An official website of the United States government
Abstract Despite significant study, when and how plate tectonics initiated on Earth remains contentious. Geologic evidence from some of Earth's earliest cratons has been interpreted as reflecting the formation of initial continental blocks by non‐subduction processes, which then trigger subduction initiation at their margins. Numerical models of mantle convection with a plastic yield stress rheology have shown this scenario is plausible. However, whether continents can trigger subduction initiation has not been tested with other rheologies. We, therefore, use numerical models of mantle convection with an imposed continental block to test whether continents facilitate subduction initiation with a grain‐damage mechanism, where weak shear zones form by grain size reduction. Our results show that continents modestly enhance stresses in the lithosphere, but not enough to significantly impact lithospheric damage or subduction initiation: continents have minimal influence on lithospheric damage or plate speed, nor does subduction preferentially initiate at the continental margin. A new regime diagram that includes continental blocks shows only a small shift in the boundary between the mobile‐lid and stagnant‐lid regimes when continents are added. However, as we do find that stresses are modestly enhanced at the continental margin in our models, we develop a scaling law for this stress enhancement to more fully test whether continents could trigger subduction initiation on early Earth. We find that lithospheric stresses supplied by continents are not sufficient to initiate subduction on the early Earth on their own with grain‐damage rheology; instead, additional factors would be required.
more »
« less
Evolution and demise of passive margins through grain mixing and damage
How subduction—the sinking of cold lithospheric plates into the mantle—is initiated is one of the key mysteries in understanding why Earth has plate tectonics. One of the favored locations for subduction triggering is at passive margins, where sea floor abuts continental margins. Such passive margin collapse is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin are therefore necessary. Spontaneous and accumulated grain damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks occurs. Such mixing is driven both by compositional gradients associated with petrological heterogeneity and by the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can develop, become vertically oriented, and occupy a large portion of lithosphere after about 100 million y. These bands lead to anisotropic viscosity in the lithosphere that is strong to lateral forcing but weak to bending and sinking, thereby greatly facilitating passive margin collapse.
more »
« less
- Award ID(s):
- 1853184
- PAR ID:
- 10210554
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 118
- Issue:
- 4
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- Article No. e2011247118
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
SUMMARY The occurrence of plate tectonics on Earth is rooted in the physics of lithospheric ductile weakening and shear-localization. The pervasiveness of mylonites at lithospheric shear zones is a key piece of evidence that localization correlates with reduction in mineral grain size. Most lithospheric mylonites are polymineralic and the interaction between mineral phases, such as olivine and pyroxene, especially through Zener pinning, impedes normal grain growth while possibly enhancing grain damage, both of which facilitate grain size reduction and weakening, as evident in lab experiments and field observations. The efficacy of pinning, however, relies on the mineral phases being mixed and dispersed at the grain scale, where well-mixed states lead to greater mylonitization. To model grain mixing between different phases at the continuum scale, we previously developed a theory treating grain-scale processes as diffusion between phases, but driven by imposed compressive stresses acting on the boundary between phases. Here we present a new model for shearing rock that combines our theory for diffusive grain mixing, 2-D non-Newtonian flow and two-phase grain damage. The model geometry is designed specifically for comparison to torsional shear-deformation experiments. Deformation is either forced by constant velocity or constant stress boundary conditions. As the layer is deformed, mixing zones between different mineralogical units undergo enhanced grain size reduction and weakening, especially at high strains. For constant velocity boundary experiments, stress drops towards an initial piezometric plateau by a strain of around 4; this is also typical of monophase experiments for which this initial plateau is the final steady state stress. However, polyphase experiments can undergo a second large stress drop at strains of 10–20, and which is associated with enhanced phase mixing and resultant grain size reduction and weakening. Model calculations for polyphase media with grain mixing and damage capture the experimental behaviour when damage to the interface between phases is moderately slower or less efficient than damage to the grain boundaries. Other factors such as distribution and bulk fraction of the secondary phase, as well as grain-mixing diffusivity also influence the timing of the second stress drop. For constant stress boundary conditions, the strain rate increases during weakening and localization. For a monophase medium, there is theoretically one increase in strain rate to a piezometric steady state. But for the polyphase model, the strain rate undergoes a second abrupt increase, the timing for which is again controlled by interface damage and grain mixing. The evolution of heterogeneity through mixing and deformation, and that of grain size distributions also compare well to experimental observations. In total, the comparison of theory to deformation experiments provides a framework for guiding future experiments, scaling microstructural physics to geodynamic applications and demonstrates the importance of grain mixing and damage for the formation of plate tectonic boundaries.more » « less
-
The Cenozoic landscape evolution in southwestern North America is ascribed to crustal isostasy, dynamic topography, or lithosphere tectonics, but their relative contributions remain controversial. Here we reconstruct landscape history since the late Eocene by investigating the interplay between mantle convection, lithosphere dynamics, climate, and surface processes using fully coupled four-dimensional numerical models. Our quantified depth dependent strain rate and stress history within the lithosphere, under the influence of gravitational collapse and sub-lithospheric mantle flow, show that high gravitational potential energy of a mountain chain relative to a lower Colorado Plateau can explain extension directions and stress magnitudes in the belt of metamorphic core complexes during topographic collapse. Profound lithospheric weakening through heating and partial melting, following slab rollback, promoted this extensional collapse. Landscape evolution guided northeast drainage onto the Colorado Plateau during the late Eocene-late Oligocene, south-southwest drainage reversal during the late Oligocene-middle Miocene, and southwest drainage following the late Miocene.more » « less
-
Abstract The Cenozoic landscape evolution in southwestern North America is ascribed to crustal isostasy, dynamic topography, or lithosphere tectonics, but their relative contributions remain controversial. Here we reconstruct landscape history since the late Eocene by investigating the interplay between mantle convection, lithosphere dynamics, climate, and surface processes using fully coupled four-dimensional numerical models. Our quantified depth-dependent strain rate and stress history within the lithosphere, under the influence of gravitational collapse and sub-lithospheric mantle flow, show that high gravitational potential energy of a mountain chain relative to a lower Colorado Plateau can explain extension directions and stress magnitudes in the belt of metamorphic core complexes during topographic collapse. Profound lithospheric weakening through heating and partial melting, following slab rollback, promoted this extensional collapse. Landscape evolution guided northeast drainage onto the Colorado Plateau during the late Eocene-late Oligocene, south-southwest drainage reversal during the late Oligocene-middle Miocene, and southwest drainage following the late Miocene.more » « less
-
Lithospheric shortening can be described by one of two end-member modes: indentation of the lithosphere and subduction of the lithospheric mantle. Deciphering the difference between these modes is crucial in the interpretation of past and present orogens and in predicting their structural architecture at depth. It is therefore important to establish how observable upper crustal proxies reflect deep lithospheric kinematics and dynamics. Over the last few decades, geological and geophysical data have provided valuable constraints on the northern margin of the Tibetan Plateau. This margin is defined by the Qilian Shan thrust belt, which developed in response to the far-field convergence between the Indian and Eurasian plates. The primary mechanism for this development is the southward subduction of the Asian lithospheric mantle beneath the Tibetan Plateau. We conducted numerical modelling to simulate the kinematics and response of the upper crust to the southward subduction of the lithospheric mantle. Our results show that subduction of the lithospheric mantle can result in upper crustal deformation that matches the records in the Qilian Shan, where pure shear shortening alone does not generate similar upper crust proxies, including the broad width and architecture of the bivergent orogenic wedge, the timing of fault initiation and evolution, seismicity and fault activity, the topography and geomorphology. The geometry of the subducting lithosphere impacts the width and asymmetry of the bivergent orogenic wedge. Our results demonstrate how records of crustal strain can be used to better interpret the deep structural architecture of past and present orogenic wedges.more » « less
