Transformational Faulting in Metastable Olivine, from Lab to Slab

Abstract

<p>     This year marks the 100th anniversary of the discovery of Deep Focus Earthquakes (DFEs). Despite the elaboration of several hypotheses, the mechanisms responsible for their occurrence at depths where rocks flow in a viscous way are not entirely elucidated. DFEs are far from ubiquitous and only occur in certain subducting slabs as they descend through the mantle transition zone, where olivine transforms to wadsleyite and ringwoodite. This has led to associating DFEs to the transformation of metastable olivine. Faulting induced by the olivine transformation was proven to cause brittle behavior under conditions where ductile deformation otherwise prevails [<em>Burnley et al.</em>, 1991]. It can also explain the anomalously high DFE activity in Tonga, which has been attributed to the thermal state of the subducting slab, colder slabs allowing for more metastable olivine.</p><p>     However, there are limited data regarding the conditions required for transformational faulting in terms of reaction kinetics, as well as regarding its possible propagation in ringwoodite peridotites. The seminal work of Burnley, Green and co-authors regarding transformational faulting used a Ge-olivine analogue, a material that undergoes the transition to the ringwoodite structure (Ge-spinel) at much lower pressures than the silicate counterpart [<em>Burnley et al.</em>, 1991]. Here we continue to build upon this work by combining high pressure and temperature deformation experiments with Acoustic Emission (AE) monitoring. The experiments investigate lower temperatures and strain rates to assess the extrapolation of transformational faulting towards natural conditions. Ge-olivine samples were deformed in the Ge-spinel field at 1.5 GPa and various temperatures in a modified Griggs apparatus.</p><p>     We demonstrate that transformational faulting can initiate in metastable olivine, and then continue to propagate via shear-enhanced melting in the stable high-pressure phase, which is a paramount finding since transformational faulting has been contested as the origin of DFEs on the basis that large DFEs cannot be contained within a metastable olivine wedge. The experiments yielded a range of mechanical behaviors and acoustic signals depending on the kinetics of the olivine-ringwoodite transformation. The b-values associated with the obtained AEs range from 0.6-1.5, consistent with those of DFEs. In addition, we evidence that transformational faulting is controlled by the ratio between strain rate and reaction kinetics and extrapolate this relationship to the natural conditions of DFEs. Counterintuitively, these results imply that cold slabs induce transformational faulting at higher temperatures as a result of faster descent rates. This produces more numerous small DFEs and explains the higher b-values observed.</p><p>Burnley, P. C., H. W. Green, and D. J. Prior (1991), Faulting Associated With The Olivine To Spinel Transformation In Mg2geo4 And Its Implications For Deep-Focus Earthquakes, <em>Journal of Geophysical Research-Solid Earth and Planets</em>, <em>96</em>(B1), 425-443.</p>