FW: Plate tectonics, damage and inheritance

Damage mechanics is a continuum theory for how deformation affects material strength, usually by the formation of micro-cracks or reduction of grain size, either of which weakens the material16. The pervasiveness of mantle-lithosphere peridotitic mylonites—where localized deformation correlates with reduced mineral grain sizes—at tectonic margins17, 18 indicates that a self-weakening feedback between grain reduction by damage and grain-size-dependent strength (for example, diffusion creep)19, 20 at mid-lithospheric depths is key to plate-boundary formation. This grain-damage feedback mechanism is most plausible and evident in multi-phase or polycrystalline lithospheric rocks (for example, peridotite, which is a mixture of mostly olivine and pyroxene): grain growth in such rocks—which strengthens and heals the material—is known from laboratory and field observations to be impeded by pinning (that is, blocking of grain-boundary migration) by the interface between phases17, 21, 22. Thus, our lithospheric damage mechanism is a coupled evolution model that describes the competition between damage and healing for both grains and interfaces in two-phase assemblages, and where interface pinning both limits grain growth and healing, and promotes damage and shear-localization1, 2 (see Methods for further details).

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Posted on: Sunday, 6 April 2014 10:00 AM
Author: David Bercovici
Subject: Plate tectonics, damage and inheritance


Plate tectonics, damage and inheritance

Nature 508, 7497 (2014). doi:10.1038/nature13072<http://dx.doi.org/10.1038/nature13072>

Authors: David Bercovici & Yanick Ricard

The initiation of plate tectonics on Earth is a critical event in our planet's history. The time lag between the first proto-subduction (about 4 billion years ago) and global tectonics (approximately 3 billion years ago) suggests that plates and plate boundaries became widespread over a period of 1 billion years. The reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics. Here we suggest that when sufficient lithospheric damage (which promotes shear localization and long-lived weak zones) combines with transient mantle flow and migrating proto-subduction, it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone. We simulate this process using a grain evolution and damage mechanism with a composite rheology (which is compatible with field and laboratory observations of polycrystalline rocks), coupled to an idealized model of pressure-driven lithospheric flow in which a low-pressure zone is equivalent to the suction of convective downwellings. In the simplest case, for Earth-like conditions, a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate, with passive spreading and strike-slip margins that persist and localize further, even though flow is driven only by subduction. But for hotter surface conditions, such as those on Venus, accumulation and inheritance of damage is negligible; hence only subduction zones survive and plate tectonics does not spread, which corresponds to observations. After plates have developed, continued changes in driving forces, combined with inherited damage and weak zones, promote increased tectonic complexity, such as oblique subduction, strike-slip boundaries that are subparallel to plate motion, and spalling of minor plates.



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