Alain Chauvet

Mongolian summits: An uplifted, flat, old but still preserved erosion surface

In Gobi Altay and Altay, Mongolia, several flat surfaces, worn through basement rocks and uplifted during the ongoing tectonic episode to a similar altitude of 4000 m, suggests disruption of a single large-scale surface. New thermochronology and field data show that the plateau surfaces represent uplifted parts of an ancient peneplain that formed during Jurassic time. The Gobi Altay and Altay flattopped massifs are tectonically and geomorphologically unique. Their preservation for ~150 m.y. implies that no further tectonic movements occurred before the onset of the last deformation episode, 5 ± 3 m.y. ago. It also suggests that very low erosion rates were maintained by a dry climate over millions of years.

Transpressional tectonics and stream terraces of the Gobi‐Altay, Mongolia

We studied the patterns, rates and evolution of fluvial terraces and fault system during the building process of an intracontinental transpressional mountain in the Gobi-Altay (Mongolia). By analyzing incisions and offsets of fluvial terraces and alluvial fans, we show that the massif has grown by outward migration of thrust faults through time. On the northern flank, the present bounding thrust fault began its activity ~600 ka ago, while a more internal sub-parallel fault was still active until ~200-100 ka. Vertical offset of an alluvial fan abandoned ~100 ka ago allows an estimate of 0.1 mm/yr Upper Pleistocene - Holocene uplift rate. The morphology of the catchment-piedmont system strongly suggests a periodical formation of the alluvial surfaces, controlled by the climatic pulses, at the beginning of the wet interglacial periods. The abandonment of the alluvial terraces lags by several thousand years the abandonment of the alluvial fans, showing a diachronous incision propagating upstream. The incision rate deduced from the different elevations of straths exceeds of one order of magnitude the rock uplift rate. This excess is mostly due to ongoing drainage network growth at the core of the massif, and incision due to alluvial apron entrenchment near the outlet. This implies that fluvial response is mainly controlled by drainage growth, interaction with piedmont and cyclic climatic variations, rather than by rock uplift.

Structural evolution of the southernmost segment of the West European Variscides: the South Portuguese Zone (SW Iberia)

Abstract The South Portuguese Zone (SPZ) represents the southernmost unit of the Iberian Massif. It is mainly composed of three structural domains, from north to south, the Beja–Acebuches Ophiolitic Complex (BAOC), The Pulo do Lobo Antiform (PLA) and the Iberian Pyrite Belt (IPB). This study proposes a structural analysis of the Spanish part of the SPZ that allows us to point out two main kinds of deformation; one accommodated by early top-to-the-south and following sinistral strike-slip tectonics in the northern part of the SPZ and the other by top-to-the-south thrusting in a wide southern branch. This transition, underlining the strain partitioning, is analysed by lattice preferred orientation of quartz using the texture goniometry method. It shows that the deformation is accommodated in the PLA at low to middle temperature by basal and prismatic 〈a〉 slip. Quartz textures suggest increasing thermal conditions of deformation from thrust to strike-slip tectonics. Our work within the IPB allows us to present a sequence of deformation showing a primary south-verging ductile thrusting and coeval crustal thickening in response to the thin-skinned tectonics. The progressive deformation generated backthrusts while it turns shallower southward. These features are summarised in an interpretative cross-section of the SPZ that underlines the main structural style of deformation, the fore-mentioned southward propagating thin-skinned thrusts.

A three-stage structural evolution of the Quadrilátero Ferriífero: consequences for the Neoproterozoic age and the formation of gold concentrations of the Ouro Preto area, Minas Gerais, Brazil

Abstract The Quadrilatero Ferrifero occurs in the southern part of the Sao Francisco Craton (Brazil), where three major Proterozoic supracrustal units (the “Rio das Velhas”, “Minas” and “Itacolomi” Supergroups) are exposed together with Archaean to Palaeoproterozoic granito-gneissic basement. Microtectonic and kinematic analyses have been undertaken in the area close to Ouro Preto in order to precise the tectonic evolution and to distinguish the tectonic features related to the Transamazonian and Brasiliano orogenies (respectively ∼ 2000 and ∼600 Ma ). Three main tectonic events have been recognized. The earlier event, De, is clearly expressed within basement rocks of the Bacao Complex and surrounding supracrustal units. A stretching lineation (Le lineation), roughly oriented N—S, can be observed. Numerous shear criteria such as drag folds and asymmetric pressure shadows indicate a top-to-the-south sense of movement along the southern border of the Bacao Complex. This event was developed within amphibolite facies conditions, fact confirmed by quartz 〈c〉 axis fabrics. This earlier deformation is related to plutonic activity and reactivation of the Archaean basement expressed by the occurrence of scarce leucocratic granitoid rocks intruded within the Bacao gneisses. It is here attributed to the Transamazonian orogeny in agreement with recent UPb dating ( ∼ 2000 Ma ). The large-scale folds (i.e. Mariana anticline, Dom Bosco syncline) which define the dominant structural geometry of the Quadrilatero Ferrifero were formed during this event. The second tectonic event, Dm, has been defined as a major thrusting towards the WNW related to the Brasiliano cycle. A well-expressed stretching and mineral lineation, oriented E—W to NW—SE (Lm lineation) and associated NW-verging shear criteria characterize this event. Occurrence of muscovite, biotite and more scarcely sillimanite indicate that deformation developed in upper greenschist/lower amphibolite facies. Quartz 〈c〉 axis fabrics confirm the sense of shearing but the large scattering of several fabrics traduces a weak intensity of strain. All supracrustal and gneiss formations were affected by the thrusting which is developed on a folded surface previously formed during the De event. The third event, Dm′, is characterized by hangingwall down movement of the nappe pile in response to relaxation of the Brasiliano compressional Dm forces. This event, developed in greenschist facies, is marked by the occurrence of metre-scale drag-folds, sometimes associated to the development of a new lineation Lm′. Gold mineralization, which occurs along the Main Mineralized Contact (located between the “Minas” and “Rio das Velhas” Supergroups), is found within late veins which comprise large recrystallized quartz, carbonate, sulphide, chlorite and tourmaline. Microscopic analysis and veins geometry allow us to conclude that the veins were emplaced mostly after the ductile Dm deformation. It is proposed that the veins were formed by filling of open cavities created during Dm′, tectonics. This study highlights that the northwest-verging thrusting, which is the main tectonic event of the Quadrilatero Ferrifero, is only relevant of the Brasiliano orogeny. A model of structural evolution which involves plutonic activity at ∼ 2000 Ma within the Archaean Bacao Complex, followed by Neoproterozoic thrust tectonics (Brasiliano) is proposed and discussed. The age of gold concentration, assumed to be achieved during the relaxation that occurred in response to the thrusting event, is re-interpreted and related to the late stage of the Brasiliano tectonics.

Eutectic mixtures for pharmaceutical applications: A thermodynamic and kinetic study

The thermodynamics and kinetics of the solidification process of several mixtures of SR 33557 and PEG 6000 have been analyzed by differential scanning calorimetry. The calculated phase diagram showed a negative interaction energy between the constituents in the liquid phase. A unified description for solidification accounting for isothermal and continuous cooling is presented. The onset of solidification shifts to higher temperatures on decreasing the cooling rate and to longer times on decreasing the annealing temperature under continuous cooling and isothermal holding, respectively. The analysis is based on the fact that nuclei have to be created prior to any crystal growth. The driving force for nucleation is considered proportional to the undercooling, ΔT (= TL − T). By coupling the isothermal and continuous cooling experiments, the high temperature part of the time-temperature transformation and temperature-cooling rate transformation diagrams are constructed under a wide range of conditions.

Tectonic evolution of the Cevennes para-autochthonous domain of the Hercynian French Massif Central and its bearing on ore deposits formation

The Cevennes area belongs to the para-autochthonous domain of the Hercynian Belt of the French Massif Central. Three lithological series, namely: sandstone-pelite, black micaschist and gneiss-micaschist, are identified. They form an imbrication of five tectonic units which overthrust the unmetamorphosed Viganais Paleozoic units to the south and the gneissic Mamejean Unit to the north. The structural, metamorphic and magmatic evolution of the Cevennes area is characterized by three events, namely: (1) southward shearing coeval to a MP/MT metamorphism dated around 340 Ma; (2) post nappe anatexis (T 5 kb); (3) Namurian (ca 315 Ma) E-W extensional tectonics and plutonism. The structure of the Mt-Lozere-Borne granitic complex is constrained by new AMS and gravimetric data. The plutons are the driving power of the hydrothermal convective circulations responsible for an early deposition of diffuse arsenopyrite in the thermal aureole. Gold bearing sulfides are afterwards concentrated in quartz veins along brittle normal and wrench faults around the granite. Lastly, ore bearing quartz pebbles are sedimented in the Stephanian Ales coal basin.

Influence of fault rock foliation on fault zone permeability: The case of deeply buried arkosic sandstones (Gres d'Annot, southeastern France)

We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195C to 268C. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sandstone (10–2 md for Y against 10–3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions.

Focal mechanism of prehistoric earthquakes deduced from pseudotachylyte fabric

Fault pseudotachylytes form by frictional melting during seismic slip and therefore are widely interpreted as “earthquake fossils.” Rapid movement along a rupture surface typically forms a pseudotachylyte generation vein, the thickness of which increases with earthquake magnitude. The direction and sense of seismic slip cannot always be determined due to the generally complex geometry of pseudotachylyte veins. Here we show, for the first time, that the orientation of the magnetic fabric of fault pseudotachylytes indicates both direction and sense of seismic slip. The magnetic fabric, acquired in a manner similar to that of other magmas, arises in this case from the asymmetric preferred orientation of paramagnetic grains during viscous shear of the friction melt. This kinematic information, coupled with fault plane orientation and generation vein thickness, provides new and critical insight for the earthquake focal mechanism. The magnetic fabric of pseudotachylytes therefore not only constitutes a valuable kinematic criterion for these fault rocks, but also could expand our knowledge of prehistoric seismic events.

Structural, mineralogical, and paleoflow velocity constraints on Hercynian tin mineralization: the Achmmach prospect of the Moroccan Central Massif

The Achmmach tin mineralization (NE of the Moroccan Central Massif) is associated with tourmaline-rich alteration halos, veins, and faults hosted in sandstones and metapelites of the Upper Visean-Namurian. These deposits are reported to be late Hercynian in age and related to the emplacement of late-orogenic granite not outcropping in the studied area. Structural and paragenetic studies of the Achmmach tin deposit were conducted in order to establish a general model of the mineralization. From field constraints, the late Hercynian phase is marked by a transition from transpression to extension with deformation conditions evolving from ductile to brittle environments. The transpression (horizontal shortening direction roughly trending E-W) is coeval with the emplacement of the first tourmaline halos along several conjugated trends (N070, N020, and N120). Thereafter, a tourmaline-rich breccia formed in response to the fracturing of early tourmaline-altered rocks. Subsequently, during the extensional phase, these structures were reactivated as normal faults and breccias, allowing the formation of the main tin mineralization (cassiterite) associated with a wide variety of sulfides (arsenopyrite, chalcopyrite, sphalerite, galena, pyrrhotite, bismuthinite, pyrite, and stannite). This evolution ends with fluorite and carbonate deposition. The hydrothermal fluid flow velocity, calculated by applying statistical measures on the tourmaline growth bands, varies with the lithology. Values are lower in metapelites and higher in breccia. In the general evolution model proposed here, tourmaline alteration makes the rock more competent, allowing for brittle fracturing and generation of open space where the main Sn mineralization was precipitated.

Reply to comment by C. A. Boulter on “A new geodynamic interpretation for the South Portuguese Zone (SW Iberia) and the Iberian Pyrite Belt genesis”

[1] Our paper [Onezime et al., 2003] was basically aimed at providing a new, and thus debatable, geodynamic interpretation at a regional scale of the South Portuguese Zone, of which the Iberian Pyrite Belt (IPB) is part. Our study relied mainly on new structural data and facies analysis, with some emphasis on the volcanic and related facies of the Volcano-Sedimentary Complex (VSC) of the IPB, on which Boulter’s [2005] comments concentrate. Boulter complains that his ideas about the IPB and more specifically his interpretations of the VSC volcanic and volcaniclastic facies [Boulter, 1993a, 1993b, 1996] were not correctly rendered and were partly misinterpreted in our paper. A huge literature exists about the IPB geology, and various authors have dealt in more or less detail with the volcanological aspects of this belt. There was reasonably no room in our paper for an in-depth discussion of all relevant previous interpretations on this topic, including Boulter’s sill-sediment model. Some other workers in the IPB have already stressed the poor consistency of this model with the field observations at the regional scale [e.g., Carvalho et al., 1999], and recent reviews of the IPB geology and metallogeny do not rely much on this model [e.g., Leistel et al., 1998; Saez et al., 1996, 1999]. We restrict our reply to the issues raised by Boulter in his comment. [2] We acknowledge the fact that Boulter [2005] reported extrusive volcaniclastics and resedimented volcanic deposits in the VSC. We thus acknowledge that our quoting of his model as ‘‘a complete intrusive model’’ was a bit loose in this regard. However, the claim by Boulter that ‘‘he did recognize the importance of stratified volcaniclastic rocks in the Pyrite Belt’’ does not give any support to the sillsediment model he in fine continues to defend. It rather tends to weaken his case since such volcaniclastic facies are not normal components of intrusive magma-sediment systems. Boulter interprets them as mostly ‘‘extrusive hydroclastic breccias’’ derived from disruption, eventually explosive, of the sediment cover above the high-level intrusive system. In this interpretation, there is implicit assumption that such volcaniclastic deposits should be volumetrically trivial in the VSC, if this is still to be described as a sill-sediment system. We thus envisioned the extrusive component as being subsidiary in Boulter’s model and not worth to be quoted in our paper. It is concerning, and rather contradictory to his claim, that Boulter himself states that ‘‘the majority of the volcanic facies in the Rio Tinto district were peperitic intrusions that did not supply detritus to the sedimentary basin.’’ [3] Boulter [2005] pretends to understand that we are unaware of the possible formation of volcaniclastic rocks in intrusive conditions. This is rather unfair misreading of our writing. The exact wording of his comment is ‘‘Despite important amounts of volcaniclastic ‘‘deposits’’ (quotation marks added), a complete intrusive model has been proposed. . .’’. ‘‘Volcaniclastic deposits’’ in our writing referred to material that has been transported and deposited (either pyroclastic or resedimented), according to the common meaning. It was not to refer to the intrusive autoclastic facies reported and emphasized by Boulter in his papers. Intrusive (and extrusive as well) autoclastic facies in submarine environment of relevance here are mostly formed by quench fragmentation (granulation in our paper). We have made due recognition of such facies in the VSC (e.g., Figure 8 of our paper, our ‘‘autoclastic brecciated facies’’). Although locally conspicuous at the outcrop scale, they do not represent a prominent volumetric part of the volcanic province as a whole. [4] The issues raised by Boulter [2005] and discussed above should not obscure some more significant differences between his description of the VSC and ours. A major point is the evidence gained from our fieldwork that volcaniclastic deposits, both primary and resedimented, are, along with volcanogenic sedimentary deposits, a major component of the VSC in terms of volume. The volcaniclastic deposits are dominated by sandstones and lapillistones. In the perspective of the issues raised by Boulter in his comment, it is clear that this evidence does not match with the sillsediment hypothesis. There is obvious contradiction between a model where volcanics are dominated by intrusions, TECTONICS, VOL. 24, TC1010, doi:10.1029/2004TC001775, 2005