Strain weakening enables continental plate tectonics

Abstract

Much debate exists concerning the strength distribution of the continental lithosphere, how it controls lithosphere-scale strain localization and hence enables plate tectonics. No rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid-like behaviour of plate interiors. Here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution. Geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle, respectively. The positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remain strong. 2 1/ IntroductionThe extrapolation of laboratory flow laws to geological scale suggests a complex layering of brittle and ductile layers within the continental lithosphere (Brace and Kohlstedt, 1980;Sawyer, 1985). For classical continental geotherm, the upper lithospheric mantle is expected to be brittle and support high stresses. Many analogue and numerical experiments indicate that such a rheological layering is important to reproduce first-order patterns of lithosphere deformation (Brun, 1999;Burov and Yamato, 2008). In particular, the presence of a brittle uppermost mantle is needed to explain strain localisation at lithospheric scale (Buck, 1991;Gueydan et al., 2008).However, recent geophysical studies question this classical view of the continental strength layering. Based on earthquakes distribution and elastic thicknesses of the continental lithosphere, including cratons, it has been proposed that the uppermost mantle could behave as ductile instead of brittle (Déverchère et al., 2001;Jackson, 2002;Maggi et al., 2000). However, the mechanical stability of cratons requires that the uppermost mantle supports high stresses (Burov, 2010). In addition, the post-seismic displacement field, i.e., the pattern of deformation at the surface of the Earth within weeks to years following an earthquake, suggests that the deep crust is stronger than the lithospheric mantle. (Bürgmann and Dresen, 2008;Thatcher and Pollitz, 2008). Note however that post seismic displacement field may also result from a complex combination of poro-elasticity and fault creep in the seismogenic layer and viscous flow in the lower crust (Barbot and Fialko, 2010). In this case, it may not be used to constrain strength ratios between the ductile crust 3 and lithospheric mantle. Finally, the recent discovery of non-volcanic tremors, i.e., long duration seismic events with small amplitudes, below the San Andreas fault suggests a zone of localized and easily modulated faulting in the lower crust (Nadeau and Guilhem, 2009;Thomas et al., 2009). Assuming high temperatures in plate boundaries such as the San Andreas...

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…References [31,32] postulate that mylonitic shear zones in the crust become continually weaker owing to the rising temperatures within the shear zone. Reference [33][34][35][36][37] argue that faults within crystalline basement become weak because of reaction softening: phyllosilicate-rich mylonites (phyllonites) form from the mechanical breakdown of feldspars, which is followed by a chemical breakdown in the presence of a fluid, or grain-size reduction allowing grain-boundary sliding. Reference [38] report that the crystalline basement in the external parts of the Western Alps deformed in a brittle-ductile regime with distributed shear within spaced shear zones or shear bands that are up to a few hundred meters thick.…”

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…[30.31] postulate that mylonitic shear zones in the crust become continually weaker owing to the rising temperatures within the shear zone. [32][33][34] argue that faults within crystalline basement become weak because of reaction softening: phyllosilicate-rich mylonites (phyllonites) form from the mechanical breakdown of feldspars, which is followed by a chemical breakdown in the presence of a fluid, or grain-size reduction allowing grain-boundary sliding. [35] found that the crystalline basement in the external parts of the Western Alps deformed in a brittle-ductile regime with distributed shear within spaced shear zones or shear bands that are up to a few hundred meters thick.…”

Asymmetric vs. symmetric deep lithospheric architecture of intra-plate continental orogens

“…Upper crustal thickness increases at the location of the pop-down and cold upper crustal material is pushed down at great depths. If such lithospheric swelling continues growing, a gravitational instability (Rayleigh-Taylor) may be triggered (Houseman and Molnar, 1997). As predicted from numerical and analytical models, the descent of cold lithospheric material creates lateral temperature gradients and eventually the heavy lithospheric root is removed by mantle convection.…”