The syn‐rift tectono‐stratigraphic record of rifted margins (Part I): Insights from the Alpine Tethys

Abstract

The tectono‐stratigraphic evolution of rift systems is at present poorly understood, especially the one preceding the onset of oceanic seafloor spreading. Improving our understanding of the complex, polyphase tectonic evolution of the fossil Alpine Tethys, one of the best calibrated magma‐poor rift systems worldwide, offers great potential for the interpretation of the distal part of global rifted margins, which notoriously lack data. In this paper, we propose a tectono‐stratigraphic model for the former Alpine Tethys, whose remnants are exposed in the Alps of Western Europe. We first review the historical evolution of some concepts grounding present knowledge on the Alpine‐ and global magma‐poor rifted margins. Then we present a spatial and temporal template for the Alpine Tethys rift system using a ‘building block (BB)’ approach. This new approach is powerful in that it can integrate high‐resolution, structural, stratigraphic, thermochronological, and petrological observations from dismembered outcrops into a margin‐scale template compatible with the first‐order seismic architecture defined at present‐day, magma‐poor rifted margins. The detailed analysis of the syn‐rift sequence of the Alpine Tethys margins demonstrates that extension migrated and localised while evolving from the stretching to the necking phase. During hyperextension, rifting was asymmetric and controlled by in‐sequence detachment faulting. Then, the rift system evolved back into a more symmetric, embryonic spreading system. With this contribution, we aim to allow readers without knowledge of the Alps to access this unique ‘archive’ preserving some of the world's best‐exposed rift structures. Recognising these structures is critical to understand the origin of some new concepts used to explain present‐day, deep‐water rifted margins, and to interpret and predict the tectono‐stratigraphic evolution during advanced stages of rifting.

The syn‐rift tectono‐stratigraphic record of rifted margins (Part II): A new model to break through the proximal/distal interpretation frontier

“…Rifting may not always initiate on a thermally and gravitationally equilibrated lithosphere (Manatschal, Chenin, Lescoutre, et al., 2021; Zhang et al., 2020) and/or the pre‐rift sediments may display variable thicknesses inherited from previous tectonic events. In Western Europe, for instance, the Alpine Tethys rifting overprinted an area that was formerly affected by the Meliata‐Vardar rifting, which is expressed by a thickening of the pre‐Alpine Tethys rifting sedimentary sequence to the east (Manatschal, Chenin, Ghienne, et al, 2021 and references therein). In Grand Banks, the Jeanne d’Arc stretching basin underwent post‐Iberia–Newfoundland rifting extension related to the opening of the Orphan Basin (e.g.…”

“…A broadly distributed extensional event affected a large part of Western Europe and eastern North America from the Permo‐Triassic onward (Leleu et al., 2016, and references therein), and is here regarded as the stretching phase of both the Alpine Tethys and Iberia–Newfoundland rifting. In the Alpine Tethys rift system, the onset of localized deformation (necking phase) started at 185 ± 5 Ma (Sinemurian–Pliensbachian), evolved to hyperextension from 175 ± 5 Ma (Toarcian–Aalenian) and culminated with mantle exhumation from 165 ± 5 Ma onward (Bajocian–Bathonian; Manatschal, Chenin, Ghienne, et al, 2021). Rifting presumably stopped in the Alpine Tethys realm before steady‐state seafloor spreading occurred in any branch of the Alpine rift system (Picazo et al., 2016).…”

“…constrained by either outcrop observations or drilling) are by far those of the fossil Alpine Tethys exposed in the Alps. This example is described in detail in the companion paper by Manatschal, Chenin, Ghienne, et al (2021) and only briefly summarized here (Figure 7c,d). Another wellcalibrated example is the conjugate transect across the Iberia-Newfoundland margins imaged by the TGS-LG 12 and SCREECH 2 reflection seismic profiles respectively.…”

“…In the present section, we describe the characteristics of the TSs of our idealized model (Figure 5a–g) without going through the details of construction. The observations on which this model was built derive essentially from the Alps and are presented in Part I (companion paper by Manatschal, Chenin, Ghienne, et al, 2021). The present contribution aims to present our idealized model and show its application.…”

“…Schematic evolution of an idealized magma‐poor rifting (crustal surface and pre‐rift sediment length are conserved among the different stages): (a) pre‐rift setting; (b) stretching phase; (c) early necking phase; (d) late necking phase; (e) hyperextension phase; (f) mantle exhumation phase; (g) seafloor spreading phase. Abbreviation LP stands for Lower Plate and UP for Upper Plate; insets g1–g5 zoom on characteristic ‘Building Blocks’ (BB1–BB7) of rifted margins (see Manatschal, Chenin, Ghienne, et al, 2021, for a description of the different Building Blocks)…”