Articles | Volume 7, issue 2
Solid Earth, 7, 355–366, 2016

Special issue: Deformation mechanisms and ductile strain localization in...

Solid Earth, 7, 355–366, 2016

Research article 08 Mar 2016

Research article | 08 Mar 2016

Strain localization in ultramylonitic marbles by simultaneous activation of dislocation motion and grain boundary sliding (Syros, Greece)

A. Rogowitz1,2, J. C. White3, and B. Grasemann1 A. Rogowitz et al.
  • 1Department for Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
  • 2Natural History Museum, Burgring 7, 1010 Vienna, Austria
  • 3Department of Earth Sciences, University of New Brunswick, 3 Bailey Dr, Fredericton, NB E3B 5A3, Canada

Abstract. Extreme strain localization occurred in the centre of the cross-cutting element of a flanking structure in almost pure calcite marbles from Syros, Greece. At the maximum displacement of 120 cm along the cross-cutting element, evidence of grain size sensitive deformation mechanisms can be found in the ultramylonitic marbles, which are characterized by (1) an extremely small grain size ( ∼  3 µm), (2) grain boundary triple junctions with nearly 120° angles, (3) a weak crystallographic preferred orientation with very low texture index (J = 1.4), (4) a random misorientation angle distribution curve and (5) the presence of small cavities. Using transmission electron microscopy, a deformation sequence is observed comprising recrystallization dominantly by bulging, resulting in the development of the fine-grained ultramylonite followed by the development of a high dislocation density ( ∼  1013 m−2) with ongoing deformation of the fine-grained ultramylonite. The arrangement of dislocations in the extremely fine-grain-sized calcite differs from microstructures created by classical dislocation creep mediated by combined glide and thermally activated climb. Instead, it exhibits extensive glide and dislocation networks characteristic of recovery accommodated by cross-slip and network-assisted dislocation movement without formation of idealized subgrain walls. The enabling of grain boundary sliding to dislocation activity is deemed central to initiating and sustaining strain softening and is argued to be an important strain localization process in calcite rocks, even at a high strain rate ( ∼  10−9 s−1) and low temperature (300 °C).

Short summary
This paper discusses the processes resulting in extreme strain localization (gamma up to 1000) in an almost pure calcite marble located in Syros, Greece. We observed grain size reduction by bulging recrystallization, followed by the simultaneous activation of grain boundary sliding and a dislocation motion in conditions (high differential stress, high strain rate, low temperature) in which brittle deformation rather than ductile flow might be suspected.