Michel Corsini

STRUCTURAL CONSTRAINTS ON THE EVOLUTION OF THE CENTRAL ASIAN OROGENIC BELT IN SW MONGOLIA

We provide a detailed description of the structures along a 300 km long and 50 km wide transect across the Central Asian Orogenic Belt (CAOB) in southwestern Mongolia, covering the Precambrian Dzabkhan continental domain with overthrust Neoproterozoic ophiolites in the north (Lake Zone), a Silurian-Devonian passive margin association (Gobi-Altai Zone) and oceanic domain (Trans-Altai Zone) in the center, and a continental area (South Gobi Zone) in the south. Structural analysis suggests late Cambrian collapse of the thickened Lake Zone continental crust, leading to stretching of the lithosphere and followed by Silurian-Devonian formation of oceanic crust in the Trans-Altai domain. Subsequent emplacement of Devonian-Carboniferous and late Carboniferous magmatic arcs occurred on the Gobi-Altai and South Gobi Zone crusts, respectively, during E-W shortening. Finally, the entire system was affected by N-S convergence from the Permian to Jurassic, leading to heterogeneous shortening of the orogenic domain. The model best fitting these observations is one of generalized westward drift of the Tuva-Mongol-Dzabkhan-Baydrag ribbon continents during the Silurian-Devonian, associated with westward-subduction of the Mongol-Okhotsk Ocean and sequential growth of syn-convergent magmatic arcs. Back-arc basins opened during this period in the area of the western Paleoasian Ocean. The present-day shape of the CAOB in southern Mongolia was probably formed during Permian to Mesozoic anticlockwise rotation and folding of the Tuva-Mongol-Dzabkhan-Baydrag continental ribbons, combined with a strike-slip (transpressional) reactivation of ancient transform boundaries in the Paleoasian oceanic domain. All continental and oceanic crustal domains were reactivated and intensely deformed during this convergence in a style controlled by crustal rheology and a heterogeneous Permian magmatic-thermal input. The sequence of tectonic events is tested against published paleomagnetic data, paleogeographic reconstructions and tectonic models, leading to a revised model for the accretion of juvenile crust to a continental margin in the CAOB of southern Mongolia.

The Pombal granite pluton: Magnetic fabric, emplacement and relationships with the Brasiliano strike-slip setting of NE Brazil (Paraiba State)

Abstract The Pombal pluton (500 km 2 ), a suite of diorite, syenite and porphyritic granite bodies, is here used to constrain kinematics of Brasiliano-age tectonic episodes in northeast Brazil. The pluton intrudes high-grade to migmatitic gneiss forming the western basement of the Serido belt, and is located at the intersection between two sets of continental-scale dextral strike-slip shear zones. The northern set of shear zone strikes NE-SW and branches, southwards, into the E-W Patos mega-shear zone. A detailed microstructural and low-field magnetic susceptibility study was performed to unravel the relationships between solid-state deformation in the country rocks and magma emplacement. Porphyritic granite and syenite have quite high magnetic susceptibilities (10 −3 –10 −2 SI units) indicative of magnetite as the principal carrier of susceptibility. The magnetic fabric is remarkably homogeneous in orientation throughout the pluton. It is characterized by a shape-preferred alignment of magnetite, itself parallel to the shape fabric of mainly biotite (±amphibole), i.e. to the magmatic fabric. Even close to the contact with the high-temperature mylonites of the Patos shear zone, south of Pombal, no imprint of the E-W-trending structures is observed in the fabrics of either the granite or the host rocks. Granite emplacement and its internal fabric development is concluded to be independent of the movement of the Patos shear zone. In the southwestern border of the pluton, a low-dip foliation bearing a NE-SW-striking lineation is shared in both the magmatic fabric of the pluton and the solid-state fabric. Farther to the north, approaching the NE-SW strike-slip shear zone, the magmatic fabric is characterized by a steeply dipping NE-striking foliation carrying a subhorizontal lineation. Transition from low to steep dips of the planar fabrics is progressive. Two models are proposed for emplacement of the Pombal pluton. One considers magma injection during an early episode of tangential tectonics, responsible for the gently dipping foliations, evolving later to strike-slip deformation. The other model considers that the pluton was emplaced in a pull-apart domain developed in the overlapping sector of a right-hand en echelon system of a dextral shear zone. Compatibility of these models with the tectonic evolution of the Serido belt is discussed.

Strain transfer at continental scale from a transcurrent shear zone to a transpressional fold belt: The Patos-Seridó system, northeastern Brazil

During the Brasiliano-pan-African orogeny, a complex continental-scale pattern of east-west transcurrent shear zones and northeast-trending fold belts formed in the northern and central Borborema province of northeastern Brazil. The east-west shear zones have been usually regarded as slightly younger features, but the study of the most spectacular case of intersection between these two structures, the Patos shear zone and the Serido transpressional belt, leads to a different tectonic model. Satellite imagery and structural, petro-logical, and geophysical data support the interpretation that these structures (1) are in structural continuity and (2) formed simultaneously under amphibolite facies metamorphic conditions that led to partial melting. This suggests a model of strain transfer at the scale of the orogen: at the eastern end of the Patos east-west dextral shear zone, the strain that accommodated the relative motion of the northern block was transferred to the northeast-trending Serido belt, where it resulted in folding, strike-slip faulting, and stretching parallel to the strike of the belt.

Crustal influx, indentation, ductile thinning and gravity redistribution in a continental wedge: Building a Moldanubian mantled gneiss dome with underthrust Saxothuringian material (European Variscan belt)

[1] The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica-Snieznik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Tepla-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material.

Thermal history of the Pan-African/Brasiliano Borborema Province of northeast Brazil deduced from 40Ar/39Ar analysis

Abstract Detailed step heating 40 Ar/ 39 Ar analyses were performed on different minerals from magmatic and metamorphic rocks of the Borborema Province. This region, located in northeast Brazil, belongs to the Pan-African/Brasiliano belt which is about 1000 km wide and 600 km long. Twenty-six single grains of amphibole, muscovite and biotite were extracted from eighteen samples selected in an area of about 17,500 km 2 (250 km by 70 km) in the Patos region. This region has been affected by a continental-scale shear zone system. Well-defined 40 Ar/ 39 Ar plateau ages and an U Pb analysis on zircon [Leterrier, J., Jardim de Sa, E.F, Bertrand, J.M. and Pin, C., 1994. Ages U Pb sur zircon de granitoides brasilianos de la ceinture du Serido (Province Borborema, NE bresil). C.R. Acad. Sci., Paris, 318: 1505–1511] allow the definition of a homogeneous and unusual slow cooling history (3–4°C/Ma) and suggest a rather slow uplift rate between 580 and 500 Ma, followed by a fast cooling of the whole studied area around 500 Ma. Rapid cooling is suggested by concordant plateau ages on muscovite and biotite. The existence of such an event at the end of the belt history is in agreement with data obtained on other major belts such as the Alpine-Himalayan system or the Hercynian belt.

40 AR/ 39 AR dating of synkinematic white mica; insights from fluid-rock reaction in low-grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps

Abstract This paper highlights the use of synkinematic white mica, biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low-grade metamorphic conditions. The Mont Blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow, resulting in substantial differences in mineralogy and whole-rock geochemistry. On the basis of their synkinematic alteration assemblages and geographic distribution within the Mont Blanc Massif, three main metamorphic zones are distinguished within the network of shear zones. These are: (i) epidote±white mica-bearing assemblages; (ii) chlorite–phlogopite-bearing assemblages; and (iii) white mica±biotite±calcite±actinolite±epidote- bearing assemblages. 40Ar/39Ar age spectra of biotite and phlogopite are complex, and reflect significant variations in chemical composition. In biotite, this is partly due to inheritance from precursor Variscan magmatic biotite. In contrast, new white mica grew at the expense of feldspar during Alpine deformation and its Ar spectra do not show any excess 40Ar. On the SE side of Mont Blanc, ages of shear zone phengites have a narrow range of 15.8–16.0±0.2 Ma, which is in the same age range as 40Ar/39Ar ages of minerals from kinematically related veins. The top-to-SE sense of shear is consistent with initiation of a Mont Blanc flower-structure within a dextral transpressional system by 16 Ma. On the NW side, mini-plateaux ages of 14.5±0.3 and 23.4±0.4 Ma are preserved in the same sample, suggesting the possibility of two phases of deformation. This is also supported by partly preserved ages of 18–36.6 Ma in biotites and phlogopites. Ages between 36 and 18 Ma might reflect ongoing top-to-NW thrusting, following Penninic Front activation, in a context of nappe stacking and crustal thickening. NW-directed thrusting on the NW side of Mont Blanc continued after 18 Ma, synchronous with SE-directed thrusting on the SE side of the massif. These divergent movements produced the overall pop-up geometry of the Mont Blanc Massif, which may correspond to a positive flower structure developed within a zone of regional dextral transpression extending SW from the Rhone valley into the Mont Blanc area.

Ductile duplexing at a bend of a continental-scale strike-slip shear zone: example from NE Brazil

Abstract During the Pan-African orogeny, the Borborema Province in NE Brazil developed a continental-scale shear-zone system that comprises NE- and EW-trending ductile strike-slip shear zones. Remote sensing and structural mapping has revealed a pattern of arcuate anastomosing strike-slip shear zones separating sigmoidal lenses of less deformed material, located at the western end of the EW-trending Patos shear zone, which is one of the largest shear zones of the Province. This structure of imbricate shear zones was initiated under high-temperature deformation conditions. It is interpreted as a ductile strike-slip duplex and may represent a kinematic pattern for strain accommodation in response to a bend of a ductile mega-shear zone.

Comment on “Alpine thermal and structural evolution of the highest external crystalline massif : The Mont Blanc” by P. H. Leloup, N. Arnaud, E. R. Sobel, and R. Lacassin

In this comment we discuss the approach used and the significance of Ar-Ar dating of synkinematic phengite within low-grade Alpine shear zones, and we comment the geodynamic models that can be derived from this method. The paper by Leloup et al. [2005] is a good step forward in the tectonic comprehension of the Mont Blanc area and provides a good synthesis of preexisting data. Leloup et al. [2005] have proposed a polyphase Alpine history for the Mont Blanc Massif (west Alps) based on a multidisciplinary approach: Ar-Ar on biotite for the higher pressure-temperature events of the Mont Blanc, and Ar-Ar on K-feldspar, fission tracks (FT) on zircon and apatite for its later exhumation stages. However, at this point of our knowledge of Alpine deformation in the Mont Blanc Range, the polyphased tectonic evolution, in particular the timing of thrust and back thrust events are not in agreement with recently obtained Ar-Ar data.

Variscan evolution of the Tanneron massif, SE France, examined through U–Pb monazite ages

The Tanneron massif belongs to the southernmost segment of the Variscan belt in France and is composed of migmatitic orthogneisses and paragneisses cross-cut by a main tonalite–granite complex. Late Carboniferous detrital basins are bounded by north–south-trending ductile to brittle normal faults and delineate three distinct zones. Our new isotope dilution U–Pb monazite dating indicates a pre-Variscan history in the central part of the massif recorded by monazites from an orthogneiss yielding ages from 440 to 410 Ma, whereas monazites from a migmatitic paragneiss record a late Carboniferous high-Tevent at 317 ± 1 Ma. In the eastern part, a migmatization event is recorded by monazites from a synkinematic leucogranitic layer and a mylonitic orthogneiss yielding ages of 309 ± 5 and 310 ± 2 Ma, respectively. Later post-collisional magmatism related to lithospheric thinning is recorded by the intrusion of undeformed granitic bodies at 302 ± 4 and 297 ± 5 Ma, and marks the final stage of the Variscan evolution. The data indicate the presence of two crustal blocks, displaying different levels of exposure of the Variscan crust. The overall tectonomagmatic evolution of this massif suggests a strong affinity with the internal part of the Variscan belt.

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian‐Variscan orogeny

Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310-295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.