The Alpine Tethys rifted margins: Reconciling old and new ideas to understand the stratigraphic architecture of magma‐poor rifted margins

Abstract

This study focuses on the assessment of the stratigraphic architecture of the proximal and distal Jurassic Alpine Tethys rifted margins. The aim of the study was to reconcile the major observations performed in the Alps through time with what is observed in present‐day magma‐poor rifted margins. The proximal and distal rifted margins reflect two evolutionary phases of rifting involving different structures and isostatic evolutions leading to different stratigraphic records. The proximal rifted margins formed from Upper Triassic to Pliensbachian times and resulted in the formation of classical half graben basins. Rift evolution changed drastically during Pliensbachian to Toarcian times when extension started to localize in the future distal rifted margins. Low‐angle detachment faults become the new elementary structure controlling final crustal tapering and eventually mantle exhumation to the sea floor. New structural elements controlling the stratigraphic architecture of the distal rifted margins include extensional allochthons and breakaway blocks, both resulting from the delamination of a keystone block, also referred to as hangingwall block, during hyper‐extension processes. To define the stratigraphic architecture of the proximal and distal rifted margins, three stratigraphic marker horizons are used, namely the top of the pre‐rift sequences, the top of the proximal margin syn‐tectonic sequence and the base of post‐rift sequences. Based on the distribution of these stratigraphic marker horizons across the Alpine units and a structural re‐evaluation of the rift structures, a synthetic tectono‐stratigraphic evolution of the Alpine Tethys domain is proposed that reconciles old and new ideas about how rift systems may evolve in space and time.

Geological setting of the southern termination of Western Alps

“…Ligurian-Piemonte basin and the Dauphinois and Provençal domains. In the northern part of the Alps, the Briançonnais domain was separated from the proximal margin by the Valais oceanic basin that progressively wedged out towards the south (Mohn et al 2010;Masini et al 2013) finally disappearing in the SW Alps. The stratigraphy of the Briançonnais starts with Lower Triassic fluvial to littoral conglomerates, quartzarenites and lagoonal mudrocks, followed by a Middle Triassic peritidal carbonate succession.…”

The role of inheritance in structuring hyperextended rift systems: Some considerations based on observations and numerical modeling

“…General situation. The Err detachment system exposed in the Lower Austroalpine Err nappe in SE Switzerland is one of the world's few exposed and preserved rift-related detachment systems (e.g., Masini et al, 2013). This structure can be mapped over about 200 km 2 exhibiting primary relationships with pre-rift Permian structures and Alpine thrust faults.…”

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

“…Froitzheim and Eberli, 1990;Masini et al, 2011 Mohn et al, 2012). This domain was followed to the west by the Adriatic hyper-extended domain, floored locally by exhumed crust and overlain by extensional allochthons, now sampled in the Lower Austroalpine nappes in the Eastern Swiss Alps [Err nappe (Froitzheim and Eberli, 1990;Manatschal and Nievergelt, 1997;Masini et al, 2011Masini et al, , 2013 and Bernina nappe ] and in the Canavese Zone in the Southern Alps (Ferrando et al, 2004). The areas floored by exhumed mantle belonging to the South Penninic basin are now best preserved in the Forno/Malenco, Platta and Totalp units in the Eastern Swiss Alps (Figs.…”

“…As mentioned above, a second marker horizon should be chosen within the post-tectonic sedimentary sequence, which seals the allochthons and/or syn-tectonic sediments [note that post-tectonic sediments, in the context of the evolving margin, may be both syn-and post-rift (for terminology see Masini et al, 2013)]. The marker layer should lie as close as possible to the base of the post-tectonic sedimentary sequence.…”