Evolution of a Paleoproterozoic “weak type” orogeny in the West African Craton (Ivory Coast).

International audienceThe Paleoproterozoic domain of Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of long-lived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of “domes and basins” geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of longlived Paleoproterozoic “weak type” orogen

Metamorphic core complexes vs. synkinematic plutons in continental extension setting: insights from key structures (Shandong Province, eastern China)

International audienceContinental extension is an important geodynamical process mostly diagnostic of a peculiar behaviour of the crust accommodated by geological structures that highlight these specific conditions in the crust. Though a Metamorphic Core Complex (MCC) reveals a much stronger crustal extension than a synkinematic pluton, the nature of those two structures implies different implications in terms of crustal extension mechanisms and geodynamic significations. In eastern Asia, a major continental extensional event occurred during Mesozoic and early Cenozoic times. The resulting various extensional events described in previous studies consist in large intracontinental basins, important volcanism, emplacement of plutons in the upper-crust and exhumation of MCCs. An efficient description of MCCs and plutons in eastern Asia is essential to discriminate important differences of those two structures in terms of strain amount undergone by continental crust and geodynamic significations. In that way, an integrated structural and geophysical study has been realized to precisely discriminate the structure in the Jiaodong Peninsula (Shandong Province, eastern China), previously regarded as simple sheared plutons. A succession of three main stages have been identified all pertaining to a NW-SE extensional setting: (1) the exhumation of the Late Jurassic-Early Cretaceous Linglong MCC below the SE-dipping Linglong detachment fault, (2) the emplacement of the Guojialing syntectonic pluton below the N-dipping extensional Guojialing intracrustal shear zone (130-124 Ma) and (3) a penetrative brittle normal faulting associated with gold mineralizations (not, vert, similar120 Ma). As a result, the maximum amount of extension in Jiaodong Peninsula, characterized by partially-melted lower to middle crust upward into the Linglong MCC should be revised to Late Jurassic-Early Cretaceous period

The Mesozoic palaeorelief of the northern Tian Shan (China)

International audienceThe Tian Shan range offers a natural laboratory to study orogenic processes. Most of the previous studies focused on either the Palaeozoic evolution of the range or its Cenozoic intracontinental evolution linked with the India-Asia collision. In this study, it is shown that detailed field investigations on the relationship between sedimentary cover and basement constrain the Mesozoic evolution of the northern Tian Shan. Sedimentological observations argue for a limited transport distance for Lower and Uppermost Jurassic deposits. Geological sections presented in this paper show that, in preserved locations, Triassic to Jurassic sedimentary series present a continuous onlap type sedimentary unconformity on the top of the basement. At different scales, observations clearly evidence the existence of major palaeorelief during the Mesozoic. According to the present study, the topography of the current Tian Shan range and the associated movements along its northern front structures cannot be considered as the consequence of Cenozoic reactivation alone

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

Palaeomagnetic constraints from granodioritic plutons (Jiaodong Peninsula): New insights on Late Mesozoic continental extension in eastern Asia.

International audienceMechanism and kinematics of the Late Mesozoic continental extension event of eastern Asia are still debated. In order to better constrain its evolution, two granodioritic plutons of the Jiaodong Peninsula have been chosen as targets for a time-constrained palaeomagnetic study. Indeed, plutons are devoid of visible deformation, did not experience rotation along horizontal axis and are precisely dated by U/Pb and 40Ar/39Ar methods. Multidomain (MD) magnetite has been identified as the principal magnetic remanent carrier. The interpolation of existing and new U/Pb and 40Ar/39Ar ages revealed that characteristic remanent magnetisation was acquired in a narrow range of 116 ± 2 Ma. Twenty out of 27 sites present stable magnetic directions calculated from high-temperature or high-coercive components. The observations of the solo normal magnetic polarity for this palaeomagnetic collection and of the magnetic remanent age consistent with the Cretaceous Normal Superchron (CNS) argue that the characteristic magnetic remanence may be considered as primary. Detailed field observations of the intrusive relationship between the plutons and country rocks and Anisotropy of Magnetic Susceptibility (AMS) study reveal the absence of the subsequent deformation of plutons, or rigid rotation of plutons along a horizontal axis. Two palaeomagnetic poles have been therefore calculated from these plutons. Among 12 out 15 Cretaceous palaeomagnetic poles, including the two new poles provided by this study, from the Jiaodong Peninsula and on both sides of Tan-Lu and Muping-Jimo faults are statistically consistent. As a result, the Jiaodong Peninsula behaved as a rigid block as internal deformation appears negligible. The remaining three derived poles are probably due to the secular variation or/and the vicinity of fault zones near of the palaeomagnetic sampling site. Thus, they can not be applied to the peninsula-scaled tectonics. Comparison of these time52 constrained Cretaceous palaeomagnetic results of the Jiaodong Peninsula with those of North China Block (NCB) indicate that the Jiaodong Peninsula was rigidly attached to NCB since, 3 at least, the Cretaceous as residual difference is clearly insignificant with respect to the error brackets. These new palaeomagnetic results confirm that the relative clockwise (CW) rotation of Eastern Liaoning-Korea Block (ELKB) with respect to NCB does not affect the Jiaodong Peninsula as a rigid block, nevertheless, some tectonic structures have been reactivated since Cenozoic and local rotations could be produced. This palaeomagnetic study reveals that the Late Mesozoic extension in Eastern Asia is heterogeneously expressed in time and space. The complete understanding of the mechanism(s) of this extensional event needs further multidisciplinary investigations

New Insights on the Sorbas Basin (SE Spain): the onshore reference of the Messinian Salinity Crisis

The Sorbas Basin is the land reference of the Messinian Salinity Crisis (MSC) that affected the Mediterranean Sea in the latest Miocene. Its stratigraphy has been re-visited using calcareous nannofossils and planktonic foraminifers, which provide a reliable biostratigraphic frame and lead to particularly specify the relationships between the Sorbas and Zorreras members with Yesares evaporites. The evaporites overlie a shallowing upward sequence ending with the deposition of the Reef Unit and Terminal Carbonate Complex (TCC) on the periphery of the basin. The reefal carbonates of the TCC are overlain by clastic deposits that are foreset beds of post-MSC Gilbert-type fan deltas developed on the northern edge of the basin. These sedimentary structures are separated from reefal carbonates and the Reef Unit by the Messinian Erosional Surface (MES). The various facies of the Sorbas Member have been correlated with the bottomset beds of the Gilbert-type fan deltas despite some differences in palaeobathymetry. In the southeastern periphery of the basin, the MES separates the Sorbas Member from the Yesares gypsums. In the central part of the basin, a hiatus characterizes the contact between these members. The Zorreras Member postdates the MSC and entirely belongs to Zanclean. Its white "Lago Mare" layers are lagoonal deposits, the fauna of which is confirmed to result from Mediterranean-Paratethys high sea-level exchange after the post-MSC marine reflooding of the Mediterranean Basin. This study allows to re-assert the two-step scenario of the MSC (Clauzon et al., 1996) with the following events: - at 5.971-5.600 Ma, minor sea-level fall resulting in the desiccation of this peripheral basin with secondary fluctuations; - at 5.600-5.460 Ma, significant subaerial erosion (or lack of sedimentation) caused by the almost complete desiccation of the Mediterranean Sea; - instantaneous marine reflooding, accepted at 5.460 Ma, followed by continuing sea-level rise

New insights on the Sorbas Basin (SE Spain): the onshore reference of the Messinian Salinity Crisis

International audienceThe Sorbas Basin is the land reference of the Messinian Salinity Crisis (MSC) that affected the Mediterranean Sea in the latest Miocene. Its stratigraphy has been re-visited using calcareous nannofossils and planktonic foraminifers, which provide a reliable biostratigraphic frame and lead to particularly specify the relationships between the Sorbas and Zorreras members with Yesares evaporites.The evaporites overlie a shallowing upward sequence ending with the deposition of the Reef Unit and Terminal Carbonate Complex (TCC) on the periphery of the basin. The reefal carbonates of the TCC are overlain by clastic deposits that are foreset beds of post-MSC Gilbert-type fan deltas developed on the northern edge of the basin. These sedimentary structures are separated from reefal carbonates and the Reef Unit by the Messinian Erosional Surface (MES). The various facies of the Sorbas Member have been correlated with the bottomset beds of the Gilbert-type fan deltas despite some differences in palaeobathymetry. In the southeastern periphery of the basin, the MES separates the Sorbas Member from the Yesares gypsums. In the central part of the basin, a hiatus characterizes the contact between these members. The Zorreras Member postdates the MSC and entirely belongs to Zanclean. Its white “Lago Mare” layers are lagoonal deposits, the fauna of which is confirmed to result from Mediterranean–Paratethys high sea-level exchange after the post-MSC marine reflooding of the Mediterranean Basin.This study allows to re-assert the two-step scenario of the MSC (Clauzon et al., 1996) with the following events:- at 5.971–5.600 Ma, minor sea-level fall resulting in the desiccation of this peripheral basin with secondary fluctuations;- at 5.600–5.460 Ma, significant subaerial erosion (or lack of sedimentation) caused by the almost complete desiccation of the Mediterranean Sea;- instantaneous marine reflooding, accepted at 5.460 Ma, followed by continuing sea-level rise

Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and ⁴⁰Ar/³⁹Ar investigations

New structural, microstructural, and 40Ar/39 Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop-up structure. The Galician region, in the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones. Copyright 2007 by the American Geophysical Union