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Transform continental margins – Part 1: Concepts and models Christophe Basile

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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￿

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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 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 across the transform . But this model is compatible neither with numerical computation including flexural behavior of the 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 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). Drilling margins (Mercier de Lepinay et came from the definition of

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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). ‘’ 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 ) 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 (- transform plate boundary. West to East, the Liberia divergent related) and Pacific (- 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 result d’Ivoire-Ghana margin, one can (Rabinowitz and Labrecque, 1979; from divergent movements but are distinguish a western segment Scrutton, 1979) and ‘transform located within a lithospheric plate

Figure 1: Geodynamic map of the northern side of the Gulf of Guinea. MP: Marginal Plateau; TM: Transform Margin; DM: Divergent Margin; CP DM: Cape Palmas Divergent Margin. The trends of margins are drawn from the bathymetry only, and do not reflect the continent-ocean boundary.

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(intra-plate), while the latters between the two plates (Freund, transform margin: the closer the result from convergent movements 1974), and ‘terminates abruptly at margin segment to the inner at lithospheric plate boundaries. both ends’ (Wilson, 1965). At the corner, the briefer intra- Transform margins were first edge of a rifted area, the relative continental transform faulting and described in the Equatorial displacement decreases from one active transform margin stages are. , where they are side of the rift to the other. Both intra-continental transform actually located intra-plate. They Horsetail splays typically and active transform margin were consequently referred to as a accommodate this progressive stages last the time t = din/SR after specific class of Atlantic-type or change of relative displacement the start of oceanic accretion, passive margins (Mascle, 1976a). and the connection with rifted where din is the distance between In a divergent margin the structures (Christie-Blick and the given point and the continent- displacement at plate boundary Biddle, 1985, Basile et al., 1993). ocean boundary in the inner corner, (active stage) is restricted to a During the intra-continental and SR is the oceanic spreading previous intra-continental (rift) transform stage, the syn-rift rate (Figures 2 and 3). The stage, and the continent-ocean transform fault connects the spreading rate SR may present transition is tectonically inactive corners of individual (Figure temporal variations for very long (intra-plate) at the margin stage. 2-1), while the post-rift transform transform faults, but can be However, as explained in the next fault connects the incipient considered as a constant for most section, a transform margin oceanic accretion axes within the transform margins where the experiences several stages of rifts (Figure 2-2). Consequently, active stage duration was shorter evolution, one of them being a the post-rift transform fault is than 10 My (Mercier de Lepinay tectonically active plate boundary. longer than the syn-rift one, by et al., this volume). This time also For example, some transform propagation in the outer corner. equals to (L-dou)/SR, where L is margins are seismically active, During the active transform the length of the transform margin such as those in the Gulf of margin stage, the continental between the continent-ocean California (Goff et al., 1987) or lithosphere slides against the boundaries at inner and outer those of the oceanic lithosphere, defining an corners, while dou is the distance (Leroy et al., 2000). Consequently, active transform margin (Figures between the given point and the the Atlantic/Pacific and 2-4 to 2-6 for the blue star). This continent-ocean boundary at the passive/active terminology that stage ends when the oceanic outer corner (Figures 2 and 3). If predated the identification of accretion axis passes along the transform faulting predates the transform margins is clearly transform margin (Figure 2-6 for oceanic accretion, then the inadequate. the blue star). Then starts the duration of the intra-continental passive transform margin stage transform stage increases by the 3. A kinematic model for (Figure 2-7 to 2-9 for the blue duration of rifting. transform continental margins star). The first two stages are The last contact between the two The kinematic evolution of a tectonically active and correspond continental lithospheres is lost transform margin can be divided to an active transform plate when the intra-continental into three stages (Mascle and boundary. During the third and transform stage ends at the Blarez, 1987), which are defined last stage, the transform margin is extremity of the transform margin by the position of the active intra-plate. (continent-ocean boundary at the transform fault and the of The comparison of various outer corner, Figure 3). This last the lithosphere on its sides, locations along the transform contact occurs at the time t = L/SR including: 1) intra-continental margin (blue, purple or red stars in after the start of the oceanic transform fault, 2) active Figure 2) shows that the described accretion. Along a continent, the transform margin, and 3) passive three stages are not coeval along lengths of the transform margins transform margin (Figure 2). the margin. For example the can be highly variable in a same A specific point on the future purple star ends its active area (e.g. Figure 1) and the last transform margin (blue star in transform margin stage in Figure continental lithosphere contact Figure 2) first experiences intra- 2-3, while the blue and red stars occurs at the extremity of the continental transform faulting are still within the intra- longest transform margin. The last (Figure 2-1 to 2-3), which ends continental transform stage. In contact between the two with its movement against a Figure 2-4, the red star continental lithospheres also stretched continental lithosphere experiences the intra-continental marks the starting time for the (Figure 2-3). It is noteworthy that transform stage, the blue star the intra-oceanic transform faulting, the syn-rift transform fault does active transform margin stage, and and the connection of deep not limit the edge of the rift. The the purple one the passive oceanic basins. However, the transform fault is characterized, transform margin stage. connection of surface marine along its entire length, by a The timing of these three stages waters is expected to have constant relative displacement depends on the location along the occurred a bit earlier, when the

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Figure 2: Schematic evolution of a transform margin and adjacent divergent margins, modified from Le Pichon and Hayes (1971), Scrutton (1979), Mascle and Blarez (1987) and Basile et al. (2013). Timing is given as a function of the ratio between the active transform fault length (L, equals the accretion axis offset) and the spreading rate (SR). This sketch assumes inherited plate boundaries, constant displacement rates, and excludes oblique displacements, as opposed to the scenario shown in Figure 6. It also neglects the sphericity of , where plate boundaries are arcs of circles. See text for further explanations.

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two divergent margins moved and normal faults at the tip of the prolonged by transform margins along each other (Figure 2-4). intra-continental rift zone (Figures and fracture zones (Wilson, 1965; The passive transform margin 2-1 and 3). In fact, the inner Wright, 1976; Mascle, 1976a; stage starts at time t = 2din/SR (or t corner does not belong to the Sibuet and Mascle, 1978, = 2(L-dou)/SR) after the beginning transform margin itself, and Bellahsen et al., 2013). However, of oceanic accretion. The records only the post-rift it seems difficult to use these beginning of this stage should be unconformity (Figure 3). On the observations as a rule, as there are recorded by the post-transform contrary, the outer corner (red star also many examples where unconformity, which is in Figures 2 and 3) is the part of transform faults crosscut older diachronous along the transform the transform margin that structures (e.g. Basile et al., 2005 - margin, and postdates the post-rift experiences the longest transform the Equatorial Atlantic). It is also unconformity at the outer corner fault activity after the end of difficult to imagine that plate transform-divergent intersection rifting. The outer corner also displacement is systematically (Figure 3). contains a active parallel to inherited structures. The described schematic during the rifting, which was cut More recently, based on the evolution implies along-strike by the transform fault after the detailed study of the back-arc asymmetry of the transform rifting. Here, both post-rift and Woodlark Basin, Taylor et al. margin. Each part of the transform post-transform unconformities are (2009) demonstrated that margin experiences the three same expected to occur and should be transform faults only appeared stages, but the duration of each clearly separate (Figure 3). when oceanic accretion started. In stage increases from the inner to this basin, rift segments were not the outer corners. This results in 4. Initiation of transform connected by strike-slip faults diachronic evolution. margins during the intra-continental rift Moreover, the two rift-transform Initiation of transform stage. No transform fault could be intersections have very different continental margins remains an defined for this stage. In fact, behavior. The inner corner (green open question. Most of the early transform fault formed after the star in Figures 2 and 3) is not studies assumed that transform beginning of oceanic accretion to affected by the transform fault faults re-activate older continental connect propagating oceanic itself, but only by the transfer structures, providing some spreading axes. structures such as horsetail examples of onshore tectonic It is noteworthy that the same structures that connect strike-slip structures lined up with or hypothesis and discussion arise for the initiation of oceanic transform faults. For example, Gerya (2012) basically opposed their inheritance from the geometry of the continental break-up, to their post- rift formation during the oceanic spreading.

5. Deformation partitioning in divergent setting It is possible to look at the debate about the transform margin initiation from another point of view, changing the investigation scale from a single plate boundary situation to regional scale situation. In the oceanic lithosphere, alternating transform faults and spreading axes accommodate the obliquity between the direction of relative plate displacement and the regional plate boundary strike Figure 3: The three stages of evolution in a transform margin, (Figure 4A). Some slow to very depending on time and location along strike. The red, blue, purple slow spreading axis, such as the and green stars are located along transform margin strike in Figure Mohns and Reykjanes ridges in 2. SR: Spreading Rate; PRU: Post-Rift Unconformity; PTU: Post- the northern Atlantic, are devoid Transform Unconformity; COB: Continent-Ocean Boundary. See text of transform faults (Dauteuil and for comments. Brun, 1996; Peyve 2009). With

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these few exceptions, where between rift segments are mainly Cerro Prieto transform in the oceanic accretion is oblique accommodated by transfer fault northern part of the Gulf of relatively to the plate zones (Gibbs, 1984; Milani and California, shows that transform displacement, the deformation of Davison, 1988), in some cases faulting can also occur in intra- the oceanic lithosphere is referred to as transform faults continental setting. commonly accommodated by (Chorowicz, 1989). However, for From these observations it plate boundaries either parallel the East African Rift, these appears that divergent (spreading axis) or perpendicular transfer structures are oblique to partitioning, while obvious in the (transform fault) to the relative the relative plate displacement oceanic domains, does not always plate displacement. (Rosendahl, 1987; Versfelt and occur during continental rifting. Partitioning of deformation at Rosendahl, 1989; Bird, 2003), and Accordingly, several theoretical plate boundaries is well described partitioning does not appear to be cases can be considered (Figures 4 for convergent settings (Fitch, efficient in this setting. As and 5): 1972), where oblique convergence suggested by Dauteuil and Brun a) Transform faults can predate is accommodated by strike-slip (1996), the mechanical layering of oceanic accretion. This case faulting within the the lithosphere may be involved, should be more frequent for low and by frontal convergence at the and partitioning may be associated angles between the regional trend trench. Similarly, oblique with highly localized deformation. and the relative plate motion, in divergence is accommodated in A very weak lithosphere, such as which case the length of transform the oceanic lithosphere by spatial the oceanic one close to the faults exceeds both the length and partitioning of the deformation accretion axis, should allow this width of individual rifts (Figure (Figure 4A). localisation. The existence of long 4B). But it can also occur in the In intra-continental divergent intra-continental transform faults, case of narrow rifts with no setting, relative displacements like the Dead transform or the overlap at their tips (Figure 5-1A).

Figure 4: Obliquity between relative plate motion and the regional trend of a divergent plate boundary (intra-oceanic: 4A; intra-continental: 4B). The regional trend is defined from the alignment of homologous structures, such as the transform/accretion axis intersection (4A), or the edges of rifted basins (4B). 0° obliquity is a pure transform fault, 90° obliquity a pure divergent plate boundary. Same symbols as in Figure 2. Note that because the divergent plate boundary is narrower in the oceanic lithosphere (4A) than in the continental one (4B), the intra-oceanic transform fault is longer than the syn-rift intra-continental one for the same obliquity. See text for further comments.

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b) Continental rifting can evolve the outer corner. area of stretched continental without any transform fault. In In these two models, transform lithosphere may form a marginal this case, the obliquity between faults may appear at the start of continental plateau (Mercier de the regional trend of the rift and oceanic accretion in the Lepinay et al., this volume) the relative plate displacement is prolongation of transfer zones (Figures 6-3A and 6-4A). accommodated either by oblique (Figure 5-2B) or within the most For these two cases, the rifting (Figure 6-1) or by oblique parts (Figure 6-2A). In the structural variations during the rift overlapping en echelon rift case of oblique rifting, these stage do not modify the temporal segments when the width of transform faults define transform evolution previously described individual rifts exceeds the offset margins that postdate oblique (Figures 2 and 3). The only with the adjacent rift segment rifting (Figures 6-3A and 6-4A), difference can be the lack of (Figure 5-1B). In this last setting, with strike-slip faulting cross- localisation of the deformation in transfer zones separate rift cutting the normal or a transform plate boundary before segments and both inner and outer transtensional faults developed the beginning of oceanic corners are deformed within these during the oblique rifting. This accretion. In any case, the transfer zones by strike-slip or also induces an asymmetry obliquity (Figure 4) and/or the transtensional faulting, with strike between a narrow inner corner and width of the rifted zone control the slip increasing from the inner to a wide outer corner, where large length of the transform fault, and consequently the timing and duration of each stage of evolution (Figure 3). c) One more case may occur if no partitioning occurs when the oceanic accretion starts (Figure 6- 2B). In such a situation, oblique oceanic accretion following oblique rifting results in the formation of an oblique margin (Figure 6-3B). Later partitioning and formation of intra-oceanic transform faults do not affect the margin (Figure 6-4B). In this case, there is no transform margin, and the term ‘oblique margin’ does not refer to the obliquity of bathymetric or rift structures, but to the oblique oceanic accretion and lack of oceanic transform faults at the beginning of oceanic accretion. Finally, the length of transform margins is controlled by three parameters. The first one is the obliquity between regional trend and relative plate motion direction (Figure 4B), where the length of the transform decreases with obliquity. The second is the width of the continental rift (Figure 5-2), where the length of the transform decreases when the rift widens. The third is the timing of deformation partitioning, where Figure 5: Influence of the width of the intra-continental rift on the the transform is longer when length and evolution of transform margins. For a similar obliquity partitioning occurs after the rifting between the regional trend and the relative plate displacement (65°, (compare Figures 6 and 5-2). cf. Figure 4), narrow intra-continental rifts allow intra-continental transform faults to appear (5-1A) while wide rifts overlap and are devoid of intra-continental transform faults (5-1B). When oceanic accretion starts, narrow rifts (5-2A) are connected by longer transform faults than the wide ones (5-2B). Same symbols as in Figure 2. 7 Tectonophysics, 661, p. 1-10, http://dx.doi.org/10.1016/j.tecto.2015.08.034

Figure 6: Oblique rifting and influence of the timing of deformation partitioning, either when oceanic accretion starts (Figure 6-2A to 6-4A) or after the beginning of oceanic accretion (Figure 6-2B to 6- 4B). The obliquity between the regional trend and the relative plate displacement (65°) is the same as in Figure 5. Same symbols as in Figure 2. The dotted line in Figure 6-4B represents the position of the oceanic accretion axis prior to the onset of partitioning. The new oceanic accretion axis probably does not appear everywhere simultaneously, but probably results from rift propagation. See text for comments.

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6. Models for vertical lithosphere to the continental one generated by heat transfer from displacements across the transform fault. The the oceanic to the continental One of the most striking contact of the oceanic lithosphere lithosphere decreases from the characteristic features of transform against the continental one across transform fault towards the margins is a frequent occurrence the transform margin produces a continent. Assuming local of a marginal ridge, parallel to the horizontal thermal gradient, which isostasy, the maximum uplift transform fault, at the outer corner results in the continental computed at the time of the of the transform margin, i.e. where lithosphere being heated by the oceanic accretion at the foot of the the transform margin experienced oceanic one. This heating reduces transform margin can fit the the maximum duration of strike- the thickness of the continental observed shape and elevation of a slip deformation. One of the best lithosphere, and induces a thermal marginal ridge. This is not the examples and best-studied site is uplift. The subsequent cooling of case when regional isostasy is the marginal ridge of the Côte the lithosphere results in thermal assumed. Then, the maximum d’Ivoire-Ghana transform margin subsidence. uplift can reach only few hundred (Basile et al., 1993; Basile et al., This thermal uplift is supposed meters (Figure 7E). In both cases, 1996 and references herein; to be at its maximum when the subsequent thermal subsidence Mascle et al., 1997; Basile et al., hottest oceanic lithosphere, i.e. the reduces the uplift to few tens to 1998) (Figure 7A). Other spreading axis, moves along the few hundred meters, and cannot fit examples of marginal ridge are transform margin. Scrutton (1979) the observed marginal ridges. shown and discussed in Mercier first proposed this mechanism, Ocean Drilling Program (ODP) de Lepinay et al., this volume. which was also supported by Leg 159 was designed (among Many models were proposed to Mascle and Blarez (1987), and other topics) to test the heat- explain the development of modeled by Todd and Keen transfer model by investigating the marginal ridges. The first group of (1989) and Lorenzo and Vera vertical displacements through models is related to the crustal (1992) for Newfoundland and time and space along the Côte thickness variations. Le Pichon Exmouth margins, respectively. d’Ivoire-Ghana transform margin and Hayes (1971) proposed that a Reid (1989) and Vågnes (1997) (Mascle et al., 1996). It indicated marginal ridge is a continental also proposed thermo-mechanical that no thermal event was sliver, transported within the models of the uplift associated associated with the passing of the transform fault from a place with with lithospheric ductile flow oceanic accretion axis along the relatively thick continental crust along the transform fault, and transform margin (Wagner and towards a rifted basin where the inducing uplift close to the plate Pletsch, 2001), and that the adjacent crust is thinned and boundary at the time of oceanic vertical displacements are not therefore deeper (Figure 7C). accretion. correlated with this passing, Some authors (Huguen et al., However, these thermo- because the uplift of the ridge was 2001; Attoh et al., 2004) proposed mechanical models were all based interpreted as starting earlier a related explanation, where the on constant heating by the oceanic during the intra-continental stage marginal ridge results from crustal lithosphere, i.e. they did not take (Basile et al., 1998). thickening by transpression in the into account the cooling of the The third group of models that intra-continental transform zone oceanic lithosphere by the explain the morphology and (Figure 7B). These models imply continental one. Furthermore they vertical displacements of the a crustal root below the marginal cannot explain the stability of the marginal ridge is based on the ridge. There is only one described marginal ridges through time, flexural response of the case where the crustal thickening which remain elevated several lithosphere to unloading by can be associated with marginal tens of million years after the erosion along the transform fault ridge. It is the southern transform transform margin became inactive, (Basile and Allemand, 2002) boundary of the Exmouth Plateau, despite of the fact that the thermal (Figure 7D). The driving where a wide marginal ridge is subsidence should have erased the mechanism is the difference in associated with magmatic syn-transform thermal effects. elevation between the two underplating at depth (Lorenzo et Nemčok et al. (2012) also adjacent lithospheres across the al., 1991). However, mapping of suggested that heat transfer alone transform margin. It induces the the Moho depth by wide angle induces a maximum uplift limited erosion of the edge of the seismic evidences a flat and to few hundred meters, which continental plate, which is higher. horizontal Moho below the cannot fit the observations When the active transform fault marginal ridges in most examples Finally, Gadd and Scrutton acts as a free border for vertical (e.g. Sage et al., 2000) (Figure (1997) discussed the validity of displacements, this erosion 7A). the thermal uplift models. As discharges the edge of the plate The second group of models is previous studies (Todd and Keen, and allows for a flexural uplift associated with the lateral heat 1989; Lorenzo and Vera, 1992), similar in shape to a rift shoulder. transfer from the oceanic they showed that the uplift Both the flexural shape (Basile

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Figure 7: Conceptual and numerical models explaining the marginal ridge and vertical displacements along transform continental margins. All sections are displayed with no vertical exaggeration, and are superposed to the crustal section of the Côte d’Ivoire- Ghana marginal ridge (7A, from Sage et al., 1997) (red dotted lines). Figure 7B is modified from Huguen et al., 2001. In Figure 7D, the black dotted line represents the shape of the marginal ridge restored at Santonian times, and fitted by the flexural uplift of the continental border (Basile and Allemand, 2002). The present day shape results from the loading by the sedimentary infilling in the deep Ivorian basin north of the marginal ridge and unloading by erosion of the continental slope. The continental and oceanic lithospheres are supposed to be decoupled before Santonian (active transform fault), coupled since then (passive transform margin). Figure 7E displays the flexural uplift induced by thermal conduction, as computed by Gadd and Scrutton, 1997. The thin dotted line indicates the horizontal for reference. Figure 7F shows the downward flexure of the transform margin coupled (padlock) across the inactive transform fault with the thermally subsiding oceanic lithosphere (inspired from Lorenzo and Wessel, 1997).

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and Allemand, 2002; ten Brink et coupled plates (more likely in the models proposed for the vertical al., 1997; Vågnes, 1997), and the stage) and lead to displacements along transform timing of uplift and erosion of the downward flexure of the margins are based on very few marginal ridge (Basile et al., 1998; continental edge driven by the natural case studies, while the Bigot-Cormier et al., 2005) thermal subsidence and exhaustive investigation of support this flexural model. sedimentary loading of the Mercier de Lepinay et al. (this However, the shape of the Moho adjacent oceanic plate (Lorenzo volume) identified 79 transform does not show the uplift that and Wessel, 1997) (Figure 7F). margins worldwide. It provides should be induced by the flexural new insights for the morphology bending (Figure 7D). 7. Conclusion of transform margins and their However, a systematic study of The concept of transform margin geodynamic setting (Mercier de vertical displacements is still emerged from the prolongation of Lepinay et al., this volume), and lacking for transform margins, and oceanic transform faults towards can be a guideline for new the bathymetric shape can be a the continental margins (Le investigations on transform useful tool for it (Mercier de Pichon and Hayes, 1971; Mascle margins. Lepinay et al., this volume). As 1976a). The kinematic of the continental lithosphere is not transform margins has been Acknowledgments thinned or poorly thinned along similarly derived from the This work has been supported by transform margins, thermal kinematic of oceanic transform CNRS/INSU Action Marge subsidence is not expected to faults (Le Pichon and Hayes, program, and by Labex occur. But the continental 1971; Scrutton, 1979; Mascle and OSUG@2020 (ANR10 LABX56). lithosphere is in contact with the Blarez, 1987), using the I am particularly indebted to M. oceanic lithosphere, and even if assumption that inherited tectonic Mercier de Lépinay and my heat transfer is not efficient, the features localize the incipient colleagues L. Loncke, A. vertical displacements of the transform faults. However, this Maillard, M. Patriat and W. Roest margin should be strongly inheritance hypothesis appears for their help and comments at controlled by the coupling or questionable, and an alternative various stages of this work, and to decoupling of the two plates approach can be proposed, based two anonymous reviewers and G. across the transform fault. As on the partitioning of deformation Iaffaldano who contributed to discussed above, flexural uplift between divergent and transform improve this paper. I also thank can be related to decoupled plates plate boundaries. It implies a the other participants of across the transform fault, variability of the kinematic IGUANES cruise who pushed me expected when the transform fault evolution that is controlled by to publish this work, and the is active (Basile and Allemand, variations in both geometry and participants of Action Marge 2002). On the other hand, flexural timing of partitioning. In any case, workshops for the fruitful subsidence may result from these kinematic models as the discussions we had. ______References Arens, G., Delteil, J.R., Valery, P., Damotte, B., Basile, C., Mascle, J., Benkhelil, J., Bouillin, J.P., Montadert, L., Patriat, P., 1971. The continental 1998. Geodynamic evolution of the Côte margin of the Ivory Coast and Ghana. In: The d’Ivoire-Ghana transform margin: an overview geology of the East Atlantic continental margin, of Leg 159 results. In Mascle, J., Lohmann, Delany F.M. (Ed.), Rep. 70/16, 4, . Inst. G.P., Clift, P.D., eds, Proc. ODP Sci. Res., 159, Geol. Sci. London, 61-78. p. 101-110. Attoh, K., Brown, L., Guo J., Heanlein, J., 2004. Basile, C., Mascle, J., Guiraud, R., 2005. Seismic stratigraphic record of transpression Phanerozoic geological evolution of the and uplift on the Romanche transform margin, Equatorial Atlantic domain. J. Afric. Earth Sci., offshore Ghana. Tectonophysics, 378, 1-16. 43, 275-282. Basile, C., and Allemand, P., 2002. Erosion and Basile, C., Mascle, J., Popoff, M., Bouillin, J.P., flexural uplift along transform faults. Geophys. Mascle, G., 1993. The Côte d’Ivoire-Ghana J. Int., 151, 646-653. transform margin: a marginal ridge structure Basile, C., Maillard A., Patriat M., Gaullier V., deduced from seismic data. Tectonophysics, Loncke, L., Roest, W., Mercier de Lepinay, M., 222, 1-19. Pattier, F., 2013. Structure and evolution of the Basile, C., Mascle, J., Sage, F., Lamarche, G., Demerara Plateau, offshore French Guiana : Pontoise, B., 1996. Pre-cruise and site surveys: rifting, tectonic and post-rift tilting at a synthesis of marine geological and transform-divergent margins intersection. geophysical data on the Côte d’Ivoire-Ghana Tectonophysics, 591, 16-29. Doi: transform margin. In Mascle, J., Lohmann, G.P., 10.1016/j.tecto.2012.01.010 Clift, P.D., eds, Proc. ODP Init. Rep., 159, 47- 60.

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