Recognizing remnants of magma-poor rifted margins in high-pressure orogenic belts: The Alpine case study

Abstract

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...

Record of Jurassic mass transport processes through the orogenic cycle: Understanding chaotic rock units in the high-pressure Zermatt-Saas ophiolite (Western Alps)

“…Preorogenic reconstruction of the LMO implies tectonic denudation of lithospheric mantle at the seafloor of the JLPO, as it has been documented in modern slow-and ultraslow-spreading ridges (e.g., Cannat, 1993;Dick et al, 2003;Escartin et al, 2008) and inferred for some areas of the Alps and Apennines (e.g., Tricart and Lemoine, 1991;Lagabrielle, 2009;Tartarotti et al, 2011;Sansone et al, 2012;Beltrando et al, 2014;Balestro et al, 2015a). Metaophicalcite records the early history of mantle exhumation by extensional tectonics and concurrent hydrothermal fluid circulation on the seafloor.…”

“…However, the local occurrence and mixing and/or juxtaposition of oceanic-and continent-derived material and units allowed the different interpretation of the ZSO as either the remnant of an hyperextended continental margin in an ocean-continent transition zone (e.g., see Beltrando et al, 2014, and references therein) or the result of the Alpine tectonic juxtaposition between oceanic and continental margin units (e.g., Fassmer et al, 2016, and references therein).…”

The tectonic significance of the Cabo Frio Tectonic Domain in the SE Brazilian margin: a Paleoproterozoic through Cretaceous saga of a reworked continental margin

“…The reactivation of continental margins is due to inherited heterogeneities that control the rifting processes (Manatschal et al 2007;Beltrando et al 2010Beltrando et al , 2014. In this context, the differences in structure and rheology associated with the CFTD could have influenced the pattern of segmentation and magmatism of the southeast Brazilian margin during its early stages.…”

“…Usually, this reactivation of continental margins is due to inherited heterogeneities that control the rifting processes (Manatschal et al 2007;Beltrando et al 2010Beltrando et al , 2014. Former continental margins may be later involved in a collisional event, becoming the core of an orogenic belt, according to the Wilson cycle.…”

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

“…1a). It is thought to derive from the distal portion of the Adriatic passive margin that was involved in the Alpine subduction (Dal Piaz, 1999;Beltrando et al, 2014). The Sesia Zone is bounded to the west by blueschist to eclogite facies subunits derived from the Piemonte-Liguria Ocean (Bearth, 1967;Dal Piaz and Ernst, 1978;Martin et al, 1994;Cartwright and Barnicoat, 2002;Bucher et al, 2005;Groppo et al, 2009;Rebay et al, 2012;Negro et al, 2013) and to the east by the Insubric Line.…”