ALBERT

Description: The chemical composition of gold within the Archaean metasedimentary rocks of the Witwatersrand Supergroup displays significant heterogeneity at the micro-, meso-and regional scales. A detailed electron microbeam analytical and petrological study of the main auriferous horizons in the Central Rand Group throughout the Witwatersrand Basin indicates that gold has been remobilized late in the paragenetic sequence over distances of less than centimetres. Contemporaneous chlorite formation was strongly rock-buffered. Gold mobilization occurred under fluid-poor conditions at temperatures that did not exceed 350 {degrees}C. Widespread circulation of mineralizing fluids within the Central Rand Group is not supported by the gold and chlorite chemical data. Brittle deformation that affects most of the paragenetic sequence of the Central Rand Group late in its post-depositional history is followed by sequences of mineral growth and dissolution that appear throughout the Central Rand Group and have consistent textural relationships with gold. The consistent location within the paragenetic sequence, the wide regional and stratigraphic extent of the brittle deformation, together with mineral chemical and petrological data suggest that the Vredefort Impact Event (2.02 Ga) was the cause of this late deformation, and that post-impact fluid-poor metamorphism resulted in crystallization of a significant proportion of the gold on and within mineral grains that were deformed during this event.

Description: Locating sinkholes in Pinellas County, Florida, is confounded by the presence of a cover of Quaternary sediments that mute the surface appearance of these sinkholes. As a first step in addressing the sinkhole hazard in the county, we analysed aerial photographs from 1926 and 1995 that covered the entire county. We digitized all identifiable sinkholes in each set of photographs in a GIS (Geographical Information System) using a set of criteria established to differentiate between karst depressions and depressions resulting from other geological processes. The 1926 photographs, although of low quality, helped to establish a baseline prior to urbanization. The 1995 photographs provided a post-urbanization distribution of natural sinkholes and man-made depression features (e.g. retention ponds). From these two data sets, we are able to assess natural and anthropogenic changes in the karst landscape of the study area. In particular, we discovered that 87% of the sinkhole features identified in the 1926 photographs are no longer present in the photographs from 1995. Many of the lost depressions have been incorporated into retention ponds.

Description: The low anisotropies of paramagnetic granites, due to magnetocrystalline anisotropy, require a statistical treatment of the anisotropy of magnetic susceptibility (AMS) data when systematic fabric studies are performed. Absence of statistical information on these data makes evaluation of their quality difficult. The statistical significance of magnetic fabric in granites is evaluated in this paper. Jelinek's elliptical confidence angles for the three principal susceptibility axes (E13, E12, E23) of a specimen are used as markers of the quality of the AMS data. Comparing these markers at sample-, site- and massif-scale with the mean AMS axes that result from spherical statistical models helps clarify the reliability of the AMS data. This analysis is presented in detail for the plutons of Veiga and Trives (Spain). It is then applied to seven other massifs from the Pyrenees. We propose the following guides: (1) fabrics with E13 between 10{degrees} and 20{degrees} tend to isotropy; the directional data and the shape parameter should be considered with great care; (2) lineation is not reliable when E12 > 25{degrees}, i.e. when Kmax is almost the same as Kint; (3) similarly, foliation is considered as not reliable when E23 > 25{degrees}, i.e. Kmin does not easily differentiate from Kint. Errors attached to the mean Kmax and Kmin axes should always be produced, thus allowing further interpretation. In Trives and Veiga, perfect' triaxiality cannot be automatically assumed since foliation and lineation could be defined simultaneously in only 53% of the cases. Finally, a minimum of three cores (9 specimens) per site would considerably increase the proportion of reliable orientation data.

Description: The region of the gold-rich Ashanti belt in southern Ghana was chosen as the subject for a detailed regional thermal modelling study. Geological studies, in addition to laboratory measurements of thermal properties and heat-production rates, allow us to constrain a finite-element thermal modelling. Scenarios intergrating variations of the structure of the crust and various chronological settings were examined. We calculated the thermal regime before and after the thrust tectonism that affected the region during the Eburnean orogeny (2130-2095 Ma), just before ore deposit formation. This gives a new insight into the regional thermal state of the crust before the mineralizing events. To satisfy the thermobarometric observations, the most probable mantle heat flow must be 60 mW m-2, which is at least three times greater than the present-day value. At shallow depths, our results also indicate anomalies of lateral heat flow reaching 25 mW m-2, focused on the margins of each lithological unit, including the Ashanti belt. These anomalies are related to the distortion of the isotherms in the first few kilometres that can be explained mostly by lateral contrasts in thermal conductivity. Such anomalies could be of importance for the mineralizing events, as they would favour fluid circulation locally.

Description: Well performance is extremely variable in the stacked sequence of relatively tight Devonian and Mississippian carbonates in the northern part of the Waterton Complex, Alberta, Canada. This is despite having an extensively developed fracture system present in all the wells. In order to determine why some wells penetrated more permeable fractures than others, a full reinterpretation of the geophysical, structural, stress, matrix and dynamic data sets was carried out. Flow simulations at sector scales using discrete fracture network models and fullfield continuum modelling were used to test a range of geological and dynamic scenarios. One of the most northwestern fields of the Waterton complex, the West Carbondale field, is the focus of the work presented. For this field the best-fit dynamic models consist of a major fracture zone, corresponding to either a seismic scale lineament or zone of enhanced curvature, trending through the area of most prolific wells. Outside this zone, the vast majority of the fracture system makes little contribution to the flow in the wells, other than slightly enhancing the reservoir permeability.

Description: In order to improve predictions of coastal morphological response to sea-level rise and sustainably manage dredged sediment there is an urgent need to develop a field methodology that can measure accurately transport pathways of the 〈63 {micro}m sediment fraction in coastal and estuarine environments. Techniques such as sediment trend analysis and sediment tracing using fluorescent sands are well established for the silt and sand fraction but are unsuitable for clay sediments due to their cohesive nature. Geochemically labelled clays have been used as fine sediment tracers in freshwater environments with some success, although little is known about their chemical or physical behaviour once released in saline environments. A number of pure clays and natural estuarine sediments were labelled with La following agitation in a 0.01 M solution of La Cl3. In order to examine the retention of La on the clay mineral surface the labelled sediment was washed sequentially four times using both de-ionised water and artificial seawater. A labelled bentonite retained 43000 {micro}g g-1 La and this was only reduced to 36000 {micro}g g-1 La after washing in seawater. This suggests that retention of La is good even in saline conditions and concentrations of La are high enough to enable detection after considerable signal dilution. Sorption of La is dependent predominantly upon the cation exchange capacity of the sediment.

Description: An area of focused fluid venting off NE Sakhalin, Sea of Okhotsk, was investigated in 2003 during the 31st and 32nd international expeditions of R/V Akademik M. A. Lavrentyev within the framework of the CHAOS Project. More than 40 structures related to seafloor gas venting were discovered and gas hydrates were sampled from three of these: CHAOS, Hieroglyph and Kitami. Geochemical analyses were used to define the mechanisms of gas hydrate accumulation and the sources of fluids involved. Chemical and isotopic analyses of the interstitial and hydrate waters suggest that hydrates were formed from seawater (or in-situ pore water) and an ascending fluid enriched in salts. Hydrate formation occurs at locations of the most intensive saline water upflow, and this is probably a function of the gas solubility in water in equilibrium with hydrate. The water involved in gas hydrate formation consists of about 70% pore water derived from the host sediment and 30% from the ascending fluid. The overall isotopic composition of the fluid' taking part in hydrate formation was calculated as{delta} 2H{approx}-11{per thousand} and {delta}18O{approx}-1.5{per thousand}.

Description: In the North American Cordillera, crustal thickening, magmatism and flow of deep crust created an orogenic plateau, or series of related plateaux, in the Late Mesozoic-Early Cenozoic. From west to east, the plateaux extended from the continental arcs to the inboard crystalline belts of the Omineca-Sevier belt. From north to south, the plateaux ranged from British Columbia/SE Alaska to Baja California, Mexico. Although a vast region of western North America was characterized by thickened crust (60-70 km), unroofing of deep crust from 〉30 km was largely confined to the edges of the plateaux: the arcs and the eastern margins. Comparison of the unroofing histories of the Cordilleran arcs reveals that they differed dramatically from each other in the amount and style, but not timing, of exhumation. The northern Cordilleran arc and northern interior (Omineca) belt were exhumed from deep mid-crustal levels, with regional-scale Eocene extension accompanied by magmatism. In contrast, the central (Sierra Nevada) and southern (Peninsular Ranges) arcs were unroofed to much shallower levels (typically 〈15 km), primarily by erosion and local deformation. North to south differences in exhumation style and magnitude in the Cordilleran arcs may reflect differences in the degree of coupling between the subducting plate and the thickened continental lithosphere in the north v. south. In the northern Cordillera, relationships between Pacific-region plate activity and Tertiary continental extension/magmatism and deep exhumation suggest continued geodynamic coupling between subducting plates and orogenic crust following crustal thickening and plateau formation. In contrast, the central and southern Cordilleran arcs do not contain evidence for mechanical links with the subducting plate after the Late Cretaceous.

Description: We used analogue models to study the fault evolution produced by extension through a heterogeneous crust. In the experiments, the heterogeneous crust consisted of a gently dipping silicone layer surrounded by brittle material. The viscous silicone level simulates a weak, upper crustal nappe stack that formed during a previous phase of shortening. X-ray scanner facilities allowed us to acquire 3D images of the experimental models at regular time invervals and hence to study the fault pattern development and the location of the main depocenters during rifting. The experimental results show that the inherited weak nappe stack acts as a decollement and localizes deformation. In the early stages of extension a system of conjugate high-angle normal faults initiates close to the upper tip of the gently dipping silicone layer near the free surface and propagates upwards, resulting in an initial symmetrical graben configuration. Further extension results in (1) a progressive asymmetry of the rifted zone, due to migration of its right margin down the nappe, (2) a shift of the main depocentre downward along the decollement, and (3) the simultaneous activity of several normal faults within the rifted zone. When the pre-existing silicone layer is oblique to the extension, the normal faults develop in an en echelon array, with a strike intermediate between the azimuth of the gently dipping silicone layer and the extension direction. The experiments also show how rheological differences between areas with potential intracrustal weak layers and adjacent domains without decollement level can lead to significant differences in fault pattern, dimension and orientation of the rifted zone. Complete asymmetry of a rift and switches in fault dip direction between adjacent domains can be explained by the presence of pre-existing upper crustal heterogeneities.

Description: The conclusions of most previous studies on the eastward extension of the collision zone between the Sino-Korean and Yangtze Blocks can generally be divided into two categories: (1) the collision zone is connected to the Imjingang belt and crosses the Korean Peninsula; (2) the eastward extension of the collision zone does not enter the Korean Peninsula, nor is connected to the Imjingang belt. Recent geophysical studies on gravity and velocity tomography in the Yellow Sea and adjacent regions, respectively, have provided geophysical evidence for a nearly northsouth-trading dextral strike-slip fault in the eastern margin of the Yellow Sea, which we have named the East Marginal Fault of the Yellow Sea (EMFYS). The geophysical evidence indicates that the EMFYS extends to great depth. It dips westward, and within 100 km depth the dip angle is very steep. The geophysical characteristics on its two sides show that they belong to different tectonic units. This fault is connected to the WulianQingdao Fault in the north, and to the South Marginal Fault of Jeju Island (SMFJI) in the south (the SMFJI is a part of the collision zone). Therefore the EMFYS is considered as a part of the junction zone between the Sino-Korea and Yangtze Blocks, with the Korean Peninsula being part of the Sino-Korea Block. In the Late Triassic, it is inferred that dextral strike-slip movement took place on the EMFYS, and in the same geological period sinistral strike-slip took place on the Tan-Lu Fault Zone. Under northsouth tectonic stress the Yangtze Block was translated northward and inserted into the Sino-Korea Block. Therefore the junction zone between the two blocks formed a gigantic Z-shaped tectonic belt. According to the gravity data and seismic tomography it is also inferred in this paper that the junction zone between the Yangtze and Cathaysia Blocks extends from the Jiangshao Fault eastward to the southern edge of the Hida Block, Japan.