Drainage integration and sediment dispersal in active continental rifts:A numerical modelling study of the central Italian Apennines

Progressive integration of drainage networks during active crustal extension is observed in continental areas around the globe. This phenomenon is often explained in terms of headward erosion, controlled by the distance to an external base‐level (e.g. the coast). However, conclusive field evidence for the mechanism(s) driving integration is commonly absent as drainage integration events are generally followed by strong erosion. Based on a numerical modelling study of the actively extending central Italian Apennines, we show that overspill mechanisms (basin overfilling and lake overspill) are more likely mechanisms for driving drainage integration in extensional settings and that the balance between sediment supply vs. accommodation creation in fault‐bounded basins is of key importance. In this area drainage integration is evidenced by lake disappearance since the early Pleistocene and the transition from internal (endorheic) to external drainage, i.e. connected to the coast. Using field observations from the central Apennines, we constrain normal faulting and regional surface uplift within the surface process model CASCADE (Braun & Sambridge, 1997, Basin Research, 9, 27) and demonstrate the phenomenon of drainage integration, showing how it leads to the gradual disappearance of lakes and the transition to an interconnected fluvial transport system over time. Our model results show that, in the central Apennines, the relief generated through both regional uplift and fault‐block uplift produces sufficient sediment to fill the extensional basins, enabling overspill and individual basins to eventually become fluvially connected. We discuss field observations that support our findings and throw new light upon previously published interpretations of landscape evolution in this area. We also evaluate the implications of drainage integration for topographic development, regional sediment dispersal and offshore sediment supply. Finally, we discuss the applicability of our results to other continental rifts (including those where regional uplift is absent) and the importance of drainage integration for transient landscape evolution.publishedVersio

Comportement mécanique des lithosphères continentales chaudes Evolution des cratons Néoarchéens et Paléoprotérozoïques de Terre Adélie (Antarctique Est) et du Gawler (South Australia)

The East Antarctic Shield is made of several geological domains amalgamated at about 530 Ma, during the Ross orogeny. All this domains but one recorded Grenvillian and Pan-African events : the Terre Adelie Craton. The Terre Adelie Craton and its northern extension, the Gawler Craton (in South Australia), are both parts of a same block : the Mawson continent. Also, they display identical geological histories prior the opening of the Southern Ocean basin at around 90 Ma. These cratons are made of a polymetamorphic basement structured during two main orogenesis dated to Neoarchaean (∼ 2.5 Ga) and Paleoproterozoic (∼ 1.7 Ga) times. The present work deals with the study of the tectonic mechanisms responsible for the structuration of the Mawson Paleo-continent. Field investigations and laboratory work (petrology, Ar–Ar and U–Th–Pb geochronology) on samples from both cratons allowed (1) to precise the age and the origin of the Neoarchaean deformation and the mechanical behaviour of the continental lithosphere at that time, and (2) to quantify the magnitude of the Paleoproterozoic deformation within the Archaean crust and in adjacent Paleoproterozoic domains. This work provides new constraints regarding Precambrian tectonics. Our results allowed us to evidence and to numerically model in 3D at the lithospheric-scale, the strain evolution during waning of convergence force into a warm Archaean lithosphere. We show that tectonic and gravitationnal forces compete in order to drive horizontal constrictive flow of the continental lithosphere, in a direction perpendicular to the convergence. Furthermore, we illustrate rheological duality between a stabilized cratonic domain and its autochtonous sedimentary cover during later tectonic reactivation.Le bouclier Est Antarctique est constitué de nombreux domaines géologiques accrétés autour de 530 Ma. Tous ces domaines enregistrent la trace des événements Grenvillien et Pan-Africain à l'exception d'un seul : le Craton de Terre Adélie. Le Craton de Terre Adélie et son prolongement septentrional, le Craton du Gawler (en South Australia), font partie d'un même bloc : le Mawson Continent. Ils présentent donc une histoire géologique commune avant l'ouverture du domaine océanique Austral il y a environ 90 Ma. Ces cratons sont constitués d'un socle métamorphique polyphasé formé et structuré lors de deux événements géologiques majeurs datés au Néoarchéen (∼ 2.5 Ga) et au Paléoprotérozoïque (∼ 1.7 Ga). Cette étude présente les mécanismes tectoniques à l'origine de la structuration de ce paléo-continent. Les campagnes de terrain et les travaux en laboratoire (pétrologie, géochronologie Ar–Ar et U–Th–Pb) réalisés sur des roches provenant des deux cratons ont permis (1) de préciser l'âge et l'origine de la déformation néoarchéenne ainsi que le comportement de la lithosphère continentale à cette époque et (2) de quantifier l'importance de la déformation paléoprotérozoïque au sein du noyau archéen et dans les domaines paléoprotérozoïques adjacents. Ces travaux apportent de nouvelles contraintes sur la tectonique précambrienne. Nos travaux ont permis de mettre en évidence et de modéliser numériquement en 3D l'évolution à l'échelle lithosphérique de la déformation lors de l'affaiblissement des contraintes tectoniques convergentes appliquées à une lithosphère archéenne chaude. Sous l'influence de la gravité, la lithosphère chaude va fluer dans une direction perpendiculaire à celle de convergence, principalement par des mécanismes de constriction horizontale. De plus, nous mettons en évidence une dualité rhéologique entre un noyau cratonique stable et sa couverture autochtone lors de processus tardifs de réactivation tectonique

The role of surface processes in basin inversion and breakup unconformity

International audienceIn the context of continental extension, transient compressional episodes (stress inversion) and phases of uplift (depth inversion) are commonly recorded with no corresponding change in plate motion. Changes in gravitational potential energy during the rifting process have been invoked as a possible source of compressional stresses, but their magnitude, timing, and relationship with depth inversions remain unclear. Using high-resolution two-dimensional numerical experiments of the full rifting process, we track the dynamic interplay between the far-field tectonic forces, loading and unloading of the surface via surface processes, and gravitational body forces. Our results show that rift basins tend to localize compressive stresses; they record transient phases of compressional stresses as high as 30 MPa and experience a profound depth inversion, 2 km in magnitude, when sediment supply ceases, providing an additional driver for the breakup unconformity, a well-documented phase of regional uplift typically associated with continental breakup

Rotation, narrowing, and preferential reactivation of brittle structures during oblique rifting

Occurrence of multiple faults populations with contrasting orientations in oblique continental rifts and passive margins has long sparked debate about relative timing of deformation events and tectonic interpretations. Here, we use high-resolution three-dimensional thermo-mechanical numerical modeling to characterize the evolution of the structural style associated with varying geometries of oblique rifting in a layered continental lithosphere. Automatic analysis of the distribution of active extensional shear zones at the surface of the model demonstrates a characteristic sequence of deformation. Phase 1 with initial localization of deformation and development of wide moderately oblique en-échelon grabens limited by extensional shear zones oriented close to orthogonal to σ3 trend. Subsequent widening of the grabens is accompanied by progressive rotation of the phase 1 extensional shear zones to an orientation sub-orthogonal to the plate motion direction. Phase 2 is characterized by narrowing of active deformation resulting from thinning of the continental mantle lithosphere and development of a second-generation of extensional shear zones. During phase 2 deformation localizes both on plate motion direction-orthogonal structures that reactivate rotated phase 1 shear zones, and on new moderately oblique structures parallel to σ2. Finally, phase 3 consists in the oblique rupture of the continental lithosphere and produces an oceanic domain where oblique ridge segments are linked with highly oblique accommodation zones. We conclude that while new structures form and trend parallel to σ2 in an oblique rift, progressive rotation and long-term slip along phase 1 structures promotes orthorhombic fault systems, which accommodate upper crustal extension and control oblique passive margin architecture. The distribution, orientation, and evolution of frictional-plastic structures observed in our models consistent with documented fault populations in the Main Ethiopian Rift and the Gulf of Aden conjugate passive margins, both of which developed in moderately oblique extensional settings

Constrictional flow and strain partitioning during oblique deformation: insights from the Variscan Tanneron massif, SE France

International audienceStructural analysis through precise digital mapping combined with microstructural and quantitative finite strain data were used to investigate strain partitioning and strain shape evolution during the late-stage oblique tectonic collapse of a hot orogen. The Tanneron massif in SE France was structured in an oblique tectonic regime at the end of the Variscan orogeny, leading to the exhumation of lower to middle crustal migmatite terrains. Strain patterns show prominent stretching lineations associated with L>S tectonites and dextral strike-slip SZ compatible with subsimple shear deformation. The overall kinematic with pure shear sub-horizontal constrictional flow and sub-vertical simple shear-dominated transcurrent corridors depict a transtensional regime. The progressive transtensional deformation evolves through two successive phases, the first characterised by a dominant sub-horizontal flow of the ductile crust represented by gently dipping foliation and widespread sub-horizontal stretching lineations followed by a second plane strain flow associated with vertical foliation and S-L tectonites. Finite strain analysis confirms the monotony of the L>S and S-L tectonites and highlights a lithological control on the finite strain ellipsoid shape with meta-igneous units defining L>S fabrics while meta-sedimentary units depict S-L fabrics. Microstructural observations also constrain the temperature evolution of the progressive transtensional deformation. Sub-horizontal flow starts at supra-solidus conditions and progresses to sub-vertical shear down to greenschist facies solely in hydrated meta-sedimentary units. We propose a rheologically driven strain path partitioning during the progressive exhumation of this deep crust during a two-phase transtensional regime

AuScope's Use of Standards to Deliver Earth Resource Data

AuScope is an integrated national geosciences framework being built as a component of the Australian National Collaborative Research Infrastructure Strategy. The AuScope Grid element is a geoinformatics network using open standards to allow real time access to data, information and knowledge stored in distributed repositories. AuScope Grid draws together information from new initiatives and existing sources in academia, industry and government. The network is being deployed using infrastructure components from multiple open source projects in various domains. Together these provide a complete suite of tools for spatial data interoperability. The tools have been used to deploy OGC Web Map and Web Feature Services (WMS/WFS), service registration (CSW) and vocabulary services, and for the development of community application schemas. The key to linking resources in this Community Earth Model is web-service access to geoscience information holdings and computational services, using common service interfaces and standard (i.e. community agreed) information models. For example, EarthResourceML has been developed under the leadership of the Australian Government Geoscience Information Committee. EarthResourceML is an extension of GeoSciML, the IUGS developed language for exchange of geological map features. Each State and Territory Geological Survey has an earth resource database, storing information on mineral occurrences, commodities, historical production, reserves and resources, deposit classification and the like, each with its own format and sets of attributes and vocabularies. An OGC Web Feature Service has been deployed in each jurisdiction, which maps the local database to the common exchange model, thus allowing the AuScope Discovery Portal to query and consume the earth resource data from the distributed databases. This approach to interoperability is predicated on (a) community acceptance of the exchange model (b) ability and commitment by each data provider to deploy and maintain a conformant service. In addition to standard information models, interoperability is further enhanced by use of common vocabularies. AuScope has built a service based on SKOS vocabularies, which is used by the Discovery Portal to match user searches against preferred and alternative names. Each service provider matches their local terms to the unique identifier for the same concept in the vocabulary service, which enables the portal to find all responses that match the concept the user specified. This overcomes differences due to language, spelling, synonyms and local variations.JRC.DDG.H.6-Spatial data infrastructure

Looking beyond kinematics: 3D thermo-mechanical modelling reveals the dynamics of transform margins

International audienceTransform margins represent ∼ 30 % of nonconvergent margins worldwide. Their formation and evolution have traditionally been addressed through kinematic models that do not account for the mechanical behaviour of the lithosphere. In this study, we use high-resolution 3D numerical thermo-mechanical modelling to simulate and investigate the evolution of intra-continental strain localization under oblique extension. The obliquity is set through velocity boundary conditions that range from 15˚ (high obliquity) to 75˚ (low obliquity) every 15˚ for rheologies of strong and weak lower continental crust. Numerical models show that the formation of localized strike-slip shear zones leading to transform continental margins always follows a thinning phase during which the lithosphere is thermally and mechanically weakened. For low-(75˚) to intermediate-obliquity (45˚) cases, the strike-slip faults are not parallel to the extension direction but form an angle of 20˚ to 40˚ with the plate motion vector, while for higher obliquities (30˚ to 15˚) the strike-slip faults develop parallel to the extension direction. Numerical models also show that during the thinning of the lithosphere, the stress and strain reorient while boundary conditions are kept constant. This evolution, due to the weakening of the lithosphere, leads to a strain localization process in three major phases: (1) initiation of strain in a rigid plate where structures are sub-perpendicular to the extension direction; (2) distributed deformation with local stress field variations and formation of transtensional and strikeslip structures; (3) formation of highly localized plate boundaries stopping the intra-continental deformation. Our results call for a thorough re-evaluation of the kinematic approach to studying transform margins

Fluage en constriction et partitionnement de la déformation au sein d'un système tectonique oblique : le cas du massif tardi-varisque du Tanneron (SE France)

International audienceLe démantèlement progressif des domaines orogéniques est généralement associé à des régimes tectoniques obliques. Ces cinématiques imposent une vision tri-dimensionnelle de la déformation et sont représentées par deux régimes majeurs : la transpression et la transtension. Ces systèmes complexes peuvent être à l'origine d'une grande diversité de fabriques tectoniques dont l'évolution dans le temps et dans l'espace peuvent se révéler difficile à analyser. L'étude détaillée du partitionnement, de la nature et de la cinématique des fabriques à l'échelle régionale peut permettre de déterminer si le régime oblique est compatible avec un régime transpressif ou transtensif et de mieux déceler les structures qui leurs sont associées.Notre étude, centrée sur le domaine interne du massif varisque des Maures-Tanneron est basée sur une nouvelle cartographie géologique et structurale à haute résolution du socle migmatitique combinée à des mesures en laboratoire de l'ellipsoïde de la déformation finie (ASM et analyse tri-dimensionnelle des fabriques microscopiques). Ces analyses ont permis de mettre en évidence la superposition de deux fabriques ductiles. La première, particulièrement bien préservée dans les unités d'orthogneiss migmatitique, est marquée par une dominance de la linéation d'étirement des leucosomes sur l'aplatissement (tectonite L>S) et souligne un important fluage sub-horizontal longitudinal de la croûte partiellement fondue. La seconde est représentée par des zones de cisaillements décrochantes mylonitiques sub-verticales, majoritairement visibles dans les unités de paragneiss et en bordure du bassin Carbonifère du Reyran. Les contrastes rhéologiques observés à travers l'étude de la déformation finie soulignent un partitionnement de la déformation qui se localise durant le refroidissement de la croûte et son amincissement progressif. Des plis (à multiples échelles) à axes parallèles à la linéation d'étirement soulignent également le caractère oblique de la déformation. Cette constriction régionale accompagnée par des zones de cisaillement décrochantes et l'ouverture d'un bassin intracontinental étroit semble être compatible avec un régime tectonique en transtension.De nouvelles données thermobarométriques pour cette zone complètent et confortent nos observations structurales et interprétations tectoniques. Cette partie interne du massif des Maures-Tanneron a été structurée par une déformation progressive associée à l'amincissement de la chaîne de 330 à 300 Ma, dans un régime oblique transtensif

The role of asthenospheric flow during rift propagation and breakup

International audienceContinental rifting precedes the breakup of continents, leading to the formation of passive margins and oceanic lithosphere. Although rifting dynamics is classically described in terms of either active rifting caused by active mantle upwelling, or passive rifting caused by far-field extensional stresses, it was proposed that a transition from passive to active rifting can result from changes in buoyancy forces due to localized thinning of the lithosphere. Three-dimensional numerical experiments of rifting near an Euler pole allow the quantification of these buoyancy forces and show that gravitational stresses are strong enough not only to sustain rifting and drive axis-parallel motion in the asthenosphere dome, but also to promote along-axis asthenospheric flow and to drive the propagation of the rift tip toward its rotation pole. We show that gradients of gravitational potential energy due to the presence of the dome of asthenosphere induce time-dependent phases of compressional and transcurrent stress regimes, despite an overall divergent plate setting. Our experiments predict overdeepened bathymetry at the tip of the propagating rift, as well as the variability of focal mechanisms of shallow seismic events similar to those observed in such a setting. We also explain the episodes of basin inversion documented in many rifted continental margins