Uplift and strength evolution of passive margins inferred from 2D conductive modelling

International audiencePost-breakup vertical motions of passive margins are seen here as a result of the post-rift 2-D thermal evolution. A 2-D finite element numerical model is performed to evaluate both the vertical and horizontal conduction that drive the thermal evolution of continental passive margins, from breakup to post-breakup states. Initial temperature configurations corresponding to non-volcanic and volcanic margins are tested, and lead to different thermal evolution of the lithosphere. For both margins, a thermal thickening is observed in the stretched lithosphere, whereas the unstretched lithosphere undergoes first (0­80 Ma) a thermal thinning and secondly (after 80 Ma) a thermal thickening. In comparison with non-volcanic margins, volcanic margins show a slower thermal thickening and a greater thermal thinning in stretched and unstretched lithosphere, respectively. The variations with time of lithosphere thickness are then translated into isostatic vertical movements and reveal 'seaward' thermal induced subsidence and 'landward' thermal induced uplift. The estimated uplift reaches up to 250 m in volcanic margins and 120 m in non-volcanic margins. The modelled timing and amount of uplift in both margins are consistent with present-day topography of volcanic passive margins that stand two to three times higher than non-volcanic margins. Using these thermal models, we finally show that the 2-D strength of the margins drastically evolves with time from a seeward dominant strength (0­80 Ma) toward a landward dominant strength (time larger than 80 Ma). These lateral strength evolution could have strong effect on the flexural response of the margin through time

Brittle-ductile coupling : Role of ductile viscosity on brittle fracturing

International audienceLocalized or distributed deformations in continental lithosphere are supposed to be triggered by rheological contrasts, and particularly by brittle-ductile coupling. A plane-strain 2D finite-element model is used to investigate the mechanical role of a ductile layer in defining the transition from localized to distributed fracturing in a brittle layer. The coupling is performed through the shortening of a Von Mises elasto-visco-plastic layer rimed by two ductile layers. By increasing the viscosity of the ductile layers by only one order of magnitude, the fracturing mode in the brittle layer evolves from localized (few faults) to distributed (numerous faults), defining a viscosity-dependent fracturing mode. This brittle-ductile coupling can be explained by the viscous resistance of the ductile layer to fault motion, which limits the maximum displacement rate along any fault connected to the ductile interface. An increase of the viscosity will thus make necessary new faults nucleation to accommodate the boundary shortening rate

Strain weakening enables continental plate tectonics

International audienceMuch debate exists concerning the strength distribution of the continental lithosphere, how it controls lithosphere-scale strain localization and hence enables plate tectonics. No rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid-like behaviour of plate interiors. Here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution. Geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle, respectively. The positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remai

Tectonics of the Syros blueschists (Cyclades, Greece): From subduction to Aegean extension

On Syros, high‐pressure metamorphism affects a lithological pile that is composed of, from base to top: (1) the Komito‐Vari granitic basement, (2) a margin sedimentary sequence that is predominantly made of marbles and schists (the Pyrgos and Kastri units), and (3) the Kambos metaophiolitic mélange. The tectonic history occurred in three main stages. During the first stage, in the mid‐Eocene, the Kambos oceanic unit was thrust southward on top of the sedimentary pile. Top‐to‐the‐south‐southwest ductile senses of shear are synchronous with prograde high‐pressure metamorphism and associated with this thrusting event. The second stage corresponds to a top‐to‐the‐northeast ductile shear that affects the whole metamorphic pile and is synchronous with the metamorphic retrogression from eclogite to greenschist facies. However, the Kambos oceanic unit remained partly undeformed, as shown by significant volumes containing undeformed lawsonite pseudomorphs. No major extensional detachment related to this exhumation event outcrops on the island. The localized semibrittle to brittle deformation of the third stage is associated with the postmetamorphic development of (1) a ramp‐flat extensional system at the island scale, whose southward minimum displacement is estimated at approximately 7 km, and (2) two sets of steeply dipping strike‐slip faults with a normal component, trending either east–west or around north–south, indicating that the mean stretching and shortening directions are trending NNE–SSW and ESE–WNW, respectively. This sequence of major tectonic events and their relationship to metamorphism are interpreted within the framework of the subduction of the Pindos Ocean and then of the Adria continental passive margin

Deformation mechanisms in a continental rift up to mantle exhumation. Field evidence from the western Betics, Spain

International audienceThe identification of the structures and deformation patterns in magma-poor continental rifted margins is essential to characterize the processes of continental lithosphere necking. Brittle faults, often termed mantle detachments, are believed to play an essential role in the rifting processes that lead to mantle exhumation. However, ductile shear zones in the deep crust and mantle are rarely identified and their mechanical role remains to be established. The western Betics (Southern Spain) provides an exceptional exposure of a strongly thinned continental lithosphere, formed in a supra-subduction setting during Oligocene-Lower Miocene. A full section of the entire crust and the upper part of the mantle is investigated. Variations in crustal thickness are used to quantify crustal stretching that may reach values larger than 2000% where the ductile crust almost disappear, defining a stage of hyper-stretching. Opposite senses of shear top-to-W and top-to-E are observed in two extensional shear zones located close to the crust-mantle boundary and along the brittle-ductile transition in the crust, respectively. At locations where the ductile crust almost disappears, concordant top-to-E-NE senses of shear are observed in both upper crust and serpentinized mantle. Late high-angle normal faults with ages of ca. 21 Ma or older (40Ar/39Ar on white mica) crosscut the previously hyper-stretched domain, involving both crust and mantle in tilted blocks. The western Betics exemplifies, probably better than any previous field example, the changes in deformation processes that accommodate the progressive necking of a continental lithosphere. Three successive steps can be identified: i/a mid-crustal shear zone and a crust-mantle shear zone, acting synchronously but with opposite senses of shear, accommodate ductile crust thinning and ascent of subcontinental mantle; ii/hyper-stretching localizes in the neck, leading to an almost disappearance of the ductile crust and bringing the upper crust in contact with the subcontinental mantle, each of them with their already acquired opposite senses of shear; and iii/high-angle normal faulting, cutting through the Moho, with related block tilting, ends the full exhumation of the mantle in the zone of localized stretching. The presence of a high strength sub-Moho mantle is responsible for the change in sense of shear with depth. Whereas mantle exhumation in the western Betics occurred in a backarc setting, this deformation pattern controlled by a high-strength layer at the top of the lithosphere mantle makes it directly comparable to most passive margins whose formation lead to mantle exhumation. This unique field analogue has therefore a strong potential for the seismic interpretation of the so-called “hyper-extended margins”

The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene

International audienceThe Aegean domain is a key area for understanding the processes of back-arc extension. Observed deformation pattern and present day kinematics result from the interaction between the southward retreat of the Hellenic trench and the westward escape of Anatolia. Lithosphere-scale analogue models were employed to display that the overall pattern of Aegean extension requires not only the combination of trench retreat and Anatolia escape since middle Miocene but also the presence of an inherited lithosphere-scale mechanical discontinuity: the Vardar Suture Zone (VSZ). The reactivation in dextral shear of the eastern branch of the VSZ accommodates both the trench retreat (NS stretching) and the westward escape of Anatolia (EW shortening) in the Cyclades area since middle Miocene. Additionally, our model shows that the North Anatolian Fault (NAF) is a late structure in the evolution of the Aegean, initiated around 10 Ma after the onset of Anatolia escape. The displacement field at the surface of the model allows the identification of sub-domains, which result from strain partitioning instead of being "rigid microplates", directly comparable to the present-day displacement field (GPS) of the Aegean and western Anatolia. Our model provides a simple but powerful way to look at the dynamics of Aegean extension in two main stages. From middle Eocene to middle Miocene, extension was only driven by the southward retreat of the Hellenic trench at a rate lower than 1 cm*y− 1. Since middle Miocene, the combination of slab rollback with Anatolia westward escape resulted in a southwest direction of trench retreat, with an accelerating rate of up to 3 cm*y− 1

Oligo-Miocene thinning of the Beni Bousera peridotites and their Variscan crustal host rocks, Internal Rif, Morocco†

International audienceDeciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied are

Upper Cretaceous exhumation of the western Rhodope Metamorphic Province (Chalkidiki Peninsula, northern Greece)

International audienceThe Vertiskos Unit of northern Greece is an elongated basement belt with a complex poly-metamorphic history. It extends from Greece (Chalkidiki peninsula), to the south, up to Serbia, in the north, and arguably represents the westernmost part of the Rhodope Metamorphic Province (northern Greece to southern Bulgaria). The Vertiskos Unit experienced a medium pressure lower amphibolite-facies metamorphic overprint during the Alpine Orogeny. The available medium-temperature geochronology implies that it remained at temperature of approximately 300°C (or slightly higher) during Lower Cretaceous. In order to constrain its post-Lower Cretaceous thermal history, until near-surface exposure, we applied apatite fission track analysis. The central ages obtained range from 68.5 ± 3.8 to 46.6 ± 3.6 Ma (uppermost Cretaceous to Middle Eocene) and mean track lengths between 13 and 13.5 μm. We applied two inverse thermal modeling approaches using either each sample independently (high degree of freedomin the thermal history, better data fit) or all samples together interpreting them as a vertical profile (simpler thermal history, worse data fit). Irrespective of the modeling approach, we conclude that the bulk thermal history of the Vertiskos Unit crosses the high-temperature limit of the apatite partial annealing zone by the uppermost Cretaceous and reaches near-surface conditions as early as lower/middle Eocene. These results contrast with the thermal history of the other domains of the Rhodope Metamorphic Province further east (namely the Southern Rhodope Core Complex and the Northern Rhodope Complex) and establish the Vertiskos basement complex as the oldest exhumed coherent basement fragment of the Rhodope Metamorphic Province and Greece

The two-stage aegean extension, from localized to distributed, a result of slab rollback acceleration

International audienceBack-arc extension in the Aegean, which was driven by slab rollback since 45 Ma, is described here for the first time in two stages. From Middle Eocene to Middle Miocene, deformation was localized leading to i) the exhumation of high-pressure metamorphic rocks to crustal depths, ii) the exhumation of high-temperature metamorphic rocks in core complexes and iii) the deposition of sedimentary basins. Since Middle Miocene, extension distributed over the whole Aegean domain controlled the deposition of onshore and offshore Neogene sedimentary basins. We reconstructed this two-stage evolution in 3D and four steps at Aegean scale by using available ages of metamorphic and sedimentary processes, geometry and kinematics of ductile deformation, paleomagnetic data and available tomographic models. The restoration model shows that the rate of trench retreat was around 0.6 cm/y during the first 30 My and then accelerated up to 3.2 cm/y during the last 15 My. The sharp transition observed in the mode of extension, localized versus distributed, in Middle Miocene correlates with the acceleration of trench retreat and is likely a consequence of the Hellenic slab tearing documented by mantle tomography. The development of large dextral NE-SW strike-slip faults, since Middle Miocene, is illustrated by the 450 Km-long fault zone, offshore from Myrthes to Ikaria and onshore from Izmir to Balikeshir, in western Anatolia. Therefore, the interaction between the Hellenic trench retreat and the westward displacement of Anatolia started in Middle Miocene, almost 10 Ma before the propagation of the North Anatolian Fault in the North Aegean