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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.
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The dependence of planetary tectonics on mantle thermal state: Applications to early Earth evolution
For plate tectonics to operate on a planet, mantle convective forces must be capable of forming weak, localized shear zones in the lithosphere that act as plate boundaries. Otherwise, a planet's mantle will convect in a stagnant lid regime, where subduction and plate motions are absent. Thus, when and how plate tectonics initiated on Earth is intrinsically tied to the ability of mantle convection to form plate boundaries; however, the physics behind this process are still uncertain. Most mantle convection models have employed a simple pseudoplastic model of the lithosphere, where the lithosphere "fails" and develops a mobile lid when stresses in the lithosphere reach the prescribed yield stress. With pseudoplasticity high mantle temperatures and high rates of internal heating, conditions relevant for the early Earth, impede plate boundary formation by decreasing lithospheric stresses, and hence favor a stagnant lid for the early Earth. However, when a model for shear zone formation based on grain size reduction is used, early Earth thermal conditions do not favor a stagnant lid. While lithosphere stress drops with increasing mantle temperature or heat production rate, the deformational work, which drives grain size reduction, increases. Thus the ability of convection to form weak plate boundaries is not impeded by early Earth thermal conditions. However, mantle thermal state does change the style of subduction and lithosphere mobility; high mantle temperatures lead to a more sluggish, drip-like style of subduction. This "sluggish lid" convection may be able to explain many of the key observations of early Earth crust formation processes preserved in the geologic record. Moreover, this work highlights the importance of understanding the microphysics of plate boundary formation for assessing early Earth tectonics, as different plate boundary formation mechanisms are influenced by mantle thermal state in fundamentally different ways.
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- Award ID(s):
- 1723057
- PAR ID:
- 10065516
- Date Published:
- Journal Name:
- Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
- ISSN:
- 1471-2962
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1098/rsta.2017.0409
