The power spectrum of rich clusters of galaxies on large spatial scales

We present an analysis of the redshift-space power spectrum, P ( k ), of rich clusters of galaxies based on an automated cluster catalogue selected from the APM Galaxy Survey. We find that P ( k ) can be approximated by a power law, P ( k )∝ kⁿ , with n ≈−1.6 over the wavenumber range 0.04< k <0.1 h Mpc⁻¹. Over this range of wavenumbers, the APM cluster power spectrum has the same shape as the power spectra measured for optical and IRAS galaxies. This is consistent with a simple linear bias model in which different tracers have the same power spectrum as that of the mass distribution, but shifted in amplitude by a constant biasing factor. On larger scales, the power spectrum of APM clusters flattens and appears to turn over on a scale k ∼0.03 h Mpc⁻¹. We compare the power spectra estimated from simulated APM cluster catalogues with those estimated directly from cubical N -body simulation volumes, and find that the APM cluster survey should give reliable estimates of the true power spectrum at wavenumbers k ≳0.02 h Mpc⁻¹. These results suggest that the observed turnover in the power spectrum may be a real feature of the cluster distribution, and that we have detected the transition to a near-scale-invariant power spectrum implied by observations of anisotropies in the cosmic microwave background radiation. The scale of the turnover in the cluster power spectrum is in good agreement with the scale of the turnover observed in the power spectrum of APM galaxies.     

The linkage between fault throw and footwall scarp erosion patterns: an example from the Bremstein Fault Complex,offshore Mid‐Norway

Studies of normal fault systems in modern extensional regimes (e.g. Basin and Range), and in exhumed, ancient rift basins (e.g. Gulf of Suez Rift) have shown a link between the evolution of fault‐related footwall topography and associated erosional drainage systems. In this study, we use 3D seismic reflection data to image the footwall crest of a gravity‐driven fault system developed during late Middle Jurassic to Early Cretaceous rifting on the Halten Terrace, offshore Mid‐Norway. This 22‐km‐long fault system lacks significant footwall uplift, with hangingwall subsidence accommodating throw accumulation on the fault system. Significant erosion has occurred along the length of the footwall crest and is defined by 96 catchments characterized by erosional channels. These erosional channels consist of small, linear systems up to 750 m long located along the front of the fault footwall. Larger, dendritic channel systems extend further back (up to 3 km normal to fault strike) into the footwall. These channels are up to 7 km long, up to 50 m deep and up to 1 km wide. Fault throw varies along strike, with greatest throw in the centre of the fault decreasing towards the fault tips; localized throw minima are interpreted to represent segment linkage points, which were breached as the fault grew. Comparison of the catchment location to the throw distribution shows that the largest catchments are in the centre of the fault and decrease in size to the fault tips. There is no link between the location of the breached segment linkage points and the location and size of the footwall catchments, suggesting that the first‐order control on footwall erosion patterns is the overall fault‐throw distribution.     

Intraplate deformation adjacent to the Macquarie Ridge south of New Zealand—The tectonic evolution of a complex plate boundary

The distribution of relocated seismicity and the evolving shape of fracture zones through time in the oceanic crust of the Australian Plate adjacent to the Australia:Pacific plate boundary south of New Zealand are used to constrain the deformation of this region of the Australian Plate, here called the Puysegur Block. Relocated seismicity reveals a broad distribution of earthquakes in the Puysegur Block on both inter- and intraplate structures, including two great (M8+) earthquakes in the region over the past twenty years, one of which occurred over 130 km from the plate boundary. Plate reconstructions from the Late Oligocene through Early–Mid Miocene allow us to determine the undeformed shape of fracture zones in the Puysegur Block, formed during the Paleogene when the plate boundary was dominantly a divergent mid-ocean ridge system. Comparing these reconstructions to the present-day shape of the fracture zones allows us to map the deformation that has occurred within the Puysegur Block since the fracture zones formed. These two sets of independent observations delineate a broad zone of deformation extending ~ 150 km into the plate interior from the Macquarie Ridge Complex, the modern plate boundary structure through the region. The persistence of this deformation through time indicates a link with the evolution of the plate boundary over the past ~ 25 Ma from divergence to translation and subduction of the Australian Plate further north at the Puysegur Trench. We infer that this deformation may be a result of stresses in the Puysegur Block resulting from the impingement of the subducting plate on the thickened lithosphere of southern New Zealand. Such a collision may resist subduction, and if resistance remains substantial, further deformation internal to the Puysegur Block may lead to a southward migration of the Australia:Pacific subduction interface and the capturing of this section of lithosphere onto the Pacific Plate.     

Photometry and Imaging of Comet C/2000 Wm1 (Linear)

Earth, Moon, and Planets -     

Comparison of giant cutgrass productivity in tidal and impounded marshes with special reference to tidal subsidy and waste assimilation

Aboveground live standing crop of giant cutgrass (Zizaniopsis miliacea) populations in similar freshwater tidal and impounded nontidal marshes were almost identical (peaking at 1,039 g per m² in each). The mortality, however, was greater in the tidal marsh resulting in significantly (95% level) greater annual production of aboveground cutgrass in the tidal (1,530±103 g per m² per yr) than the impounded (1,172±88 g per m² per yr) marsh, a 31% difference which we consider to be a measure of tidal subsidy. Belowground production also was found to average higher in the tidal marsh, but estimates were not as satisfactory as the aboveground results due to sampling difficulties. Combined annual above and belowground net production comes to an estimated 2,048 ±101 g per m² per yr for the tidal and 1,481±219 for the impounded cutgrass marsh. The potential of freshwater tidal marshes for tertiary treatment of wastes is briefly discussed.     

Resonant Vibrational Instabilities in Magnetized Stellar Atmospheres

We perform linear stability analysis on stratified, plane-parallel atmospheres in uniform vertical magnetic fields. We assume perfect electrical conductivity and we model non-adiabatic effects with Newton's law of radiative cooling. Numerical computations of the dispersion diagrams in all cases result in patterns of avoided crossings and mergers in the real part of the frequency. We focus on the case of a polytrope with a prevalent, relatively weak, magnetic field with overstable modes. The growth rates reveal prominent features near avoided crossings in the diagnostic diagram, as has been seen in related problems (Banerjee, Hasan, and Christensen-Dalsgaard, 1997). These features arise in the presence of resonant oscillatory bifurcations in non-self adjoint eigenvalue problems. The onset of such bifurcations is signaled by the appearance of avoided crossings and mode mergers. We discuss the possible role of the linear stability results in understanding solar spicules.     

Petrogenesis of some palaeozoic rhyolites in Victoria

Thick rhyolite sheets occur within the middle Palaeozoic cauldron subsidence complexes in Victoria. Well‐preserved eutaxitic and vitroclastic textures in the basal regions of the Rubicon, Jemba, and Ryans Creek rhyolites provide evidence of an ash‐flow origin. Subsequent recrystallization during cooling has masked these primary textures towards the central and upper parts of the rhyolitic sheets. All three rhyolites show dense welding throughout and the major zonation observed is in the degree of granophyric recrystallization.

Despite the turbulent ash‐flow emplacement and the possibility of post‐eruptive chemical modification, the mineralogical and chemical trends within vertical sequences in the Rubicon and Jemba Rhyolites are essentially continuous and represent the inverted variation within the original magma body. The bulk compositions of the rhyolites suggest that conditions of water saturation were closely approached or attained within the magma chamber. The chemical variation with increasing height and phenocryst content in the rhyolites can be summarized as: increasing K, Ca, Ba and Sr, and decreasing Na and Rb. Whereas this variation is in accord with frac‐tionation theories, it is more likely to have resulted from the partial fusion process believed to have been responsible for the generation of the acid magmas.