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Contractional relay zones between pairs of normal faults are sometimes associated with multiple antithetic faults in a geometry similar to that found in Riedel shear zones. Detailed fault displacement profiles of outcrop examples of this geometry demonstrate that the antithetic faults accommodate the transfer of displacement between the synthetic faults that bound the relay zones. The throw on individual antithetic faults, or R′ shears, is typically constant across relay zones while the throw profile on the synthetic faults, or R shears, is stepped; the steps occurring across branchpoints with abutting R’ shears. Transfer of fault displacement occurs by a combination of block rotation and irrotational block translation within the relay zone. As fault throw increases, contractional relay zones are by-passed by the linkage of the synthetic faults, in a manner analogous to the formation of P-shears by the linkage of R shears in classic Riedel shear experiments, but with the original relay zone structure still preserved within the fault zone. With yet further strain bedding may rotate into near-parallelism with the fault surface, with the original geometrical configuration of the relay zone difficult to unravel.

International audience; Slab tearing induces localized deformations in the overriding plates of subduction zones and transfer zones accommodating differential retreat in back-arc regions. Because the space available for retreating slabs is limited in the Mediterranean realm, slab tearing during retreat has been a major ingredient of the evolution of this region since the end of the Eocene. The association of detailed seismic tomographic models and extensive field observations makes the Mediterranean an ideal natural laboratory to study these transfer zones. We review in this paper the various structures in back-arc regions differential retreat from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models to better understand the partitioning of deformation between high-angle and low-angle faults, as well as the 3-D kinematics of deformation in the middle and lower crusts. Simple, archetypal, crustal-scale strike-slip faults are in fact rare in these contexts above slab tears. Transfer zones are in general instead wide deformation zones, from several tens to several hundred kilometers. A partitioning of deformation is observed between the upper and the lower crust with low-angle extensional shear zones at depth and complex association of transtensional basins at the surface. In the Western Mediterranean, between the Gulf of Lion and the Valencia basin, transtensional strike-slip faults are associated with syn-rift basins and lower crustal domes elongated in the direction of retreat (a-type domes), associated with massive magmatic intrusions in the lower crust and volcanism at the surface. On the northern side of the Alboran Sea, wide E-W trending strike-slip zones in the brittle field show partitioned thrusting and strike-slip faulting in the external zones of the Betics, and E-W trending metamorphic core complexes in the internal zones, parallel to the main retreat direction with a transition in time from ductile to brittle deformation. On the opposite, the southern margin of the Alboran Sea shows short en-échelon strike-slip faults. Deep structures are not known there. In the Aegean-Anatolian region, two main tear faults with different degrees of maturity are observed. Western Anatolia (Menderes Massif) and the Eastern Aegean Sea evolved above a major left-lateral tear in the Hellenic slab. In the crust, the differential retreat was accommodated mostly by low-angle shear zones with a constant direction of stretching and the formation of a-type high-temperature domes exhumed from the middle and lower crust. These low-angle shear zones evolve through time from ductile to brittle. On the opposite side of the Aegean region, the Corinth and Volos Rift as well as the Kephalonia fault offshore, accommodate the formation of a dextral tear fault. Here, only the brittle crust can be observed, but seismological data suggest low-angle shear zones at depth below the rifts. We discuss the rare occurrence of pure strike-slip faults in these contexts and propose that the high heat flow above the retreating slabs and more especially above slab tears favors a ductile behavior with distributed deformation of the crust and the formation of low-angle shear zones and high-temperature domes. While retreat proceeds, aided by tears, true strike-slip fault system may localize and propagate toward the retreating trench, ultimately leading to the formation of new plate boundary, as shown by the example of the North Anatolian Fault.; Les déchirures des panneaux plongeants dans les zones de subduction induisent des déformations localisées dans les domaines arrière-arcs des plaques chevauchantes sous la forme de zones de transfert qui accommodent les différences latérales de vitesse de retrait des fosses. Parce que l’espace disponible pour ce retrait est limité dans le domaine méditerranéen, les déchirures ont joué un rôle majeur dans l’évolution de cette région depuis la fin de l’Eocène. L’association de modèles tomographiques détaillés et de nombreuses observations de terrain font de la Méditerranée un laboratoire naturel idéal pour étudier ces zones de transfert. Nous proposons ici une revue des diverses structures des domaines arrière-arc accommodant le retrait différentiel depuis la Mer d’Alboran jusqu’à la région égéenne et nous les discutons grâce à des modèles numériques 3-D pour mieux comprendre le partitionnement de la déformation entre les failles à fort et faible pendage et la cinématique 3-D dans la croûte moyenne et la croûte inférieure. Nous montrons que les failles localisées purement décrochantes d’échelle crustale sont rares au-dessus de ces zones de déchirure. Les zones de transfert sont au contraire plutôt des zones de déformation larges de quelques dizaines à quelques centaines de kilomètres. On y observe un partitionnement de la déformation entre la croûte supérieure et la croûte inférieure avec des zones de cisaillement extensives à faible pendage en profondeur et une association complexe de bassins en transtension et de détachements dans la croûte supérieure. En Méditerranée occidentale, entre Golfe du Lion et bassin de Valence, des failles décrochantes transtensives sont associées à des bassins syn-rift et des dômes de croûte inférieure allongés dans la direction du retrait (dômes de type « a »), associés à des intrusions magmatiques massives dans la croûte inférieure et du volcanisme en surface. Sur la marge septentrionale de la Mer d’Alboran, des zones décrochantes E-O larges dans le domaine cassant montrent un partitionnement entre des chevauchements et des décrochements dans les zones externes et des dômes métamorphiques allongés E-O dans les zones internes, parallèlement à la direction principale de retrait, avec une évolution du ductile vers le cassant au cours du temps. La rive sud de la Mer d’Alboran montre à l’opposé des failles décrochantes en échelon courtes et les structures profondes sont mal connues. Dans la région Egée-Anatolie, deux zones larges de degré de maturité différents sont observées. L’ouest de l’Anatolie (massif du Menderes) et l’est de la Mer Egée ont évolué au-dessus d’une déchirure majeure du panneau plongeant hellénique. Dans la croûte, le retrait différentiel est principalement accommodé par des zones de cisaillement extensives à faible pendage avec des directions d’étirement cohérentes régionalement et des dômes métamorphiques de type « a » et de haute température exhumés depuis la croûte moyenne et la croûte inférieure. Ces zones de cisaillement à faible pendage évoluent du ductile au cassant au cours du temps. Du côté opposé du domaine égéen, le rift de Corinthe, le rift de Volos, ainsi que la faille de transfert de Céphalonie accommodent la formation d’une large zone de transfert dextre. Dans cette région, seule la croute supérieure peut être observée, mais les données sismologiques suggèrent l’existence de zones de cisaillement à faible pendage sous les rifts actifs. Nous discutons la rareté des grandes failles de transfert purement décrochantes dans ces contextes et proposons que le flux de chaleur élevé au-dessus des panneaux plongeants en recul et plus encore au-dessus des zones de déchirures favorise un comportement ductile avec déformation distribuée de la croûte et la formation de zones de cisaillement à faible pendage et de dômes de haute température. Pendant le retrait, favorisé par ces déchirures, de véritables failles décrochantes lithosphériques peuvent ensuite se localiser et évoluer en limites de plaques, comme le montre l’exemple de la Faille Nord Anatolienne.

Mechanical heterogeneity of a sedimentary sequence exerts a primary control on the geometry of fault zones and the proportion of offset accommodated by folding. The Wildensbuch Fault Zone in the Swiss Molasse Basin, with a maximum throw of 40 m, intersects a Mesozoic section containing a thick (120 m) clay-dominated unit (Opalinus Clay) over- and underlain by more competent limestone units. Interpretation of a 3D seismic reflection survey indicates that the fault zone formed by upward propagation of an east–west-trending basement structure, through the Mesozoic section, in response to NE–SW Miocene extension. This configuration formed an array of left-stepping normal fault segments above and below the Opalinus Clay. In cross-section a broad monoclinal fold is observed in the Opalinus Clay. Folding, however, is not ubiquitous and occurs in the Opalinus Clay where fault segments above and below are oblique to one another; where they are parallel the fault passes through the Opalinus Clay with little folding. These observations demonstrate that, even in strongly heterogeneous sequences, here a four-fold difference in both Young9s modulus and cohesion between layers, the occurrence of folding may depend on the local relationship between fault geometry and applied stress field rather than rheological properties alone.

Normal faults often comprise arrays of fault segments. The boundaries between adjacent fault segments, relay zones and relay ramps can be of crucial importance to fluid migration pathways across and along faults and for across-fault reservoir juxtaposition. However, despite the increased number of studies of faults in high quality 3D seismic data, the three dimensional geometric or kinematic properties of relay zones or the controls on their 3D geometry are still unclear. This contribution investigates the characteristics of three dimensional segmentation based on an extensive collection of normal faults and relay zones in different geological settings. Our analysis shows that relay zones most often develop by bifurcation from a single fault surface but can also arise from the formation of segments which are disconnected in 3D from their inception. Relay zones generally occur between fault segments that step in the dip or strike direction, and intermediate oblique relay zones are less frequent. When combined together, these different geometries can form a complex spectrum of fault segmentation, whose characteristics can be related to underlying geological controls, such as the mechanical heterogeneity of the faulted sequence and the influence of the basement structure