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 inher- ited 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 ap- parent 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, © 2019 The Authors 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