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 20-200MPa, 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 behavior to stable sliding at depths between 10 and 40 km, in association with water-rock interactions forming phyllosilicates. We explored the rheological behavior of lizardite, an archetype 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 plycrystalline lizardite – the low temperature serpentine variety – using a D-DIA apparatus at a variety of pressure and temperature conditions form 1 to 8GPa and 150 to 400 degree C for strain rates between 10-4and 10-6 s-1.  Recovered samples show plastic deformation features and no evidence of brittle failure.

Images of the deformed sample : (a) X-ray radiography of the lizardite sample in the press during deformation at P=8GPa and T=250 degrees C; the sample is lined with two gold foils appearing as black horizontal lines. (b) ESEM image of fine grained sample deformed at 8GPa showing homogeneous deformation marked by alignment of lizardite (001) flakes perpendicular to compression. (c) FIB thin section and (d) TEM image showing lizardite deformation by glide along (001) and kinking. Maximum apparent deformation is calculated by unkinking the traces of (001) planes (red broken lines) parallel to compression axis (thick arrows).

Elodie Amiguet, Bruno Reynard, Razvan Caracas, Bertrand Van de Moortèle, Nadège Hilairet, Yanbin Wang, Creep of phyllosilicates at the onset of plate tectonics, Earth and Planetary Science Letters, Volumes 345–348, 2012, pages 142-150, ISSN 0012-821X,