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Evolution and Demise of Passive Margins Through Grain Mixing And Evolution and demise of passive margins through grain mixing and damage INAUGURAL ARTICLE David Bercovicia,1 and Elvira Mulyukovaa aDepartment of Earth & Planetary Sciences, Yale University, New Haven, CT 06511 This article is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2018. Edited by David L. Kohlstedt, University of Minnesota, Minneapolis, MN, and approved December 10, 2020 (received for review June 1, 2020) How subduction—the sinking of cold lithospheric plates into the plate boundaries and deformation belts (see ref. 19, for review). mantle—is initiated is one of the key mysteries in understand- Mylonites are an indication of extreme deformation and shear ing why Earth has plate tectonics. One of the favored locations localization correlated with intense grain-size reduction (20–23). for subduction triggering is at passive margins, where sea floor The grain-scale physics occurring in mylonites are important for abuts continental margins. Such passive margin collapse is prob- understanding processes at the global and planetary scale and lematic because the strength of the old, cold ocean lithosphere have been the subject of a number of theoretical (18, 19, 24–35) should prohibit it from bending under its own weight and sink- and observational (36–42) studies. Plate boundary mylonites and ing into the mantle. Some means of mechanical weakening of the ultramylonites typically form in the lithospheric mantle where passive margin are therefore necessary. Spontaneous and accu- there are multiple mineralogical phases, such as olivine and mulated grain damage can allow for considerable lithospheric pyroxene in peridotite, and especially where these phases mix weakening and facilitate passive margin collapse. Grain damage (40–53). This observation suggests that Zener pinning (in which is enhanced where mixing between mineral phases in lithospheric one mineral phase blocks the grain-boundary migration of the rocks occurs. Such mixing is driven both by compositional gra- other phase) plays an important role in suppressing grain growth dients associated with petrological heterogeneity and by the and keeping the medium in permanent diffusion creep (43–45, state of stress in the lithosphere. With lateral compressive stress 51, 53). Moreover, Zener pinning possibly facilitates dynamic imposed by ridge push in an opening ocean basin, bands of mix- recrystallization (i.e., formation of subgrains via the accumu- lation of dislocations) (54–57) and grain-size reduction (18). ing and weakening can develop, become vertically oriented, and EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES occupy a large portion of lithosphere after about 100 million y. Thus, pinning acts to enhance localization in two ways, by pro- These bands lead to anisotropic viscosity in the lithosphere that is moting grain damage and comminution and suppressing grain strong to lateral forcing but weak to bending and sinking, thereby growth and healing. Mixing of mineral phases has indeed been greatly facilitating passive margin collapse. shown to induce significantly different deformation and weaken- ing mechanisms (29, 34, 40–42, 49, 58) relative to where minerals passive margins j subduction j grain damage remain unmixed. Thus, the evolution of mixing in mantle het- erogeneity, i.e., mixing between mineralogical units, likely plays he sinking, or subduction, of old, cold, and heavy tectonic an important and unique role in developing lithospheric weak Tplates is considered both one of the main driving forces zones. for plate tectonics and one of the primary mechanisms for How minerals in a solid state mix at high pressures (where convectively cooling the Earth’s mantle (1–3). How subduc- cavitation is unlikely) is still a matter of active research. Mixing tion is initiated and sustained in a cold strong lithosphere is could arise from mechanical transport of one mineral’s recrys- one of the major enigmas in geosciences (4–6) and is key tallized grains along the other mineral’s grain boundaries (58), to understanding how and why plate tectonics function on Earth, but apparently not other terrestrial planets. There are Significance various possible mechanisms for initiating a new subduction zone, including, for example, nucleating from transform faults Subduction of sea floor into the Earth’s mantle is the engine or preexisting fracture zones (4, 6–11) or plume–lithosphere of plate tectonics. Yet, how subduction initiates remains a interaction (12). The initiation of subduction at passive mar- mystery. Subduction often occurs along trenches at the ocean– gins, where the oceanic lithosphere of an opening ocean basin continent margin, which implies they formed there, when abuts a continental margin, is appealing, because of the corre- the margin was once immobile or passive, but then col- lation of many existing deep ocean trenches (where subduction lapsed. Margin collapse occurs when near-surface mantle, the occurs) with continental margins. However, while passive mar- lithosphere, gets cold and heavy and founders. That cold litho- gins are where the lithosphere is oldest, coldest, heaviest, and sphere, however, should be too stiff to sink. Grain damage in most gravitationally unstable, it is also at its strongest and mantle rocks, whereby mineral grains under stress diminish in thus least likely to deform under the action of negative buoy- size, weakens the lithosphere and mostly occurs where miner- ancy (4, 5, 13, 14). Various mechanisms have been invoked to als mix with each other. Horizontal compressive stresses in a load and/or weaken the passive margin to trigger its collapse, passive margin induce mixing and damage and generate weak including, for example, tensile stress associated with sagging, bands in the lithosphere that facilitate subduction initiation. rifting, or stretching at the margin (5, 15, 16) and sediment loading (13, 17), although these still require enough weakening Author contributions: D.B. and E.M. conceived the project, developed the theory, of the lithosphere to allow the margin to detach and subduct. developed the numerical model and graphics, and wrote the paper.y Mulyukova and Bercovici (14) proposed that accumulated weak- The authors declare no competing interest.y ening due to grain damage (18, 19) could sufficiently offset This article is a PNAS Direct Submission.y thermal stiffening of the lithosphere and facilitate passive margin Published under the PNAS license.y collapse. 1 To whom correspondence may be addressed. Email: [email protected] The grain-damage mechanism has been previously proposed This article contains supporting information online at https://www.pnas.org/lookup/suppl/ to explain weakening in the strongest portion of the ductile litho- doi:10.1073/pnas.2011247118/-/DCSupplemental.y sphere, as evident in the pervasiveness of mylonites at many Published January 18, 2021. PNAS 2021 Vol. 118 No. 4 e2011247118 https://doi.org/10.1073/pnas.2011247118 j 1 of 9 Downloaded by guest on October 1, 2021 or through dissolution of one mineral and precipitation of it at Model Summary the other mineral’s grain-boundary junctions (41, 42, 49). These Our theoretical model builds on the theory for diffusive grain mechanisms are both limited by element diffusion and similarly mixing and damage (34) and the model of grain-size evolution in driven by imposed stresses. Bercovici and Mulyukova (34) pro- passive margins (14), with some adaptations; it is fully developed posed a theory whereby grain mixing itself is a form of diffusion and explained in SI Appendix, but summarized briefly here. of one mineral through the other, with diffusivity governed by Our model system is composed of three physical processes at the imposed state of stress. Such diffusive mixing between min- three different length scales; these involve 1) thermal diffusion eral phases promotes grain damage and weakening, specifically in a cooling, thickening lithosphere; 2) diffusion of petrologi- in the mixing regions; these zones correspond well to observa- cal heterogeneity through grain or phase mixing (i.e., mixing of tions of well-mixed mylonites and ultramylonites separating the olivine and pyroxene grains); and 3) mineral grain-size evolution larger-grained units of a single mineral phase (43–45, 53). via the competition between surface-tension–driven coarsening Mulyukova and Bercovici (14) demonstrated that an already and deformation-induced damage. The length scales for varia- well-mixed lithosphere leads to significant weakening by grain tions of temperature and ridge-push stress are referred to as the damage to facilitate passive margin collapse. Here we propose macroscopic scale and are much longer than the length scales for that petrological heterogeneity in an unmixed lithosphere may grain-mixing diffusion of petrological heterogeneity, referred to lead to mylonitic zones of extreme weakness at the boundaries as the mesoscopic scale, which, in turn, is larger than the grain between mineral phases, at the centimeter scale (Fig. 1). Accu- scale (Fig. 1). At physical conditions representative of Earth’s mulation of these weak zones by stress-driven grain mixing and lithosphere, the thermal diffusion length scale is LT = 112 km, grain damage may promote sufficient lithospheric weakening to the grain-mixing diffusion length scale is LG = 15 cm, and the facilitate passive margin collapse. Here we explore this hypoth- grain-size length scale is Rf = 360 µm (see SI Appendix for a esis in a model of the lithosphere
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