AbstractIn the Moldanubian domain of the Vosges massif (INK France) euperimposition of three distinct crustal units has been attributed to Middle to Late Carboniferous thrusting. Л kinematic analysis of mierostuetures within each unit suggests that extension, following the formation of a Stack of nappes, is actually responsible for the bulk structure of this region. In order to estimate the related exhumation, the temperature and pressure evolution of the lowermost unit is investigated. It is characterised by (i) a prograde evolution within the stability field of kyanite, followed by (ii) a syn-kinematic, 3-4 kbar, near-isothermal decompression before (iii) cooling. Thermal modeling shows that the isothermal decompression may be related to rapid exhumation (> I mm.a-1), which cannot be accounted for by erosion alone. Therefore, exhumation is best explained by extensional processes, possibly related to gravitational collapse of a thickened crust. 相似文献
The size and composition of microartifacts (objects less than 2 mm) and macroartifacts (objects greater than 2 mm) are utilized to interpret the formational processes of an archaeological site. Because grain-size distributions are known to reflect mode of transport and source, the attribute of size is examined by plotting each artifact class (unmodified rock, modified rock, ceramic, concretion, bone/shell, and metal) in size frequency diagrams. the plots allow the agent and mode of transport to be reconstructed, which in turn allows the formational processes of the site to be interpreted. the example used is the Pelts Site (23-DU-29) of southeastern Missouri. the site was chosen for this pilot study because the formation processes can be reconstructed from other data. the size distributions of each artifact class support those reconstructions and allow the proposed method to be evaluated as an interpretive method for other archaeological sites. 相似文献
Prediction of true classes of surficial and deep earth materials using multivariate spatial data is a common challenge for geoscience modelers. Most geological processes leave a footprint that can be explored by geochemical data analysis. These footprints are normally complex statistical and spatial patterns buried deep in the high-dimensional compositional space. This paper proposes a spatial predictive model for classification of surficial and deep earth materials derived from the geochemical composition of surface regolith. The model is based on a combination of geostatistical simulation and machine learning approaches. A random forest predictive model is trained, and features are ranked based on their contribution to the predictive model. To generate potential and uncertainty maps, compositional data are simulated at unsampled locations via a chain of transformations (isometric log-ratio transformation followed by the flow anamorphosis) and geostatistical simulation. The simulated results are subsequently back-transformed to the original compositional space. The trained predictive model is used to estimate the probability of classes for simulated compositions. The proposed approach is illustrated through two case studies. In the first case study, the major crustal blocks of the Australian continent are predicted from the surface regolith geochemistry of the National Geochemical Survey of Australia project. The aim of the second case study is to discover the superficial deposits (peat) from the regional-scale soil geochemical data of the Tellus Project. The accuracy of the results in these two case studies confirms the usefulness of the proposed method for geological class prediction and geological process discovery.
Cerro do Jarau is a conspicuous, circular morpho‐structural feature in Rio Grande do Sul State (Brazil), with a central elevated core in the otherwise flat “Pampas” terrain typical for the border regions between Brazil and Uruguay. The structure has a diameter of approximately 13.5 km. It is centered at 30o12′S and 56o32′W and was formed on basaltic flows of the Cretaceous Serra Geral Formation, which is part of the Paraná‐Etendeka Large Igneous Province (LIP), and in sandstones of the Botucatu and Guará formations. The structure was first spotted on aerial photographs in the 1960s. Ever since, its origin has been debated, sometimes in terms of an endogenous (igneous) origin, sometimes as the result of an exogenous (meteorite impact) event. In recent years, a number of studies have been conducted in order to investigate its nature and origin. Although the results have indicated a possible impact origin, no conclusive evidence could be produced. The interpretation of an impact origin was mostly based on the morphological characteristics of the structure; geophysical data; as well as the occurrence of different breccia types; extensive deformation/silicification of the rocks within the structure, in particular the sandstones; and also on the widespread occurrence of low‐pressure deformation features, including some planar fractures (PFs). A detailed optical microscopic analysis of samples collected during a number of field campaigns since 2007 resulted in the disclosure of a large number of quartz grains from sandstone and monomict arenite breccia from the central part of the structure with PFs and feather features (FFs), as well as a number of quartz grains exhibiting planar deformation features (PDFs). While most of these latter grains only carry a single set of PDFs, we have observed several with two sets, and one grain with three sets of PDFs. Consequently, we here propose Cerro do Jarau as the seventh confirmed impact structure in Brazil. Cerro do Jarau, together with Vargeão Dome (Santa Catalina state) and Vista Alegre (Paraná State), is one of very few impact structures on Earth formed in basaltic rocks. 相似文献
A total of 120 grab samples of the surficial sediments in the Cap-Breton submarine canyon and surrounding continental shelf were collected and analyzed by grain-size sieving. A Q-mode Factor Analysis was made on the grain-size data in order to define the most meaningful facies types. Four distinct lithological facies were found to exist: silt and clay, very fine sand, fine sand, and coarse sand. Comparison with previous work and a 14C date on the silt and clay facies showed that the facies are not contemporaneous. The sands and coarse sands on the shelf were emplaced during the pre-Würm and Würm regressions, and later probably reworked during the Holocene (Flandrian) transgression. The silty clays found in the canyon and on the shelf to the south are younger and represent sediments brought in as suspended load by the Adour and other nearby rivers during the Holocene (Flandrian) transgression. 相似文献
Minerals and the metals derived therefrom are non-renewable resources that deliver a wide range of goods and services to mankind. While their value has been recognized since thousands of years, their large-scale industrial production only really took off after World War II, thanks to the availability of efficient industrial production processes and of a rapidly rising demand, driven by demographic growth, urbanization and growing economic affluence in developed countries. The fast development of the Chinese economy, with about 300 million people reaching middle-class status and migrating to cities, drove a boom demand for minerals and metals of unprecedented magnitude in terms of tonnages consumed, that lasted from 2002 to 2013. Over a century, from 1914 to 2013, the production and consumption of pig iron, manganese and copper grew by a factor of 20, that of chromium by a factor of 182 and that of phosphate by a factor of nearly 42, while humanity grew only by a factor of about 4. Driven by demographic growth, continuing urbanization and growth of the global middle class the demand for minerals and metals will continue to exponentially grow. A scenario is proposed assessing what the theoretical requirements could be up to 2050 for minerals and metals commonly used in the construction industry, to develop infrastructure, for the mass production of metal intensive goods such as cars or in agriculture, to provide our food. The scenario, built on the trends observed since 1950, foresees that, by 2050, the demand for aluminium demand could be eight times the current production levels and the production of cement would need to grow by a factor of 7 to meet the 2050 level of demand. The lead demand would double by 2050. While geological scarcity does not appear to be an issue to feed such a demand, many factors are likely to limit the amount of minerals and metals that can be sustainably produced. Energy and water, both essential to mining, ore processing and metallurgy are likely to be limiting factors, as well as the sustainable management of the enormous amount of waste that would be generated. Public acceptance and access to exploration and mine development financing are other limiting factors to be considered. Inter-generational equity among all the stakeholders is concerned by mining, and intra-generational equity is necessary to ensure that the use of non-renewable natural capital provides benefits that last well beyond the closure of mining operations. This requires equity among stakeholders, including with nature, the silent stakeholder our lives depend on. Overcoming the challenges of the twenty-first century will require the following factors:
Decoupling economic growth from its unsustainable negative environmental impacts (UNEP 2011a), inter alia to reduce the pressure on natural resources;
Rapid development of a more circular economy based on resource efficiency and the systematic minimization of waste. Flanking policy measures (for instance: European Commission 2016) are needed to close the natural resources loop and avoid the dire predicaments of resources depletion;
Institutional strengthening and capacity building to ensure that sustainability agendas are developed and well integrated in corporate strategies and public policies; and
Trust among the stakeholders, based on corporate accountability, transparency, stakeholder engagement and reporting of the economic, environmental, governance and social performance of companies.
Failure to act on the above requirements at the global scale is likely to be a source of deep crisis where all humanity could lose.
Geoscientists are faced with a number of complexities that represent obstacles to the development of realistic simulation
of deep earth processes. Realistic 4D thermo-mechanical simulation using software packages like Underworld and Gale, when
combined appropriately with geoscientific expertise, can lead to novel insights into the deformation of geological structures
at a wide range of time and spatial scales. The challenge for end-user geoscientists lies in applying their knowledge within
the framework of the software’s input specification, including initial, internal, and boundary conditions and output visualization
parameters. We have built a Graphical User Interface (GUI) to remove many of the difficulties related to editing the Extensible
Markup Language (XML) encoded input files of Underworld/Gale geomodels and therefore, to greatly broaden the user base of
these software packages. By helping Underworld/Gale to meet a large audience, we provide a tool to the geoscience community
that helps to move from untested conceptual models to physically valid, properly scaled modelling. Furthermore, the UnderworldGUI
offers a mechanism for storing and retrieving experimental models in a centralised database, thus providing the geoscience
community with a means to share the outcomes of its experimental research. Further details of the UnderworldGUI are available
at the web site .
In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension-driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high-pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well-preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites record P ~ 1.5 ± 0.2 GPa at T ~ 700 ± 20°C, but differ from each other in whole-rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c. 310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome-margin eclogite zircon retains a widespread record of protolith age (c. 470–450 Ma, the same as host gneiss protolith age), whereas dome-core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust at c. 310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low-P/high-T conditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome. 相似文献