Surficial and Deep Earth Material Prediction from Geochemical Compositions

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.

     

Shock deformation confirms the impact origin for the Cerro do Jarau,Rio Grande do Sul,Brazil, structure

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 30^o12′S and 56^o32′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.     

Direct dating of thick‐ and thin‐skin thrusts in the Peruvian Subandean zone through apatite (U–Th)/He and fission track thermochronometry

Although the structure of the central Peruvian Subandean zone is well defined, the timing of thrust‐related exhumation and Cenozoic sedimentation remain poorly constrained. In this study, we report new apatite (U–Th)/He (AHe) and fission track (AFT) ages from thrust‐belt and foreland strata along three published balanced cross sections. AHe data from the northern, thick‐skinned domain (i.e. Shira Mountain, Otishi Cordillera and Ucayali Basin) show young AHe ages (ranging from 2.6 ± 0.2 to 13.1 ± 0.8 Ma) compared with AFT ages (ranging from 101 ± 5 to 133 ± 11 Ma). In the southern Camisea Basin, where deformation is mainly thin‐skinned, AHe and AFT ages have been both reset and show young cooling ages (3.7 ± 0.8 Ma and 8 ± 2 Ma respectively). Using low‐temperature thermochronology data and the latest fission track annealing and He diffusion codes, the thermal history of the study area has been reconstructed using inverse modelling. This history includes two steps of erosion: Early Cretaceous and late Neogene, but only Neogene sedimentation and exhumation varies in the different sectors of the study area. From a methodological point of view, large AHe data dispersion point to the need for refinement of AHe damage and annealing models. The influence of grain chemistry on damage annealing, multiple age components and the possibility of fission tracks as traps for He need further consideration. For the central Peruvian Subandes, AHe and AFT ages combined with balanced cross sections emphasize the dominant control of Paleozoic inheritance rather than climate on Cenozoic infilling and exhumation histories. Finally, our data provide the first field example of how thick‐skinned thrust‐related deformation and exhumation in the Subandes can be directly dated through AHe thermochronology.     

A 3D gravity data interpretation of the Matagami mining camp, Abitibi Subprovince, Superior Province, Québec, Canada: Application to VMS deposit exploration

Two inversions, unconstrained and constrained, of a gravity survey of the Matagami mining camp (Abitibi Archaean Subprovince, Canada) have been performed in order to identify the downward extension of a rhyolitic horizon hosting VMS-type base metals deposit and the morphologies of the major felsic plutons. A comparative study exhibits the similarities between measured and calculated densities from chemical compositions of the Matagami lithologies. This allows building an initial 3D geodensity model which integrates densities and available structural and geological surface mapping data. This model is integrated during the iteration process of the constrained inversion in the objective function. The resulting true density model and two derived cross-sections upgrade the 3D imaging of this area. Also, the model gives new insight for regional geological interpretation exposing possible shapes of the main geological units at depth and suggests the potential existence of deep fertile geological bodies.     

Middle and Late Carboniferous extension In the Variscan Belt: structural and petrological evidences from the Vosges massif (Eastern France)

Abstract

In 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.     

Towards a More Equitable Use of Mineral Resources

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.

     

Increasing the usability and accessibility of geodynamic modelling tools to the geoscience community: UnderworldGUI

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 .

Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire,French Massif Central)

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.     

LINET—An international lightning detection network in Europe

During the past years a VLF/LF lightning detection network (LINET) was developed at the University of Munich, which provides continuous data for both research and operational purposes. In particular, the network introduces five new features: a) total lightning capability: both cloud-to-ground strokes (CG) and cloud lightning (IC) are measured; b) low-amplitude reporting: weak lightning events from discharge channel with currents well below 5 kA are detected within the central part of the network, whereby IC events dominate; c) new 3D-discrimination: a time-of-arrival method is utilized to separate CG from IC with good reliability, provided that the sensor baseline does not exceed ~ 250 km; d) IC emission height: for each cloud event a height is determined which is thought to reflect the central region of the involved channel; and e) optimised location accuracy: due to precision and combined action of all influential network components, complemented by site-error corrections, the position accuracy of strokes reaches an average value as small as ~ 150 m, whereby false locations (‘outliers’) rarely occur. During international co-operations LINET has been deployed in four continents: Europe (initially Germany), South America (area of Bauru, Brazil), Australia (around Darwin), and Central Africa (Benin). Since the features quoted above could be verified in the tests, a 65-sensor network was established in Europe and started on May 1, 2006, in co-operation with the service company nowcast. LINET covers a wide area approximately from longitude − 10° to 25° to latitude 35° to 66°; it is available for scientific projects and officially utilized by the German Weather Service for operational purposes. Meanwhile, the network was extended by deployment of additional sites so that it comprises about 90 sensors in 17 countries.