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

International audienceA long-standing question in Earth Sciences is related to the importance of inheritance in controlling tectonic processes. In contrast to physical processes that are generally applicable, assessing the role of inheritance suffers from two major problems: firstly, it is difficult to appraise without having insights into the history of a geological system; and secondly all inherited features are not reactivated during subsequent deformation phases. Therefore, the aim of this paper is to give some conceptual framework about how inheritance may control the architecture and evolution of hyperextended rift systems.In this paper, we use the term inheritance to refer to the difference between an “ideal” layer-cake type lithosphere and a “real” lithosphere containing heterogeneities. The underlying philosophy of this work is that the evolution of hyperextended rift systems reflects the interplay between their inheritance (innate/“genetic code”) and the physical processes at play (acquired/external factors). Thus, by observing the architecture and evolution of hyperextended rift systems and integrating the physical processes, one may get hints on what may have been the original inheritance of a system.We first define 3 types of inheritance, namely structural, compositional and thermal inheritance and develop a simple and robust terminology able to describe and link observations made at different scales using geological, geophysical and modeling approaches. To this, we add a definition of rift-induced processes, i.e. processes leading to compositional changes during rifting (e.g. serpentinization or decompression melting). Using this approach, we focus on 3 well-studied rift systems that are the Alpine Tethys, Pyrenean–Bay of Biscay and Iberia–Newfoundland rift systems. However, as all these examples are magma-poor, hyperextended rift systems that evolved over a Variscan lithosphere the concepts developed in this paper cannot be applied universally. For the studied examples we can show that:1) the inherited structures did not significantly control the location of breakup2) the inherited thermal state may control the mode and architecture of rift systems, in particular the architecture of the necking zone3) the architecture of the necking zone may be influenced by the distribution and importance of ductile layers during decoupled deformation and is consequently controlled by the thermal structure and/or the inherited composition of the crust4) conversely, the deformation in hyperextended domains is strongly controlled by weak hydrated minerals (e.g. clay, serpentinite) that result from the breakdown of feldspar and olivine due to fluid and reaction assisted deformation5) inherited structures, in particular weaknesses, are important in controlling strain localization on a local scale and during early stages of rifting6) inherited mantle composition and rift-related mantle processes may control the rheology of the mantle, the magmatic budget, the thermal structure and the localization of final rifting.These key observations show that both inheritance and rift-induced processes played a significant role in the development of the magma-poor southern North Atlantic and Alpine Tethys rift systems and that the role of inheritance may change as the physical conditions vary during the evolving rifting and as rift-induced processes (serpentinization; magma) become more important. Thus, it is not only important to determine the “genetic code” of a rift system, but also to understand how it interacts and evolves during rifting. Understanding how far these new ideas and concepts derived from the well-studied hyperextended rift systems of the southern North Atlantic and Alpine Tethys can be translated to other less explored hyperextended rift systems will be one of the challenges of future research in rifted margins

Plagioclase Peridotites in Ocean-Continent Transitions: Refertilized Mantle Domains Generated by Melt Stagnation in the Shallow Mantle Lithosphere

The existence of different mantle domains exposed in ocean-continent transition zones provides a framework for understanding the generation of ultramafic seafloor along magma-poor rifted margins. In this study we present detailed petrological and geochemical data on peridotites from the Eastern Central Alps ophiolites in Switzerland and Italy to identify different mantle domains, to estimate the extent of refertilization, and to test whether refertilization is associated with a thermal signature that has important implications for geophysical interpretations of ocean-continent transitions. The compositions of clinopyroxene, orthopyroxene and spinel clearly reflect the different mantle domains. Relative to clinopyroxenes from spinel peridotites, clinopyroxenes from plagioclase peridotites have lower Na2O and Sr contents, but higher middle to heavy rare earth element ratios and Zr concentrations, and different Sc-V relationships. Spinels in plagioclase peridotites have higher TiO2 and lower Mg-numbers compared with those in spinel peridotites. Mineral-mineral trace element partitioning suggests that spinel peridotites equilibrated at substantially lower temperatures than plagioclase peridotites. The temperature difference between the spinel and plagioclase peridotites indicates an important thermal boundary between the two. The geochemical data show systematic spatial variations. A heterogeneous, ‘subcontinental domain' with no syn-rift melt imprint is separated from a ‘refertilized domain' that exhibits a complex history of regional-scale melt infiltration and melt-rock reaction, which has erased most of the ancient history. Simple calculations suggest that up to 12% of mid-ocean ridge basalt-type melt can be stored in plagioclase peridotite, relative to a depleted residue. Such a ‘lithospheric sponge' provides an explanation for the fertile compositions of the peridotites and the rare occurrence of volcanic rocks in magma-poor rifted margins. We suggest that magma-poor vs magma-rich margins are largely determined by the efficiency of melt extraction and not so much by melt generation processes, given a similar initial composition of the upwelling mantle. It is proposed that refertilization increases textural diversity and chemical heterogeneity related to shallow crystallization in the mantle lithospher

Correction to "Extreme crustal thinning in the Bay of Biscay and the Western Pyrenees: From observations to modeling"

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Reactivation of a hyperextended rift system: the Basque‐Cantabrian Pyrenees case.

This contribution investigates the role of a hyperextended rift system in the formation of the Basque-Cantabrian Pyrenees by discussing their present-day architecture as well as the inherited rift template. Moreover, this work attempts to decipher the onset of reactivation of a hyperextended system and to discuss the related processes during collision. To carry out this study, two regional, crustal-scale cross-sections are presented that provide geological and geophysical information and interpretations across the Central and Western Basque-Cantabrian Pyrenees. Moreover, the two sections are restored back to the Cenomanian and Barremian, corresponding to the end of two independent rift stages respectively. The two sections document different structural styles observed along the orogenic belt. The Central section, involving the Iberian and European plates, shows a thin-skinned structural style, where the Upper Triassic salt acted as a decoupling level between the sedimentary cover and the underlying basement during both extension and reactivation. The Western section, by contrast, crosses only the Iberian plate (i.e., intra-plate section) and displays a hybrid situation showing both thin- and thick-skinned structural styles that were conditioned by the irregular distribution of Triassic salt. Extensional deformation was localised in the north (i.e., Bay of Biscay) and less important in the south. Despite compressional reactivation, the northern part of the Western section preserves its rift template, which provides key insights to restore the internal part of the Central section. In contrast to the Western section, the Central section shows stacked depocenters, resulting from overprinted Mesozoic rift events that had a first order control on the subsequent reactivation. This study corroborates the importance of rift inheritance during the onset of convergence by reactivating the most distal and weak part of the rift system (i.e., serpentinised mantle) before starting the collision phase. A key learning is that the understanding of the nature and distribution of decoupling levels at a crustal scale is fundamental to reconstruct the structural evolution during the formation and reactivation of a hyperextended rift system

Upper-plate magma-poor rifted margins: stratigraphic architecture and structural evolution

International audienceAlthough it is generally accepted that many distal, magma-poor rifted margins are asymmetric and can be divided into lower and upper plate margins, little is known about the details of how and when this asymmetry evolves and how upper and lower plate margins can be distinguished. This is due to the fact that most papers focused on the so called lower plate margins, while the upper plate margins remained less well understood, mainly due to the lack of public accessible drill hole data. The aim of this paper is to provide a first order description of the global architecture and stratigraphic evolution of an upper plate, magma-poor rifted margin. In order to provide such a template, we focused on 2 seismic sections, the ION-1000 line (East Indian margin), and the SCREECH 2 line (Newfoundland margin) and describe key, km-scale outcrops from the fossil European margin exposed in the Western/Central Alps, all of which document classical upper plate margins. Based on these data we show that upper plate magma-poor rifted margins can be characterized by a staircase type architecture with terraces (T1, T2, T3) and ramps (R1, R2) that result as a consequence of an evolution through a coupling, exhumation and breakup stage. We also defined key stratigraphic levels that we try to link with the evolution of the margin which enables us to link the tectonic evolution with the creation of accommodation space and formation of the staircase architecture that characterizes the upper plate margin. From these observations we develop a conceptual model for the evolution of upper-plate margins and discuss the applicability of this model for different strain rates, rates of subsidence and sedimentation rates

Assessing the impact of orogenic inheritance on the architecture, timing and magmatic budget of the North Atlantic rift system: a mapping approach: Orogenic inheritance and the North Atlantic rift

International audienceIn order to investigate the impact of orogenic inheritance on the characteristics of hyperextended rift systems we develop new mapping methods highlighting the first-order architecture and timing of hyperextended rifts, as well as the distribution of heterogeneities inherited from previous orogenies. We use these to characterize the North Atlantic rift system and adjacent areas affected by the Palaeozoic Caledonian and Variscan orogenies. Comparison of these maps demonstrates major differences in the behaviour of the North Atlantic rift relative to both orogens, the Variscan front appearing to be a major limit. Indeed, the rift cuts through the Caledonian orogen and parallels its structural grain, while it circumvents the core of the Variscides. In addition, rifting is protracted and polyphase, and breakup is magma-rich North to the Variscan front, as opposed to the South where a single, apparently continuous extensional event lead to magma-poor breakup in less than 50 Myr. These observations point to a major influence of orogenic inheritance on the characteristics of hyperextended rift systems. On the other hand, our study supports that rifts reactivate sutures corresponding to former large (> 2 000 km) oceans, while leaving sutures of small (< 500 – 1 000 km) oceanic basins little affected, suggesting a significant impact of the pre-orogenic histories on subsequent extensional processes

Determining the COB location along the Iberian margin and Galicia Bank from gravity anomaly inversion, residual depth anomaly and subsidence analysis

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Testing the Extensional Detachment Paradigm: A Borehole Observatory in the Sevier Desert Basin

Low-angle normal faults or detachments are widely regarded as playing an important role in crustal extension and the development of rifted continental margins (Manatschal et al., 2007). However, no consensus exists on how to resolve the mechanical paradox implied by the gentle dips of these faults and by the general absence of evidence for associated seismicity (Sibson, 1985; Wernicke, 1995; Axen, 2004). As part of a new initiative to rationalize geological and geophysical evidence and our theoretical understanding of how rocks deform, a group of forty-seven scientists and drilling experts from five countries met for four days on 15–18 July 2008 to discuss the present status of the paradox and a borehole-based strategy for resolving it. The workshop was held at two venues in Utah (the Utah Department of Natural Resources in Salt Lake City, and Solitude Mountain Resort in the adjacent Wasatch Range), with a one-day field trip to the Sevier Desert basin of west-central Utah to examine the general setting of potential drill sites and the footwall geology of the Sevier Desert detachment (Canyon Range)

The Basque - Cantabrian Pyrenees: report of data analysis

This contribution presents the analysis of a data set that was put together in the PhD thesis of Jordi Miró which is part of the OROGEN Project. The Basque - Cantabrian Pyrenees, that are the focus of this report, have been extensively studied over the last years. Several open debates in the Earth Science community aroused from this realm regarding the formation and reactivation of rift domains and formation of fold and thrust belts. This report summarizes the main tectonic models proposed to explain both the extension and reactivation history over this area and compile a series of data to consider for further discussions and interpretations. This report includes a thematic map of the Basque - Cantabrian Pyrenees showing an analysis of the tectono-stratigraphic evolution of the area. The map covers an area of more than 33 000 km2 and is a graphic representation of the geology of the region based on a large geodatabase including previous published maps and field observations. A composite reflection seismic line crossing the entire Basque - Cantabrian Pyrenees from the Ebro foreland basin to the offshore Landes High is also presented. This section enables to present a continuous dataset along the entire area with the projection of few drill holes, which are presented with the stratigraphic logs following the same tectono-stratigraphic legend obtained from the previous analysis. The main goal of this data report is to provide a coherent and complete dataset to the community, which enables to propose, discuss and test some of the new concepts related to the formation and reactivation of rifted margins. This data report is complementary to the contributions of Lescoutre and Manatschal (2020) and Cadenas et al. (in prep) that are part of the same special volume. Cette contribution présente l'analyse d'une compilation de données acquises par Jordi Miró lors de sa thèse de doctorat qui s'inscrit dans le projet OROGEN. Les Pyrénées Basco-Cantabriques, qui sont l'objet de ce rapport, ont été largement étudiées au cours de ces dernières années. Cette chaîne orogénique a alimenté de nombreuses discussions concernant la formation et la réactivation des domaines de rift ainsi que la formation des chaînes d'avant-pays (« fold-and-thrust belt »). Ce rapport résume les principaux modèles proposés dans la zone d'étude pour expliquer l'évolution tectonique de l'extension à la réactivation, et compile une série de données à prendre en considération pour les discussions et interprétations futures. Il comprend notamment une carte thématique des Pyrénées Basco-Cantabriques montrant les différentes unités tectono-stratigraphiques de la zone. La carte, qui couvre une superficie de plus de 33 000 km2, est une représentation graphique de la géologie régionale s'appuyant sur une large quantité de données comprenant des cartes publiées précédemment ainsi que de nouvelles observations de terrain. Un profile de réflexion sismique composite traversant la totalité des Pyrénées Basco-Cantabriques (du bassin d'avant-pays de l'Ebre au Haut des Landes) est également présentée. Cette section permet de présenter un ensemble de données continu sur toute la zone via notamment la projection des informations extraites des puits de forage tels que les logs stratigraphiques. L'objectif principal de ce rapport est de fournir à la communauté scientifique un ensemble de données cohérent et complet permettant de proposer, discuter et tester certains des nouveaux concepts liés à la formation et à la réactivation des marges hyper-étirées. Ce rapport de données fournit des informations complémentaires aux études disponibles dans cette publication spéciale du projet OROGEN