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In recent years, an increasing number of studies have used single-grain age dating of detrital sediments to obtain information on sediment provenance, the thermal history and exhumation of sediment source areas, and landscape evolution. In order to highlight the value of this current development in geological research, we based this volume on papers presented in a session on detrital thermochonology at the 2002 Geological Society of America Annual Meeting held in Denver, Colorado. This Special Paper provides a short overview of the different, nowadays commonly used, dating techniques in detrital thermochronology, followed by discussion of some of the methodological aspects of detrital thermochronology (in the first two manuscripts) and of a variety of applications of detrital thermochronology, demonstrated in several regional case studies. <...>
The United States and the individual states have followed a legal system based on Roman law, which allows private ownership of oil and gas interests. Nevertheless, the federal and state governments control the leasing of about one-third of the oil and gas interests underlying the onshore landmass of the United States (estimated at 2.2 billion acres). They also control 100% (97% federal and 3% state) of the offshore areas of the United States (estimated at 885.6 million acres). In additional, 54 million acres of Indian lands are subject to U.S. government leasing control, and leasing rights to 44 million acres in Alaska are controlled by Alaskan natives (Mineral Management Service, 1989; Petroleum Independent, 1991). <...>
The physical and mechanical properties of clay materials required to scale the tectonic analogue experiments are analysed based on the modern soil mechanics concepts. A practical procedure for the determination of these properties is presented. Scaled clay experiments under Riedel shear mechanism produced a series of typical pull-apart structures. At the initial stages of the development of the deformation zone within the clay slab, the synthetic shear fractures (Riedel shears) display dilatational behaviour.
Activities in mineral deposit modeling have continued to develop on several fronts since the publication of “Mineral Deposit Models,” edited by Cox and Singer (1986). That bulletin is a collection of 87 descriptive deposit models and 60 grade and tonnage models prepared by many authors both from within and outside of the U.S. Geological Survey. The present bulletin continues that effort with the addition of new or revised models. Before these models are introduced, a review of modeling as used here is provided as well as an overview of mineral deposit modeling since the publication of Cox and Singer (1986).
Clay minerals occur in all types of sediments and sedimentary rocks and are a common constituent of hydrothermal deposits. They are the most abundant minerals in sedimentary rocks perhaps comprising as much as 40% of the minerals in these rocks. Half or more of the clay minerals in the earth's crust are illites, followed, in order of relative abundance, by montmorillonite and mixed-layer illite-montmorillonite, chlorite and mixed-layer chlorite-montmorillonite, kaolinite and septachlorite, attapulgite and sepiolite. The clay minerals are fine-grained. They are built up of tetrahedrally (Si, A l , Fe3+) and octahedrally (Al, Fe3+, Fez+, Mg) coordinated cations organized to form either sheets or chains. All are hydrous. <...>
The Devonian was a critical period with respect to the diversification of early terrestrial ecosystems. The geotectonic setting was characterized by the switch from the post-Caledonian to the pre-Variscan situation. Plant life on land evolved from tiny tracheophytes to trees of considerable size in combination with a global increase in terrestrial biomass, and vertebrates started to conquer the land. Extensive shallow-marine areas and continental lowlands with a wide range of different habitats existed. These are preserved in a large number of basins all around the world. Climate change finally led from greenhouse to icehouse conditions towards the end of the Devonian. Rapid evolution of terrestrial ecosystems and climate change had a pronounced influence on sedimentation and biodiversity, not only in the terrestrial, but also in the marine realm. This volume contains case studies from Australia, China, Europe, South America and North America, and individual palaeoecosystems and their components have been investigated in different palaeogeographic settings that contribute to a much better understanding of the Devonian Period. This is a contribution to the IGCP 499 project on "Devonian land-sea interaction: evolution of ecosystems and climate”. <...>
The Devonian was a peculiar time in the Phanerozoic evolution of the Earth. Most continents, including the large Gondwana and Laurussia cratons, formed a Pangea-type assembly around the tropical Prototethys and an increasingly hot, global, greenhouse climate prevailed, with a complete lack of major ice sheets, even in polar areas. There was gradual and increasing flooding of the continents, creating huge epicontinental seas that have no modern analogues. Under these conditions the plants finally conquered the land, with the innovation of deep roots in the Emsian, the appearance of seed precursors and trees in the Givetian, and the spread of vegetation into dry uplands in the late Famennian. In the marine realm, the largestknown Phanerozoic tropical reef belts surrounded craton margins and tropical islands. It was the time of the sudden radiation of early ammonoids, of the earliest episodic blooms of calcareousshelled, pelagic zooplankton (tentaculitoids), the rise to dominance of fishes, mostly of armoured forms and with giants reaching 10 m in length, but also including the first sharks, and the appearance of earliest tetrapods in marginal settings. However, the tropical and subtropical areas reaching up to 458 latitude were hardly a paradise. <...>
The Dexing porphyry copper field in Jiangxi, China, is defined by three porphyry copper deposits which are, from southeast to northwest, Fujiawu, Tongchang and Zhushahong respectively, and by the Guanmaoshan gold deposit which lies between Fujiawu and Tongchang. Technically, the field lies on the southeastern edge of the Jiangnan Anteclise, and is controlled by the NE-trending, deep-seated, Gandongbei fracture zone. The emplacement of the ore-bearing Fujiawu, Tongchang and Zhushahong granodiorite porphyry intrusions, dated at 184-172 Ma (Zhu et ah, 1983; Zhu et al9 1990), was also controlled by NW-trending structures. Mineralisation and alteration continued from 172 Ma to 100 Ma, and are characterised by symmetric zoning centred on the contacts between granodiorite porphyries and the enclosing country rocks of the Mesoproterozoic Shuangqiaoshan Group jrfryllites.
The absolute and relative time of subduction of rocks is crucial information for subduction-exhumation models. We investigated the timing of subduction in one of the oldest ultra-high pressure (UHP) localities worldwide: the Kokchetav massif in Kazakhstan. SHRIMP ion microprobe dating of monazite from coesite-bearing micaschists of the Kulet unit indicates that subduction occurred between ~500-520 Ma. This new data provides evidence that the Kulet unit underwent UHP metamorphism 10-15 Ma later than the diamond-facies rocks in the nearby Kumdy-Kol unit. This time constrain excludes models that argue for a simultaneous evolution of coesite- and diamond-facies rocks, it suggest that subduction continued well after continental crust was involved, and that exhumation was not initiated by a single event such as slab break-off. The dynamic of this UHP massif also indicates that Cambrian tectonic was similar to that of recent orogenic belts.
Diamond deposits can be classified as primary (kimberlites and lamproites) and secondary (alluvial and marine). In 1995, their relative production contributions in South Africa were, primary (kimberlite) 8.63 million carats (89% of total), alluvial 960,000 carats (10%). and marine 90.000 carats (1%). The percentage of gems in kimberlites is highly variable. Published figures suggest approximately 40% for the Kimberley mines and 55% for Premier mine. The proportion of gems in the West Coast marine deposits is over 98%. The MIBA mine in the Congo and the Argyle mine in Australia have an average gem content of only 5%. The dispersal of diamonds from their primary sources into streams and rivers and ultimately to the sea is generally accompanied by an increase in average value per carat.| as flawed stones are progressively destroyed with greater and greater transport.
