Sequence and style of major post-nappe structures, Simplon—Pennine Alps

Milnes, A. G.; Greller, M.; Müller, R. D.

doi:10.1016/0191-8141(81)90041-9

Cited by 65 publications

(59 citation statements)

References 6 publications

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“…Evidently, this feature goes along with vertical shortening, given the flat-lying axial planes of the latestage mega-folds mentioned above. Note that the flat-lying axial planes of all these folds are in contrast with the steeply dipping axial planes of the classical backfolds, that are related to backthrusting along the Insubric line (Schmid et al 1987), such as the Vanzone antiform, formed later in respect to the Mischabel backfold (Milnes et al 1981 that no evidence for back-thrusting or normal faulting was found anywhere in the working area. While the interpretation of the present geometry can largely be explained by D3 nappe refolding, differences in P-T conditions found amongst the different tectonic units of the working area ) are unrelated to this folding.…”

Section: Discussionmentioning

confidence: 83%

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Tectonic evolution of the Briançonnais units along a transect (ECORS-CROP) through the Italian-French Western Alps

Bucher¹,

Ulardic

Bousquet³

et al. 2004

Eclogae geol. Helv.

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Based on new structural data from an area in the Italian-French Western Alps, situated between the Petit Saint Bernard pass and the Gran Paradiso massif, the large-scale geometry of the tectonic features is established for the tectonic units derived from the Briançonnais paleogeographic domain. Based on this, and other new data on the metamorphic evolution and geochronology of the area, a consistent model for the tectonic evolution for the Briançonnais domain is proposed. A nappe stack consisting of, from bottom to top, Zone Houillère unit, Ruitor unit, Internal unit and Piemont-Ligurian oceanic unit, was affected by three deformation phases. The third phase of deformation (35-31 Ma), referred to as post-nappe folding, is responsible for the formation of two large-scale folds: the Ruitor and Valsavaranche mega-folds. An overturned nappe stack, characterized by foreland-dipping foliations, is found between the axial planes of these two mega-folds. No evidence was found for back-thrusting, postulated by many previous authors. The individual thrusts between the tectonic units were active during a second phase of deformation (43-35 Ma), characterized by top-NW to -NNW shearing. This D2 is related to nappe stacking and accompanied by substantial exhumation. Thrusting becomes relatively younger towards the external parts of the study area. We emphasize that extension played no significant role in the exhumation of the highpressure units of the Italian-French Alps. The contacts between the tectonic units were already active during the first phase of deformation (50-43 Ma), which is related to subduction and peak pressures.

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Section: Discussionmentioning

confidence: 83%

“…Classically, two phases of backfolding are known from the Penninic units of Western Switzerland and adjacent Aosta valley (e.g. Milnes et al 1981). During a first phase of backfolding (D3) the Mischabel backfold, characterized by a flat-lying axial plane, develops.…”

Section: Discussionmentioning

confidence: 99%

Tectonic evolution of the Briançonnais units along a transect (ECORS-CROP) through the Italian-French Western Alps

Bucher¹,

Ulardic

Bousquet³

et al. 2004

Eclogae geol. Helv.

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“…Ar, Rb/Sr and oxygen stable isotopes). Sm, Simplon mylonites; S1/2, foliation related to the D1 and D2 deformation phases [Milnes et al, 1981]; Penninic M. cover, Penninic Mesozoic cover; Camug.-Monc., CamugheraMoncucco unit. MC492, MC497, and MC498 come from boulders in the Isorno River.…”

Section: Methods and Sample Descriptionmentioning

confidence: 99%

“…Kinematic indicators from both the ductile and brittle components consistently show a top down to the SW shear sense [Mancktelow, 1985[Mancktelow, , 1987[Mancktelow, , 1990Grosjean et al, 2004]. In contrast, older structures are generally well preserved in the hanging wall, showing overprinting relationships between several deformation phases that correspond well in their orientation and style to the four regional phases, D1 to D4 [Milnes et al, 1981]. These ductile structures are nonpervasively overprinted by brittle faults.…”

Section: Simplon Fault Zonementioning

confidence: 99%

“…This gradual change in plunge of the stretching lineation defines an overall domal shape [Mancktelow, 1985]. The penetrative mylonitic overprint in the Monte Leone unit (regions II and III) is gradual over 100-200 m. To the north, parasitic F 2 folds (related to D2 deformation phase [e.g., Milnes et al, 1981]) are recognized. The mylonitic foliation seems to be axial plane of these F 2 fold structures and therefore would correspond to an S 2 fabric.…”

Section: Se Prolongation Of the Simplon Fault Zonementioning

confidence: 99%

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Geochronological evidence for continuous exhumation through the ductile‐brittle transition along a crustal‐scale low‐angle normal fault: Simplon Fault Zone, central Alps

et al. 2010

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[1] Major low-angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation. Field relationships alone cannot establish whether these different responses to deformation represent (1) parts of a single process of exhumation along the detachment or (2) two separate events, with the later, more discrete brittle detachment exhuming a fossil ductile shear zone from depth. These two general models are critically assessed for the lowangle normal Simplon Fault Zone (SFZ) in the central Alps. The SFZ shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics. The age of the ductile shear zone and ductile-brittle transition is controversial. We present a detailed geochronological study based on fission track, 40 Ar/ 39 Ar, and Rb/Sr microsampling dating, coupled with structural, petrological, and chemical analyses that provides tight constraints on SFZ timing. Discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 Ma. On the basis of synkinematic white mica in low-temperature shear zones and necks of foliation boudinage, the brittle-ductile transition in the footwall could be dated at ∼14.5-10 Ma. Overall, the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework, over a period of ∼15 Ma. The SFZ is therefore an example of a telescoped crustal section within a single major lowangle fault, involving a continuous period of exhumation rather than a two-stage structure.

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Nappe stack of Piemonte–Ligurian units south of Aosta Valley: New evidence from Urtier Valley (Western Alps)

Ellero

Loprieno²

2017

Geological Journal

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New results of structural and lithostratigraphic investigations performed in the Urtier Valley, located south of Aosta Valley at the northern flank of the Gran Paradiso Massif, shed new light on the nappe stack architecture of the Western Alps. Two distinct Piemonte–Ligurian units were recognized in the area: (a) the blueschist facies Broillot unit, with a well‐preserved lithostratigraphic succession that includes ophiolites and their metasedimentary cover and (2) the eclogitic Bardonney unit with clasts and/or slices of ophiolites embedded in a carbonate/quarzitic and/or metabasic matrix. Three major deformation stages have been recognized (D1, D2, and D3). D1 phase occurred during the peak metamorphic conditions (i.e., eclogite or blueschist facies). D2 and D3 phases are related to multistage exhumation processes, from early stacking to late refolding of former nappe contacts. Despite the intensity of deformation and metamorphism, the original lithostratigraphic succession of the Broillot unit allowed us to use a lithostratigraphic approach to reconstruct the structures in the study area. For the first time, differences in the nappe stack exposed south and north of the Aosta Valley are reported. In fact, contrary to what is observed in the north, the eclogitic units exposed in the Urtier Valley lay above and not below the blueschist units. A new model for the tectonic evolution, which ranges from stages of subduction to large‐scale nappe refolding, is proposed. It is able to explain the difference in the architecture of the north and south of the Aosta Valley.

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Sequence and style of major post-nappe structures, Simplon—Pennine Alps

Cited by 65 publications

References 6 publications

Tectonic evolution of the Briançonnais units along a transect (ECORS-CROP) through the Italian-French Western Alps

Tectonic evolution of the Briançonnais units along a transect (ECORS-CROP) through the Italian-French Western Alps

Geochronological evidence for continuous exhumation through the ductile‐brittle transition along a crustal‐scale low‐angle normal fault: Simplon Fault Zone, central Alps

Nappe stack of Piemonte–Ligurian units south of Aosta Valley: New evidence from Urtier Valley (Western Alps)

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