Creep of phyllosilicates at the onset of plate tectonics

Plate tectonics is the unifying paradigm of geodynamics yet the mechanisms and causes of its initiation remain controversial. Some models suggest that plate tectonics initiates when the strength of lithosphere is lower than 20–200 MPa, below the frictional strength of lithospheric rocks (>700 MPa). At present-day, major plate boundaries such as the subduction interface, transform faults, and extensional faults at mid-oceanic ridge core complexes indicate a transition from brittle behaviour to stable sliding at depths between 10 and 40 km, in association with water–rock interactions forming phyllosilicates. We explored the rheological behaviour of lizardite, an archetypal phyllosilicate of the serpentine group formed in oceanic and subduction contexts, and its potential influence on weakening of the lithospheric faults and shear zones. High-pressure deformation experiments were carried out on polycrystalline lizardite—the low temperature serpentine variety—using a D-DIA apparatus at a variety of pressure and temperature conditions from 1 to 8 GPa and 150 to 400 °C and for strain rates between 10−4 and 10−6 s−1. Recovered samples show plastic deformation features and no evidence of brittle failure. Lizardite has a large rheological anisotropy, comparable to that observed in the micas. Mechanical results and first-principles calculations confirmed easy gliding on lizardite basal plane and show that the flow stress of phyllosilicate is in the range of the critical value of 20–200 MPa down to depths of about 200 km. Thus, foliated serpentine or chlorite-bearing rocks are sufficiently weak to account for plate tectonics initiation, aseismic sliding on the subduction interface below the seismogenic zone, and weakening of the oceanic lithosphere along hydrothermally altered fault zones. Serpentinisation easing the deformation of the early crust and shallow mantle reinforces the idea of a close link between the occurrence of plate tectonics and water at the surface of the Earth.