The Austroalpine Sesia±Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier±Rafray, Etirol±Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt±Saas) tectonic elements, or within the latter. Rb±Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia±Lanzo inlier, is of Eocene age (49±40 Ma), like the underlying Zermatt±Saas ophiolite (45±42 Ma) between the Aosta valley and Gran Paradiso massif. 40 Ar± 39 Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt±Saas nappe (46±43 Ma) are consistent with Rb±Sr dating, whereas that on the Austroalpine Glacier±Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt±Saas nappe, 20±25 Ma later than the eclogitic Sesia±Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte±Ligu-rian ocean far from the Dent Blanche±Sesia domain and Adriatic margin.Keywords 40Ar±39Ar´Eocene eclogitic metamorphism´Italian Western Alps´Rb±SrŹ ermatt±Saas ophiolite

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. Magma-poor rifted margins are being increasingly recognized in present-day Atlantic-type systems. However, findings of fossil areas floored by exhumed mantle or hyper-extended crust are comparatively rare within orogenic belts that were originated through the inversion of pre-existing rifted margins. This discrepancy may be due to the common reactivation of lithological contacts during subduction/orogeny, potentially masking pre-orogenic relationships, and, most importantly, to the frequent lack of a preorogenic layer-cake architecture, hindering retro-deformation of multiply deformed tectonic units. This study outlines a methodology to detect sections of magma-poor, hyper-extended rifted margins without a layer-cake architecture in multiply deformed/metamorphosed terrains. This approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of Alpine orogens. In the latter domains, continental basement and hydrated peridotites were exhumed at the basin floor during Jurassic rifting along long-offset detachment systems. Extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre-rift sediments from the hanging wall blocks. Lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn-to post-rift meta-sediments and/or serpentinized subcontinental mantle are widespread within sections of Alpine-type orogenic belts that underwent high-to ultra-high-pressure metamorphism. However, similar associations may arise from a variety of processes other than rift-related lithospheric thinning in magma-poor environments, including subduction mélange dynamics or deposition of sedimentary mélanges along convergent/divergent margins. The partial preservation of rift-related lithostratigraphic associations may still be assessed, despite the lack of biostratigraphic evidence, by (1) the consistency of the lithostratigraphic architecture over large areas, despite pervasive Alpine deformation, which rules out chaotic mixing during subduction/ exhumation, (2) the presence of clasts of basement rocks in the neighboring meta-sediments, indicating the original proximity of the different lithologies, (3) evidence of brittle deformation in continental basement and ultramafic rocks pre-dating Alpine metamorphism, indicating that they were juxtaposed by fault activity prior to the deposition of post-rift sediments, and (4) the similar Alpine tectono-metamorphic evolution of ophiolites, continental basement and meta-sediments. A re-assessment of basement-cover relationships in...

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The boundary between the Helvetic and the Penninic (=Briançonnais) Zones has long been recognized as a major fault (“Penninic Front”) in the Western Alps. A narrow oceanic domain has been postulated at least along part of this boundary (the Valaisan Basin). However, the information provided by the pre‐Triassic basement has not been fully exploited and will be discussed here in detail. The igneous and metamorphic history of the pre‐Triassic basement shows significant differences between the External Massifs from the Helvetic Zone, with abundant Late Carboniferous granites, and the basement of the Briançonnais Zone, including the Internal Massifs (Dora‐Maira, Gran Paradiso, and Monte Rosa), devoid of Carboniferous granites. A major coal‐bearing basin, the “Zone Houillère,” opened along this boundary. This limnic intramontane basin has never been properly investigated. The Zone Houillère is not comparable with the external, paralic, flexural, basins on both sides of the Variscan belt but shows similarities with the Saar‐Saale Basin. Like the latter, we interpret the Zone Houillère as a transtensional basin opened along a major, crustal‐scale, fault zone, namely, the East Variscan Shear Zone. The Permian magmatism and sedimentation displays contrasting distributions, being absent or very localized in the Helvetic Zone, and widespread in the Penninic Zone. The above data indicate that the structural inheritance from the Variscan belt plays a major role in defining the future location of the Valaisan Basin, that is, the boundary between the European paleomargin and the Briançonnais microcontinent.

Eclogites occur in the Monte Rosa and Gran Paradiso Penninic nappes of the Western Alps as small bodies within metasedimentary rocks and, inMonte Rosa, rarely, as boudinaged dikes within late-Variscan granitoids. Minor but characteristic constituents of these eclogites are glaucophane and paragonite. Bulk composition of the protoliths ranges from basaltic to Fe-Ti basaltic.High-pressure assemblages comprising chloritoid, phengite, garnet ± kyanite, talc, Mg-chlorite and glaucophane also occur in micaschists of both nappes. The Early Alpine high-pressure assemblages postdate the intrusion of late-Variscan granitoids and are overprinted by the greenschist, mid-Tertiary Lepontine metamorphism.Garnet-clinopyroxene geothermometry and phase compatibilities suggest highpressure metamorphic temperatures ranging from 440°C to 530°C in the Monte Rosa nappe, with pressure in excess of 8-10 Kb but lower than approximately 14 Kb. These conditions are in the same range as established for the overlying Piedmont ophiolite nappe and for parts of the Austroalpine basement nappe, implying a similar tectonometamorphic evolution during Early Alpine time. Garnet-omphacite pairs from the amphibole eclogites of the Gran Paradiso nappe yield lower equilibration temperatures (T =400°C), possibly recording a later stage in the P-T-t path of this unit.

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