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Plate kinematics in the Cantabrian domain of the Pyrenean orogen
Full text linkThe Cantabrian domain represents the western portion of the Pyrenean orogen, in the area where the Iberian continental lithosphere was subducted toward the north underneath the transitional to oceanic lithosphere of the Bay of Biscay. There, the about 100 km of orogenic convergence have been mostly accommodated in the northern portion of the orogen (i.e. the retro wedge) developed in the Bay of Biscay abyssal plain, while only crustal-scale folding with limited internal deformation occurred in the Cantabrian southern wedge (pro-wedge). Integrated meso- and macrostructural analyses and a reappraisal of available information from the transitional area between the Pyrenean and Cantabrian domains are presented in this work, allowing to set geometric and kinematic constraints on the entire Meso-Cenozoic history of the northern portion of the Iberian Plate, including subduction initiation and evolution in the western portion of the Pyrenean orogen. <br><br> The structural record of the Late Jurassic to Early Cretaceous deformation stage, which was associated with rifting and seafloor spreading in the Bay of Biscay, indicates a ridge perpendicular (NNE-SSW oriented) extension, with no evidence of relevant strike-slip components during rifting. A Cenozoic NNW-SSE oriented shortening stage followed, related to the limited (about 100 km) north-directed subduction of the Iberian continental lithosphere underneath the transitional to oceanic lithosphere of the Bay of Biscay. Subduction led to the formation of the poorly-developed Cantabrian pro-wedge, which is laterally juxtaposed to the well-developed Pyrenean pro-wedge to the east. During this convergence stage, the structural framework in the Cantabrian pro-wedge, and particularly along its transition with the Pyrenean wedge to the east, was severely complicated by the reactivation of Paleozoic and Mesozoic inherited structures. <br><br> Data presented in this work fully support the development of the Cantabrian Mountains as related to indentation and consequent thickening of the Bay of Biscay transitional lower crust during north-directed subduction of Iberian continental lithosphere. In essence, the Cantabrian pro-wedge is a lithospheric south-verging fault-propagation anticline developing above the subduction plane. The structural record in the area indicates that a lithospheric fault-propagation folding stage was predated, during the very early stages of orogenic shortening, by the development of a lithospheric-scale open syncline overlying the nucleation point of lithosphere sinking. Such a syncline is today partially preserved and represents one of the few natural examples of subduction initiation
Along-strike evolution of folding, stretching and breaching of supra-salt strata in the Plataforma Burgalesa extensional forced fold system (northern Spain)
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Mesozoic rifting in the Basque-Cantabrian Basin (Spain): Inherited faults, transversal structures and stress perturbation
No full textModelling complex geological structures byFORC2 implementation of the HCA numerical technique
No full textWe present results of the implementation of the HCA method to describe the evolution of complex geological cross-sections by the FORC2 software. The Hybrid Cellular Automata (HCA) is a numerical forward modelling algorithm that follows an hybrid methodology between the cellular automata (CA) and the finite element method (FEM) philosophies. Layered rock units are simulated by a mesh of a very large number of semi-independent cells. Cells are related by three types of links that simulate the natural behaviour of layered rocks: a) intralayer relations; b) interlayer relations, and c) discontinuity relations. Intralayer relations link cells belonging to the same layer. Interlayer relations regulate the relationships among adjacent layers. These relations take into account the weaker rheologies of interlayer material, physical boundary conditions, and volume preservation conditions, while partial freedom is given to surface variations. Discontinuity relations correspond to the presence of ruptures such as faults. Volume and surface preservation is accomplished by the large amount of cells (typically >100,000) and by preserving the average distance among adjacent cells. The forward modelling pace is selected small enough to ignore not-adjacent cells in the computation and to reduce the links to first order equations. Units are simulated by grouping elementary cells with identical link properties into a mechanical unit. A multilayer of different mechanical units constitutes the mechanical stratigraphy. The physical parameters and type of links within the mechanical stratigraphy can be modified or are modified by the local conditions at any step during the run, thus resulting a self-constraining algorithm. No other conditions are imposed to the model that “self-decide” its evolutionary pathway. The conceptual architecture of the modelling tool is independent of the fold kinematics and geometry, and can simulate also complex tectonic evolutionary paths. The HCA-FORC implementation used in the modelling allows to compute at each step the stress and (brittle) deformation conditions at each cell. Three kinematic effects were considered to compute the latter component: a) torsion-induced fibre stress; b) flexural slip (i.e. interlayer slip) induced stress; and c) slip along the discontinuities. By varying the rheology, the elastic parameters, and the bedding geometry for each cell, it is possible to replicate the mechanical stratigraphy on the model multilayer as well as along strike variations. Example applications include: fault-related folding either in thrusts and extensional environments, complex duplex structures, syntectonic sedimentation and erosion, and salt diapirs
Curvilinear fault-bend folding: geometrical, kinematical and deformational implications.
No full textFault-bend folding is a common folding mechanism in thrust and fold belts worldwide. The widely used kink-band geometric model of fault-bend folding (Suppe, 1983) necessitates complex ramp segmentations to reproduce the rounded shape of many natural thrust related anticlines. Curvilinear hinge sectors provide a geometric and kinematic alternative solution to kink bands for modelling curved-hinge folds. We developed an analytical solution for modelling fault-bend folding using circular hinge sectors, named curvilinear fault-bend folding. The velocity field of this kinematic solution is different from that associated to the classical, kink-style model. Our solution predicts the development of curvilinear anticlines above staircase fault geometries and the occurrence of diachronous limb rotation in the forelimb (Step I) and in the crest (Step I). Different kinematic evolution of curvilinear fault-bend folding with respect to the classical kink-style model, implies different growth strata pattern and different expected deformation distribution. In particular, the development of rotational syngrowth wedges on both the forelimb and the crest of curvilinear fault-bend anticline is predicted, while it is neglected in the kink-style model (e.g. Suppe et al., 1992). Comparison between expected deformation pattern in the two model, when an homogeneous mechanical stratigraphy is assumed, show how deformation intensity in curvilinear fault-bend folding is lower than that expected in the kink-style model, and it is dependent on both the layer parallel and layer orthogonal position, whereas in the kink-style model it is dependent only on the layer-parallel position.ReferencesSuppe, J., 1983. Geometry and kinematics of fault-bend folding. American Journal of Sciences. 283, 684-721.Suppe, J., Chou, G.T., Hook, S.C., 1992. Rates of folding and faulting determined from growth strata. In: McClay, K.R. (Ed.), Thrust Tectonics. Chapman abd Hall, London, pp. 105-121
Application of HCA numerical models to simulate complex geological structures.
No full textWe present results of the implementation of the HCA method to describe the evolution of complex geological cross-sections by the FORC2 software. The Hybrid Cellular Automata (HCA) is a numerical forward modelling algorithm that follows an hybrid methodology between the cellular automata (CA) and the finite element method (FEM) philosophies. Layered rock units are simulated by a mesh of a very large number of semi-independent cells. Cells are related by three types of links that simulate the natural behaviour of layered rocks: a) intralayer relations; b) interlayer relations, and c) discontinuity relations. Intralayer relations link cells belonging to the same layer. Interlayer relations regulate the relationships among adjacent layers. These relations take into account the weaker rheologies of interlayer material, physical boundary conditions, and volume preservation conditions, while partial freedom is given to surface variations. Discontinuity relations correspond to the presence of ruptures such as faults. Volume and surface preservation is accomplished by the large amount of cells (typically >100,000) and by preserving the average distance among adjacent cells. The forward modelling pace is selected small enough to ignore not-adjacent cells in the computation and to reduce the links to first order equations. Units are simulated by grouping elementary cells with identical link properties into a mechanical unit. A multilayer of different mechanical units constitutes the mechanical stratigraphy. The physical parameters and type of links within the mechanical stratigraphy can be modified or are modified by the local conditions at any step during the run, thus resulting a self-constraining algorithm. No other conditions are imposed to the model that “self-decide” its evolutionary pathway. The conceptual architecture of the modelling tool is independent of the fold kinematics and geometry, and can simulate also complex tectonic evolutionary paths. The HCA-FORC implementation used in the modelling allows to compute at each step the stress and (brittle) deformation conditions at each cell. Three kinematic effects were considered to compute the latter component: a) torsion-induced fibre stress; b) flexural slip (i.e. interlayer slip) induced stress; and c) slip along the discontinuities. By varying the rheology, the elastic parameters, and the bedding geometry for each cell, it is possible to replicate the mechanical stratigraphy on the model multilayer as well as along strike variations. Example applications include: fault-related folding either in thrusts and extensional environments, complex duplex structures, syntectonic sedimentation and erosion, and salt diapirs
Fault-bend folding as an end-member solution of (double-edge) fault-propagation folding.
No full textFault-bend folding is the most commonly used kinematic mechanism to interpret the architecture and evolution of thrust-related anticlines in thrust wedges. However, its basic requirement of an instantaneous propagation of the entire fault before hangingwall deformation, limits its kinematic effectiveness. To overcome this limitation, we used the interdependence between fold shape and fault slip versus propagation rate (S/P ratio) implemented in double-edge fault-propagation folding. We show that very small S/P values produce fault-propagation anticlines that, when transported forelandward along an upper décollement layer, closely resemble fault-bend anticlines. Accordingly, if small geometric discrepancies between the two solutions are accepted, transported double-edge fault-propagation provides an effective kinematic alternative to fault-bend folding. Even at very low S/P values, it in fact predicts a fast but finite propagation rate of the fault. We thus propose that double-edge fault-propagation folding provides a broadly applicable model of fault-related folding that includes fault-bend folding as an end-member kinematic solution
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