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Transform Continental Margins – Part 1: Concepts and Models Christophe Basile Transform continental margins – Part 1: Concepts and models Christophe Basile To cite this version: Christophe Basile. Transform continental margins – Part 1: Concepts and models. Tectonophysics, Elsevier, 2015, 661, pp.1-10. 10.1016/j.tecto.2015.08.034. hal-01261571 HAL Id: hal-01261571 https://hal.archives-ouvertes.fr/hal-01261571 Submitted on 25 Jan 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Tectonophysics, 661, p. 1-10, http://dx.doi.org/10.1016/j.tecto.2015.08.034 Transform continental margins – Part 1: Concepts and models Christophe Basile Address: Univ. Grenoble Alpes, CNRS, ISTerre, F-38041 Grenoble, France.cbasile@ujf-grenoble. Abstract This paper reviews the geodynamic concepts and models related to transform continental margins, and their implications on the structure of these margins. Simple kinematic models of transform faulting associated with continental rifting and oceanic accretion allow to define three successive stages of evolution, including intra- continental transform faulting, active transform margin, and passive transform margin. Each part of the transform margin experiences these three stages, but the evolution is diachronous along the margin. Both the duration of each stage and the cumulated strike-slip deformation increase from one extremity of the margin (inner corner) to the other (outer corner). Initiation of transform faulting is related to the obliquity between the trend of the lithospheric deformed zone and the relative displacement of the lithospheric plates involved in divergence. In this oblique setting, alternating transform and divergent plate boundaries correspond to spatial partitioning of the deformation. Both obliquity and the timing of partitioning influence the shape of transform margins. Oblique margin can be defined when oblique rifting is followed by oblique oceanic accretion. In this case, no transform margin should exist in the prolongation of the oceanic fracture zones. Vertical displacements along transform margins were mainly studied to explain the formation of marginal ridges. Numerous models were proposed, one of the most used is being based on thermal exchanges between the oceanic and the continental lithospheres across the transform fault. But this model is compatible neither with numerical computation including flexural behavior of the lithosphere nor with timing of vertical displacements and the lack of heating related to the passing of the oceanic accretion axis as recorded by the Côte d’Ivoire- Ghana marginal ridge. Enhanced models are still needed. They should better take into account the erosion on the continental slope, and the level of coupling of the transform continental margin with the adjacent oceanic lithosphere. Keywords transform continental margin; sheared margin; strike-slip margin; transform fault; partitioning; oblique margin __________________________________________________________________________________________ 1. Introduction Program Leg 159 has been the al., this volume). The tectonic and Transform continental margins only scientific drilling dedicated sedimentary processes involved in first appeared as a scientific topic to a transform margin (Mascle et their formation and evolution are during the 1970’s (Fail et al., al., 1996 and 1997). Very few specific and deserve scientific 1970; Le Pichon and Hayes, 1971; synthetic studies or reviews interest. Mascle, 1976a, 1976b; Rabinowitz (Scrutton, 1979 and 1982; Mascle The goals of this paper are to and Labrecque, 1979; Scrutton, et al., 1987; Lorenzo, 1997; Reid present a review of the 1979), but since then they have and Jackson, 1997; Bird, 2001) geodynamic concepts and models only been sparsely studied, were produced on this topic. The used to understand transform especially when compared with reasons why these margins were continental margins, including the other types (divergent or poorly studied can certainly be their initiation and vertical convergent) of continental found in their complexity. These displacements. This paper is margins. Regional case studies margins are not cylindrical at all associated with that of Mercier de were mostly performed and (in their morphology, crustal Lepinay et al. (this volume), published during the seventies and structure or vertical which presents the first worldwide eighties (Agulhas: Scrutton, 1973 displacements: for examples see catalogue of transform margins and 1976; Dingle, 1973; Ben- Mercier de Lepinay et al., this and synthesizes the observations Avraham et al., 1997; Côte volume), and their structure and from regional case studies. d’Ivoire-Ghana: Arens et al., evolution cannot be understood 1971; Delteil et al., 1974; Blarez from a few cross-sections. But 2. Definition, historical and Mascle, 1988; De Caprona, until the Jubilee discovery in 2007 perspective and terminology 1992; Basile et al., 1993; Exmouth offshore Ghana they also lacked of From a geodynamic point of Plateau: Lorenzo et al., 1991; Gulf large hydrocarbon discoveries, view, a continental margin of California: Bischoff and and were not a priority target for represents the transition zone Henyey, 1974; Moore and Curray, oil industry. However, transform between continental and oceanic 1982; Lonsdale, 1985; margins represent 16% of the lithospheres. The concept of Newfoundland: Todd et al., 1988; cumulated length of continental transform continental margins Keen et al., 1990). Ocean Drilling margins (Mercier de Lepinay et came from the definition of 1 Tectonophysics, 661, p. 1-10, http://dx.doi.org/10.1016/j.tecto.2015.08.034 transform faults (Wilson, 1965) as defined by a bathymetric marginal margin’ (Lonsdale, 1985; Mascle a ‘new class of faults’, where a plateau, a central segment devoid and Blarez, 1987) have been used. lithospheric plate boundary is of marginal plateau but sharing the Among these terms, ‘transform parallel to the relative plate same N65° trend for the margin’ appears to be the most displacement. Transform continental slope, and an eastern relevant to link these margins to continental margins consequently segment with a N40°-trending the processes that controlled their refer to the juxtaposition, in the continental slope (Figure 1). A formation. Although ‘strike-slip’ same location, of a continental transform margin intersects a correctly describes the margin with an active or divergent margin, at its both ends. displacement that occurred along previously active transform fault. These intersections are named these margins, it is not linked to It is noteworthy that this outer corner at the end of the any specific scale, such as upper terminology is based on the margin located towards the crustal, crustal or lithospheric, dynamics of the lithosphere, and oceanic accretion axis and inner whereas ‘transform’ implies a not on geographic or corner at the opposite end, lithospheric scale (Freund, 1974). physiographic characters. As a towards the continent (Figure 1). ‘Shear’ should be avoided in consequence, in this terminology a Over the years, several different description of these margins, margin can be defined from the terms were used to describe because shearing is the main associated geodynamic process as transform continental margins: tectonic process involved in the transform, divergent or convergent. Wilson (1965) introduced the term development of all margin settings, A segment of margin then should ‘transform fault’, but it was first with various geometries (strike- be a part of a margin that can be used in the oceanic domain. Fail et slip, dip-slip, normal or reverse, in defined by characteristics al. (1970) or Le Pichon and Hayes steep or flat shear zones). For (structure, sedimentation, (1971) emphasized the extension transform margins, the adjective bathymetry) other than the of oceanic fracture zones along the ‘shear’ reflects horizontal shear geodynamic setting. For example, trend of some segments of the along vertical faults. Therefore, the northern side of the Gulf of continental margin. Mascle (1976a) ‘transform margin’ should be used Guinea contains several first distinguished these segments to name the continent-ocean alternatively transform and as related to transform faulting, transition derived from a divergent margins, such as, from distinct from the Atlantic (rift- transform plate boundary. West to East, the Liberia divergent related) and Pacific (subduction- Another terminology has also margin, St Paul transform margin, related) types. Following Keen been comprehensively used. It was Cape Palmas divergent margin, and Keen (1973), Mascle (1976a) primarily based on the geographic the Liberia-Côte d’Ivoire defined this third continental location (Atlantic- versus Pacific- transform margin, the Ivorian margin type as ‘transform faulted type margins), then on the divergent margin, and the Côte or strike-slip margin’. Since then, lithospheric plate boundary d’Ivoire-Ghana transform margin the terms ‘strike-slip margin’ (e.g. character, including passive versus (Figure 1). Within the Côte Nagel et al., 1986), ‘shear margin’ active margins. The formers
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