The present-day strain partitioning of the Western Alps and its relationships with the crustal scale geometry
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Strain partitioning
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This paper compares results from two‐dimensional finite element dynamic modeling with the kinematic evolution of the Swiss Alps during the collision phase. In particular, we investigate the role of inherited lateral strength heterogeneities on orogenesis. A number of first‐order characteristics are directly comparable at crustal scales. In the models the entry of continental crust into the convergent margin marks the end of near‐perfect subduction. Accretion of material of the subducting plate to the upper plate creates an orogenic wedge on the incoming (pro)side and initiates a retroshear zone (or model backthrust). The addition of material to the upper plate builds a bivergent orogen. Heterogeneities in the pro‐crust focus shear and lead to the development of “nappe structures” The combined action of pro‐shear (nappe stacking) and retroshear (backthrusting) uplifts a plug between the two shear zones. Subsequent focusing of shear along the retroshear zone results in rotation of the plug and overlying units, leading to crustal‐scale backfolds as observed in the Swiss Alps. The model experiments predict features relevant to Alpine dynamics, including (1) similar crustal thicknesses and exhumation patterns to those observed in the Swiss Alps today for erosion rates comparable to natural ones (1 mm yr −1 ), (2) continued accretion and subduction of upper crustal fragments allowing high‐pressure metamorphic conditions, (3) tilting and exhumation of lower crust when a midcrustal weak zone is present, and (4) “shunting” of material across the strong lower crustal wedge of the upper plate.
Accretionary wedge
Convergent boundary
Continental Margin
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Reflection
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Abstract Partitioning of crustal shortening between the colliding continental plates is highly variable in nature. Physical controls of such variability remain largely enigmatic and require quantitative understanding. In this study, we employ 2‐D thermomechanical numerical modeling to investigate the influence of the rheological properties of the continental crust on the dynamics and distribution of crustal shortening during continental collision. Three major physical parameters, (i) the mechanical strength of the upper crust, (ii) the Moho temperature, and (iii) the convergence rate, are investigated, and their influences on crustal shortening partitioning between the lower and upper plates are systematically documented. Numerical modeling results suggest that a strong upper crust of the lower plate, high Moho temperature, and slow convergence rate favor migration of crustal shortening from the lower to the upper plate. Our numerical modeling results compare well with natural observations from the Alpine orogenic system where variable partitioning of crustal deformation between the plates is documented.
continental collision
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Summary Crustal-scale thrusts of the western Alps are characterized by W to WNW directed movements of several hundreds of kilometres. In the internal zones, prominent lineations mark these slip directions. In the central Alps, coeval right-lateral strike slip (Insubric fault) and N-NNW vergent thrusting (Helvetic and more external thrusts) occurred during the upper Tertiary, on crustal-scale faults. It is proposed that WNW directed convergence between NW Apulia and Europe was accommodated mainly by the combination of movements on three first-order faults: in the central Alps, this oblique convergence is partitioned in two components (thrust and strike slip) at a deep fault bifurcation; in the western Alps, it accounts for the observed thrusts. This direction of convergence in the western and central Alps is markedly different from the coeval relative motion of Africa and Europe (N to NNW directed convergence). It implies either specific movements of Apulia with respect to Africa, or the lateral extrusion of NW Apulia during the indentation of the African promontory during the collision.
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The Zagros fold and thrust belt formed during the collision between the Africa/Arabia and Eurasian plates from Miocene times. The region is characterized by intermediate seismicity, a probable detached subducting slab, a deep foreland basin, and an irregularly folded sedimentary cover. Despite extensive acquisition of geophysical data major unknowns are related to i) the nature of the crustal deformation during collision and the resulting crustal structure; ii) the existence of a mantle root and the possible strain partitioning between crust and mantle lithosphere; and iii) the basement deflection caused by the building of the Zagros mountains and the associated deep geometry of the foreland basin. These items are addressed in two ways. An integrated approach, combining the use of gravity, geoid and absolute elevation allows us to infer the 3D regional crustal and lithospheric structure. A range of input parameters, e.g. the isostatic compensation depths and the elastic properties of the lithosphere, are tested to separate the regional and local field components which, in turn, allows for a more detailed 2D lithospheric modelling along selected geotransects. These geotransects contain existing and partly new data including seismic profiles, surface elevation, gravity, geoid, and magnetics. The crustal and lithospheric structure is modelled by using a numerical code that simultaneously solves the geopotential, lithostatic, and heat transport equations. Temperature distribution in the lithosphere is used to constrain the lithospheric strength and add information on the location of the transition from brittle to ductile deformation.
Collision zone
Mountain formation
continental collision
Lithospheric flexure
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Heat flow
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Present day
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Extensional tectonics
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Abstract The Periadriatic fault system (PFS) is an array of late orogenic faults (35-15 Ma) in the retro-wedge of the Alpine orogen that accommodated dextral transpression during oblique indentation by the southern Alpine crust. Decoupling along the leading edges of the southern Alpine indenter occurred where inherited lithological and rheological contrasts were accentuated by lateral thermal gradients during emplacement of the warm orogenic retro-wedge next to the cold indenter. In contrast, decoupling within the core and retro-wedge of the orogen occurred in a network of folds and mylonitic faults. In the Eastern Alps, this network comprises conjugate sets of upright, constrictional folds, strike-slip faults and low-angle normal faults that accommodated nearly coaxial NNE-SSW shortening and E-W extensional exhumation of the Tauern thermal dome. The dextral shear component of oblique convergence was taken up by a discrete, brittle fault parallel to the indenter surface. In the Central and Western Alps, a steep mylonitic backthrust, upright folds, and low-angle normal faults effected transpressional exhumation of the Lepontine thermal dome. Mylonitic thrusting and dextral strike-slip shearing along the steep indenter surface are transitional along strike to low-angle normal faults that accommodated extension at the western termination of the PFS. The areal distribution of poles to mylonitic foliation and stretching lineation of these networked structures is related to the local shape and orientation of the southern Alpine indenter surface, supporting the interpretation of this surface as the macroscopic shearing plane for all mylonitic segments of the PFS. We propose that mylonitic faults nucleate as viscous instabilities induced by cooling, or more often, by folding and progressive rotation of pre-existing foliations into orientations that are optimal for simple shearing parallel to the eigenvectors of flow. The mechanical anisotropy of the viscous continental crust makes it a preferred site of decoupling and weakening. Networking of folds and mylonitic fault zones allow the viscous crust to maintain strain compatibility between the stronger brittle crust and upper mantle, while transmitting plate forces through the lithosphere. Decoupling within the continental lithosphere is therefore governed by the symmetry and kinematics of strain partitioning at, and below, the brittle-to-viscous transition.
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Upper crust
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