a decrease of wave velocities and a dispersion of propagation directions, of both compressional and shear waves. Since the preferred orientation of quartz and muscovite can be directly related to the main macroscopic structures in these rocks (foliation, lineation, and pole of foliation) and the anisotropic seismic properties are related to the preferred orientation, it is possible to determine the propagation directions in terms of these structures. Due to the relatively high muscovite content, many of the maximum propagation velocities are parallel/subparallel to the foliation and some parallel to the lineation of the reference frame. On the other hand, directions of minimum propagation cannot be directly related to the foliation pole. The presence of folds in the mid-to lower crust can exert changes in the propagation directions due to the foliation variation around such structures, mainly in the P-waves.

Late Cretaceous paleolatitude of the Hawaiian Hot Spot: New paleomagnetic data from Detroit Seamount (ODP Site 883)

Paleomagnetic data from Detroit Seamount of the Emperor seamount chain (northwestern Pacific Ocean) provide a direct way to estimate the latitudinal position of the Hawaiian hot spot in the Late Cretaceous (75–81 Ma), providing important constraints on geodynamic models for Hawaiian plume motion. Crystalline basement of Detroit Seamount was sampled at four drill sites by Ocean Drilling Program (ODP) Legs 145 (Sites 883 and 884) and 197 (Sites 1203 and 1204). Here we report detailed thermal and alternating field (AF) demagnetization data collected from basalt lava flows recovered at ODP Site 883. Most samples showed two nearly antiparallel components of natural remanent magnetization (NRM). AF demagnetization was often insufficient to separate the magnetization components; therefore thermal demagnetization data were used to define their directions. Both magnetization components are chemical remanent magnetizations (CRMs) carried by single‐domain to pseudo‐single‐domain titanomaghemite produced by low‐temperature oxidation of titanomagnetite. Rock magnetic experiments suggest that the normal polarity component is a CRM which pseudomorphs the primary thermal remanent magnetization (TRM) of parental titanomagnetite. In contrast, the reversed polarity component is a CRM self‐reversed with respect to a primary TRM [Doubrovine and Tarduno, 2004]. Inclination data analysis of characteristic remanence (normal polarity component) results in the five distinct inclination groups and an average inclination of 57.7°−13.8°+12.3° (95% confidence limits quoted). The angular dispersion of paleomagnetic directions suggests that the Site 883 basalts adequately average paleosecular variation and that the inclination average represents the time‐averaged geomagnetic field. Combining all available paleomagnetic data from the four sites on Detroit Seamount, a revised paleolatitude of 34.4°−5.8°+3.2° was calculated. This estimate is significantly north of the present latitude of the Hawaiian hot spot (∼19°N), suggesting that the hot spot moved southward from Late Cretaceous to Paleogene times (81–47 Ma) at rates of ∼33–67 mm/yr. If the inclination results from ODP Site 883 represent the best estimate among the data available from Detroit Seamount, rates at the higher range of values are favored. Because such rates are comparable to the velocities of lithospheric plates, the new data from ODP Site 883 present a strikingly different view on hot spot stability.

Record of deformation by secondary magnetic remanences and magnetic anisotropy in the Nar/Phu valley (central Himalaya)

Secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility (AMS) were studied in low-grade metamorphic carbonates of the Tethyan Himalaya in Nar/Phu valley (central Nepal) and used for interpretation of tectonic deformations. The characteristic remanence (ChRM) is likely of thermomagnetic origin related to post-peak metamorphic cooling occurring after the Eohimalayan phase (35–32 Ma). The ChRM postdates small-scale folding (main Himalayan folding F1 and F2) as shown by a negative fold test of site mean directions at 99% confidence level, and has been probably acquired between 32 and 25 Ma. Late-orogenic long-wavelength folding associated with the Chako antiform (CA) is recorded by the spatial dispersion of ChRM directions and the distribution of the main axes of the AMS tensor. The mean tilting of the ChRM direction since remanence acquisition (≈20–30°) approximately coincides with the tilting of the CA (31°) at the study area indicating that the pyrrhotite remanence predates the CA (CA formed at <18 Ma according to preliminary U/Pb dating). However, comparison of tilt angles of remanence directions and AMS tensor axes suggests that remanence acquisition was not completed before the onset of the CA formation. This could imply a younger age (Early Miocene or even younger) of the ChRM. Using the distribution of remanence directions along a small-circle as well as the distribution of AMS tensor axes, a clockwise mean rotation of 16° is obtained for a remanence age of ≈30 Ma. An Early Miocene remanence age would not change this result substantially. Compilation of rotations in the Tethyan Himalaya deduced from secondary pyrrhotite remanences reveals an increasing clockwise rotation from the Hidden valley in the W to the Shiar valley in the E (≈150 km distance), incompatible with an oroclinal bending model.

Paleoenvironmental analysis of the late Neoproterozoic Mistaken Point and Trepassey formations, southeastern Newfoundland

The Mistaken Point and Trepassey formations (Conception and St. John's groups, respectively) comprise a terminal Neoproterozoic, deep-marine succession of fine-grained turbidites and volcanogenic deposits that are part of the Avalonian Terrane. Debris-flow beds, slumped units, the low dispersion of turbidity-current paleoflow directions, and the absence of wave-generated structures together indicate that the sediment was deposited on a deep-water, southeast- facing slope. Channels were not present in the study area. The upward increase in the abundance of slump structures suggests that these units represent toe-of-slope and mid-slope environments, respectively. These units prograded over basin-floor deposits of the Drook and Briscal formations, which have (axial) paleocurrent directions that are orthogonal to the inferred downslope flow that characterized the overlying deposits. Within the Mistaken Point and Trepassey formations, a diverse assemblage of soft-bodied, non-phototrophic Ediacaran organisms is preserved beneath volcanic ash layers on more than one hundred surfaces. Individual fossiliferous surfaces can be correlated up to several kilometres. The felling orientations of frondose fossils indicate that contour currents, as well as up- and downslope currents of tidal and (or) wind-forced origin, influenced the sea floor in the intervals between event beds when the organisms lived. The contour currents may have been responsible for sustaining the organisms in this deep-water setting. The current-produced inclination of the frondose organisms at the time of ash deposition allowed their preservation by preventing the accumulation of ash beneath them.

Analysis of the early Jurassic geomagnetic data recorded at the Breggia Gorge (Ticino, Switzerland)

Geomagnetic directions recorded in a lower Jurassic stratigraphic sequence that outcrops at the Breggia Gorge are analyzed. A new method to identify directions suitable for calculating a time-averaged dipolar geomagnetic direction has been applied to calculate a hypothetical early Jurassic geocentric axial dipole. The data were separated to identify normal/reverse directions, corresponding to periods of stable field, and intermediate directions. From an analysis of the data in the palaeogeographic coordinates of the Breggia Gorge at the time of the acquisition of the remanence, it is suggested that polarity transitions in the early Jurassic behaved in a similar manner to that described for late Cenozoic reversals, with the VGPs preferentially following longitudinal paths. The preferred paths coincide with a time-averaged location of the Jurassic subduction area of Pangea. This agreement could indicate a connection between the geometry of the reversals and the subduction zones. An analysis to test whether both N/ R and intermediate directions have similar preferred distributions suggests that the underlying structure of the early Jurassic geomagnetic field was similar to that observed for the late Cenozoic. The dispersion of directions with inclinations around 0° is higher than those belonging to N/ R polarity directions, suggesting that there was an enhancement of fluctuations of the geomagnetic field during intermediate states in the early Jurassic.

Palaeomagnetism of welded, pyroclastic-fall scoriae at Vulcano, Aeolian Archipelago

This paper deals with the magnetic fabric and remanent magnetization of welded scoriae fall deposits. Four scoriae blankets were emplaced on Vulcano (Aeolian Islands) from about 50 to 8ka. Their detailed chronostratigraphic position has been debated in the literature and they have therefore been given a different interpretation in the reconstruction of the Vulcano's eruptive history. The aims of the study were a better understanding of the magnetic features of the scoriae fall deposits, which have so far been given little attention, and to set the chronostratigraphic relationships of the sampled deposits on the ground of their palaeomagnetic directions.Overall, the magnetic properties of the four deposits are alike. Investigation of isothermal remanence and susceptibility versus temperature measurements point to low-Ti titanomagnetite as the main ferromagnetic mineral. Magnetic foliation is well developed, moderately dispersed and in most cases close to horizontal. Lineation is either clustered or dispersed within the foliation plane. Stepwise thermal and alternating field demagnetization shows that secondary components are negligible in two units. In the other two, they are always completely removed in the first steps, below 20mT or 400°C. Thereafter, the characteristic component (ChRM) is clearly isolated and the within-site dispersion of its direction is small (α95<4° at 16 out of 23 sites). The within-site consistency of the directional features of both magnetic fabric and remanence shows that no en masse movement of welded scoria occurred nor did individual spatter move with respect to one another after their emplacement. The scoriae therefore welded and cooled in situ. The between-site dispersion of the ChRM directions also is very small (α95<3.5°) for three units, and their mean palaeomagnetic direction is consistent with the palaeosecular variation curve for Aeolian Islands. Statistical analysis of the results from two units whose isotopic ages are indistinguishable within the error range (Quadrara: 21.3±2.4ka; Spiaggia Lunga: 24.0±5.0ka) shows that they were emplaced at different times, because their ChRM directions are different at the 95% confidence level. Moreover, the Spiaggia Lunga scoriae were emplaced by a single eruption, since their mean-site directions are statistically indistinguishable at five out of six sites.The palaeomagnetic results corroborate the hypothesis that the investigated scoriae deposits correspond to different volcanic phases, related to recurrent arrival of mafic magma, and show that those of Spiaggia Lunga and Quadrara were produced during two distinct periods of volcanic activity with little time separation between them.

Clockwise block rotations along the eastern border of the Cordillera de Domeyko, Northern Chile (22°45′–23°30′S)

We report new paleomagnetic results from a study of 38 sites in Cretaceous to Early Tertiary red beds and volcanic rocks belonging to the Purilactis Group, which outcrop along the western border of the Salar de Atacama Basin. After detailed thermal demagnetization, characteristic directions were determined for 32 units. In most cases, red bed sediments from the lower members of the Purilactis Group have a well-defined normal polarity magnetization probably carried by hematite forming an early diagenetic cement. A large decrease in the dispersion of the paleomagnetic directions upon tilt correction demonstrates that this magnetization is a pre-tectonic magnetization. We interpret the dominant occurrence of the normal polarity direction as evidence for acquisition of the magnetization during the Cretaceous normal polarity superchron. Large deviations of the paleomagnetic declinations from the expected ones for stable South America provides new evidence for clockwise tectonic rotations associated with Tertiary deformation in the Cordillera de Domeyko. These data confirm that clockwise tectonic rotations are one of the most significant structural characteristics of the north Chilean Andes. This study, however, indicates spatial variation in the magnitude of the rotation with rotations >60° in the Cerro Totola area. These rotations have occurred in conjunction with transpressional deformation that affected large tracts of the Cordillera de Domeyko during Eocene deformation. The systematic observation of clockwise rotations contemporaneous with sinistral displacements in the Cordillera de Domeyko can be explained by shear-traction at the base of the brittle crust.