Dimitrios Sokoutis

Analogue modelling of continental extension: a review focused on the relations between the patterns of deformation and the presence of magma

Abstract Continental extension may occur in two main different modes, narrow and wide rifting, which mainly differ in the width of the deformed region. A third mechanism, the core complex, has been considered either a distinct mode of extension or a local anomaly within wide rifts. In terms of causative processes, continental rifting may be explained by both active or passive mechanisms, which also differ in the volume of magmatic products and in the rheological properties and stratification of the extending lithosphere. Both numerical and analogue models have investigated the main parameters controlling the extension of a rheologically layered lithosphere. In particular, analogue models have highlighted that the style of deformation is mainly controlled by the competition between the total resistance of the lithosphere and the gravitational forces; this competition, in turn, is mainly controlled by boundary conditions, such as the applied strain rate and the rheological characteristics of the extending lithosphere. Magmatic bodies eventually present within the continental lithosphere may significantly affect the process of extension. Both the thermal and mechanical effects related to the presence of magma strongly weaken the lithosphere and localise strain; this effect may have important implications for the mode of continental extension. At a crustal scale, magmatic intrusions may affect significantly the local fault pattern also favouring the development of core complex structures. Results of analogue models, performed taking into account the presence of an initially underplated magma and reproducing various continental extensional settings, suggest a close interaction between deformation and magma emplacement during extension. Particularly, magmatic underplating influences deformation localising strain in correspondence to the low-viscosity body, while on the other hand, rift kinematics and associated deformation has a major control on the pattern of magma emplacement. In particular: (1) During orthogonal rifting, magma is passively squeezed from an axial position towards the footwall of the major boundary faults; emplacement occurs in a lateral position in correspondence to lower crust domes. This process accounts for the close association between magmatism and the development of core complex structures, as well as for the occurrence of off-axis volcanoes in continental rifts. (2) During oblique rifting, deformation causes magma to emplace within the main rift depression, giving rise to intrusions with oblique and en echelon patterns. In nature, these patterns are found in continental rifts and also in some oceanic ridges. (3) Polyphase first orthogonal–second oblique rifting models suggest lateral squeezing and off-axis emplacement in the first phase and oblique en echelon intrusions in the successive oblique rifting phase. This evolution matches the magmatic and tectonic history of the Main Ethiopian Rift. (4) Development of transfer zones between offset rift segments has a great influence on both magma migration and deformation. Particularly, magma accumulates in correspondence to the transfer zone, with a main flow pattern that is perpendicular to the extension direction. This pattern may explain the concentration of magmatism at transfer zones in continental rifts. Overall, analysis of centrifuge models and their comparison with nature suggest that deformation and magma emplacement in the continental crust are intimately related, and their interactions constitute a key factor in deciphering the evolution of both continental and oceanic rifts.

Analogue modeling of detachment fault systems and core complexes

Laboratory experiments on detachment systems in an extending brittle-ductile crust showed that the rise of a domal core complex in the ductile layer required that shear be localized above a soft inclusion in a laterally inhomogeneous ductile layer. Gravity spreading of models consisting of laterally homogeneous layers of sand over ductile silicone resulted in homogeneous patterns of tilted blocks. Systematic and reproducible patterns of our models simulate features classically observed in natural detachment systems. The extensional system is composed principally of a main detachment fault with a convex-upward shape and at least one listric accommodation fault, which facilitates progressive steepening of the footwall during the rise of the core complex. All faults begin as steeply dipping normal faults that rotate to low dips during extension.

A major Oligo-Miocene detachment in southern Rhodope controlling north Aegean extension

Plutonic and metamorphic rocks of the southwest part of the Rhodope massif in Greece correspond to ductile lower crust exhumed and deformed along a major detachment during post-thickening extensional tectonics. Extension started during the Oligocene and is responsible for the development of Miocene–Quaternary sedimentary basins. Both brittle and ductile deformations result from gravity collapse of previously thickened lithosphere, as proposed for others large extended terranes. This interpretation disagrees with the previous models which attributed Tertiary ductile deformation to Alpine thrusting and brittle extensional deformation to back arc tectonics above a subduction zone.

Mechanical interactions between rigid particles in a deforming ductile matrix. Analogue experiments in simple shear flow

Abstract The mechanical interaction between two or more particles is investigated in Newtonian simple shear flow. The experimental models allow observation of both the particle rotation and the deformation pattern around the particles. The finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric, with high finite strain zones aligned to the direction of maximum finite stretch. These correspond to local flow with low vorticity number. Heterogeneous strain patterns around rigid particles spread over a distance of 1–2 times the particle length and this distance increases with increasing bulk strain. Where rigid particles are more concentrated, these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions. In rocks, the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion, which becomes unreliable with high concentrations of rigid particles. Particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length, that is only in very concentrated suspensions of rigid particles. In a composite shape fabric, the development of the sub-fabric corresponding to the smaller minerals will be more disturbed.

45 m.y. of Aegean crust and mantle flow driven by trench retreat

The available seismic anisotropy data in the Aegean shallow mantle and their relationship with crustal deformation are used for deciphering the lithosphere-scale flow pattern driven by rollback of the Hellenic subduction slab. In the north and central Aegean, the directions of mantle seismic anisotropy trend parallel to stretching lineations in core complexes of the overlying crust, suggesting that crust and mantle have undergone the same direction of flow. At the scale of the entire Aegean, crustal extension is controlled by dextral rotation around a pole located at Scutary-Pec, Albania, that is related to the Hellenic trench retreat. The intensity of mantle anisotropy increases as a function of distance from the pole and of amount of rotation. Surface geology reveals that this rotating flow pattern resulted in strains accumulated since 45 Ma. A slab tear model is suggested to integrate the observed variations of geological events in time and space to subduction dynamics.

Transition from continental break‐up to punctiform seafloor spreading: How fast, symmetric and magmatic

[1] We present a comparison between numerical and analogue models focusing on the role of inherited lithospheric structures in influencing the process of continental break-up. Our results highlight that the presence of pre-existing anisotropies localizes strain and favors continental break-up and formation of a new ocean. For a fixed strain rate, the pre-rift lithosphere configuration influences rift duration, melt production and width and symmetry of the continental margin pair. Model results show a mainly two-phase tectonic history from continental extension to oceanization. In the first phase extension affects contemporaneously the whole rift structure, while in the second phase asthenosphere upwelling occurs into punctiform regularly-spaced spots sequentially propagating in an extension-orthogonal direction. INDEX TERMS: 8109 Tectonophysics: Continental tectonics—extensional (0905); 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; 8150 Tectonophysics: Plate boundary—general (3040). Citation: Corti, G., J. Van Wijk, M. Bonini, D. Sokoutis, S. Cloetingh, F. Innocenti, and P. Manetti, Transition from continental break-up to punctiform seafloor spreading: How fast, symmetric and magmatic, Geophys. Res. Lett., 30(12), 1604, doi:10.1029/2003GL017374, 2003.

Analogue models of orogenic wedges controlled by erosion.

Abstract Indentation in the upper brittle crust where one plate is stiffer than the other produces vertical extrusion of a doubly vergent orogenic wedge. Sandbox models of this process show that erosion with or without deposition of the eroded material onto one or both margins significantly changes the internal patterns of orogenic shear and compaction within the orogens. Erosion decreases the vertical stress and changes the criticality of the orogenic wedge, whereas redeposition increases the vertical stress on its margins. Effective indenters of accreted sand, which develop in models without erosion if the rigid indenter face dip is 75° (e.g., >45°) or ≤15 are strongly affected by erosion. Rapid erosion favors thrusting over compaction and decreases both the size and the relevance of the effective indenters. Redeposition of eroded material on the margins also expands the lifetime of the active shear as the additional load delays initiation of underlying new shears.

Experimental modelling of mantled porphyroclasts

Abstract Mantled porphyroclasts are commonly used as sense of shear indicators in mylonites. Analogue experiments have been carried out to model the development of such objects. Rigid spheres mantled by a viscous Plasticene-putty mixture with power-law behaviour were suspended in an optically transparent Newtonian polymer matrix. The samples were deformed between two cylinders, one placed within the other, and with coinciding axes. This induces a non-coaxial flow between the cylinders and allows the achievement of high finite strain. The viscous mantle around the rigid sphere remained undeformed, deformed into an ellipsoid, or developed wings. The final geometry depends mainly on the induced viscosity contrast between the mantle and the matrix, which is a function of the flow-induced stress on the surface of the mantle. Winged mantles only develop if the induced viscosity contrast between the mantle and the matrix polymer is sufficiently small. If this ‘critical’ induced viscosity contrast is exceeded, the mantled sphere remains undeformed or obtains a stable ellipsoidal shape, even at high finite strain. σ-shaped winged mantles only develop in simple shear flow if the mantle around a rigid sphere is thin.

Modelling hanging wall accommodation above rigid thrust ramps

Abstract Experimental models are used to study the role of material rheology in hanging wall accommodation above rigid flat–ramp–flat thrust footwalls. The deformation in the hanging wall was accomplished by forwards sliding along a rigid basal staircase trajectory with a variable ramp angle, α, ranging from 15° to 60°. We model different ramp angles to examine hanging wall accommodation styles above thrust ramps of overthrust faults (α ranging from 15° to 30°), as well as above pre-existing normal faults (α ranging from 45° to 60°). For the hanging walls we used stratified frictional (sand) and viscous (silicone putty) materials. In this paper we study three types of models. Type 1 models represent purely frictional hanging walls where accommodation above thrust ramps was by layer-parallel thickening and by generating a series of back thrusts. Type 2 and 3 models represent stratified frictional/viscous hanging walls. In these models, accommodation was by a complex association of reverse and normal faults, mainly controlled by the rheological anisotropy as well as by the ramp inclination angle α. In Type 2 models the silicone covered only the lower flat, while in Type 3 models it also covered the rigid ramp. For α≤30° in Type 2 models and α≤45° in Type 3 models, the viscous layer inhibited the development of back thrusts in the frictional hanging wall, instead the silicone thickened to develop a ‘ductile ramp’. For α-values higher than 30° in Type 2 models and α=45° in Type 3 models, back thrusts develop in response to the bulk compression. The experiments simulate many structures observed above natural thrust ramps with α≤30° and pre-existing normal faults with α≥45°. The models emphasise the importance of a basal ductile layer, which allows the hanging wall to step-up over the rigid ramp by building up its own ductile ramp. The models also emphasise that foreland-directed normal faulting can develop at a thrust front in the case that the vertical stress due to gravity exceeds the horizontal stress due to end-loading within a thrust wedge.

Laboratory modelling of strain variation across rheological boundaries

Abstract Laboratory models using viscous and viscoplastic silicones to simulate strain variation across competence contrasts are presented. This provides a test for earlier theoretical modelling of strain refraction in layers with Newtonian viscosity contrast, and a method of examining refraction rules for non-Newtonian materials. The main purpose is to test the theoretical rule that the finite shear strain ratio across a boundary is equivalent to the inverse viscosity ratio. Results for simple-shear experiments confirm this within the errors of viscometry and strain measurement. We investigate whether this rule applies to non-Newtonian materials, which necessarily involves a discussion on the nature of viscosity contrast for non-linear materials such as power-law fluids, and its bearing on competence contrasts in rocks. These models also provide data on strain gradients generated by viscosity boundaries, which was not included in the earlier theoretical analyses. In the simple-shear experiments, the normalized shear strain profiles indicate an approximately linear shear stress gradient from the viscosity boundaries. Using an idealized linear shear strain gradient in a Newtonian matrix approaching a contrasting layer, we can derive expressions to predict the viscosity ratio. This may be a viable method of determining approximate viscosity ratios for more general deformations.