Sedimentary geology of the Palaeoarchaean Buck Ridge (South Africa) and Kittys Gap (Western Australia) volcano-sedimentary complexes

The two Palaeoarchaean volcano-sedimentary complexes of the Buck Ridge (Barberton Greenstone Belt, South Africa) and Kittys Gap (Coppin Gap Greenstone Belt, East Pilbara, Australia) have a similar geological setting and age (∼3.45 Ga). The predominantly volcaniclastic sediments are concentrated at the top of these complexes, and experienced thorough, (very) early diagenetic silicification. In many places the silicification process has led to excellent preservation of the primary sedimentary structures. Elsewhere it has resulted in their obliteration or replacement by diagenetic structures. The Buck Ridge chert forms a regressive-transgressive succession, deposited around base level, with lacustrine and littoral marine facies. Deposition of the Kittys Gap Chert was also close to base level, almost exclusively subaqueous, with tidal influence and a regressive sequential trend.     

Repeated glacial-lake outburst floods in Patagonia: an increasing hazard?

Five similar glacial-lake outburst floods (GLOFs) occurred in April, October, December 2008, March and September 2009 in the Northern Patagonia Icefield. On each occasion, Cachet 2 Lake, dammed by the Colonia Glacier, released circa 200-million m³ water into the Colonia River. Refilling has occurred rapidly, such that further outbreak floods can be expected. Pipeflow calculations of the subglacial tunnel drainage and 1D hydraulic models of the river flood give consistent results, with an estimated peak discharge surpassing 3,000 m³ s^?1. These floods were larger in magnitude than any flood on record, according to gauged data since 1963. However, geomorphological analysis of the Colonia valley shows physical evidence of former catastrophic outburst floods from a larger glacial-lake, with flood discharges possibly as high as 16,000 m³ s^?1. Due to potential impacts of climate change on glacier dynamics in the area, jökulhlaups may increase future flood risks for infrastructure and population. This is particularly relevant in view of the current development of hydropower projects in Chilean Patagonia.     

Consistent past half-century trends in the atmosphere, the sea ice and the ocean at high southern latitudes

Simulations performed with the climate model LOVECLIM, aided with a simple data assimilation technique that forces a close matching of simulated and observed surface temperature variations, are able to reasonably reproduce the observed changes in the lower atmosphere, sea ice and ocean during the second half of the twentieth century. Although the simulated ice area slightly increases over the period 1980–2000, in agreement with observations, it decreases by 0.5 × 10⁶ km² between early 1960s and early 1980s. No direct and reliable sea ice observations are available to firmly confirm this simulated decrease, but it is consistent with the data used to constrain model evolution as well as with additional independent data in both the atmosphere and the ocean. The simulated reduction of the ice area between the early 1960s and early 1980s is similar to the one simulated over that period as a response to the increase in greenhouse gas concentrations in the atmosphere while the increase in ice area over the last decades of the twentieth century is likely due to changes in atmospheric circulation. However, the exact contribution of external forcing and internal variability in the recent changes cannot be precisely estimated from our results. Our simulations also reproduce the observed oceanic subsurface warming north of the continental shelf of the Ross Sea and the salinity decrease on the Ross Sea continental shelf. Parts of those changes are likely related to the response of the system to the external forcing. Modifications in the wind pattern, influencing the ice production/melting rates, also play a role in the simulated surface salinity decrease.     

Reconstruction of the hydrothermal history of the Cu–Ag vein-type mineralisation at Dikulushi, Kundelungu foreland, Katanga, D.R. Congo

The Kundelungu foreland, north of the Lufilian arc in the Democratic Republic of Congo, contains a number of various vein-type and stratiform copper mineralisations. The geodynamic context and metallogenesis of these mineral occurrences remain enigmatic. Currently, the vein-type Cu–Ag ore deposit at Dikulushi is the most significant deposit in the region. Mineralisation at Dikulushi comprises two major styles: 1) a polysulphide assemblage (Zn–Pb–Fe–Cu–As) within brecciated rocks along an anticlinal closure; and 2) a vein-hosted Cu–Ag assemblage. Petrographic and fluid inclusion studies indicate that the early Zn–Pb–Fe–Cu–As assemblage formed from a high-salinity Ca–Na–Cl fluid of modest temperature (135–172 °C). The later, economically more significant vein-related Cu–Ag mineralisation formed from intermediate salinity, lower temperature (46–82 °C) Na–Cl fluids. Weathering of the sulphide minerals resulted in a supergene enrichment with the formation of secondary Cu-minerals.     

The Political Geography of the Post Cold War World*

The end of the Cold War is marked by dramatic events like the reunification of Germany and the crisis of the continued existence of the Soviet Union. The current changes alter the fundamental building-blocks of global international order and can be thought of as constituting a geopolitical transition. This collection of essays resulted from a panel discussion held at the annual meeting of the Association of American Geographers in Miami in April 1991. The seven essays present the views of the panel participants on the current geopolitical transition and their expectations about the nature of the new world order.     

Upscaling of Hooke''s Law for Imperfectly Layered Rocks

In this paper, we consider the upscaling of Hooke's law and its parameters on the fine scale, to a similar law with upscaled parameters on a larger scale. It is assumed that the fine scale material properties of the rock are imperfectly layered. In the governing equations, the deviations from perfect layering introduce a small parameter that can be used in perturbation series expansions for the stress, the strain, and the displacement. In the approximation of order zero the upscaled compliance matrix contains the well-known Backus parameters; this approximation holds exactly for a perfect layering. However, many natural rock types are imperfectly layered and in that case the approximation of order zero may not be sufficiently accurate. Therefore, we consider also the first order corrections. The derivation and results are presented both for the most general case and for the much simpler case in which the fine scale Poisson ratio may be assumed constant. From thermodynamic principles, it follows that the compliance tensor is symmetric on the fine scale. However, it is shown that the argument for symmetry cannot be extended to upscaled rigidities. One of the most important conclusions is that upscaled compliance tensors are nonsymmetric when there are trends in the deviations from perfect layering.