Tectonic Regime as a Control Factor for Crustal Fault Zone (CFZ) Geothermal Reservoir in an Amagmatic System: A 3D Dynamic Numerical Modeling Approach

Crustal fault zones provide interesting geological targets for high-temperature geothermal energy source in naturally deep-fractured basement areas. Field and laboratory studies have shown the ability of these systems to let fluid flow down to the brittle–ductile transition. However, several key questions about exploration still exist, in particular the fundamental effect of tectonic regimes on fluid flow in fractured basement domains. Based on poro-elasticity assumption, we considered an idealized 3D geometry and realistic physical properties. We examined a model with no tectonic regime (benchmark experiment) and a model with different tectonic regimes, namely a compressional, an extensional and a strike-slip tectonic regime. Compared to the benchmark experiment, the results demonstrate that different tectonic regimes cause pressure changes in the fault/basement system. The tectonic-induced pressure changes affect convective patterns, onset of convection as well as the spatial extent of thermal plumes and the intensity of temperature anomalies. Driven by poro-elastic forces, temperature anomalies around vertical faults in a strike-slip tectonic regime have a spatial extent that should be considered in preliminary exploratory phases

Corrugations plutoniques : génèse, fracturation et gisements porphyriques

National audienceLa plupart des gisements dits "porphyriques" se localisent aux sommets de petits stocks ou plutons cylindriques mis en place en subsurface, et issus d'une chambre magmatique plus profonde, généralement de grande taille horizontale. De nombreux travaux se sont focalisés sur la description géologique détaillée des systèmes porphyriques, ou sur les processus magmatiques au sein d'un réservoir acide, mais peu d'équipes ont regardé le problème de la génèse et du devenir de ces petits corps magmatiques, si souvent cartographiés en exploration minière. L'intérêt de comprendre ces structures réside non seulement dans une meilleure connaissance des processus magmatiques, mais également dans l'optique d'améliorer les guides de prospection minière à partir de la modélisation de ces systèmes particuliers. En effet, les fractures et les failles qui se concentrent au sommet de ces "apophyses" plutoniques permettent aux fluides hydrothermaux d'y déposer leurs minéraux et de former des gisements à cuivre et or d'importance mondiale. Il est largement reconnu que le degré de fracturation autour de ces stocks magmatiques joue un rôle primordial dans le potentiel minéralisateur de ces structures. L'étude présente tout d'abord un scénario suggérant la formation de corrugations aux toits des réservoirs lorsque ceux-ci sont définitivement installés dans la croûte supérieure. Lors de l'ascension d'un grand volume de magma, la présence d'un encaissant de plus en plus froid ralentit l'ascension, et les interactions mécaniques entre le réservoir et son encaissant prennent de plus en plus d'importance. La couche limite où se produit la cristallisation du magma ("mushy layer") se trouve alors soumise aux contraintes thermiques (conduction et convection solutale) mais aussi aux contraintes mécaniques imposées par l'interaction encaissant-magma. Pour les réservoirs de grande taille, l'ondulation de la limite fragile-ductile au cours de l'ascension du réservoir facilite donc la formation d'instabilités morphologiques lors de la cristallisation du magma. Les corrugations ainsi créées peuvent atteindre jusqu'à quelques kms de haut. Le deuxième aspect de l'étude consiste à examiner, au travers d'une modélisation numérique où les fractures ne sont pas prédéfinies, les conditions de fracturation au toit d'une apophyse plutonique, plus légère que les roches encaissantes. Le comportement mécanique des roches peut varier aussi bien avec la température qu'avec le régime de contraintes local (rhéologie de type cassant-élastique-ductile). Différentes valeurs pour la viscosité minimale équivalente du magma ductile et pour le contraste thermique sont testées, de même que le rôle de l'extension régionale. Le cas de deux apophyses rapprochées montre que seule l'une d'entre elles va concentrer les fractures, ce qui peut expliquer la présence simultanée de deux plutons similaires et voisins, l'un d'entre eux étant minéralisé et l'autre stérile. Une loi d'échelle reliant la distance entre deux apophyses et l'épaisseur de la couche fragile, permet de mieux comprendre la répartition des gisements porphyriques au-dessus d'une chambre magmatique sous-jacente de grande taille. Il reste cependant à savoir déterminer la profondeur de la transition fragile-ductile, qui ne dépend pas uniquement du régime thermique, mais également du champ de contraintes et des taux de déformation locaux

Spatial and temporal distribution of ore deposits, in relation with thermal anomalies

International audienceOre deposits have formed over billion years of Earth's history with a discontinuous distribution in time and space.In time, their formation is episodical, and the data concerning the main deposit types show an alternation of periods with mineralisation followed by time gaps, with no large amount of ore generation. The "Snowball Earth" theory suggests that Banded Iron formations (BIF) are in relation with periods of intense global glaciations of Paleoproterozoic (2500-1600 Ma) and Neoproterozoic (1000-540 Ma) ages. During global glaciations, surface temperatures were of the order of-50 deg C and an ice layer of about 1km covered almost the entire Earth. We have performed analytical and numerical modelling of such events that shows the development of high thermal anomalies in the crust. These anomalies can reach several tens of degrees for a time lapse that depends on the duration of the glaciation. If the thermal impact is clear, the mechanical impact of the glaciations on the crustal rocks still needs to be evaluated to precisely assess the role of glaciations as a possible cause to some of the gaps in the temporal distribution of ore deposits.Ore deposits have also a heterogeneous spatial distribution: they are located in specific places of the Earth's crust, where thermo-mechanical and hydrothermal conditions have triggered their formation. The temperature pattern corresponding to these cases can prove to be particularly favourable to ore deposits. A good example of such deposits, is the Ashanti belt in Ghana of which we present a detailed study. It is the key district of gold mineralisation in the Paleoproterozoic terrane of West Africa. This is the second giant concentration of gold deposits after South Africa with a potential of about 2500 tons of gold. The Eburnean orogeny operated between 2.13 and 1.98 Ga. Two tectonic phases affected the area, a period of thrusting and a second one corresponding to tran-scurrent tectonism. A numerical modelling was performed to constrain the thermal evolution of the belt. Conditions of pressure and temperature given by metamorphism data, indicate that a temperature higher than 600 deg C is required at depth close to 18 km to produce the expected partial melting. According to regional lithology, crustal structure and thermal properties, such conditions can only be achieved when the basal heat flow is greater than 60mW/m-2. Lateral variations between rock units have to be considered to explain anomalous heat transfer of this area

Plume head-lithosphere interactions near intra-continental plate boundaries.

Plume-lithosphere interactions (PLI) have important consequences both for tectonic and mineralogical evolution of the lithosphere: for example, Archean metallogenic crises at the boundaries of the West African and Australian cratons coincide with postulated plume events. In continents, PLI are often located near boundaries between younger plates (e.g., orogenic) and older stable plates (e.g., cratons), which represent important geometrical, thermal and rheological barriers that interact with the emplacement of the plume head (e.g., Archean West Africa, East Africa, Pannonian – Carpathian system). The observable PLI signatures are conditioned by plume dynamics but also by lithosphere rheology and structure. We address the latter problem by considering a free-surface numerical model of PLI with two stratified elasto-viscous-plastic (EVP) lithospheric plates, one of which is older and thicker than another. The results show that: (1) plume head flattening is asymmetric, it is blocked from one side by the cold vertical boundary of the older plate, which leads to the mechanical decoupling of the crust from the mantle lithosphere, and to localized faulting at the cratonic margin; (2) the return flow from the plume head results in sub-vertical down-thrusting (delamination) of the lithosphere at the margin, producing sharp vertical cold boundary down to the 400 km depth; (3) plume head flattening and migration towards the younger plate results in concurrent surface extension above the centre of the plume and in compression (pushing), down-thrusting and magmatic events at the cratonic margin (down-thrusting is also produced at the opposite border of the younger plate); these processes may result in continental growth at the “craton side”; (4) topographic signatures of PLI show basin-scale uplifts and subsidences preferentially located at cratonic margins. Negative Rayleigh-Taylor instabilities in the lithosphere above the plume head provide a mechanism for crustal delamination. Inferred consequences of PLI near intra-continental plate boundaries, such as faulting at cratonic edges and enhanced magmatic activity, could explain plume-related metallogenic crises, as suggested for West Africa and Australia

Ressources minérales dans un monde durable

Mineralization processes in ultramafic laterites are partly controlled by fluid flow regimes in these porous and permeable systems. During weathering of ultramafic rocks, exothermic chemical reactions, such as hydration of olivine, may generate shallow subsurface (< 200 m) temperatures of several tens of °C. The reaction-induced fracturing leads to further reactions and increases the permeability. For more than 20 Myr, the peridotite massifs of New Caledonia have undergone intense weathering, and the observable undulations along the weathering front, attest to a corrugated bedrock topography. Combined together, the excess heat (up to 70-90°C) and high permeability (10^-14 to 9 10^-13 m²) might have potentially triggered hydrothermal convection, as confirmed by Rayleigh number estimates. This was numerically modeled by accounting for temperature-dependent fluid properties, and for time-dependent and spatially varying parameters simulating the deepening of the weathering front. Modeling the transient evolution of the thermal and flow velocity fields over 10 Myr reveals that hydrothermal convection can develop in the New Caledonian laterites, even on sloped surfaces where topography-driven flow prevails. Convective cells develop above the weathering front, and the models reveal two-dimensional corrugations below which weathering is no longer efficient