On fault misorientation in exhumed metamorphic complexes: slip-tendency analysis of faults in the Alpine orogenic wedge
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Misorientation
Wedge (geometry)
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Multiphase, decoupled faulting in the southern German Molasse Basin – evidence from 3-D seismic data
Abstract. We use three-dimensional seismic reflection data from the southern German Molasse Basin to investigate the structural style and evolution of a geometrically decoupled fault network in close proximity to the Alpine deformation front. We recognise two fault arrays that are vertically separated by a clay-rich layer – lower normal faults and upper normal and reverse faults. A frontal thrust fault partially overprints the upper fault array. Analysis of seismic stratigraphy, syn-kinematic strata, throw distribution, and spatial relationships between faults suggest a multiphase fault evolution: (1) initiation of the lower normal faults in the Upper Jurassic carbonate platform during the early Oligocene, (2) development of the upper normal faults in the Cenozoic sediments during the late Oligocene, and (3) reverse reactivation of the upper normal faults and thrusting during the mid-Miocene. These distinct phases document the evolution of the stress field as the Alpine orogen propagated across the foreland. We postulate that interplay between the horizontal compression and vertical stresses due to the syn-sedimentary loading resulted in the intermittent normal faulting. The vertical stress gradients within the flexed foredeep defined the independent development of the upper faults above the lower faults, whereas mechanical behaviour of the clay-rich layer precluded the subsequent linkage of the fault arrays. The thrust fault must have been facilitated by the reverse reactivation of the upper normal faults, as its maximum displacement and extent correlate with the occurrence of these faults. We conclude that the evolving tectonic stresses were the primary mechanism of fault activation, whereas the mechanical stratigraphy and pre-existing structures locally governed the structural style.
Molasse
Thrust fault
Normal fault
Echelon formation
Stress field
Growth fault
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Echelon formation
Normal fault
Transform fault
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Lineament
Wrench
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Basement
Echelon formation
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Protolith
Transform fault
Fault gouge
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Thrust fault
Echelon formation
Metamorphic core complex
Overprinting
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Photogrammetric Digital Outcrop Model analysis of a segment of the Centovalli Line (Trontano, Italy)
The Centovalli Line is a complex network of brittle faults developing between Domodossola (West) and Locarno (East), where it merges with the Canavese Line (western segment of the Periadriatic Lineament). The Centovalli Line roughly follows the Southern Steep Belt which characterizes the inner or “root” zone of the Penninic and Austroalpine units, which underwent several deformation phases under variable P‐T conditions over all the Alpine orogenic history. The last deformation phases in this area developed under brittle conditions, resulting in an array of dextral-reverse subvertical faults with a general E‐W trend that partly reactivates and partly crosscuts the metamorphic foliations and lithological boundaries. Here we report on a quantitative digital outcrop model (DOM) study aimed at quantifying the fault zone architecture in a particularly well exposed outcrop near Trontano, at the western edge of the Centovalli Line. The DOM was reconstructed with photogrammetry and allowed to perform a complete characterization of the damage zones and multiple fault cores on both point cloud and textured surfaces models. Fault cores have been characterized in terms of attitude, thickness, and internal distribution of fault rocks (gougebearing), including possibly seismogenic localized slip surfaces. In the damage zones, the fracture network has been characterized in terms of fracture intensity (both P10 and P21 on virtual scanlines and scan-areas), fracture attitude, fracture connectivity, etc.
Lineament
Outcrop
Fracture zone
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Cataclastic rock
Transform fault
Fault gouge
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Abstract Structural and microstructural analyses of the Argentat fault, combined with sedimentological and structural analyses of the associated Hospital basin allow us to discuss the tectonic control of coal basins by crustal-scale faults during the late Palaeozoic evolution of the Variscan lithosphere in the French Massif Central. The brittle Argentat fault zone consists of first- and second-order strike-slip faults, with dominant NNW-sinistral faults, NNE-dextral or sinistral faults and secondary ENE-dextral faults. Several experimental and theoretical models explain the observed fault patterns, like en echelon faults, A-type secondary faults, conjugate faults and Riedel shears. Strike-slip faulting is responsible for folding of the metamorphic formations characterized by N-S to NE-SW-trending axis. The regional-scale geometry of brittle faults and associated folds corresponds to a positive flower structure centered on the brittle Argentat fault, combined to a negative flower structure centered on the coal basin. Using tectonic inversion software, we show that these structures result from a left-lateral movement of the brittle Argentat fault in relation to a tectonic regime intermediate between extension and strike-slip, with a horizontal NE-SW to NNE-SSW-trending maximum stretching axis. Detailed map and cross-sections, and sedimentological interpretations of the late Stephanian Hospital basin show the occurrence of intra-basin syn-sedimentary strike-slip faults and progressive overlaying, indicating that sedimentation occurs during left-lateral strike-slip faulting and folding of basement along the Argentat fault. These data are consistent with a model of N-S to NE-SW-trending postorogenic extension proposed to account for the late Carboniferous evolution of the Variscan lithosphere. They also point out the complexity and the variety of structures developed along a regional brittle strike-slip fault zone and the necessity to take into account all the structures and the resulting geometry of the basement in order to better constrain the tectonic setting of intra-continental deposits.
Massif
Pull apart basin
Echelon formation
Transtension
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Thrust fault
Classification of discontinuities
Extensional tectonics
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