ALBERT

Description: We review a set of geological and geophysical observations that strongly support a coherent deformation of the entire lithosphere in major intracontinental wrench faults. Tectonic studies of wrench faults eroded down to the middle to lower crust show that, even in cases in which the lower to middle crust is partially melted, strain remains localized (although less efficiently) in transcurrent shear zones. Seismic profiling as well as seismic tomography and magnetotelluric soundings provide strong argument in favour of major wrench faults crosscutting the Moho and deforming the upper mantle. Pn velocity anisotropy, shear-wave splitting and electric conductivity anisotropy measurements over major wrench faults and in transpressional domains support that a wrench fault fabric exists over most or even the entire lithosphere thickness. These seismic and electrical anisotropies are generated by a crystallographic preferred orientation of olivine and pyroxenes developed in the mantle during the fault activity, which is frozen in the lithospheric mantle when the deformation stops. The preservation of such a wrench fault type' fabric within the upper mantle may have major effects on the subsequent tectonothermal behaviour of continents, because olivine is mechanically and thermally anisotropic. Indeed, the association of numerical models and laboratory data on textured mantle rocks strongly suggests that the orogenic continental lithosphere is an anisotropic medium with regards to its stiffness and to heat diffusion. This anisotropy may explain the frequent reactivation, at the continents scale, of ancient lithospheric-scale wrench faults and transpressional belts during subsequent tectonic events.

Description: Peridotite xenoliths exhumed by Quaternary alkaline magmatism in the Tahalgha district, southern Hoggar, represent fragments of the subcontinental lithospheric mantle beneath the boundary between the two major structural domains of the Tuareg Shield: the ‘Polycyclic Central Hoggar’ to the east and the ‘Western Hoggar’, or ‘Pharusian Belt’, to the west. Samples were collected from volcanic centres located on both sides of a major lithospheric shear zone at 4°35' separating these two domains. Although showing substantial variations in their deformation microstructures, equilibrium temperatures and modal and chemical compositions, the studied samples do not display any systematic changes of these features across the 4°35' fault. The observed variations rather record small-scale heterogeneities distributed throughout the study area and reflecting the widespread occurrence of vein conduits and metasomatized wall-rocks related to trans-lithospheric melt circulation during the Cenozoic. These features include partial annealing of pre-existing deformation microstructures, post-deformation metasomatic reactions, and trace-element enrichment, coupled with heating from 750–900°C (low-temperature lherzolites) to 900–1150°C (intermediate- T lherzolites and high- T harzburgites and wehrlites). Trace-element modelling confirms that the range of rare earth element (REE) variations observed in the Tahalgha clinopyroxenes may be accounted for by reactive porous flow involving a single stage of basaltic melt infiltration into a light REE (LREE)-depleted protolith. Whole-rock compositions record the final entrapment of disequilibrium metasomatic melts upon thermal relaxation of the veins–wall-rock system. The striking correlations between equilibrium temperatures and trace-element enrichment favor a scenario in which the high-temperature peridotites record advective heat transport along melt conduits, whereas the intermediate- and low-temperature lherzolites reflect conductive heating of the host Mechanical Boundary Layer. This indicates that the lithosphere did not reach thermal equilibrium, suggesting that the inferred heating event was transient and was rapidly erased by thermal relaxation down to the relatively low-temperature present-day geotherm. The low- T (〈900°C) deformed lherzolites (porphyroclastic to equigranular) are characterized by only incipient annealing and LREE-depleted clinopyroxene compositions. They were only weakly affected by the Cenozoic events and could represent relatively well-preserved samples from rejuvenated Pan-African lithosphere. Extensive lithospheric rejuvenation occurred either regionally during the Pan-African orogeny, as a result of lithospheric delamination or thermomechanical erosion after thickening, or more locally along the meridional shear zones. The low- T Tahalgha lherzolites are comparable with lherzolites from Etang de Lherz, southern France, interpreted as lithospheric mantle rejuvenated by melt-induced refertilization during a late stage of the Variscan orogeny.

Description: 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.

Description: 〈span〉〈div〉Abstract〈/div〉Extensive partial melting of the middle to lower crustal parts of orogens, such as of the current Himalaya-Tibet orogen, significantly alters their rheology and imposes first-order control on their tectonic and topographic evolution. We interpret the late Proterozoic Araçuaí orogen, formed by the collision between the São Francisco (Brazil) and Congo (Africa) cratons, as a deep section through such a hot orogen based on U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon ages and Ti-in-zircon and Zr-in-rutile temperatures from the Carlos Chagas anatectic domain. This domain is composed of peraluminous anatexites and leucogranites that typically exhibit interconnected networks of garnet-rich leucosomes or a magmatic foliation. Zirconium-in-rutile temperatures range from 745 to 820 °C, and the average Ti-in-zircon temperature ranges from 712 to 737 °C. The geochronologic and thermometry data suggest that from 597 to 572 Ma this domain was partially molten and remained so for at least 25 m.y., slowly crystallizing between temperatures of ∼815 and 〉700 °C. Significant crustal thickening must have occurred prior to 600 Ma, with initial continental collision likely before 620 Ma, a time period long enough to heat the crust to temperatures required for widespread partial melting at middle crustal levels and to favor a “channel flow” tectonic behavior.〈/span〉

Description: Detailed structural and petrological mapping in the Beni Bousera peridotite (Rif Belt, northern Morocco) shows that this orogenic peridotite massif is composed of four tectono-metamorphic domains with consistent kinematics, marked by a pervasive, shallowly dipping foliation with a NW–SE stretching lineation that progressively rotates towards a NNE–SSW orientation in the lowermost part of the massif. From top to bottom, these domains are garnet–spinel mylonites, Ariègite subfacies fine-grained porphyroclastic spinel peridotites, Ariègite–Seiland subfacies porphyroclastic, and Seiland subfacies coarse-porphyroclastic to coarse-granular spinel peridotites. Microstructures and crystal preferred orientations point to deformation dominantly by dislocation creep in all domains, but the continuous increase in average olivine grain size indicates decreasing plastic work rates from top to bottom. This evolution in deformation conditions is consistent with the change in synkinematic pressure and temperature conditions, from 900°C at 2·0 GPa in garnet–spinel mylonites to 1150°C at 1·8 GPa in the Seiland domain. A pervasive diffuse dunitic–websteritic layering subparallel to the foliation suggests deformation in the presence of melt in the Seiland domain. Gravitational instabilities owing to local melt accumulation may account for 〈200 m wide areas exhibiting a vertical lineation in this domain. To account for the consistent kinematics and the tectono-metamorphic evolution, which implies a temperature gradient of c. 125°C km –1 preserved across the Beni Bousera massif, we propose that the entire massif records the functioning of a low-angle shear zone, a few kilometres wide, which accommodated exhumation of the base of the lithosphere from ~90 to ~60 km depth. Partial melting in the Seiland domain may be explained by fast decompression of the footwall, without the need for exotic heat sources. Moreover, if the present-day orientation of the shear zone is similar to that when it was active in the mantle, the stretching lineations at high angle to the metamorphic gradient imply that shearing parallel to the trend of the belt accompanied thinning; that is, a transtensional deformation of the margin.