Effect of Contrasting Strength from Inherited Crustal Fabrics on the Development of Rifting Margins

Research Paper

GEOSPHERE Effect of contrasting strength from inherited crustal fabrics on the development of rifting margins

1, 2, GEOSPHERE, v. 15, no. 2 S. Jammes * and L.L. Lavier * 1Department of Geography, Texas State University, San Marcos, Texas 78666, USA 2Department of Geological Sciences and Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA https://doi.org/10.1130/GES01686.1

5 figures; 1 supplemental file ABSTRACT deformation, many fundamental questions remain about the effects of inherited geological conditions on localization processes at rifted margins. Several CORRESPONDENCE: [email protected] To investigate the effect of crustal heterogeneities inherited from previous studies suggest that inheritance is a key control on the development of rift tectonic phases on magma-poor rifting processes, we performed numerical structures (Dunbar and Sawyer, 1989; Ring, 1994; Piqué and Laville, 1996; Corti CITATION: Jammes, S., and Lavier, L.L., 2019, Effect of contrasting strength from inherited crustal experiments of lithospheric extension with initial conditions that included et al., 2007; Clerc et al., 2015; Manatschal et al., 2015). Inheritances are the fabrics on the development of rifting margins: Geo‑ strength variations from inherited crustal fabrics. Crustal fabrics were intro- result of the successive tectonic events that affect the continental lithosphere sphere, v. 15, no. 2, p. 407–422, https://doi.org/10.1130 duced in the model by using an element-wise bimineralic composition in during its complex geological history. Although they are interrelated, geol- /GES01686.1. which mineral phases were distributed in a way that was compatible with the ogists usually distinguish three types of inheritances: compositional, struc- orientation and distribution of kilometric-scale heterogeneities observed in tural, and thermal. In the literature, most of the studies focus on the effect Science Editor: Raymond M. Russo seismic reflection data. Our numerical models show that strength variations of structural inheritances (Ring, 1994; Corti et al., 2004, 2007; van Wijk, 2005;

Received 24 February 2018 from inherited crustal fabrics strongly influence the mechanisms of deforma- Autin et al., 2013; Chenin and Beaumont, 2013) and thermal inheritances (Buck, Revision received 18 November 2018 tion in the stretching and thinning phases of rifting. The strength variations 1991; Brune et al., 2014, 2017; Svartman Dias et al., 2015) on rifting localization. Accepted 10 January 2019 also generate alternative models for the evolution of faulting during distrib- Structural inheritances are defined as mechanically weak shear zones inher- uted stretching and localized thinning phases that are usually associated ited from previous orogenic events. Studies suggest that they can control the Published online 8 February 2019 with detachment or sequential faulting models. During the stretching phase, localization of deformation from the beginning of rifting and rejuvenate litho- inherited strength variations control the distribution and the processes of spheric structures that are properly oriented with respect to the direction of deformation. Vertical fabrics favor the formation of horst-and-graben struc- extension (Harry and Sawyer, 1992; Ring, 1994; Corti et al., 2004, 2007; Autin tures. Horizontal and dipping fabrics favor the formation of detachment faults et al., 2013; Chenin and Beaumont, 2013). However, according to Manatschal and core complexes. During the thinning phase, processes differ depending et al. (2015), structural inheritances do not significantly control the location of on the orientation of the crustal fabrics and involve either a combination of breakup. Thermal inheritances can cause variations in the degree of coupling detachment faults and sequential normal faults or an alternative model in between crustal and mantle deformation, which in turn controls the long-term which deformation remains decoupled between the upper crust and litho- evolution and architecture of rifts (e.g., Manatschal et al., 2015). The rifting spheric mantle, with the formation of high-angle faults in the upper crust and of old, cold lithosphere, with strong coupling between the upper brittle crust a low-angle detachment fault in the upper mantle. As a consequence, strength and mantle, results in the formation of narrow rifts, whereas rifting of a young, variations inherited from crustal fabrics also control the resulting geometry of warm lithosphere with a thick decoupled lower crust results in the formation of the margin and the width of the necking and hyperextended domains. Finally, a wide rift (Bassi, 1991; Bassi et al., 1993; Bassi, 1995; Buck, 1991; Brune et al., our models demonstrate that inherited crustal fabrics do not control breakup 2014, 2017; Svartman Dias et al., 2015). Field and seismic observations clearly and mantle exhumation. These processes are ubiquitously associated with demonstrate that the composition of the crust is compositionally heteroge- the development of new detachment faults exhuming mantle to the seafloor. neous (Smithson, 1978; Rudnick and Fountain, 1995). However, due to their apparent complexity, little attention has been given to the effects of compositional inheritances on the rifting process. Indeed, in most numerical experiments of INTRODUCTION lithospheric extension, the composition of the crust and mantle is assumed to be layered and homogeneous and composed of wet or dry plagioclase, quartz, Passive margins define about half of Earth’s coastlines and have been the or olivine (Buck, 1991; Lavier and Buck, 2002; Huismans and Beaumont, 2003, focus of many geological and geophysical studies in the last decades. While 2007, 2011; Huismans et al., 2005; van Wijk and Blackman, 2005; Gueydan et great progress has been made in understanding the mechanics of extensional al., 2008; Rosenbaum et al., 2010; Duretz et al., 2016). To take into account This paper is published under the terms of the the heterogeneities of the lithosphere, some numerical models use a labo- CC‑BY-NC license. *E-mails: [email protected], [email protected] ratory-determined flow law for polymineralic rock like granite, quartz-diorite,

GEOSPHERE | Volume 15 | Number 2 Jammes and Lavier | Effect of inheritances on rifting processes Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/2/407/4663900/407.pdf 407 by guest on 01 October 2021 Research Paper

diabase, or gabbro (Dunbar and Sawyer, 1989; Lavier and Manatschal, 2006; approximating rheological heterogeneities in the crust and mantle succeeds van Wijk and Blackman, 2005). By using a bulk strength envelope for the poly- in reproducing the following structural features related to the formation of mineralic aggregate, these studies do not explicitly take into consideration the magma-poor rifted margins: (1) the absence of a sharp deformation zone at interaction between the different minerals and imply, as for monomineralic the brittle-ductile transition; (2) the initiation of the rifting process as a wide assemblages, that rheology is either elastoplastic, in order to simulate a brittle delocalized rift system with multiple normal faults dipping in both directions; upper crust and upper lithospheric mantle, or viscous/viscoelastic to simulate (3) the development of anastomosing shear zones in the middle/lower crust a ductile middle to lower crust and lower lithospheric mantle. Consequently, and the upper lithospheric mantle similar to the crustal-scale anastomosing in numerical studies, the role of compositional inheritance has mainly been patterns observed in the field (Carreras, 2001; Fusseis et al., 2006) or in seis- tested by comparing models in which the globally averaged crustal or mantle mic data (Clerc et al., 2015); and (4) the preservation of undeformed lenses compositions vary. For example, Svartman Dias et al. (2015) compared models of material leading to lithospheric-scale boudinage structures and resulting in which the crust is made of either dry quartz or plagioclase, and the mantle in the formation of continental ribbons, as observed along the Iberian-New- composition is wet or dry olivine. The main problem with such approaches foundland margin. is that the overall lithospheric composition remains homogeneous and lay- Following these results, we believe that using an explicit bimineralic as- er-caked, and deformation at the brittle-ductile transition is constrained to semblage is a better approximation of the rheological complexity of the lith- occur at the sharp transition between brittle and ductile material. osphere and yields a better understanding of rifting processes. However, this Observations of the brittle-ductile transition show strong evidence of se- previous work (Jammes et al., 2015; Jammes and Lavier, 2016) used a random mibrittle deformation at the scale of rock or outcrop. One can observe that distribution of heterogeneities unconstrained by any observations. Hetero- over the brittle-ductile transition, porphyroclasts remain slightly deformed or geneities in the crust are not completely random; they preserve a structural exhibit localized fractures, while the surrounding matrix shows evidence of pattern (fabric) inherited from a complex tectonic history. Here, we designed ductile deformation (e.g., Wakefield, 1977; Mitra, 1978; White et al., 1980; Handy, numerical experiments with initial conditions that included strength variations 1990, 1994; Jammes et al., 2015). At the mesoscale (meters to kilometers), un- inherited from crustal fabrics. Crustal fabrics were parameterized as variations deformed lenses of material surrounded by mylonitic shear zones lead to the in the orientation and mineralic composition of the crust and were derived formation of large anastomosing patterns or meter-scale boudinage structure. from two-dimensional (2-D) seismic observations. The objective of this study Such structures are well described at the fossil brittle-ductile transition exposed was to understand how strength variations from inherited crustal fabrics in- in Cap de Creus, Spain (Carreras, 2001; Fusseis et al., 2006) or in seismic images fluence the mechanisms of deformation during rifting processes and how of the middle to lower crust along the Uruguayan margin (Clerc et al., 2015). they affect the resulting geometry of the margins. Magmatic intrusions are Such observations demonstrate that a natural strength contrast between min- also known to play an important role during extensional processes, but their eral phases or mineral aggregates exists, not only at the microscale, but also basic physics still need to be clarified before they can be consistently included between meter- to kilometer-scale units of significantly different composition. in the mechanics of rifting (e.g., Qin and Buck, 2005; Davis and Lavier, 2017). The rheology and deformation processes at lithospheric scale are consequently We therefore focused our study on the effect of inherited strength variations controlled by the interaction between metric to kilometric blocks of strong or on rifting processes in magma-poor rift settings. weak average composition—an observation that emphasizes the importance After reviewing prerift crustal structures, rifting mechanisms in magma-poor of compositional inheritances in tectonic deformation processes. In a previous margins, and geometry, we present and compare the results of our numerical study, to test the effect of mesoscale compositional heterogeneities on defor- study to observations of magma-poor margins and the most recent models mation processes, we performed numerical experiments of rifting using an attempting to explain the mechanics of rifting. We show that strength variations explicit bimineralic composition in the crust and/or the mantle with a random inherited from crustal fabrics strongly affect rifting processes, including: (1) the distribution of mineral phases among particles located in each element of the distribution of strain during the initial phase of deformation, (2) the mecha- model (Jammes et al., 2015; Jammes and Lavier, 2016). This complex rheology nism of thinning during the rifting process, and (3) the mechanism leading to was introduced to take into account the interaction between kilometric blocks mantle exhumation and/or the formation of oceanic crust. of strong material (approximated by the rheology of plagioclase for the crust or olivine minerals for the mantle lithosphere) and weak material (approximated by the rheology of quartz for the crust or orthopyroxene minerals for PRERIFT CRUSTAL STRUCTURE the mantle lithosphere), representing, for example, the presence of mafic clus- ters in a globally felsic crust. By comparing bimineralic models to numerical Field and seismic observations clearly demonstrate that the composition simulations using an implicit bimineralic composite (an average viscous flow of the crust is highly heterogeneous at the mineralic scale, but also at the law for a two-phase aggregate; Tullis et al., 1991) in the crust and the mantle, lithospheric scale, where kilometer-scale units of significantly different com- Jammes and Lavier (2016) demonstrated that an explicit bimineralic approach position may be juxtaposed (Smithson, 1978; Rudnick and Fountain, 1995).

Structural inheritances, defined as mechanically weak zones inherited from controlled by pure shear processes: Brittle faults initiated in the upper crust previous orogenic events, also contribute in defining the complexity of the are rooted in the ductile middle and lower crust and remain decoupled from crustal structure. They may affect the entire lithospheric thickness, but brittle the deformation accommodated in the brittle upper mantle. During this phase, structures are limited to the shallowest part of the crust (Ring, 1994; Corti et rifting appears to be a largely symmetric process on a crustal scale. The subse- al., 2004, 2007), while anastomosing ductile shear zones affect the deeper part quent localized thinning phase involves stretching of the crust, resulting in its of the crust. The two seismic profiles presented in Figure 1 are onshore deep complete embrittlement, and faulting in the brittle mantle lithosphere that can seismic profiles illustrating the complexity of crustal composition and structure connect with the faults formed in the remaining brittle crust. During the final of eastern Canadian continental crust (for location, see Fig. 2). Located onshore, exhumation phase, circulation of water through the fractures into the mantle these profiles depict continental crust that is not strongly affected by extensional lithosphere leads to its serpentinization. As a result, large asymmetric detach- processes. We can therefore use them as analogues of prerift crustal structure. ment faults develop in the weak mantle serpentinites, producing a late-stage These two profiles (Figs. 1A and 1B) show strong variability in seismic asymmetry and exhuming serpentinized mantle on the seafloor (Reston, 2009; reflectivity. Using theoretical models and numerical simulation, Hurich and Péron-Pinvidic and Manatschal, 2009; Jammes et al., 2010). This late asymmet- Smithson (1987) and Hurich (1996) demonstrated that the scale of composi- ric rifting process results in the formation of an “upper-plate margin” in the tional variability can be measured effectively in seismic reflection profiles. hanging wall and a “lower-plate margin” in the footwall (Lister et al., 1991). These results are consistent with other studies demonstrating that deep seis- The upper-plate margin is described as a narrow, sharp margin, constituted mic reflectivity is strongly controlled by lithological variation (Green et al., of faulted upper crustal blocks (Reston, 2009; Péron-Pinvidic and Manatschal, 1990; Ji et al., 1997). For ductile shear zones, studies show that they may be 2009; Péron-Pinvidic et al., 2017), whereas the lower-plate margin is highly transparent, unless they are compositionally layered (Hurich et al., 1985) or structured, faulted, and hyperextended as a result of movements along ex- contain significant volumes of foliated phyllosilicates (Jones and Nur, 1984). huming detachment faults overlain by tilted crustal blocks and extensional As a consequence, the seismic variability observed in Figure 1 can be ap- allochthons (Reston, 2009; Faleide et al., 2010; Huismans and Beaumont, 2011; propriately parameterized as compositional heterogeneities, interpreted as Péron-Pinvidic et al., 2017). While published magma-poor rifting models glob- crustal structures of variable orientation and scale. The apparent dips of the ally agree on these first-order rifting mechanics, many differences between crustal structures vary from horizontal to dipping (20° to 50° dip) to vertical, the models remain at second order. whereas the scales of structures range from 5 km to 25 km long. The aim of In the magma-poor margin literature, two main polyphase rifting models our numerical experiment was to understand the effects of the orientation are discussed. In these two models, deformation mechanisms differ mostly of strength variations inherited from crustal fabrics on rifting processes in a during the pre-exhumation phases and more precisely during the thinning magma-poor setting. We therefore focused on end-member models in which phase. In the model deriving from the work of Pérez-Gussinyé et al. (2003), crustal fabrics are 10–15 km long and horizontal, vertical, or dipping (30° dip). Pérez-Gussinyé (2013), Reston (2005, 2007, 2009), and Ranero and Pérez-Guss- inyé (2010), extreme crustal thinning is interpreted to occur by sequential normal faulting. On the other hand, in the model deriving from the work of RIFTING MECHANISMS IN MAGMA-POOR SETTINGS Whitmarsh et al. (2001), Manatschal et al. (2001), Manatschal (2004), Lavier and Manatschal (2006), and Péron-Pinvidic and Manatschal (2009), extreme crustal Forty years ago, two end-member mechanisms were proposed to explain thinning is interpreted to occur by detachment faulting. In the following, we extension of the lithosphere in magma-poor settings: pure shear with uniform will describe these two models in order to use them as a basis for discussion stretching (McKenzie, 1978), and simple shear, where extension is accommo- in the description and interpretation of our results. dated by a low-angle detachment fault (Wernicke and Burchfiel, 1982). Since then, progress has been achieved in understanding extensional deformation, and it is now commonly accepted that rifting in magma-poor settings is a Sequential Normal Faulting Models (Figs. 3A–3D) polyphase process that involves different deformation mechanisms (i.e., both pure shear and simple shear processes; Lavier and Manatschal, 2006; Reston, In this model (Reston, 2007, 2009), the mechanism of pre-exhumation defor- 2009; Péron-Pinvidic and Manatschal, 2009; Péron-Pinvidic et al., 2017). Lavier mation is divided into two main phases. During the first phase of deformation, and Manatschal (2006) showed that three main phases of deformation during extension is distributed over a broad area along high-angle faults of different the rifting process can be identified: distributed stretching, localized thinning, orientations. During the second phase of deformation, extension localizes, and exhumation. Deformation processes during each phase are characterized and boudinage of the lower crust occurs, while upper-crustal deformation is by the degree of coupling of deformation between the crust and mantle lith- accommodated along a series of high-angle faults mostly dipping in the same osphere (Reston, 2009; Péron-Pinvidic and Manatschal, 2009; Jammes et al., direction (Fig. 3A). Fault slip and rotation cause further thinning of the crust 2010). In the initial stretching phase, the deformation is decoupled and mostly on the first generation of faults until they lock (first generation of faults lock

A) Crustal structure of the Grenville Province NS 0 50 100 150 0

5 15

ime (s ) 10 30 Upper Crust T Depth (km ) 30 45 Lower Crust Groswater Bay Pinware NSTerrane Terrane Mantle 0 10 Moho 20 Seismic reflector 30 Moho Depth (km ) 40 050 100 150 Distance (km) B) Crustal structure of the Appalachian Province NW SE 0

ime (s ) 12 T 16

NW SE 0 Laurentia Ganderia Avalonia 10 20 30 40 Moho Depth (km ) 50 050 100 150 200 250 300 Distance (km)

Figure 1. Illustration of the complexity of prerift crustal structure: (A–B) Deep seismic reflection profiles from the (A) Lithoprobe East transect in the Grenville Province (modified from Gower et al. (1997) and (B) Lithoprobe Eastern Canadian Shield Onshore-Offshore transect in the Appalachian Province (modified from van der Velden et al., 2004). See Figure 2 for location.

A 70W 60W 50W 40W 30W 20W 10W 0 Greenland

Fig. 2B 60N 88R2 Labrador Age of Orogen and cratons 90R1 Sea Canada Ireland Archean early Paleoproterozoic SC 50N REECH 1 SCREECH 3 Fig. 2C Paleo-Mesoproterozoic Paleozoic ISE1 IAM-9 40N Iberia Lithoprobe seismic reflection profile

IBERSEIS seismic reflection profile 500 km 500 km Seismic reflection/refraction profile

B 70W 60W 50W C 10W 60N Nain Pr

SE Churchill ovinc Province e 90R1 Fig. 1A lle Front ISE1 Superior Grenvi Pr. IAM-9 Variscan Grenville Province 40N Fig. 1B Orogen 50N SCREECH 1 A pp nt alachian Fro Appalachian Province SCREECH

3 250 km 250 km 250 km 250 km

Figure 2. Location maps for the data presented in the study. (A) Relief map of the North Atlantic Ocean. (B) Detail of the northeastern Canadian margin. (C) Detail of the west Iberian margin. Loca- tions of the seismic lines presented in Figures 1 and 4 are indicated in red and black, respectively.

A) Thinning : 1st generation of faults lock-up i) Stretching 0 0 Upper crust 10 10 Lower crust 20 20

30 30 Lithospheric mantle 40 40 Asthenospheric mantle km km B) Thinning : 2nd generation of faults lock-up ii) Thinning Serpentinized mantle 0 0 Embryonic oceanic crust 10 10

20 20 Faults

30 30 UP Upper plate

40 40 LP Lower plate km km C) Exhumation : Serpentine detachment iii) Exhumation 0 0

10 10

20 20

30 30

40 40 km km D) Seafloor spreading iv) Seafloor spreading 0 0

10 10

20 20

30 30

UP LP 40 UP LP 40 km km Figure 3. Schematic models describing the mechanism of rifting presented in the literature. Left column: sequential normal faulting model (modified from Reston, 2009), showing (A–B) two steps of the thinning phase, (C) exhumation phase, and (D) seafloor spreading. Right column: detachment faulting model (modified from Péron-Pinvidic and Manatschal, 2009), showing (i) stretching phase, (ii) thinning phase, (iii) exhumation phase, (iv) seafloor spreading.

up). Locking occurs when the dip and static friction angle of the faults are not Detachment Faulting Model (Figs. 3i–3iv) compatible with slip. As a result, a second generation of faults forms at more favorable high-angle dips, thinning the crust further. The previously formed In this model, the stretching phase is characterized by high-angle faults tilted blocks are then truncated by high-angle normal faults (Fig. 3B). The same associated with classical half-graben subsidence. During this phase, deforma- pattern is then repeated, resulting in a gradual thinning of the crust toward tion is distributed over a broad region in which continental crust is slightly the future ocean (Pérez-Gussinyé and Reston, 2001; Reston, 2007, 2009). When stretched (Fig. 3i). As extension continues, deformation localizes along two complete embrittlement of the crust has been achieved, water can diffuse conjugate faults that are decoupled from the mantle along a midcrustal décol- through the deformed crust, leading to the serpentinization of the underlying lement. These faults delimit an upper-crustal block (identified as the H block in mantle. Mantle unroofing is the result of continued extension as large asym- Lavier and Manatschal, 2006) and evolve from high-angle faults to low-angle metric detachment faults can develop in the serpentinized mantle during the detachment faults, exhuming deep crustal and/or mantle material underneath exhumation phase (Fig. 3C). As extension continues, crustal separation occurs the H block (Fig. 3ii). Through this process, detachment faulting results in along the detachment fault, leading to the initiation of seafloor spreading and local exhumation of basement rocks and extreme and localized thinning of the formation of new oceanic crust (Fig. 3D). the crust at the transition between the proximal and distal margins (e.g., the

pronounced necking zone observed on the Newfoundland margin; Péron-Pin- Comparison between Upper-Plate Margins vidic and Manatschal, 2009). This extreme thinning is associated with an embrittlement of the crust that can lead to the exhumation phase. Detachment Both the western Greenland (Fig. 4A) and the southern Newfoundland mar- faults can crosscut the remaining crust and exhume serpentinized mantle gins (SCREECH 3 profile; Fig. 4C) present a sharp necking zone and a relatively rocks at the seafloor (Fig. 3iii). Final seafloor spreading is defined by the irre- wide hyperextended domain in which the lower crust seems to be missing vocable localization of thermal and mechanical processes in a narrow zone beneath a thin remnant of stretched upper crust (Reston, 2009). While the ar- corresponding to a protoridge (Fig. 3iv). chitecture of both margins seems to be similar, they differ in scale, since the In these two models, the final exhumation phase is controlled by the ini- hyperextended domain is ~115 km long in the southern Newfoundland margin tiation of a large detachment fault that exhumes serpentinized mantle to the and only 56 km long in the western Greenland margin. In contrast, the northern seafloor, resulting in an asymmetric conjugate margin. The upper-plate mar- Newfoundland margin (SCREECH 1 profile; Fig. 4B) presents a different ge- gin corresponds to the left-side margin of both models, and the lower-plate ometry, with a more gradual necking zone leading to a narrow hyperextended margin corresponds to the right-side margin (Fig. 3). As previously described, domain (31 km long). It appears therefore that if the characteristics of the the upper plate is sharp and narrow and composed of faulted upper-crustal western Greenland margin correspond to an archetypical upper-plate margin blocks resulting from the sequential faulting models (Fig. 3D) or from the dis- (narrow, sharp, and devoid of structure), the same is not the case for the wide mantling of the H block (detachment faulting model, Fig. 3iv). On the other southern Newfoundland margin or for the northern Newfoundland margin. side, the lower plate is more extended as a result of detachment faults overlain by tilted crustal blocks and extensional allochthons. Comparison between Lower-Plate Margins

VARIABILITY OF MARGIN ARCHITECTURE The necking domain is widely distributed in the northern Iberian margin (ISE 1 profile; Fig. 4B) and is narrowly distributed in the southern Iberian mar- The majority of magma-poor margins display a number of common fea- gin (IAM 9 profile; Fig. 4C), thus showing significant variations in width and tures, such as extreme crustal thinning from 30 km to a few kilometers thick total extension along strike. In contrast, the width of hyperextended domain is over length scales of 100–200 km, normal faults in the upper crust, and a surprisingly constant along the entire margin (41 km and 49 km respectively; zone of serpentinized mantle exposed under an extremely thin crust (Reston, Sutra et al., 2013). In comparison, the Labrador margin (Fig. 4A) presents the 2009). They also exhibit significant variability in their architecture, which has opposite geometry, with a wide hyperextended domain (122 km long) and a not yet been explained by the two faulting models previously discussed. This narrowly distributed necking domain (~68 km). It appears therefore that only variability is particularly pronounced in the width and characteristics of the the Labrador margin seems to correspond to the archetype of a lower-plate necking and hyperextended domains. When defined for magma-poor conti- margin, with a narrow necking domain and a wide hyperextended domain. nental margins, the necking domain corresponds to a wedge-shaped struc- These comparisons demonstrate that not only does a first-order asymme- ture in which the continental crust is abruptly thinned from 30 km to <10 km try exist between conjugate sides of a magma-poor margin, but also there is thick, whereas the hyperextended domain corresponds to a crustal wedge significant variability in terms of the width of the necking and hyperextended tapering to 0 km thickness, which may be succeeded oceanward by a zone of domains and in the architecture of the “upper-plate” and “lower-plate” mar- exhumed mantle (Péron-Pinvidic et al., 2013, 2017; Sutra et al., 2013; Tugend et gins. Although sequential and detachment faulting models can explain the al., 2015). Formation of the hyperextended domain is followed by the eventual first-order asymmetry, they do not provide any explanation for the variability exhumation of serpentinized mantle lithosphere, which itself constitutes the in the width of the “upper-plate” and “lower-plate” margins (Fig. 4). Numerous most distal exhumation domain. In Figure 4, three conjugate margins located studies have demonstrated that margin architecture is largely the mechanical between Europe and Canada are presented with their respective necking and consequence of the variability in crustal thickness, lithospheric thermal struc- hyperextended domains (for locations, see Fig. 2). From north to south, exam- ture, rheological properties of the crust and mantle, finite strain, and extension ples include the Labrador–western Greenland conjugate margin (Chian et al., rates (England, 1983; Kusznir and Park, 1987; Bassi, 1991, 1995; Buck, 1991; Buck 1995); the northern Newfoundland–Iberia margin (Funck et al., 2003; Zelt et al., et al., 1999; Huismans et al., 2005; Lavier and Manatschal, 2006; Gueydan et al., 2003; Sutra et al., 2013), and the southern Newfoundland–Iberia margin (Van 2008; Huismans and Beaumont, 2011; Brune et al., 2014; Svartman Dias et al., Avendonk et al., 2006; Lau et al., 2006; Dean et al., 2000; Sutra et al., 2013). The 2015). The presence of preexisting fabrics is also thought to be an additional Labrador and the Iberian margins are usually identified as lower-plate margins, control (Dunbar and Sawyer, 1989; Ring, 1994; Piqué and Laville, 1996; Corti whereas the western Greenland and Newfoundland margins are described et al., 2007; Clerc et al., 2015; Manatschal et al., 2015), but their effect on the as upper-plate margins (Péron-Pinvidic and Manatschal, 2009; Reston, 2009; development of rift structures is not very well constrained. Our numerical Sutra et al., 2013) and should be compared separately. experiments tested whether or not preexisting fabrics can affect deformation

A) Labrador-Greenland margin (90R1-88R2)

0 Labrador Labrador sea Greenland Postrift sediments Hyperextended domain 20 Continental crust Necking domain LP UP Depth (km ) ~60 km 122 km 56 km ~35 km Mantle UP Upper plate 40 0 100 200 300 400 Serpentinized mantle LP Lower plate Distance (km) Oceanic crust B) Newfoundland-Iberia margin (SCREECH 1-ISE1)

0 Flemish Cap Galicia Bank Galicia

20 UP LP Depth (km ) ~72 km 31 km 41 km ~228 km 40 0 100 200 300 400 500 600 Distance (km) C) Newfoundland-Iberia margin (SCREECH 3-IAM 9) Grand Bank Central Newfoundland basin Southern Iberia abyssal plain Iberia 0

20 UP LP Depth (km) ~145 km 115 km 49 km ~168 km 40

Supplemental file for 0 100 200 300 400 500 600 700 800 900 1000 Effect of contrasting structural and compositional Distance (km) inheritances on the development of rifting margins Figure 4. (A–C) Crustal transects across the (A) Labrador-Greenland margin (90R1–88R2; modified from Chian et al., 1995), (B) northern Newfoundland–Iberian margin (SCREECH 1-ISE 1; modified from Funck et al., 2003; Zelt et al., 2003; Sutra et al., 2013), and (C) the southern Newfoundland–Iberia margin (modified from Van Avendonk et al., 2006; Lau et al., 2006; S. Jammes1, and L. Lavier2 1Department of geography, Texas State University, San Marcos, USA, Sutra et al., 2013). UP—upper-plate margin, LP—lower-plate margin. [email protected] 2 Institute for Geophysics, University of Texas at Austin, USA, [email protected]

INTRODUCTION processes, and if they do, whether sequential normal faulting or detachment deform by Maxwell viscoelastic thermally activated creep, approximated as This supporting information present the evolution of the models discussed in the paper. For each model, the plastic strain (brittle deformation), the material and the strain rate are plotted for the different phases of deformation. In these models, crustal fabrics are either randomly distributed (reference model, Model 1, Figure S1), horizontal (Model 2, Figure faulting is favored. The numerical experiments can also explain the variability a nonlinear temperature- and strain rate–dependent flow (Choi et al., 2013; S2), vertical (Model 3, Figure S3) or oblique (30° dip; Model 4, Figure S4). Model5 is similar to model M4 but the crustal fabrics are dipping in the opposite direction. The parameters used in the models (mechanical, thermal, resolution parameters and boundary in “upper-plate” and “lower-plate” margin architecture in terms of the width Svartman Dias et al., 2015). The mechanism of deformation that requires less conditions) are presented in table S1. of the necking and hyperextended domains. energy or effective stress (square root of the second invariant of the stress tensor) is favored. As in our previous study (Jammes and Lavier, 2016), the explicit polymineralic rheological composition was generated by distributing NUMERICAL STUDY the different mineral phases in each element among Lagrangian particles. In each element, one fraction of particles is assigned one mineral phase while Method the remnant fraction is assigned the other. The fraction of each mineral phase 1 corresponds to the percentage of each mineral phase in the elements. The Experiments were performed with an extended version of the numeri- friction, cohesion, and viscosity in each element correspond to their geometric cal code PARAVOZ for elasto-visco-plastic material (EVP), called geoFLAC average, weighted by the ratio of the particles (Jammes et al., 2015; Jammes 1 Supplemental File. Presents parameters and evolution of models discussed in the paper. For each model, plas- (Fast Lagrangian Analysis of Continua; Poliakov et al., 1993; Tan et al., 2012; and Lavier, 2016). For each phase, the parameters assigned to the viscous tic strain (brittle deformation), material, and strain rate Svartman Dias et al., 2015). At low temperatures, the material behaves elas- flow law were obtained from laboratory measurements. We used “wet” quartz are plotted for different phases of deformation. Please tically until it reaches its yield strength, as described by a Mohr-Coulomb (Brace and Kohlstedt, 1980) and plagioclase (Shelton and Tullis, 1981) for the visit https://doi.org/10.1130/GES01686.S1 or access the full-text article on www.gsapubs.org to view the failure criterion. Subsequently, the material flows plastically (Choi et al., 2013). crust and “dry olivine” (Goetze and Poirier, 1978) and “orthopyroxene” (Raleigh Supplemental File. When temperatures are high enough to activate dislocation creep, materials et al., 1971) for the mantle (see Table S1 for the parameters1). In the literature

(Fountain and Christensen, 1989), the composition of a granite or granodiorite Model 1 with Random Distribution (Fig. 5A; Fig. S1) is described as being composed on average of 20% to 40% weak minerals (quartz and biotite) and 80% to 60% strong minerals (plagioclase, feldspar, and In this model, the initial phase of deformation is characterized by distributed amphibole). Following these conventions, we used in this study 20% quartz normal faulting in the upper crust and distributed anastomosing shear zones and 80% plagioclase for the crustal composition and 70% dry olivine and 30% in the lower crust and lithospheric mantle (Fig. 5A, part i; Fig. S1B [footnote 1]). orthopyroxene for the lithospheric mantle peridotite. In the crust, mineral The plastic strain (Fig. S1B) and strain rate field (Fig. 5A; Fig. S1B) show that phases are either randomly distributed amongst the particles of each element brittle shear zones cut through the brittle upper crust and sole out at midcrustal or statistically distributed using an algorithm initially developed to create a 2-D levels. After 50 km of extension, two types of faults can be identified in the synthetic velocity field (Holliger and Levander, 1992; Holliger et al., 1993; Goff upper crust: 60°–50°-dipping normal faults and low-angle normal faults that et al., 1994). In this study, mineral phases were distributed with a horizontal sole out in the middle crust and exhume crustal material (see distribution of scale of 7 km and vertical scale of 1 km. This distribution was then rotated by plastic strain in Fig. S1B). In the lower part of the crust and in the lithospheric 30° or 90° to be compatible with the orientation of the structures observed in mantle, extension is accommodated along distributed ductile shear zones, the seismic reflection data (Fig. 2). In the lithospheric mantle, mineral phases forming an anastomosing pattern preserving undeformed crustal or mantle were randomly distributed. Due to a lack of constraints, the asthenospheric blocks (Fig. 5A, part i). This combination of brittle and ductile deformation mantle is considered as depleted and was modelled as only dry olivine. processes results initially in a uniformly stretched lithosphere. After ~100 km In each model, the domain was 250 km in depth and 400 km in length. Ve- of extension, strain weakening promotes localization of deformation in one of locity boundary conditions were imposed on both sides of the models (velocity the basins. This basin becomes the main rift basin, under which the lithospheric = 0.5 cm yr–1, in extension), the top surface was free, while at the base of the mantle is progressively thinned by the action of persistent ductile shear zones model, a Winkler foundation was imposed to maintain isostatic equilibrium. forming an anastomosing pattern (see strain rate field in Fig. S1B). As a result, The crust was initially 35 km thick. The temperature at the crust-mantle bound- the asthenospheric mantle upwells (Fig. 5A, part ii). After 200 km of extension, ary was taken to be at 500 °C, increasing to 1330 °C at 100 km depth, and was deformation remains mostly localized in and next to the main rift basin (Fig. set at 10 °C at the surface. Strain softening was introduced in the models to S1C). Left and right of the main basin, high-angle brittle detachment struc- account for the formation of faults and occurred when the plastic strain was tures alternate and exhume crustal material (see distribution of plastic strain higher than 0.1, corresponding to a linear decrease of cohesion (from = 40 MPa in Fig. S1C). However, deformation processes in the crust and mantle remain to = 4 MPa) and friction angle (from φ = 30° to φ = 15°) with the plastic strain decoupled, and there is no detachment structure crosscutting the entire crust (Lavier et al., 2000; Lavier and Manatschal, 2006). All the parameters used are and potentially exhuming mantle to the seafloor. While extension continues, summarized in Table S1 (see footnote 1). we can see a basinward progression of crustal detachment systems that pro- Parameters such as the percentage, the scale, and the orientation of het- gressively thin the crust and elevate the mantle (Fig. S1D). Final exhumation erogeneities should be tested to understand the overall effect of strength occurs after 360 km of extension (Fig. 5A, part iii; Fig. S1E [footnote 1]) with variation from inherited crustal fabrics and the relative importance of crustal the development of a final low-angle detachment fault. As a result, the basin versus mantle lithosphere fabrics. However, few constraints are available is extremely asymmetric, with a gradually thinning, 95-km-long necking zone on mantle lithosphere fabrics. In this study, we focused on the effect of the and a 185-km-long hyperextended domain on the left side (forming the lower orientation of kilometer-scale crustal fabrics on rifting processes; when pos- plate), and a sharp 66-km-long necking zone with a very narrow hyperextended sible, further studies will complete our analysis in contrast with the mantle domain (34 km) on the right side (forming the upper plate). lithosphere fabrics.

Model 2 with Horizontal Crustal Fabrics (Fig. 5B; Fig. S2) Results With a horizontal crustal fabric, the initial phase of deformation is distrib- Three end-member models are presented in this paper and compared to a uted along the entire width of the model, but deformation is accommodated reference model with random strength distribution (Fig. 5A, model 1; Fig. S1 along a few normal faults and low-angle detachment faults extending to the [footnote 1]). In all the other models, the phases were randomly distributed in Moho. These faults develop in the upper crust and sole into the anastomosing the lithospheric mantle, but in the crust, they were either distributed to form shear zones created in the upper lithospheric mantle (Fig. 5B, part i). They re- a horizontal fabric (Fig. 5B, model 2; Fig. S2), a vertical fabric (Fig. 5C, model sult in the formation of several kilometer-deep basins associated with crustal 3; Fig. S3), or a 30°-dipping fabric (Fig. 5D, model 4; Fig. S4). Each model is thinning. After 100 km of extension, deformation localizes in one of the basins, presented in the Supplemental File (Figs. S1–S4 [see footnote 1]), but the main and a crustal-scale detachment fault exhumes the deeper part of the crust. results are summarized in Figure 5. As a result, the crust is quickly thinned, and the mantle is elevated (Fig. S2B

A) Model 1 B) Model 2 i) 50 km of extension i) 50 km of extension 0 -14 0 -14

)]

-1

- -15

e (s -15 50 50

rat

n rate (s

n i -16 -16

rai

st Depth (km )

[

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log[stra -17 l -17 strain rate 150 150 0100 200 300 400 -18 0100 200300 400 -18 Distance (km) ii) 150 km of extension ii) 150 km of extension Distance (km) 0 0

50 50 Depth (km )

Depth (km ) 100 100

strain rate strain rate 150 150 0100 200 300 400 0100 200300 400 Distance (km) Distance (km) iii) After breakup iii) After breakup 0 0

50 50 LP UP UP LP

Depth (km ) 100 Depth (km ) 100 Figure 5. Summary of the results. Evolution of

150 150 (A) model 1 with randomly distributed hetero- -100 0100 200300 400 500 -100 0100 200300 400 500 Distance (km) Distance (km) geneities, (B) model 2 with horizontal crustal C) Model 3 D) Model 4 fabrics, (C) model 3 with vertical crustal fabrics, (D) with dipping crustal fabrics. For each model, i) 50 km of extension i) 50 km of extension the evolution of the deformation is presented 0 -14 0 -14

)] (i) after 50 km of extension, (ii) after 150 km of

)]

- -1 extension, and (iii) after breakup. Main faults -15 e (s -15

50 50 rat are highlighted in black. Details of the strain n

n rate (s i -16 -16 rate are presented in bottom-right corner of the

g[strai Depth (km ) Depth (km ) 100 100 o

l log[stra -17 -17 models after 50 km and 150 km of extension. strain rate UP—upper-plate margin, LP—lower-plate mar- 150 -18 150 -18 0100 200300 400 0100 200300 400 gin. Different shades of green for the dry olivine Distance (km) Distance (km) ii) 150 km of extension ii) 150 km of extension are just used for visualization purposes. 0 0

50 50 Depth (km )

Depth (km ) 100 100

strain rate strain rate 150 150 0 100 200 300 400 0100 200300 400 Distance (km) Distance (km) iii) After breakup iii) After breakup 0 0

50 50 LP UP LP UP Depth (km ) 100 Depth (km ) 100

150 150 -100 0 100 200300 400 500 -100 0100 200300 400 500 Distance (km) Distance (km)

Crust : Mantle :

Plagioclase Dry Olivine UP Upper plate

Quartz Pyroxene LP Lower plate

Main/Secondary faults Last exhumation fault

[footnote 1]). After 150 km of extension, a conjugate detachment fault develops the juxtaposition of crustal allochthons on top of the newly exhumed mantle and delimits a crustal block in the center of the rifted domain (Fig. 5B, part ii; (Fig. S4C). After 200 km of extension, deformation migrates slightly to the Fig. S2C). The strain rate shows that at this stage, normal faults are develop- right with the initiation of a new detachment system (Fig. S4D). This results ing in this crustal block, presaging its future dismantlement. After 200 km of in mantle and asthenospheric exhumation after 260 km of extension (Fig. 5D, extension, a gradual thinning of the crustal block by normal faulting occurs on part iii; Fig. S4E). In this model, the lower plate is then located to the right of the left side of the margin (Fig. S2D). Breakup occurs after 260 km of extension, the upper plate. The lower plate consists of a wide, 89-km-long necking zone through the development of an antithetic detachment fault exhuming the man- and a wide, 120-km-long hyperextended domain. On the other side, the upper tle lithosphere to the seafloor (Fig. 5B, part iii; Fig. S2E). The resulting margin plate thins along a large gradual necking zone (139 km long) and a narrow is asymmetric, with the upper plate on the left side and the lower plate on the hyperextended domain (35km long). On both sides, the necking zone appears right side. The upper plate consists of a sharp 71-km-long necking zone and to be gradual toward the continent and abrupt oceanward. an 87-km-long hyperextended domain, and the lower plate is made of a sharp 72-km-long necking zone and a wide hyperextended domain (106 km). Another remarkable characteristic of this model is the presence of an aborted rift ba- DISCUSSION sin in the lower plate, resulting in boudinage at the scale of the lithosphere. Distribution of Strain during the Stretching Phase

Model 3 with Vertical Crustal Fabrics (Fig. 5C; Fig. S3) Comparison of the models after 50 km and 100 km of extension (Fig. 5; Figs. S1–S4 [footnote 1]) gives insights into the effects of strength variations With a vertical crustal fabric, the initial phase of deformation is character- inherited from crustal fabrics on the evolution of strain distribution during ized by the formation of horsts and grabens delimited by normal faults rooted the stretching phase. In the case of randomly distributed heterogeneities, the in anastomosing shear zones that develop in the upper part of the mantle stretching phase is highly distributed. Extension is accommodated by nor- lithosphere. The deformation remains distributed for 100 km of extension, mal faults formed every 20–30 km that dip in both directions, resulting in the leading to the formation of a wide domain of stretched crust (Fig. 5C, part i; formation of horsts and grabens. Similar, but wider structures are observed Fig. S3B [footnote 1]). After 150 km of extension, deformation begins to local- in model 3 (vertical fabrics), where normal faults are formed every 40–50 km. ize in the mantle lithosphere, with the formation of a low-angle detachment In models 2 and 4 (with horizontal and dipping fabrics, respectively), faults fault exhuming mantle at the base of the crust (Fig. 5C, part ii; Fig. S3C). This are more spaced out and evolve rapidly into crustal- and lithospheric-scale is accompanied by the breakup of the crust by high-angle normal faults (Fig. detachments. As a result, core-complex structures are generated instead of S3D). Final breakup occurs after 260 km of extension with the development horsts and grabens. of a synthetic detachment fault exhuming mantle to the seafloor (Fig. 5C, part Therefore, the distribution of compositional heterogeneities affects the iii; Fig. S3E). As a result, the lower plate (on the left side) is composed of a distribution of the deformation. In model 1, heterogeneities are randomly gradual necking zone, 148 km long, and a narrow hyperextended domain, 65 distributed in the entire model; as a result, deformation is widely distributed km long. On the other side, the upper plate is composed of a narrow, abrupt among numerous normal faults. In models 2, 3, and 4, heterogeneities are necking zone (31 km long) and a narrow hyperextended domain (27 km long). spaced out according to a certain scale, influencing the distribution of the However, the stretched domain, resulting in lithospheric boudinage, affects deformation among fewer and spaced out faults. the entire width of the model. The orientation of heterogeneities also affects the distribution and style of deformation during the stretching phase. Our models show that horizontal and dipping crustal fabrics result in the development of core complexes, Model 4 with 30°-Dipping Crustal Fabrics (Fig. 5D; Fig. S4) whereas vertical fabrics or randomly distributed heterogeneities result in the formation of horsts and grabens. In the case of dipping fabrics, it is clear that With a 30°-dipping crustal fabric, the initial phase of deformation is charac- detachment faults initiate along the dip of the heterogeneities. As a result, all terized by the development of lithospheric-scale low-angle detachment along the low-angle faults formed dip in directions consistent with crustal fabrics. the weak heterogeneities that extend into the upper mantle (Fig. 5D, part i; Fig. To confirm this result, a model with fabrics dipping in the opposite direction S4B [footnote 1]). Crustal deformation and mantle deformation are coupled was also tested (Fig. S5 [footnote 1]). As expected, we observed the formation in the early phase of deformation. After around 150 km of extension, defor- of low-angle detachment faults consistent with the orientations of the hetero- mation preferentially focuses on one of the detachment faults and exhumes geneities. In the case of horizontal fabrics, faults initiate along heterogeneities, the mantle in its footwall almost to the surface (Fig. 5D, part ii; Fig. S4C). The but the absence of orientation allows for the development of low-angle faults breakup of the hanging-wall block of the lithospheric detachment results in dipping in both directions.

Finally, we find that compositional heterogeneities must have a high It appears that neither the sequential or detachment faulting models de- strength contrast with the surrounding rock in order to affect deformation scribed earlier in this paper dominate the thinning process in our models. processes during the stretching phase. In the case of a random distribution Localized thinning affecting our models appears to occur via a combination of heterogeneities (model 1), the location of the heterogeneity first nucleating of processes from both models, or via a variation of either model, or even deformation is difficult to identify. However, in models 2, 3, and 4, only middle- an alternative model. In model 1 (Fig. 5A; Fig. S1 [footnote 1]), anastomosing and lower-crustal heterogeneities control the distribution of deformation. In shear zones in the middle/lower crust and upper mantle initially control the the upper part of the crust, both quartz and plagioclase are in the brittle regime phase of crustal thinning until low-angle crustal detachments thin the remain- (Jammes et al., 2015, Jammes and Lavier, 2016); for the case of equal cohesion ing crust and exhume the mantle lithosphere, as described in the detachment and frictional resistance, there is no strength contrast between compositional faulting model (Lavier and Manatschal, 2006; Péron-Pinvidic and Manatschal, heterogeneities and surrounding rock. As a result, upper-crustal heteroge- 2009). However, in contrast to this model, deformation remains decoupled, and neities do not control the localization process (models 2 and 4). However, several sequential detachments migrating toward the axis of the rift have to below 10 km, quartz becomes ductile while plagioclase remains brittle. The develop to progressively thin the crust and finally couple crustal and mantle middle and lower crust are therefore in the semibrittle deformation field, and deformation. This mechanism has been previously described by Svartman Dias a depth-dependent strength contrast arises between the weaker quartz-dom- et al. (2015), Brune et al. (2014), and Jammes and Lavier (2016) and results in inated heterogeneities and the stronger plagioclase-dominated rock. This an extremely wide and asymmetric margin. In the case of model 2 (Fig. 5B; Fig. strength contrast can explain why localization tends to initiate in the middle S2), we observe the formation of two conjugate low-angle detachment faults and lower crust. We believe that these models show that deep crustal fabrics decoupled along a midcrustal décollement bordering an isolated upper-crustal are more likely to control deformation than shallow crustal fabrics. However, block, as described in the detachment faulting model. However, as extension if shallow crustal fabrics also impart a decrease in cohesion and friction (not continues, the crustal block is gradually thinned by sequential normal faulting, accounted for in our modeling), the strength contrast between the fabrics before the development of an antithetic detachment fault exhuming upper and surrounding rock could be great enough to influence the distribution of mantle to the seafloor. The thinning process appears therefore to occur by a deformation. In summary, in the models, micromechanisms of deformation, combination of the sequential and detachment fault concepts. In the case of which are here taken into account as parameters in the Mohr-Coulomb yield model 4 (Fig. 5D; Fig. S4), the dipping heterogeneities favor coupling between criterion (cohesion and static friction angle) and dislocation creep laws (viscos- crustal and lithospheric mantle deformation. As a result, a low-angle detach- ity), determine both brittle and ductile behavior of the mineral phases present ment fault cutting the entire crust initiates in the early phase of deformation in the model. It is clear, however, that the mesoscale (here, kilometer-scale) and rapidly exhumes the mantle beneath the hyperextended crust. If this distribution of the mineral phases used to create the compositional heteroge- thinning process presents similarities with the detachment faulting model, it neities plays a dominant role in determining the locus and distribution of the differs from that model by the absence of a symmetric stretching phase and deformation during the stretching phase of rifting processes, and also during the early coupling of the deformation. Finally, given vertical crustal fabrics thinning processes (see below). (model 3; Fig. 5C; Fig. S3), crustal thinning processes differ completely from the published models. Indeed, thinning of the crust is due to the formation of high-angle faults in the upper crust and a low-angle detachment fault in Mechanism of Thinning and Exhumation the upper mantle, exhuming mantle lithosphere at the base of the crust. The crustal deformation is therefore completely decoupled from the mantle defor- The models demonstrate that rifts are likely to acquire their architectural mation during thinning processes. This mechanism, never before described in uniqueness from strength variations inherited from crustal fabrics during the literature, could explain geological observations of mantle exhumed beneath distributed stretching and localized thinning phases of deformation. The exhu- hyperextended crust (e.g., Espíritu Santo Basin; Zalán et al., 2011). mation phase does not seem to be affected by inherited structures and appears to be similar in all models. The late phase of extension is always controlled by a detachment fault (synthetic or antithetic to previous structures) that exhumes Resulting Geometry of the Margin mantle to the seafloor, leading to breakup. In their review study, Manatschal et al. (2015) suggested that inherited structures do not significantly control As observed in nature (Fig. 4), our models demonstrate that, depending the location of breakup. Our numerical study confirms this observation and on the orientation of crustal fabrics, the final structure of the margin can vary reaffirms that inherited structures have no control on the structures leading substantially. The widths of the necking and hyperextended domains vary to exhumation. However, crustal heterogeneities strongly control thinning considerably among the models (Figs. S1–S4 [footnote 1]). However, some processes and play an important role in the coupling of deformation between common trends can be observed. In three models with inherited fabrics (model the crust and the mantle lithosphere. 2, 3, and 4), the presence of aborted rift basins results in lithospheric-scale

boudinage. These basins are formed in the early phase of stretching and pres- horst-and-graben structures, whereas horizontal and dipping fabrics favor the ent different characteristics depending on the inherited fabrics. Core-complex formation of core complexes. (2) Depending on the crustal fabrics, thinning structures are formed with horizontal and dipping fabrics, whereas horst-and- processes differ and entail mechanisms involving (i) a combination of the de- graben basins are formed with vertical fabrics. tachment faulting and sequential normal faulting models, (ii) a modified ver- Upper-plate and lower-plate margins can be identified following the orien- sion of the detachment faulting model (with early or late coupling between the tation of the last detachment fault, leading to mantle lithosphere exhumation. crust and the mantle), or (iii) an alternative model in which crustal and mantle We observe that the lower-plate margin corresponds to the left-side margin processes remain decoupled and lead to the formation of high-angle faults in models 1, 3, and 4 and the right-side margin in model 2 (Fig. 5). In all in the upper crust and a low-angle detachment fault in the upper mantle. (3) our models, we show that the hyperextended domain is wider in lower-plate Strength variations inherited from crustal fabrics control the thinning processes margins than in upper-plate margins. Boudinage of the lithosphere renders and consequently the resulting geometry of the margin. (4) Finally, this study the width of the necking domain difficult to estimate precisely; however, we demonstrates that inherited structures do not significantly control the location can observe that, in general, the necking domain is narrower and sharper in of the breakup and mechanism of the exhumation phase: The late development upper-plate margins than in lower-plate margins (Fig. 5). These results are of out-of-sequence detachments in some models explains why distinguishing in agreement with the definition of “upper-plate” and “lower-plate” margins between upper-plate and lower-plate margins is not a straightforward process. (Reston, 2009; Péron-Pinvidic et al., 2017), and the observed structures of the In summary, our study demonstrates that while models do account for Labrador–western Greenland (Fig. 4A) and northern Newfoundland–Iberia micromechanical constitutive behaviors, the distribution of compositional margins (Fig. 4B). Uncertainty concerning the positions of the upper- and heterogeneities at the outcrop scale inherited from previous phases of tec- lower-plate margins remains, however, for the southern Newfoundland–Iberia tonic deformation determines the behavior of the bulk lithosphere in rifting conjugate margin (Fig. 4C). Described as an upper-plate margin in the literature settings. Thus, we propose that when determining the rheological behavior of (Péron-Pinvidic and Manatschal, 2009; Reston, 2009; Sutra et al., 2013), the the lithosphere, more attention should be given to the distribution of mineral southern Newfoundland margin presents a wider hyperextended domain than phases at the mesoscale than has been given so far in the field of rock rheology. the upper-plate margin model (115 km vs. 49 km; Fig. 4). This inconsistency could be explained by the deformation processes modeled in model 2. In this model, the detachment fault that forms during the exhumation phase is an- ACKNOWLEDGMENTS tithetic to the main structures controlling the thinning phase. Determination We thank the reviewer D.L. Harry and the Science Editor Raymond M. Russo for their constructive of the upper-plate versus lower-plate margin is consequently possible only and positive comments that significantly improved the paper. We also thank Harm van Avendonk for helpful comments and fruitful discussions. This study was supported by a National Science during the latest phase of deformation and does not involve the structures Foundation–GeoPRISMS grant for the project “Effect of contrasting structural and compositional formed during the thinning phase. The late development of out-of-sequence inheritances on the development of rifting margins.” Computations were made using the program detachments, as described by Gillard et al. (2015, 2016), could therefore explain FLAC, and data can be provided upon request by sending an e-mail to [email protected]. why distinguishing between upper- and lower-plate margins for the southern Newfoundland–Iberia conjugate margin is not straightforward, as described REFERENCES CITED by Reston (2009) and Péron-Pinvidic et al. (2017). 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