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

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

Observations from the Alpine Tethys and Iberia/Newfoundland margins pertinent to the interpretation of continental break-up, in Karner, G.D., Manatschal, G., and Pinheiro, L., eds., Imaging, Mapping and Modelling Continental Lithosphere Extension and Breakup.

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Heredity of twin births

About 1 per cent. of human births are twin births. However, there are certain families in which the proportion rises to 5, 10, or even 15 per cent. There can be little doubt then that, as in sheep, so in man, there are strains having a special tendency toward the production of twins. It is commonly believed that this tendency toward the production of twins must be wholly a maternal quality, depending upon the inherited tendency to double ovulation. The study of the heredity of twins is accompanied by certain difficulties, such as the fact that the occurrence of twins is frequently isolated, apparently haphazard, occurring perhaps in only one case in a fairly large fraternity, in which other representatives are single births. It will simplify the matter a little if we consider only those cases in which two or more sets of twins have arisen from a given mating. The study of twins is still further complicated by the fact that they are of two types, namely twins derived from a double ovulation and twins derived from a single ovulation, there being a subsequent fission or budding of the fertilized egg. Such single-egg twins are'easily distinguished clinically by being both enveloped in the same chorion. They are also always of the same sex. The statement that the mother alone determines the tendency to twins is not, however, supported by the facts. Of the births giving rise to the fraternities of twin-repeating mothers, 4.5 per cent, are twin births. Of the births giving rise to fraternities of twin-repeating fathers, 4.2 per cent are twin births. These figures depend upon 355 and 289 labors respectively. The sisters of twin-producing fathers have twins in 8.2 per cent. of labors, while the sisters of twin-producing mothers have twins in 5.5 per cent. of labors

Controls on the Thermomechanical Evolution of Hyperextended Lithosphere at Magma‐Poor Rifted Margins: The Example of Espirito Santo and the Kwanza Basins

International audienceHigh-quality, long offset seismic data from many distal rifted margins show evidence for hyper-extended, <10-km-thick crust. Direct observation of such domains is challenging as they lie, at great water depth, buried beneath thick sedimentary sequences and formed by rock-assemblages that are hydrated and geophysically indistinguishable. Only a few drill holes have penetrated basement at ultradistal rifted margins. These observations, together with outcrops of preserved analogs exposed in collisional orogens, suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or, in the serpentinized mantle controls the formation of the ocean continent transition. While depth-dependent thinning controls the early phases of rifting conforming to classical rift models, we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup. Here we develop a rheological parameterization to simulate the formation of, and slip-on, large offset normal faults rooted in growing brittle to ductile shear zones. The evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle. We also propose a simplified formulation to simulate magmatic underplating and seafloor spreading. The resulting numerical models provide a self-consistent picture for the evolution of magma-poor rifted margins from initiation of rifting to seafloor spreading. The model results are compared with first-order observations of the Kwanza and Espirito Santo conjugate margins in the South Atlantic as well as of magma-poor margins globally