The cathodoluminescence (CL) photomicrographs (both optical microscope and SEM derived) shown below provide an alternative illustration to conventional polarised light 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, CL highlights intragranular microfractures and authigenic quartz precipitation (usually dark contrast). Note that optical CL images are true colour, but SEM CL images are merely contrast (signal strength) grey scale variations.
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, SEM, EBSD, TEM, indentation.
For further details, see: Lloyd and Knipe. 1991, Journal of Structural Geology, and Knipe and Lloyd, 1994, Pure and Applied Geophysics.
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Relatively undeformed quartzite far from the main fault with a single through-going microcrack. Note: diffuse crack tip termination (symbol left) and step between grains (arrowed right). The effective crack width (double arrow) if shearing occurs will be greater than the width observed. |
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Cataclastic fault rock (right) and a relict fragment of original quartzite (left) from the main displacement plane. Note: healed (dark) through-going microcracks in the relict fragment; 'spalling' of grains on the edge of the fragment into the cataclastic zone, resulting in a weakly foliated (on the basis of original CL colour index and hence parental grain affinity) cataclasite. |
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SEM CL image of the wall rock quartzite. Note the intensity of healed intragranular microcracks and the concentration of deformation at intergrain contacts due to stress concentration. Same scale as image below. |
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SEM CL image of a narrow cataclastic 'seam' developed in wall rock quartzite. Note: foliated appearance of the cataclasite due to CL contrast affinity with the CL contrast of the parental grain; and the 'spalling' of parental grains on the edge of the seam into the cataclastic fault rock. |
Progressive development of cataclasite as imaged via optical CL |
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Undeformed quartzite |  |
The first stages of grain break-up ('shear displacement') exploiting the 'intragranular cataclasite' |
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First indications of deformation: intra-granular microcracks propagating from grain contacts |  |
Broken-up grains: proto cataclasite (note: bright specks are diamond abrassive paste used for polishing) - clast supported cataclasite |
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More intense intragranular microcrack development |  |
Fragments of original grains in a fine grained matrix - gouge supported cataclasite |
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'Intragranular cataclasite' |  |
Dominantly fine grained cataclasite with only rare fragments |
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More intense development of 'intragranular cataclasite' |  |
Pure fine grained cataclasite - the CL colour now indicates differences in size of the gouge |
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