P-wave velocity features of the lithosphere under the Eromanga Basin, Eastern Australia, including a prominent MID-crustal (Conrad?) discontinuity

The crustal structure of the central Eromanga Basin in the northern part of the Australian Tasman Geosyncline, revealed by coincident seismic reflection and refraction shooting, contrasts with some neighbouring regions of the continent. The depth to the crust-mantle boundary (Moho) of 36–41 km is much less than that under the North Australian Craton to the northwest (50–55 km) and the Lachlan Fold Belt to the southeast (43–51 km) but is similar to that under the Drummond and Bowen Basins to the east.The seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the North Australian and Lachlan provinces. In particular, there is a sharp velocity increase at mid-crustal depths (21–24 km) which has not been observed with such clarity elsewhere in Australia (the Conrad discontinuity?).In the lower crust, the many discontinuous sub-horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust, further emphasising the differences between the upper and lower crust. The crust-mantle boundary (Moho) is characterised by an increase in velocity from 7.1–7.7 km/s to a value of 8.15 + 0.04 km/s. The depth to the Moho under the Canaway Ridge, a prominent basement high, is shallower by about 5 km than the regional Moho depth; there is also no mid-crustal horizon under the Canaway Ridge but there is a very sharp velocity increase at the Moho depth of 34 km. The Ridge could be interpreted as a horst structure extending to at least Moho depths but it could also have a different intra-crustal structure from the surrounding area.The sub-crustal lithosphere has features which have been interpreted, from limited data, as being caused by a velocity gradient at 56–57 km depth with a low velocity zone above it.Because of the contrasting crustal thicknesses and velocity gradients, the lithosphere of the central Eromanga Basin cannot be considered as an extension of the exposed Lachlan Fold Belt or the North Australian Craton. The lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m, consistent with an upper crustal basement of tightly folded meta-sedimentary and meta-volcanic rocks. The crustal structure is consistent with a pericratonic or arc/back-arc basin being cratonised in an episode of convergent tectonics in the Early Palaeozoic. The seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment.     

西澳大利亚豪特曼群礁复活节岛群全新世珊瑚礁的海底胶结作用

 对澳大利亚的位于海洋过渡带和高纬度的豪特曼群礁复活节岛群上的全新世珊瑚礁进行了海底胶结作用解剖研究。研究表明,该礁像热带珊瑚礁一样,也经历了相当程度的海底胶结。胶结物包括高镁方解石的和文石的共7种类型。由于所处地理位置和海洋环境条件,使得海底胶结物中强烈贫文石而富镁方解石(分别占4%和96%),造成镁方解石胶结物中的MgCO₃的摩尔含量规律性降低(8.47%-14.29%)。特有的生物组合及其竞争,不仅构成了迎风礁和背风礁的显着岩相差异(前者以藻类粘结岩为主,后者以珊瑚骨架岩占优),并导致海底胶结物在背风礁中的发育强度远大于迎风礁这一与热带珊瑚礁海底胶结物恰好相反的分布规律。     

Determination of acoustic phonon dispersion curves in layer silicates by inelastic neutron scattering and computer simulation techniques

Inelastic neutron scattering and computer modelling techniques have been used to study acoustic phonons in several layer silicate minerals. Experimental measurements have been made on four layer silicate minerals; namely samples of muscovite, Fe-bearing muscovite, margarite and chlorite. The longitudinal acoustic modes propagating along the [0, 0, ξ] direction of muscovite and Fe-bearing muscovite were found to be the same, within experimental error. The longitudinal and transverse acoustic modes propagating along [0, 0, ξ] of muscovite have been calculated using computer simulation techniques based on lattice dynamics. The experimental and calculated longitudinal modes of muscovite are in excellent agreement, thereby demonstrating the complementary nature of both techniques. The shape of both experimental and calculated dispersion curves was found to be approximately sinusoidal, indicating that interatomic forces act principally between nearest-neighbour atoms.     

Sediments and history of the Rottnest Shelf, southwest Australia: a swell-dominated, non-tropical carbonate margin

The Rottnest Shelf is a narrow, wave-dominated open shelf on the passive continental margin of southwest Australia, adjacent to a hinterland of low relief and sluggish drainage. High physical energy, low nutrients in cool subtropical waters, and rapid postglacial transgression have limited carbonate productivity, restricted grain types, and reworked the transgressed surface to form only a thin ( < 1 m) blanket of carbonate and relict sediment, with little terrigenous influx. Subaerial weathering of the shelf during Late Pleistocene emergence was followed by postglacial drowning, erosional shoreface retreat, and generation of a transgressive lag deposit. Establishment of the warm temperate biota, dominated by bryozoans and calcareous red algae, resulted in bioerosion of the shelf disconformity surface and generation of hardground veneers and thin skeletal carbonate sheets. Linear topographic ridges of Pleistocene limestone partition the shelf into systems with varying physical energy, biota and sediment supply. The Holocene sediments are a shallowing-upward coastal sequence; wave-ripple cross-stratified grainstone (Inner Shelf); and bioturbated bryozoan grainstone to skeletal wackestone (Outer Shelf to Upper Continental Slope), characterised by seaward fining and increasing percentages of planktic carbonate sediment.

Given sufficient time, the Rottnest Shelf could recover from drowning, and form blanket-like skeletal carbonates. Thin ( < 1 m) lags overlying disconformities, which underlie shallowing-upward coastal and shelf sediments a few metres thick, will be generated by glacio-eustatic cycles of sedimentation (10⁵ y duration). Thick (several tens of metres) sediment bodies, composed of wave-rippled to bioturbated skeletal carbonate sediment with a temperate biota, will be formed during longer term (1–10 My) sedimentation cycles. Such cycles have characterised passive margins during the Cenozoic. The Rottnest Shelf thus provides a facies model for temperate shelf sedimentation along passive continental margins.     

The production of unequal risk in hazardscapes: An explanatory frame applied to disaster at the US–Mexico border

This paper advances understanding of how unequal risks to environmental hazards are generated. Marginalization is the best known explanation for the production of risk. The concept of marginalization was elaborated through studies of hazards in the global South and connotes how social inequalities constrain livelihood options of less powerful social groups. Thus, marginal groups are pressured to degrade landscapes and occupy hazardous environments while they experience decreased capacities to cope with environmental change. This paper directs analytical attention beyond the material to include the discursive realm and demonstrates that the production of unequal risk is contingent upon how hazards are differentially perceived, represented, and contested in social spaces. Findings from a flood disaster case study in the El Paso (US)–Ciudad Juárez (Mexico) border metropolis highlight how hegemonic discourses reinforce material processes of marginalization in flood-prone social spaces. Findings also reveal how socially-powerful geographical groups of people have harnessed institutional resources in their efforts to externalize risks and capitalize on environmental opportunities in some flood-prone areas. The concept of facilitation is used to explain the material-discursive production of socially-elite, flood-prone spaces. Facilitation clarifies how powerful groups are provided privileged access to institutional resources in their pursuit of environmental rewards, contributing to unjust socioenvironmental outcomes. In conclusion, I outline key aspects of how unequal risks are materially and discursively (re)produced within hazardscapes through relational processes of marginalization/facilitation.     

The potential for paleolimnology to determine historic sediment delivery to rivers

Establishment of water quality criteria to guide catchment sediment management is required by the European Union (EU) Water Framework Directive. The topic, however, is hotly contested among scientists and policy makers. Existing legislation with regard to fine sediment was set by the EU Freshwater Fish Directive. Its guideline, i.e. mean annual suspended sediment concentration, is 25 mg l⁻¹. Such a static target fails to capture the episodic nature of sediment transport. Furthermore, application of such global standards is inappropriate for a pollutant that is strongly controlled by spatial variation in key catchment drivers. Paleolimnology offers an approach for assessing background sediment pressures on watercourses, enabling determination of values for times pre-dating agricultural intensification. We propose that Modern Background Sediment Delivery to Rivers (MBSDR) across England and Wales can be determined using paleolimnology to quantify maximum feasible sediment reduction. No management programme should aim to reduce sediment loss to values below those resulting from background, natural physiographic and/or hydrological controls. Lacking generic tools to quantify process linkages between sediment pressures and biological impact, we propose that MBSDR could be taken to represent ecological demand for sediment inputs into watercourses required to support healthy aquatic habitats. In situations where generic tools exist for coupling sediment pressures and ecological impacts, assessment of MBSDR could be used to correct the gap between current or future projected sediment loss and biological condition. Existing paleolimnological data on sediment yields across England and Wales are presented to illustrate the approach and provide preliminary national estimates of MBSDR. We briefly consider the basis for reconstructing sediment yields using a paleolimnological approach and analyse temporal trends in published sediment yield, inferred for a range of landscape types. We also attempt to correlate sediment accumulation rates (SARs) with sediment yields to extend the MBSDR data base. Preliminary maps were generated to identify regions where further sediment yield data are needed to produce a more robust estimate of the spatial distribution of MBSDR across England and Wales.     

Dynamic modeling suggests terrace zone asymmetry in the Chicxulub crater is caused by target heterogeneity

We investigate the cause of terrace zone asymmetry in the Chicxulub impact crater using dynamic models of crater formation. Marine seismic data acquired across the crater show that the geometry of the crater's terrace zone, a series of sedimentary megablocks that slumped into the crater from the crater rim, varies significantly around the offshore half of the crater. The seismic data also reveal that, at the time of impact, both the water depth and sediment thickness varied with azimuth around the impact site. To test whether the observed heterogeneity in the pre-impact target might have affected terrace zone geometry we constructed two end-member models of upper-target structure at Chicxulub, based on the seismic data at different azimuths. One model, representing the northwest sector, had no water layer and a 3-km thick sediment layer; the other model, representing the northeast sector, had a 2-km water layer above a 4-km sediment layer. Numerical models of vertical impacts into these two targets produced final craters that differ substantially in terrace zone geometry, suggesting that the initial water depth and sediment thickness variations affected the structure of the terrace zone at Chicxulub. Moreover, the differences in terrace zone geometry between the two numerical models are consistent with the observed differences in the geometry of the terrace zone at different azimuths around the Chicxulub crater. We conclude that asymmetry in the pre-impact target rocks at Chicxulub is likely to be the primary cause of asymmetry in the terrace zone.