The 3D interplay between folding and faulting in a syn-orogenic extensional system: the Simplon Fault Zone in the Central Alps (Switzerland and Italy)

Extensional low-angle detachments developed in convergent or post-collisional settings are often associated with upright folding of the exhumed footwall. The Simplon Fault Zone (SFZ) is a Miocene low-angle detachment that developed during convergence in the Central Alps (Switzerland and Italy), accommodating a large component of orogen-parallel extension. Its footwall shows complex structural relationships between large-scale backfolds, mylonites and a discrete brittle detachment and forms a 3D gneiss dome reflecting upright folding with fold axes oriented both parallel and perpendicular to the extension direction. We present a regional study that investigates the interplay between folding and faulting and its implications for the resulting exhumation pattern of the gneiss dome using 3D geometric modelling (computer software GeoModeller), together with a consideration of the chronological relationships from field relationships and 40Ar/39Ar dating. The early Simplon mylonitic fabric is clearly folded by both extension-parallel and extension-perpendicular folds, forming a doubly plunging antiform, whereas the later ductile-to-brittle fabric and the cataclastic detachment are only affected by wavy extension-parallel folds. This observation, together with the interpreted cooling pattern across the SFZ, suggests that updoming of the footwall initiated at the onset of faulting during ductile shearing around 18.5Ma, due to coeval extension and perpendicular convergence. New 40Ar/39Ar dating on micas (biotite and muscovite) from a sample affected by a strong crenulation cleavage parallel to the axial plane of the Glishorn and Berisal parasitic folds establishes that these folds formed at ca. 10Ma, broadly coeval with late movement along the more discrete detachment of the SFZ. These extension-parallel folds in the footwall of the SFZ developed due to continued convergence across the Alps, accelerating ongoing exhumation of the western Lepontine dome and promoting coeval uplift of the crystalline Aar and Gotthard massifs in the late Miocene

Miocene high elevation and high relief in the Central Alps

Reconstructing Oligocene–Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth surface processes involved in the development of Alpine topography. Despite being one of the most intensively explored mountain ranges worldwide, constraints on the elevation history of the European Alps remain scarce. Here we present stable and clumped isotope measurements to provide a new paleoelevation estimate for the mid-Miocene (∼14.5 Ma) European Central Alps. We apply stable isotope δ–δ paleoaltimetry to near-sea-level pedogenic carbonate oxygen isotope (δ18O) records from the Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine phyllosilicate hydrogen isotope (δD) records from the Simplon Fault Zone (Swiss Alps). We further explore Miocene paleoclimate and paleoenvironmental conditions in the Swiss Molasse Basin through carbonate stable (δ18O, δ13C) and clumped (Δ47) isotope data from three foreland basin sections in different alluvial megafan settings (proximal, mid-fan, and distal). Combined pedogenic carbonate δ18O values and Δ47 temperatures (30±5 ∘C) yield a near-sea-level precipitation δ18Ow value of -5.8±1.2‰ and, in conjunction with the high-Alpine phyllosilicate δD value of  -14.6±0.3‰, suggest that the region surrounding the Simplon Fault Zone attained surface elevations of >4000 m no later than the mid-Miocene. Our near-sea-level δ18Ow estimate is supported by paleoclimate (iGCM ECHAM5-wiso) modeled δ18O values, which vary between −4.2 ‰ and −7.6 ‰ for the Northern Alpine Foreland Basin

The 3D interplay between folding and faulting in a syn-orogenic extensional system: the Simplon Fault Zone in the Central Alps (Switzerland and Italy)

Extensional low-angle detachments developed in convergent or post-collisional settings are often associated with upright folding of the exhumed footwall. The Simplon Fault Zone (SFZ) is a Miocene low-angle detachment that developed during convergence in the Central Alps (Switzerland and Italy), accommodating a large component of orogen-parallel extension. Its footwall shows complex structural relationships between large-scale backfolds, mylonites and a discrete brittle detachment and forms a 3D gneiss dome reflecting upright folding with fold axes oriented both parallel and perpendicular to the extension direction. We present a regional study that investigates the interplay between folding and faulting and its implications for the resulting exhumation pattern of the gneiss dome using 3D geometric modelling (computer software GeoModeller), together with a consideration of the chronological relationships from field relationships and 40Ar/39Ar dating. The early Simplon mylonitic fabric is clearly folded by both extension-parallel and extension-perpendicular folds, forming a doubly plunging antiform, whereas the later ductile-to-brittle fabric and the cataclastic detachment are only affected by wavy extension-parallel folds. This observation, together with the interpreted cooling pattern across the SFZ, suggests that updoming of the footwall initiated at the onset of faulting during ductile shearing around 18.5 Ma, due to coeval extension and perpendicular convergence. New 40Ar/39Ar dating on micas (biotite and muscovite) from a sample affected by a strong crenulation cleavage parallel to the axial plane of the Glishorn and Berisal parasitic folds establishes that these folds formed at ca. 10 Ma, broadly coeval with late movement along the more discrete detachment of the SFZ. These extension-parallel folds in the footwall of the SFZ developed due to continued convergence across the Alps, accelerating ongoing exhumation of the western Lepontine dome and promoting coeval uplift of the crystalline Aar and Gotthard massifs in the late Miocene.ISSN:1661-8734ISSN:1661-872

Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

The Silltal Fault is the northern brittle continuation of the Brenner Fault Zone, marked by a narrow zone of cataclasis and, in three locations, clay-rich fault gouges. The clay mineral composition of these gouges is dominated by higher temperature 2M1 polytype illite/muscovite, with no 1M/1Md illite or mixed layer illite/smectite detected. Smectite is limited to the northern samples from the Stephansbrücke location, whereas chlorite is present in all samples. The K-Ar ages from the different sample size fractions (<0.1, <0.4, <2, 2-6, 6-10μm, "whole rock gouge”) show a wide spread, from ca. 115 to 12Ma, with ages consistently decreasing with grain size. Although the ranges overlap, ages from the northern Stephansbrücke samples are generally older (115-36Ma) than those from the south near Matrei (55-12Ma), possibly reflecting increasing regional metamorphic temperatures to the south. The well-defined linear relationship between apparent age and hydrogen stable isotope (δD) values establishes a direct correlation between rejuvenation of the K-Ar system and increased interaction with meteoric water introduced and focussed within the fault zone. The dependence of both apparent age and δD on grain size also indicates that radiogenic and stable isotope exchange was controlled by grain size, reflecting new 2M1 illite growth, mechanical grinding of protolith muscovite during cataclastic faulting, or both. The results demonstrate the advantages of combining radiogenic and stable isotope analysis in interpretation of K-Ar ages from clay fault gouges. The combined approach was necessary to establish the crucial influence on apparent K-Ar ages of meteoric water influx focussed on the fault zone and its interaction with clay-size-fraction grains

Timing and conditions of brittle faulting on the Silltal-Brenner Fault Zone, Eastern Alps (Austria)

The Silltal Fault is the northern brittle continuation of the Brenner Fault Zone, marked by a narrow zone of cataclasis and, in three locations, clay-rich fault gouges. The clay mineral composition of these gouges is dominated by higher temperature 2M1 polytype illite/muscovite, with no 1M/1Md illite or mixed layer illite/smectite detected. Smectite is limited to the northern samples from the Stephansbrücke location, whereas chlorite is present in all samples. The K–Ar ages from the different sample size fractions (<0.1, <0.4, <2, 2–6, 6–10 μm, “whole rock gouge”) show a wide spread, from ca. 115 to 12 Ma, with ages consistently decreasing with grain size. Although the ranges overlap, ages from the northern Stephansbrücke samples are generally older (115–36 Ma) than those from the south near Matrei (55–12 Ma), possibly reflecting increasing regional metamorphic temperatures to the south. The well-defined linear relationship between apparent age and hydrogen stable isotope (δD) values establishes a direct correlation between rejuvenation of the K–Ar system and increased interaction with meteoric water introduced and focussed within the fault zone. The dependence of both apparent age and δD on grain size also indicates that radiogenic and stable isotope exchange was controlled by grain size, reflecting new 2M1 illite growth, mechanical grinding of protolith muscovite during cataclastic faulting, or both. The results demonstrate the advantages of combining radiogenic and stable isotope analysis in interpretation of K–Ar ages from clay fault gouges. The combined approach was necessary to establish the crucial influence on apparent K–Ar ages of meteoric water influx focussed on the fault zone and its interaction with clay-size-fraction grains.ISSN:1661-8734ISSN:1661-872

The 3D interplay between folding and faulting in a syn-orogenic extensional system: the Simplon Fault Zone in the Central Alps (Switzerland and Italy)

International audienceExtensional low-angle detachments developed in convergent or post-collisional settings are often associated with upright folding of the exhumed footwall. TheSimplon Fault Zone (SFZ) is a Miocene low-angle detachment that developed during convergence in the Central Alps (Switzerland and Italy), accommodating a large component of orogen-parallel extension. Its footwall shows complex structural relationships between large-scale backfolds, mylonites and a discrete brittle detachment and forms a 3D gneiss dome reflecting upright folding with fold axes oriented both parallel and perpendicular to the extension direction. We present a regional study that investigates the interplay between folding and faulting and its implications for the resulting exhumation pattern of the gneiss dome using 3D geometric modelling (computer software GeoModeller), together with a consideration of the chronological relationships from field relationships and 40Ar/39Ar dating. The early Simplon mylonitic fabric is clearly folded by both extension-parallel and extensionperpendicular folds, forming a doubly plunging antiform, whereas the later ductile-to-brittle fabric and the cataclastic detachment are only affected by wavy extension-parallel folds. This observation, together with the interpreted cooling pattern across the SFZ, suggests that updoming of the footwall initiated at the onset of faulting during ductile shearing around 18.5 Ma, due to coeval extension and perpendicular convergence. New 40Ar/39Ar dating on micas (biotite and muscovite) from a sample affected by a strong crenulation cleavage parallel to the axial plane of the Glishorn and Berisal parasitic folds establishes that these folds formed at ca. 10 Ma, broadly coeval with late movement along the more discrete detachment of the SFZ. These extension-parallel folds in the footwall of the SFZ developed due to continued convergence across the Alps, accelerating ongoing exhumation of the western Lepontine dome and promoting coeval uplift of the crystalline Aar and Gotthard massifs in the late Miocene

Thermochronology constraints for the propagation sequence of the south Pyrenean basement thrust system (France-Spain)

International audienceIn this work we combined apatite fission track and biotite/K-feldspar 40Ar/39Ar ages with tectonic data in the west central part of the Axial Zone of the Pyrenees. We discuss the exhumation ages and rates of the Néouvielle, Bordère-Louron, and Bielsa Variscan granites and their relationships with the timing and sequence of south vergent basement thrusting within the Pyrenean orogenic prism. The 40Ar/39Ar ages on K-feldspars from the Néouvielle massif (sample NV7) seem to indicate tectonic movements on the Eaux-Chaudes thrust during the early middle Eocene. Fission track results suggest that the exhumation of the Néouvielle massif occurred around 35 Ma and exhumation of the Bordère-Louron massif around 32 Ma in relation to thrusting on the Gavarnie thrust. The Bielsa massif was exhumed from around 19 Ma by out-of-sequence movements on the Bielsa thrust. We thus show that whereas most of the Pyrenean basement thrust faults (here the Eaux-Chaudes, Gavarnie, and Guarga thrusts) were active in sequence toward the southern foreland from the early Eocene to the earliest Miocene, some of them (here the Bielsa thrust) were activated out of sequence in the hinterland, later than the generally accepted Aquitanian age for the end of the Pyrenean compression. Finally, the apatite fission track modeling indicate a last cooling episode starting around 5 Ma which is most certainly related to the Pliocene reexcavation of the southern and northern flanks of the Pyrenee