Plate tectonics, damage and inheritance

Abstract

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.

Thermal evolution of the core with a high thermal conductivity

“…Nakagawa and Tackley, 2010). However, despite important progresses in the last decades, the theory of mantle convection is still far from complete (eg Tackley, 2000;Ricard, 2007;Bercovici and Ricard, 2014) and its efficiency in heat transfer is highly controversial (eg Jaupart et al, 2007). In the present paper I therefore prefer to consider several scenarios for the evolution of the CMB heat flow with time and draw implications from the resulting models.…”

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

“…Promising research paths include conceptual developments such as the understanding of lithospheric damage (Bercovici and Ricard 2014) as well as an improved treatment of plate-like behavior in spherical simulations of mantle dynamics (Bello et al 2014;Rolf et al 2012Rolf et al , 2014. In the meantime, improved reconstructions of plate velocities (Seton et al 2012) can be prescribed to mantle convection models to investigate the role of specific ingredients such as mantle rheology, phase transitions and the nature of a possible dense basal layer.…”

Abrupt tectonics and rapid slab detachment with grain damage

“…We propose a simple model for rapid slab detachment that combines triggering by entrainment of buoyant crust, and a rapid necking mechanism facilitated by the coupling of grain-sensitive rheology and grain-size evolution with damage (26)(27)(28)(29)(30)(31). In particular, grain reduction and weakening by the combination of damage and Zener pinning in a multiphase mineralogical assemblage, which is consistent with field and laboratory observations of polycrystalline rocks (27), allows rapid necking and thus abrupt detachment.…”

“…Although diffusion creep depends on the evolving grain size, the interface between the rock's mineral phases (i.e., olivine and pyroxene) induces Zener pinning, which blocks grain growth (40)(41)(42). Thus, we assume grain size evolution is slaved to the evolution of the size of the pinning bodies r, which is equivalent to the characteristic radius of curvature, or roughness of the interface (27)(28)(29). Hence, the mineral grain size is proportional to r, and using the definition for a pinnedstate (27), the rock's average grain size is approximately π 2 r. (The pinned state requires that the Zener pinning factor Z i = 1 − cð1 − ϕ i ÞðR i =rÞ 2 = 0, where R i is grain size of phase i with volume fraction ϕ i , and c = 0:87 for the grain size distributions used by refs.…”

“…27-29 and 31 for midlithosphere at temperature T ≈ 1;000 K (however, see ref. 29, Methods, for discussion of coarsening rate activation energy), which lead to the compliances and coarsening rates shown in Table 1, among other fixed parameters. For the crustal buoyancy stress, we use F c =D 0 = Δρ c gh c where h c is the effective crustal thickness, which we vary to assess the effect of different sized crustal plugs.…”