SEM images of cataclastic fault rock evolution

The SEM electron channelling (EC) photomicrographs shown below provide an alternative illustration to conventional polarised light and cathodoluminescence (CL) microscopy of the classic cataclasite fault rock evolution in Cambrian Pipe Rock Quartzite (Skiag Bridge back thrust fault) via grain size reduction due to fracturing to produce a fine grain size fault rock. In particular, EC orientation contrast highlights changes in crystal orientation and hence shows up the intracrystalline (low temperature) plasticity component inherent in cataclasis. A related image, known as electron channelling patterns (ECP), due to electron diffraction, not only defines the precise crystallographic orientation but also a qualitative indication of the amount of cold work hardening present in terms of pattern quality deterioration.

The photomicrographs are viewed towards NE and were cut normal to fault plane and parallel to movement direction; where no scale is shown the field of view is 2mm.

For other examples see: optical, CL, EBSD, TEM, indentation.

For further details, see: Lloyd and Knipe. 1991, Journal of Structural Geology; Knipe and Lloyd, 1994, Pure and Applied Geophysics 142, 229-254; Lloyd 1987, Mineralogical Magazine 51, 3-19.
 
 

Progressive development of cataclasite as imaged via SEM electron channelling (EC) orientation contrast The microstructural images of the cataclastic fault rock developemnet in quartzite in affected by the Skiag Bridge backthrust fault illustrated on this page were taken using SEM/EC/OC and reflect therefore changes in crystallographic orientation (Lloyd et al. 1987). Thus, rather than the fracture and diffusive mass transport (DMT) dominated microstructures as revealed by both optical and SEM CL, here we see the relationship between intracrystalline (low temperature) crystal plasticity, fracture and DMT deformation processes and microstructures.
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	Original Pipe Rock Quartzite unaffected by faulting. Note some intragranular deformation due to compaction, including dauphin (i.e. penetration) twins (i.e. dark-bright contrast variations.		Classic appearance of dauphine twinning due to deformation caused by stress concentrations at grain contacts due to intergrain indentation.
	Right. Intragranular LTP 'shear zone' (central grain) due to indentation by lower grain; note displacement of grain boundary accommodated by the dextral shear sense.  Left. Detail of 'shear zone'; the 'stripes' share dauphine twin relationship.		The classic intergranular LTP 'indentation-related' microstructure in quartz. An inherited and/or initial microstructure (including dauphine twins) in the central grain has been overprinted by deformation lamellae that originate from intergrain contacts; several distinct lamellae stes are therfore distinguished. The central grain has indented the top grain to produce an arcuate array of subgrains; this comminuted 'grain size' has profound implications for local DMT processes.
	LTP intragranular deformation lamellae caused by indentation of the lower right grain into the central grain. In detail, the lamellae may be microstructurally analogous to 'chevron folds' (dauphine twins?) or imbricate thrust stacks.		Cataclastic 'seam'; note variations in fragment size and relict quartz grains in the adjacent wall rock, one of which (lower right) is 'spalling' fragments into the 'seam'.
	Right. Wall rock grain adjacent to cataclastic seam (top); note intragranular fractures (cataclasite) that will eventually allow the top of this grain to 'spall' into the main cataclastic seam.  Left. Detail showing intragranular fracture array cutting learlier LTP subgrains caused by indentation (see opposite image).		Detail of images opposite, showing fracture bounded 'tilt blocks' that overprint  LTP subgrain boundaries caused by earlier indentation. The fractures have the appearance of a propagating crack-tip (from right-to-left), indicating dextral offset of the main subgrain boundary. The LTP therfore may represent crack-tip plasticity associated with an earlier stage of propagation.
	Crystal plastic microstructure of the main cataclastic fault rock. Note contrast variations  due to differences in fragment sizes that impart a 'foliated' appearance		Electron channeling patterns from different grains progressively closer (lower left, lower right, top right) to a cataclastic seam show progressively poorer quality due to increased cold work hardening.