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.
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, http://dx.doi.org/10.1016/j.epsl.2012.06.033
Images of the deformed sample:
(a) X-ray radiography of the lizardite sample in the press during deformation atP¼8 GPa and T¼2501C; the sample is lined withtwo gold foils appearing as black horizontal lines.
(b) ESEM image of fine grained sample deformed at 8 GPa showing homogeneous deformation marked by alignment oflizardite (001) flakes perpendicular to compression.
(c) FIB thin section
(d) TEM image showing lizardite deformation by glide along (001) and kinking. Maximumapparent deformation is calculated by unkinking the traces of (001) planes (red broken lines) parallel to compression axis (thick arrows).
Computed structure and critical resolved shear stress in lizardite.
(a) Crystal structure at 1 GPa.
(b) Gamma-surface on the (001) dislocation planeat 1 GPa inlizardite is obtained by displacing two layers along the (001) plane. Atoms are allowed to accommodate the displacement only in the perpendicular direction. The energydifference between the perfect crystal and sheared structures is mapped for various displacements in the basal plane.
(c) and (d) Energy curves calculated along (dashed curves) and/210S(full curves, easiest glide direction) directions at 1 and 3 GPa, respectively.