Role of the deep crustal scale geometry on Western Alps strain partitioning : Insights from S-wave velocity tomography

The recent S-wave velocity tomography undertaken at the scale of the Alps by Nouibat et al. (2021) allows a reappraisal of the deep structure of this mountain belt. These geophysical data highlight the role of crustal geometry in the strain field development observed in the Western Alps. The geophysical imagery shows a standard crustal thickness in the foreland, with slow velocities (<3.6 km.s-1) in the lower crust. The occurrence of a sharp Moho offset of 5-12 km is detected beneath the External Crystalline Massifs (ECMs). The ECMs do not show any significant crustal thickening in their frontal parts (<35 km), except for the Pelvoux ECM (35-40 km). Beneath the internal zones, east of the Penninic Frontal Thrust, the crustal geometry is more complex with the presence of an European continental slab subducting locally deeper than 80 km beneath the Adria plate. This slab is overlain by a high-pressure metamorphic orogenic prism. The lower part, corresponding to the Ivrea gravimetry anomaly, shows seismic signatures of serpentinized mantle (Vs between 3.8 and 4.3 km.s-1) whose upper limit is located at 10 km depth below the Dora Maira internal crystalline massif. This new crustal-scale image can be compared to the current deformation pattern, which appears highly partitioned at the scale of the Alpine arc. The internal zones show a transtensional deformation regime, whose activity is distributed along two major seismic lineaments (the &#8216;Piemontais&#8217; and &#8216;Brian&#231;onnais&#8217; ones). The Alpine European foreland shows a transpressional deformation that is more diffuse and associated with vertical displacements in the ECMs. Beneath the Po plain, the seismic activity is deeper (>40 km), and correlates with a transpressional deformation which is localized along sub-vertical lineaments. The deformation of the orogenic prism appears controlled by a deeper and rigid mantle indenter split in two units by a major subvertical serpentinized structure. The upper unit, which indents horizontally and vertically the crustal orogenic prism, is located between 20 and 45 km depth. The lower unit corresponds to the western boundary of the Adria mantle that pinches directly the European slab. The surface observations and geochronological data suggest that the Moho offstets are superposed on European crustal-scale faults trend inherited from the Variscan orogeny, following the East-Variscan strike-slip system. This structural anisotropy was reactivated during the Alpine orogeny as shear zones in a mainly transpressional regime since about 25-30 Ma, as documented by Ar-Ar data on syn-kinematic mica and U-Pb on monazite. The comparison of current seismicity with the kinematics of exhumed shear zones suggests a continuity of this regime since 25-30 Ma, in response to the Adria plate anticlockwise rotation.

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S-wave Velocity from P-wave Reflection Data: The Role of Surface Waves

74th EAGE Conference and Exhibition - Workshops ◽

10.3997/2214-4609.20149867 ◽

2012 ◽

Author(s):

Valentina Socco ◽

Daniele Boiero

Keyword(s):

Surface Waves ◽

Wave Velocity ◽

Wave Reflection ◽

P Wave ◽

S Wave ◽

Reflection Data

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Crystallographic and Seismic Anisotropies of Calcite at Different Depths: A Study Using Quantitative Texture Analysis by Neutron Diffraction

Minerals ◽

10.3390/min10010026 ◽

2019 ◽

Vol 10 (1) ◽

pp. 26 ◽

Cited By ~ 2

Author(s):

Michele Zucali ◽

Daniel Chateigner ◽

and Bachir Ouladdiaf

Keyword(s):

Neutron Diffraction ◽

Texture Analysis ◽

Shear Zone ◽

Shear Zones ◽

Orientation Distribution ◽

Western Alps ◽

S Wave ◽

Wide Variability ◽

Fold Belts ◽

Different Depths

Eight samples of limestones and marbles were studied by neutron diffraction to collect quantitative texture (i.e., crystallographic preferred orientations or CPO) of calcite deforming at different depths in the crust. We studied the different Texture patterns developed in shear zones at different depth and their influence on seismic anisotropies. Samples were collected in the French and Italian Alps, Apennines, and Paleozoic Sardinian basement. They are characterized by isotropic to highly anisotropic (e.g., mylonite shear zone) fabrics. Mylonite limestones occur as shear zone horizons within the Cenozoic Southern Domain in Alpine thrust-and-fold belts (Italy), the Briançonnais domain of the Western Alps (Italy-France border), the Sardinian Paleozoic back-thrusts, or in the Austroalpine intermediate units. The analyzed marbles were collected in the Carrara Marble, in the Austroalpine Units in the Central (Mortirolo) and Western Alps (Valpelline). The temperature and depth of development of fabrics vary from <100 ∘ C, to 800 ∘ C and depth from <10 km to about 30 km, corresponding from upper to lower crust conditions. Quantitative Texture Analysis shows different types of patterns for calcite: random to strongly textured. Textured types may be further separated in orthorhombic and monoclinic (Types A and B), based on the angle defined with the mesoscopic fabrics. Seismic anisotropies were calculated by homogenizing the single-crystal elastic tensor, using the Orientation Distribution Function calculated by Quantitative Texture Analysis. The resulting P- and S-wave anisotropies show a wide variability due to the textural types, temperature and pressure conditions, and dip of the shear planes.

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A lifetime of the Variscan orogenic plateau from uplift to collapse as recorded by the Prague Basin, Bohemian Massif

Geological Magazine ◽

10.1017/s0016756817000875 ◽

2017 ◽

Vol 156 (3) ◽

pp. 485-509 ◽

Cited By ~ 3

Author(s):

FRANTIŠEK VACEK ◽

JIŘÍ ŽÁK

Keyword(s):

Bohemian Massif ◽

Three Dimensional ◽

Plate Boundary ◽

Early Carboniferous ◽

Shear Zones ◽

Crustal Thickening ◽

Variscan Orogeny ◽

Prague Basin ◽

Lateral Extrusion ◽

Orogenic Plateau

AbstractThe Ordovician to Middle Devonian Prague Basin, Bohemian Massif, represents the shallowest crust of the Variscan orogen corresponding toc.1–4 km palaeodepth. The basin was inverted and multiply deformed during the Late Devonian to early Carboniferous Variscan orogeny, and its structural inventory provides an intriguing record of complex geodynamic processes that led to growth and collapse of a Tibetan-type orogenic plateau. The northeastern part of the Prague Basin is a simple syncline cross-cut by reverse/thrust faults and represents a doubly vergent compressional fan accommodatingc.10–19 % ~NW–SE shortening, only minor syncline axis-parallel extension and significant crustal thickening. The compressional structures were locally overprinted by vertical shortening, kinematically compatible with ductile normal shear zones that exhumed deep crust in the orogen's interior atc. 346–337 Ma. On a larger scale, the deformation history of the Prague Syncline is consistent with building significant palaeoelevation during Variscan plate convergence. Based on a synthesis of finite deformation parameters observed across the upper crust in the centre of the Bohemian Massif, we argue for a differentiated within-plateau palaeotopography consisting of domains of local thickening alternating with topographic depressions over lateral extrusion zones. The plateau growth, involving such complex three-dimensional internal deformations, was terminated by its collapse driven by multiple interlinked processes including gravity, voluminous magma emplacement and thermal softening in the hinterland, and far-field plate-boundary forces resulting from the relative dextral motion of Gondwana and Laurussia.

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The role of oroclinal bending in the structural evolution of the Central Anatolian Plateau: evidence of a regional changeover from shortening to extension

Geologica Carpathica ◽

10.2478/v10096-011-0026-7 ◽

2011 ◽

Vol 62 (4) ◽

pp. 345-359 ◽

Cited By ~ 13

Author(s):

Erman Özsayin ◽

Kadir Dirik

Keyword(s):

Fault Zone ◽

Structural Evolution ◽

Shear Zones ◽

Fault Zones ◽

Fault System ◽

Western Boundary ◽

Southeastern Part ◽

Anatolian Plateau ◽

Isparta Angle

The role of oroclinal bending in the structural evolution of the Central Anatolian Plateau: evidence of a regional changeover from shortening to extensionThe NW-SE striking extensional Inönü-Eskişehir Fault System is one of the most important active shear zones in Central Anatolia. This shear zone is comprised of semi-independent fault segments that constitute an integral array of crustal-scale faults that transverse the interior of the Anatolian plateau region. The WNW striking Eskişehir Fault Zone constitutes the western to central part of the system. Toward the southeast, this system splays into three fault zones. The NW striking Ilıca Fault Zone defines the northern branch of this splay. The middle and southern branches are the Yeniceoba and Cihanbeyli Fault Zones, which also constitute the western boundary of the tectonically active extensional Tuzgölü Basin. The Sultanhanı Fault Zone is the southeastern part of the system and also controls the southewestern margin of the Tuzgölü Basin. Structural observations and kinematic analysis of mesoscale faults in the Yeniceoba and Cihanbeyli Fault Zones clearly indicate a two-stage deformation history and kinematic changeover from contraction to extension. N-S compression was responsible for the development of the dextral Yeniceoba Fault Zone. Activity along this structure was superseded by normal faulting driven by NNE-SSW oriented tension that was accompanied by the reactivation of the Yeniceoba Fault Zone and the formation of the Cihanbeyli Fault Zone. The branching of the Inönü-Eskişehir Fault System into three fault zones (aligned with the apex of the Isparta Angle) and the formation of graben and halfgraben in the southeastern part of this system suggest ongoing asymmetric extension in the Anatolian Plateau. This extension is compatible with a clockwise rotation of the area, which may be associated with the eastern sector of the Isparta Angle, an oroclinal structure in the western central part of the plateau. As the initiation of extension in the central to southeastern part of the Inönü-Eskişehir Fault System has similarities with structures associated with the Isparta Angle, there may be a possible relationship between the active deformation and bending of the orocline and adjacent areas.

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Geodynamic significance of the Variscan eclogites in the External Crystalline Massifs (Western Alps): marker of a subduction or crustal thickening?

10.5194/egusphere-egu2020-7913 ◽

2020 ◽

Author(s):

Jean-baptiste Jacob ◽

Stéphane Guillot ◽

Daniela Rubatto ◽

Emilie Janots ◽

Jérémie Melleton ◽

...

Keyword(s):

High Pressure ◽

Western Alps ◽

Late Carboniferous ◽

Crustal Thickening ◽

Garnet Growth ◽

Variscan Orogeny ◽

High Grade ◽

Pressure Stage ◽

Hp Metamorphism ◽

High Pressure Stage

The Paleozoic basement exposed in the External Crystalline Massifs of the Western Alps (ECM) contains numerous relics of Variscan eclogites and high pressure granulites preserved in high grade migmatitic gneisses. These relics are taken to indicate that the ECM underwent an early HP metamorphic stage during the Variscan Orogeny. However, due to the scarcity of recent thermobarometric and geochronological data, the geodynamic significance of this high pressure metamorphism remains unclear. Based on petrological similarities with other eclogite-bearing formations in the European Variscides (especially the &#8220;leptyno-amphibolic compex&#8221; in the French Variscides), it has been suggested that the high pressure rocks from the ECM mark a mid-Devonian subduction cycle, preceding the main Carboniferous Variscan collisional stage (Fr&#233;ville et al., 2018; Guillot and M&#233;not, 2009). This interpretation mostly relies on one mid-Devonian U-Pb zircon age (395&#177;2 Ma) obtained in eclogites from the massif of Belledonne (Paquette et al., 1989), which has been interpreted as the age of eclogitization. However, dating of high pressure granulites in the Argentera Massif (Rubatto et al., 2010) yielded a Carboniferous age (ca. 340 Ma) for the high pressure stage, questioning the previous geodynamical interpretation. We present here the results of a detailed petrological and geochronological investigation of the high grade formation of the Lacs de la Temp&#234;te in NE Belledonne, where some of the eclogites dated by Paquette et al. (1989) were sampled. This area exposes mostly high-grade migmatitic metasediments with intercalated lenses of orthogneiss and garnet-bearing amphibolites, preserving locally eclogitic assemblages. Thermobarometric estimations coupling forward pseudosection modelling, Zr in rutile thermometry and garnet growth modelling constrain the minimal P conditions during the high pressure stage at ca. 1.4-1.6 GPa and 700 &#176;C. The early HP assemblage was then strongly overprinted by granulite facies metamorphism at ca. 1.0-1.2 GPa and 750 &#176;C, also recorded in the surrounding metasediments. U-Pb dating of zircon reveals that the eclogites derived from Ordovician protoliths. Zircon overgrowth in the eclogites and the surrounding metasediments constrain the age of HP metamorphism between ca. 350-305 Ma, with no evidence for a Devonian event. Rutile dating in the eclogites supports the late Carboniferous age of metamorphism. The middle-late Carboniferous corresponds to the main period of Variscan nappe stacking in the ECM, following a period of arc magmatism during late Devonian-Tournaisian (ca. 360-350 Ma, Fr&#233;ville et al., 2018). We therefore suggest that the 350-305 Ma ages recorded in the HP units of the ECM do not correspond to a Devonian subduction, but rather represent the equilibration of orogenic lower crust at HP-MT conditions during the Variscan nappe stacking events, followed by re-equilibration at lower P during late Carboniferous. This evolution presents striking similarities with the high pressure units of the Moldanubian zone in the Bohemian massif (Schulmann et al., 2009). However, deciphering the exact meaning of U-Pb ages in retrogressed eclogites remains a challenge, and further field and petrological investigation is required to produce a consistent history of the Variscan collision in the ECM.

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S-Wave Velocity Tomography: Small-Scale Laboratory Application

Geotechnical Testing Journal ◽

10.1520/gtj12638 ◽

2005 ◽

Vol 28 (4) ◽

pp. 12638 ◽

Cited By ~ 2

Author(s):

L David Suits ◽

TC Sheahan ◽

J-S Lee ◽

AL Fernandez ◽

JC Santamarina

Keyword(s):

Wave Velocity ◽

Small Scale ◽

S Wave ◽

Velocity Tomography

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Structural evolution along the Susa Shear Zone: the role of a first-order shear zone in the exhumation of meta-ophiolite units (Western Alps)

Swiss Journal of Geosciences ◽

10.1186/s00015-020-00370-6 ◽

2020 ◽

Vol 113 (1) ◽

Author(s):

Stefano Ghignone ◽

Gianni Balestro ◽

Marco Gattiglio ◽

Alessandro Borghi

Keyword(s):

Shear Zone ◽

Hanging Wall ◽

Structural Evolution ◽

Shear Zones ◽

Western Alps ◽

First Order ◽

Piedmont Zone ◽

Regional Deformation ◽

Deformation Stages

Abstract In the Western Alps, different shear zones acting at different depths have been investigated for explaining multistage exhumation of (U)HP units, and several exhumation models have been proposed for explaining present-day stacking of different tectonometamorphic units. This study aims to reconstruct the tectonic evolution of the Susa Shear Zone (SSZ), a polyphasic first-order shear zone, outcropping in the Susa Valley. The SSZ consists of a thick mylonitic zone, along which units characterized by different Alpine metamorphic P–T peaks are coupled. In the study area, the footwall of the SSZ mostly consists of oceanic units (i.e., Internal Piedmont Zone), which record eclogitic conditions, whereas the hanging wall consists of oceanic units (i.e., External Piedmont Zone), which record blueschist-facies conditions. These tectonic units were deformed during subduction- and exhumation-related Alpine history, throughout four main regional deformation phases (from D1 to D4), and were coupled along the SSZ, wherein two shearing events have been distinguished (T1 and T2). T1 occurred during early exhumation and was characterized by “apparent reverse” Top-to-E kinematics, whereas T2 occurred during late exhumation and was characterized by Top-to-W kinematics. Detailed fieldwork and structural analysis allowed us to describe the main features of the different deformation stages and define the deformation relative timing. As final result, we propose a four-step geodynamic model, focused on the different stages developed along the SSZ, from pre-T1 to syn-T2, showing the geometrical relationships between the tectonic units involved in the exhumation. The model aims at explaining the role of the SSZ in the axial sector of the Western Alps.

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