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 (
Strain localization in monomineralic rocks when associated with brittle precursors can exhibit microstructural evolution comprising grain size reduction and ensuing activation of grain-size-sensitive (GSS) deformation mechanisms such as diffusion creep, cataclastic flow and independent grain boundary sliding (GBS) (Schmid, 1976; Schmid et al., 1977; Etheridge and Wilkie, 1979; Segall and Pollard, 1983; Pennacchioni and Mancktelow, 2007; Menegon et al., 2013). The aforementioned mechanisms are typically anticipated to result in a random crystallographic orientation or decrease in the intensity of any pre-existing texture formed during plastic deformation of the original material. However, reports of fine-grained deformed polycrystalline materials showing a crystallographic preferred orientation (CPO) indicate that deformation mechanisms operative during GSS flow can develop a CPO (Schmid et al., 1977; Rutter et al., 1994; Barreiro et al., 2007; Sundberg and Cooper; 2008; Wang et al., 2010; Hansen et al., 2011; Kushnir et al., 2015).
GBS can contribute to non-uniform GSS flow behaviour (e.g. Ashby and Verall, 1973) and is able to support extremely high strains. Nevertheless, accommodation of relative grain displacement requires accommodation mechanisms to occur collaboratively in order to maintain reasonable strain compatibility. Maintenance of strain compatibility amongst grains requires grain boundary reorganization by diffusional flow and/or dislocation glide and climb within the grain boundary and the grain boundary region (Gifkins, 1976), with diffusion at relatively high homologous temperature being most commonly invoked (e.g. Boullier and Guéguen, 1975). More recently the simultaneous activity of dislocation creep and GBS has been inferred to be active in ultramylonites deformed both in nature (Vitale et al., 2007; Molli et al. 2011) and experimentally (Wang et al., 2010; Hansen et al., 2011; Kushnir et al., 2015).
The material under study is an essentially pure calcite marble layer from
Syros (Cyclades, Greece). Previous work (Rogowitz et al., 2014) has
established that the marble deformed under lower greenschist facies
conditions, resulting in a grain size of
The ultramylonite in question developed during formation of an a-type
flanking structure in an almost pure calcite marble situated on Syros
(35:00 UTM, 414 840
The pre-existing crack (cross-cutting element) associated with the flanking
structure (Fig. 1a) was initially orientated at an angle of 90
Optical micrographs of calcite marble (crossed polarizers).
Secondary electron images showing preferred locality of cavities in
fine-grained ultramylonite
Selected specimens of the shear zone were cut perpendicular to the shear zone
boundary and parallel to the stretching lineation (
In order to characterize microstructural development during progressive deformation of the fine-grained ultramylonitic layer, FIB foils were prepared along a profile across small calcite grains within the transition zone from coarser grained marble to the fine-grained ultramylonite. This enabled comparison of the defect structure within newly developed grains and those that have been further deformed within the ultramylonite layer. For analysis of microstructures within the ultramylonitic calcite, FIB foils showing a profile over at least two grains have been prepared. PIPS© was used to prepare specimens across comparable transitions in order to sample larger areas for comparison and integration with the FIB observations.
A Leica DM4500 P optical microscope has been used for selecting appropriate
thin sections for detailed microfabric analysis (Fig. 2). In order to
visualize the presence of cracks, cavities and grain boundary surface
character SEM was performed on a FEI Quanta 3-D FEG SEM equipped with an
EDAX Pegasus Apex 4 system consisting of a Digiview IV EBSD camera and an
Apollo XV silicon drift detector for EDX spectrometry at the University of
Vienna, Department of Lithospheric Research (Figs. 3, 5). Crystallographic
orientations have been measured by combined EBSD mapping and EDX spectrometry
(Fig. 4). The instrument was operated at a 10 kV accelerating voltage, a 4 nA
probe current at working distances between 10 and 14 mm. For EBSD analysis
the sample was tilted up to an angle of 70
SE images of broken specimens.
EBSD data were processed using the MATLAB© toolbox
for quantitative texture analysis MTEX (Bachmann et al., 2010). Orientation
distribution functions (ODFs) were calculated after Bunge (1982). The
orientation of the
TEM was performed with a JEOL 2011 STEM equipped with a double-tilt analytical holder and a Gatan MSC digital camera for imaging at the University of New Brunswick, Microscopy and Microanalysis Facility. The TEM was operated at an accelerating voltage of 200 kV in bright- and dark-field modes (Figs. 6, 7).
Typical TEM dislocation structures within the fine-grained
ultramylonite.
Dislocation densities were measured by use of the line-cut method. The
number of intersections between dislocations and a prescribed grid of
traverse lines was counted. The dislocation density
32 randomly chosen TEM bright field images were used in order to get a representative result. Dislocation densities were calculated for an average, maximum and minimum specimen thickness (Table S1 in Supplement).
The Syros ultramylonitic marble is characterized by quasi-equigranular to low
aspect ratio fine-grained calcite, ranging in grain size from
1 to 10
Calcite grains in the ultramylonitic layers show a weak crystallographic
preferred orientation (Fig. 4a; max.
Misorientation-angle distributions (Fig. 4b) of neighbour-pair and
random-pair grains are similar and track the calculated curve of
an ideal random misorientation-angle distribution calculated for trigonal
crystal symmetry reasonably. Minor variation in curves is visible for misorientations up
to 40
Almost no internal misorientation is visible within the grains displayed as a
misorientation deviation map wherein the misorientation over each grain with
respect to its average misorientation is plotted (Fig. 4c). Small,
(< 10
Secondary electron images (SEIs) of broken specimens show that the marble
preferentially fractures along grain boundaries with well-pronounced
crystal faces (Fig. 5a, b); only grains larger than 15
A heterogeneous grain microstructure occurs across the optical transition
zone (Fig. 6a, b). These variations correlate with distinctive defect
substructures observed by TEM. Coarser calcite grains
(> 10
In contrast to the transition zone, small grains within the fine-grained
ultramylonite contain abundant free dislocations (Fig. 7). Dislocation
multiplication by Frank–Read sources is observed throughout the ultramylonite
at grain boundaries (Fig. 7a); likewise, glide dislocations are commonly
concentrated or pinned at grain boundaries (Fig. 7b). Glide dislocations are
well developed on
The Syros ultramylonite exhibits a distinctive assemblage of attributes that require an internally consistent explanation. These important microstructural components are (1) extreme grain size reduction at low homologous temperature; (2) high-strain deformation of the resultant UFG calcite; (3) extensive dislocation glide with minimal recovery as a component of the high strain, concomitant with (4), the creation of a primary CPO in the UFG calcite; and (5) the development of nanopores. It is argued that these collectively reflect intense plastic deformation transitioning to GSS deformation comprising simultaneous operation of independent GBS and dislocation activity.
Extreme reduction in grain size can occur by brittle/frictional deformation (Blenkinsop, 1991; Mair and Abe, 2011) or solid-state dynamic recrystallization (White, 1973, 1977, 1982; Poirier and Nicolas, 1975; Etheridge and Wilkie, 1979; Sakai et al., 2014 amongst others). Although brittle fractures commonly act as loci for subsequent ductile displacement (e.g. Pennacchioni and Mancktlow, 2007) there is no indication of this behaviour during development of the ultramylonite. The microstructural evolution clearly demonstrates that dynamic recrystallization during intense plastic deformation is the grain-size-reducing mechanism (Rogowitz et al., 2014). Small calcite grains within the transition zone between the two ultramylonitic layers of contrasting grain size have low dislocation densities compared to adjacent coarser grains (Fig. 6c, d). The convex curvature of the grain boundary towards highly dislocated grains indicates grain boundary migration from the small grain towards the coarser grain indicative of bulging recrystallization, where the driving force for grain boundary movement is the contrast in internal strain energy between grains (Takeuchi and Argon, 1976; White and White, 1980; Sakai and Jonas, 1984; Platt and Behr, 2011). During grain boundary migration, the dislocation-rich volume is reorganized (consumed), resulting in a decrease in elastic strain energy. Dislocation reorganization into a new grain boundary (migration) is not inconsistent with concomitant increase in misorientation (rotation) between grains. The final “new” grain boundary will reflect the combined effect of dislocation incorporation and annihilation in the net lower energy defect configuration. Minor subgrains and bulges within coarser left-over grains in the UFG ultramylonite indicate continuous recrystallization of grains too big for sliding.
The ultra-fine-grained, recrystallized equigranular microstructure of the Syros ultramylonite is characteristic of materials exhibiting GSS deformation (Boullier and Guéguen, 1975; Schmid, 1976; Rutter, 1994). GSS flow results from the decreased diffusional mass transport distance through small grains (Nabarro–Herring creep), the enhanced diffusion rates through the integrated defects that are grain boundaries (Coble creep) and the reduced effective viscosity of grain boundaries by dislocation movements that is represented as independent GBS (Gifkins, 1976). Overall GBS reflects the role that grain boundaries play in accommodating very large macroscopic strains (Raj and Ashby, 1971; Ashby and Verrall, 1973; Gifkins, 1976; Langdon, 2009 and references therein). GSS flow has, in most materials, including rocks, been linked to a combination of sustained small grain size over large strains, low differential stress and homologous temperatures high enough to support diffusional accommodation of grain rearrangements during GBS (e.g. Ashby and Verrall, 1973; Boullier and Guéguen, 1975; Schmid, 1976). Only the small equant grain size and high strain are characteristics of the Syros ultramylonite. The primary CPO formed in the ultramylonite (Fig. 4a) contradicts the common anticipation that GSS deformation mechanisms will result in a randomization or loss in CPO intensity (Zhang et al., 1994; Bestmann and Prior, 2003; Storey and Prior, 2005).
A primary texture could, in principle, develop in one of three ways during GSS
flow. Deformation within the GSS creep field can be associated with grain
growth due to diffusion processes or surface-energy-driven coarsening
(Aktinson, 1988). In a non-isotropic stress field the growth of grains during
deformation will be stress directed and can result in a weak CPO (Schmid et
al., 1977; Bons and den Brok, 2000). The limited evidence for abundant grain
growth makes stress-directed growth a rather unlikely explanation for the
observed texture. The high deformation stress and strain rate conditions at
relatively low temperatures suppress any substantive crystal growth in the
UFG ultramylonite, with cyclic grain size reduction, maintaining the
grain size range between 10 and 1
A second explanation for the case of polyphase materials deformed by
diffusion (Coble) creep in combination with GBS is a CPO developing by
reorientation of grains towards a preferred alignment for interface reactions
(Heidelbach et al., 2000; Sundberg and Cooper, 2008). Deformation by GBS
would be consistent with the observed random misorientation-angle
distribution (Fig. 5b), indicating that no relation between neighbouring
grains exists, a typical pattern being observed for grains deformed by GBS
(Wheeler et al., 2001; Bestmann and Prior, 2003). However, the ultramylonitic
marble is essentially pure with only minor amounts of quartz, dolomite and
mica being stable over a wide range of
The third, and favoured explanation suggests the invoking of concomitant dislocation activity and GBS. Notwithstanding that much of the discussion of GBS in rocks has focussed on diffusional accommodation in the style of Ashby and Verrall (1973), there is, in principle, no preclusion of dislocation activity during GBS (Crossman and Ashby, 1975; Gifkins, 1976). At the same time, intracrystalline deformation by dislocation glide remains the most feasible explanation for CPO development.
There is long-standing evidence of rock deformation experiments that exhibit
mixed dislocation activity and GBS. Schmid et al. (1977) deformed Solnhofen
limestone at temperatures above 800
The sequence of microstructural development observed in this study constitutes mylonitization of the calcite marble, with grain size reduction by dynamic recrystallization producing relatively strain-free small grains as observed within the transition zone from coarse- to fine-grained ultramylonite (Fig. 6). Progressive deformation of fine-grained ultramylonitic layers results in extensive dislocation activity within the grains, including dislocation multiplication (Frank–Read sources) and development of high dislocation densities (Fig. 7), a microstructural combination that is distinct from the transition zone and demonstrates that the dislocations are not simply a late overprint pulse that introduced dislocations in all part of the structure. Rather, a substantive amount of intracrystalline strain must be accommodated by crystal plasticity within the fine-grained ultramylonite, which at the same time has attributes of GBS.
Minor low-angle boundaries in the fine-grained calcite are consistent with negligible thermally activated recovery associated dislocation creep. Nevertheless, the localization in the ultramylonite requires intense strain-softening. The dislocation networks and 200 nm cells observed in the ultramylonite are common to calcite deformed at low temperature (Kennedy and White, 2001; Vitale et al, 2007; Molli et al., 2011). The edge character of most network dislocations (picket fence arrays) and crystallographic alignment of cell walls is indicative of glide-mediated network formation that fills the role of climb-mediated recovery at low homologous temperature. Also, cross-slip has been argued to be active under these conditions (De Bresser and Spiers, 1990), enabling screw dislocations to overcome barriers to glide. The combined effects are to enhance strain accommodation and provide sufficient recovery to support continued ductile deformation. A substantive contribution of GBS in combination with the latter can explain the observed strain softening.
How can simultaneous dislocation activity and independent GBS be reconciled? As noted, such behaviour is not precluded by models of material behaviour, particularly that on which the core-and-mantle model of deformation (Gifkins, 1976), a central theme in geological deformation, is based. In the latter, the cores of grains act in a largely independent manner from grain mantles where dislocation densities are typically higher, as observed in our study. Within the grain mantles, dislocation motion and grain-boundary-scale deformation accommodates relative grain boundary displacements. Whereas the ensuing rotation and displacement of grains will change the crystal orientation relative to the preferred kinematically favoured orientation, there will be a concomitant impetus for reorientation by continued dislocation glide toward formation of a CPO; randomization of the fabric (controlled within grain mantles) is countered by simultaneous glide throughout the grain. The competition between GBS and glide would require continual change in the critical resolved shear stress on specific crystal planes, leading to the activation of different slip systems that would explain the various observed orientations (Fig. 7). The latter behaviour may require minerals with a sufficient number of alternate slip systems such as that provided by calcite.
The process of cavitation remains poorly understood despite the common observation of pores and cavities. Proposed mechanisms for the nucleation of cavities in polycrystalline material include vacancy condensation in a high stress region, the presence of second particles, dislocation pile-up at grain boundaries and GBS (Kassner and Hayes, 2003; Ovid'ko and Sheinerman, 2006; Fusseis et al., 2009; Rybacki et al., 2010). The preferential location of grain boundary openings orientated at a low angle to the shortening direction has already been observed by others in fine-grained polycrystalline material (Ree, 1994; Mancktelow et al., 1998) and has been associated with the activity of GBS. Shape and location of cavities within the fine-grained ultramylonite is very similar to cavities described by Ree (1994) and Ovid'ko et al. (2011) that result from grain neighbour switching during GBS. Opening of cavities occurs parallel to the shortening direction that are during further deformation partly closed due to further sliding of the grains, resulting in a small cavity at grain triple junctions. Therefore, the observed grain boundary openings and cavities at four-grain and triple junctions (Figs. 3a–d, 5) are most likely related to GBS. Dislocation glide, as described in Sect. 5.1, is able to close cavities by plastic deformation (Fig. 5e) while at the same time opening new cavities at a different locality, all of which contributes to the accommodation of GBS.
Contrasting the latter, small pores located on grain surfaces (Fig. 5e) are probably the result of minor grain boundary fluids or fluid-filled pores (Mancktelow et al., 1998). The observed small cavities having the same orientation and piling up at grain boundaries (Figs. 3g, h; 5f) might be interpreted as Zener–Stroh cracks, being the result of stress concentration due to dislocation pile-up at grain boundaries (Stroh, 1954, 1955; Fan and Xiao, 1997). Due to movement of edge dislocations on the same slip plane, the uniform orientation of cracks can be explained. Another possible explanation is stress concentration at obstacles or ledges at grain boundaries. Such features can result in stress concentration during grain sliding and lead to microcracking (Chan et al., 1986).
To test whether our interpretation of dislocation activity synchronous
with GBS resides in its compatibility with the predicted rheological
response, differential stress has been estimated using dislocation density and
recrystallized grain size. The experimental calibration of De Bresser (1996),
linking flow stress and dislocation density, has been used to estimate flow
stress conditions
Deformation mechanism map for calcite at 300
The deformation behaviour and strain rates predicted for the calculated
stresses were examined by constructing a deformation mechanism map for
calcite at 300
Detailed microstructure analysis of an ultramylonitic calcite layer has
generated a self-consistent interpretation on shear deformation, resulting in
major weakening at low temperatures and high strain rates. The processes
observed may be generally applicable to crustal deformation.
A highly strained ( The reduction in grain size enables simultaneous deformation by GBS and
dislocation activity. Network-assisted dislocation movement and cross-slip
aid in low-temperature recovery of the deforming calcite, given the
inadequacy of thermal-induced recovery (climb) to reduce strain hardening
effects. Stresses calculated by grain-size-dependent and dislocation-density-dependent
calibrations give similar estimates on the order of 180 MPa. The consistency
of the methods implies that post-deformational annealing was minimal. Strain rates on the order of 10 The switch from dislocation creep to simultaneous activation of GBS and
dislocation activity results in extreme strain softening and can be an
important strain localization process in calcite rocks, even at high strain
rate (
We thank the University of Vienna (grant number IK543002) for supporting the doctoral school DOGMA, the Austrian Science Fund (FWF): I471-N19 and the Natural Sciences and Engineering Research Council of Canada for a Discovery Grant (Fracture, Friction and Flow). We would like to thank Gerlinde Habler for FIB foil preparation and the University of New Brunswick Microscopy and Microanalysis Facility, particularly Louise Weaver, for support during TEM work. Many thanks to Luiz Morales, Luca Menegon and Benjamin Huet for valuable discussions. Detailed comments by the anonymous reviewer, Hans De Bresser and Michael Stipp are gratefully appreciated and helped a lot to improve the manuscript. Edited by: M. Stipp