Pop-down tectonics, fluid channelling and ore deposits within ancient hot orogens

International audienceMany Archaean and Paleoproterozoic deformation zones, often rich in ore resources, show particular structural patterns in particularmarked by regional vertical stretch. These zones are not restricted to greenstone-bearing Archaean domains that may have suffered gravity-driven sagduction of heavy supra-crustals, as extensively discussed since the last twenties. Structures are actually best explained by pop-down tectonics of upper-crustal unitswithin an underlyingweak crust submitted to horizontal regional shortening.Herewe present three complementary examples fromtwo Archaean greenstone belts (Abitibi sub-Province, Quebec, andMurchison belt, South Africa) and one greenstone-lacking Paleoproterozoic belt (Thompson belt,Manitoba). In the three examples, ore is concentrated along steeply dipping deformation zones, rich in syntectonic deposits and marked by substantial sub-vertical crustal stretch. On the other hand, the three regions show differences in age, in metamorphic grade (from sub-greenschist facies to upper amphibolite facies), in metal contents (gold, antimony, nickel), in metal sources, transfers and concentration histories. Our compared analysis emphasizes that pop-down tectonics associatedwith horizontal shortening of weak lithospheres may account for observed geometric patterns and provide a new and promising frame for the analysis of relationships between structural patterns and ore concentrations within old cratons

Syn-convergence flow inside and at the margin of orogenic plateaus: Lithospheric-scale experimental approach

International audienceThis study investigates three-dimensional flow modes of orogenic plateaus by means of physical modeling. Experiments consist of shortening two contiguous lithospheres of contrasting strength, one being a weak plateau-type lithosphere and the other a strong craton-type lithosphere. The lateral boundaries are either totally confined or allow escape toward a lateral foreland on one side. Two synconvergence flow regimes are distinguished, which are governed by the balance between the gravity potential and the strength of the plateau crust and the resistance of its lateral foreland. The first regime implies transversal (orogen-normal) injection of plateau lower crust into the collision zone as a result of confinement of the plateau by an increasingly stiffer lateral boundary. As a precursor mechanism to channel flow, transversal injection responds to downward thickening of the plateau crust that is forcedly extruded into the orogenic wedge. The second regime is that of collapse-driven lateral escape of the plateau. This regime is established after a threshold is attained in the interplate coupling in the collision zone, which allows the gravity potential of the plateau to overcome the resistance of its lateral boundary. Under the collapse-driven escape regime (orogen parallel), such as that governing Tibet during the last 13 Ma, most of the convergence in the plateau and the top and rear of the collisional wedge is transformed into lateral flow and extension

Metal mobility during hydrothermal breakdown of Fe-Ti oxides : insights from Sb-Au mineralizing event (Variscan Armorican Massif, France)

Hydrothermal alteration related to Sb-Au mineralization is widespread in the Variscan Armorican Massif, but mineral replacement reactions are not well characterized, in particular the hydrothermal breakdown of ilmenite-titanohematite. Based on petrography, electron probe micro-analyzer and laser ablation-inductively coupled plasma-mass spectrometer analyses, we document mineralogical change at rock- and mineral-scale and the redistribution of Sb and others trace elements during the recrystallization of ilmenite-titanohematite to hydrothermal rutile. Hydrothermal alteration is mainly potassic with associated carbonation. The replacement mechanism is interpreted to be an interface-coupled dissolution-reprecipitation process. Results show that Mn, Zn, Co, Ni, Sn, Mo and U are released during hydrothermal alteration, whereas Sb and W are incorporated in newly-formed hydrothermal rutile from the hydrothermal fluid. Furthermore, the concentration of Sb evolves through time suggesting a change in fluid composition likely related to an enrichment of fluid in Sb during rutile crystallization. Considering that Fe-Ti oxides breakdown during hydrothermal alteration is common within epithermal and mesothermal/orogenic Au-Sb mineralizing systems, results report in this study yield important constraints about metal mobility and exchanges in hydrothermal gold systems

Deep reflection seismic imaging of the internal zone of the South Armorican Hercynian belt (western France) (ARMOR 2/Géofrance 3D Program) Imagerie sismique de la zone interne de la chaîne hercynienne sud-armoricaine (projet Armor 2/programme Géofrance 3D)

International audienceWe present results and interpretation of a 72 km long deep seismic reflection profile acquired across the internal zone of the Hercynian belt of South Brittany. The profile is of excellent quality, most of the crust being highly reflective. The “ARMOR 2 South” profile, is correlated with the “ARMOR 2 North” profile that was published in 2003. Correlation of the main subsurface reflections with surface geological and structural data provides important information about the crustal structure that resulted from thickening during Late Devonian and regional-scale extension during Late Carboniferous. In particular, seismics image shows a very high reflectivity zone, lying flat over more than 40 km at about 10–12 km depth. This zone is interpreted as a major zone of ductile crustal thinning. Nous présentons les résultats et l'interprétation d'un profil de sismique réflexion en écoute longue, de 72 km de long à travers les zones internes de la chaîne hercynienne sud-armoricaine. Le profil est d'excellente qualité, avec une forte réflectivité à travers toute la croûte. Le profil, « ARMOR 2 Sud », est corrélé avec le profil « ARMOR 2 Nord » publié en 2003. Les corrélations des réflexions les plus superficielles avec les données géologiques et structurales de surface fournissent d'importantes informations sur la structure crustale qui résulte d'un épaississement débutant au Dévonien supérieur et d'une extension régionale au Carbonifère supérieur. En particulier, la sismique met en évidence une zone sub-horizontale à très forte réflectivité, de plus de 40 km de long à environ 10–12 km de profondeur. Cette zone est interprétée comme une zone d'amincissement crustal majeur

Granitoides asociados a zonas de desgarre: modelos analógicos y aplicación al macizo de Lizio (zona de cizalla surarmoricana)

Granite intrusion in the upper crust along a crustal-scale shear zone has been modeled by injecting a Newtonian fluid (low-viscosity silicone putty) into a sandpack containing a ductile layer of silicone putty which acted as a potential décollement level along which the injected material could spread. The strike-slip regime was obtained using a mobile rigid basal plate sliding horizontally. Boundary conditions were chosen in order to analyze the influence of different rheologic profiles of the upper crust on the pattern of the intrusion. Experiments showed that: 1) intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike-slip regime, 2) intrusions are asymmetric shaped in horizontal view, with the development of a sheared tail trailing behind the intrusion, 3) the strike slip environment allowed local rising of the injected fluid along faults formed in the overburden, 4) when the crustal rheologic profile is similar in both sides of the shear zone, intrusions are asymmetric drop shaped, 5) when the viscosity of materials above the feeding pipe is higher that viscosity of materials of the other side of the shear zone, the asymmetric drop shape not appears. Lizio leucogranite of the South Armorican Shear Zone (South Brittany, France) emphasize that our experiments can explain the geometry of many syntectonic' granites emplaced along strike-slip zones. They further shed some light on mechanisms of pluton intrusion in the upper crust.El emplazamiento de granitoides en zonas de desgarre de la corteza superior se ha modelizado inyectando un fluido newtoniano en un paquete de arena en el cual se intercala una capa dúctil de silicona que actúa como un nivel de despegue potencial en el cual el material inyectado puede expandirse. El régimen de desgarre se obtiene utilizando una placa basal rígida y móvil que desliza horizontalmente. Se ha analizado la influencia de diferentes perfiles reológicos de la corteza superior en la forma de la intrusión. Los experimentos muestran que: 1) las intrusiones son alargadas y su eje mayor toma la dirección principal de elongación asociada al régimen de desgarre, 2) las intrusiones presentan forma asimétrica en planta, con el desarrollo de una cola en el sentido de cizalla, 3) el régimen de desgarre permite localmente la subida del fluido inyectado a lo largo de las fallas formadas en la cobertera, 4) cuando el perfil reológico de la corteza es igual a ambos lados de la zona de cizalla, las intrusiones presentan forma de gota asimétrica, 5) cuando la viscosidad de los materiales que están por encima del canal de alimentación es superior a la de los materiales del otro lado del accidente principal, no aparece la forma en gota asimétrica. El leucogranito de Lizio en la zona de cizalla surarmoricana (Sur de Bretaña, Francia) es un ejemplo de que la modelización analógica puede dar las claves para explicar la geometría de muchos granitos sintectónicos emplazados en zonas de desgarre

Granular shear flow in varying gravitational environments

Despite their very low surface gravities, asteroids exhibit a number of different geological processes involving granular matter. Understanding the response of this granular material subject to external forces in microgravity conditions is vital to the design of a successful asteroid sub-surface sampling mechanism, and in the interpretation of the fascinating geology on an asteroid. We have designed and flown a Taylor–Couette shear cell to investigate granular flow due to rotational shear forces under the conditions of parabolic flight microgravity. The experiments occur under weak compression. First, we present the technical details of the experimental design with particular emphasis on how the equipment has been specifically designed for the parabolic flight environment. Then, we investigate how a steady state granular flow induced by rotational shear forces differs in varying gravitational environments. We find that the effect of constant shearing on the granular material, in a direction perpendicular to the effective acceleration, does not seem to be strongly influenced by gravity. This means that shear bands can form in the presence of a weak gravitational field just as on Earth

Coeval extensional shearing and lateral underflow during Late Cretaceous core complex development in the Niğde Massif, Central Anatolia, Turkey

27 p.International audienceThe Nig¢de Massif, at the southern tip of the Central Anatolian Crystalline Complex, consists of two structural units. Foliations in the lower unit define a dome cored by migmatites. The contact between the two units bears all the elements of a ductile to brittle extensional detachment. Hence the Nig¢de Massif represents an extensional metamorphic core complex. Top-to-NE/ENE shearing at higher levels of the lower unit relates to displacement along the detachment. Deeper levels of the lower unit display hightemperature top-to-SSW ductile shearing. The two shearing deformations show a difference in the mean trend of stretching lineations of up to 58. New 40Ar/39Ar ages combined with previously published data enable us to infer that the two shears were contemporaneous. In our favored interpretation, oblique shearing in the core of the dome reflects lateral underflow, i.e., horizontal flowing of the lower crust in a direction highly oblique to the direction of extension. As a result of the interaction between lateral underflow and downdip shearing along the overlying detachment, distinct structural domains are expected to exist within the migmatitic part of the core complex, with observed counterparts in the Nig¢de dome. Lateral underflow may reflect ''inward'' flow on the scale of the core complex. Regional-scale channel flow is an alternative that would better account for the record of non-coaxial deformation in the core of the dome. More generally, we suspect that the development of lateral underflow in a metamorphic core complex more likely reflects regional channel flow, rather than local inward flow

Twenty million years of continuous deformation along theKarakorum fault, western Tibet: A thermochronological analysis.

The role of the Karakorum fault zone (KFZ) is debated. South of 33°N, ongoing dextral-oblique slip along the SW edge of the Gar basin exhumes metamorphic and magmatic rocks of the Ayilari range. Minerals have recorded a continuum of deformation from temperatures >600–400°C down to 20 Ma of deformation along the fault. Greenschist facies deformation superimposed upon the medium- to high-grade deformation marks a kinematic change from pure dextral to dextral-normal motion associated with the onset of rapid cooling. At the regional scale, the coexistence of transtension in the Gar basin with transpression documented along the Pangong range farther north suggests another example of the ‘‘zipper tectonics'' model developed along the Red River fault. The kinematic shift induced the rise of the Ayilari range starting at 16–12 Ma and the incision of major river courses. The Indus River might have become captive of the relief at this time. The river's 120 km of apparent offset implies dextral motion at a long-term rate of ca 8.5 ± 1.5 mm/yr

Structure of late Variscan Millevaches leucogranite massif in the French Massif Central: AMS and gravity modelling results

In the Limousin area, Variscan leucogranitic plutons are spatially associated with normal faults and major strike-slip shear zones that are a continuation of the South Armorican shear zone. Our study focuses on the large N-S-trending Millevaches granitic massif (Massif Central, France), and intends to highlight, through gravity modelling, structural and anisotropy of magnetic susceptibility (AMS), the massif structure at depth and to discuss the mode of emplacement of granites within a strike-slip tectonic context. The mica subfabric suggests that the magnetic foliations display a general NW-SE sub-horizontal pattern on both sides of the N-S Pradines dextral wrench fault zone that deforms the core of the massif on 5 km width. The magnetic lineation trend exhibits a sigmoïdal pattern, N-S in the Pradines fault zone and NW-SE on both sides of it, which are consistent with a dextral wrench component. The horizontal magnetic foliations and lineations are consistent with the thin granite laccolith model. There is no significant imprint of the extensional Variscan belt collapse on the internal fabric of Millevaches granites than the tectonic dextral transcurrent movement prevailing in this area. © 2005 Elsevier Ltd. All rights reserved