Microstructures

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Shear Sense Indicators:
S-C mylonites and Shear Bands

Click image to enlarge

73. S-C mylonite - This example of a Type I (Lister and Snoke, 1984) S-C mylonite is from deformed granodiorite of the Borrego Springs mylonite zone. The sample contains quartz-biotite-plagioclase (An 30) - chlorite - epidote - white mica - titanite and deformation occurred under middle greenschist facies conditions. The main S foliation in the rock dips to the right in this image (ca. 60°east in outcrop) and is deflected into narrow, discontinuous C planes which are subhorizontal in the image (they dip east at ca. 30° in outcrop). Large, rounded, feldspar porphyroclasts have recrystallized tails on their top left and lower right sides (they are said to 'step up' to the left). Quartz and biotite form ribbon-like grains that help define both the S and C planes.

The term 'S-C mylonite' is used here, rather than 'shear bands', because field observations show that both the S and the C planes die away at the edge of the main deformation zone, and both are present throughout the zone. Thus there is reason to believe that the two foliations formed during the same deformation event. Where the timing of formation of the foliation and the little shear zones that cut it are not known, the structures are referred to as 'shear bands'.

FOV 6.4 mm, Plane Light.

74. sb-type porphyroclast in S-C mylonite - Detail of image #73, showing sigma grain with tails of recrystallized feldspar on upper left and lower right, consistent with left-lateral shear. Porphyroclasts that occur close together in S-C mylonites like this one may mutually interfere during deformation and are called sb-type, to distinguish them from isolated sigma grains (e.g., images #89 and #90) that interact only with their matrix.

FOV 3.2 mm, Plane Light.

75. sb-type porphyroclast in S-C mylonite - This Type I S-C mylonite from deformed granodiorite of the Borrego Springs mylonite zone, shows well-developed C-planes which are subhorizontal in top half of image and dip slightly to the left in lower half of image. S planes have rotated to near-parallelism with C planes in top half of image, but are still visible dipping to the right in image's lower half. Most of the sb feldspar grains have tails of biotite and chlorite that extend from the narrow end of the grain in a stair-stepping manner, consistent with the left-lateral shear. The feldspar porphyroclast at top center has its long axis oriented at a high angle to foliation, and is tilted in the opposite sense to the other grains in the image. Note, however, that its tails of biotite and chlorite are consistent with those on other grains, except that they are growing from the broad side of the grain rather than from its narrow ends. Deformation was at lower greenschist facies

The explanation for this anomalous grain is that it has rotated backwards with respect to the left lateral shear, whereas its tails have behaved like the matrix and like the other grains which have rotated forwards, with the shear sense (see alsoimage # 94). Back-rotation indicates that the shear zone did not undergo simple shear only, but also had a component of pure shear acting across it. In pure shear only, half of a population of rigid grains would rotate one way and half the other way. Where there is general shear (pure plus simple shear), as in this case, most of the grains rotate with the simple shear component of strain, but a few grains in appropriate orientations will rotate backward, in response to the pure shear component.

FOV 2.9 mm, Plane Light.

76. Shear bands - This shear band in mylonitic Orocopia Schist is from the Vincent-Orocopia fault zone in southeastern California. Deformation was at uppermost greenschist facies; sense of shear right lateral. Foliation planes, dipping to the left, are picked out by the bright interference colors of the muscovite. Where they are deflected into the right-lateral shear band, which runs from top left to right center of image, muscovite grains are in extinction and difficult to see.

The term 'shear band' is used here, rather than 'S-C mylonite', because the timing of formation of the foliation and of the shear band that cuts it are not known.

FOV 6.4 mm, Nicols Crossed.

77. S-C mylonite - S-C mylonite from deformed granodiorite of the Borrego Springs mylonite zone, shows the deflection of the S planes (dipping to right) into a left-lateral C plane (horizontal). Deformation was at middle greenschist facies. Quartz ribbon grains in the S planes (light colored on right of image) become narrower and undergo additional grain boundary recrystallization as they are deflected into the zone of the C plane. Note that all minerals, including mica and feldspar, are reduced in grain size as they turn into the C plane.

FOV 1.5 mm, Nicols Crossed.

Shear Sense Indicators: S-C mylonites and Shear Bands	Click image to enlarge
73. S-C mylonite - This example of a Type I (Lister and Snoke, 1984) S-C mylonite is from deformed granodiorite of the Borrego Springs mylonite zone. The sample contains quartz-biotite-plagioclase (An 30) - chlorite - epidote - white mica - titanite and deformation occurred under middle greenschist facies conditions. The main S foliation in the rock dips to the right in this image (ca. 60°east in outcrop) and is deflected into narrow, discontinuous C planes which are subhorizontal in the image (they dip east at ca. 30° in outcrop). Large, rounded, feldspar porphyroclasts have recrystallized tails on their top left and lower right sides (they are said to 'step up' to the left). Quartz and biotite form ribbon-like grains that help define both the S and C planes. The term 'S-C mylonite' is used here, rather than 'shear bands', because field observations show that both the S and the C planes die away at the edge of the main deformation zone, and both are present throughout the zone. Thus there is reason to believe that the two foliations formed during the same deformation event. Where the timing of formation of the foliation and the little shear zones that cut it are not known, the structures are referred to as 'shear bands'. FOV 6.4 mm, Plane Light.
74. sb-type porphyroclast in S-C mylonite - Detail of image #73, showing sigma grain with tails of recrystallized feldspar on upper left and lower right, consistent with left-lateral shear. Porphyroclasts that occur close together in S-C mylonites like this one may mutually interfere during deformation and are called sb-type, to distinguish them from isolated sigma grains (e.g., images #89 and #90) that interact only with their matrix. FOV 3.2 mm, Plane Light.
75. sb-type porphyroclast in S-C mylonite - This Type I S-C mylonite from deformed granodiorite of the Borrego Springs mylonite zone, shows well-developed C-planes which are subhorizontal in top half of image and dip slightly to the left in lower half of image. S planes have rotated to near-parallelism with C planes in top half of image, but are still visible dipping to the right in image's lower half. Most of the sb feldspar grains have tails of biotite and chlorite that extend from the narrow end of the grain in a stair-stepping manner, consistent with the left-lateral shear. The feldspar porphyroclast at top center has its long axis oriented at a high angle to foliation, and is tilted in the opposite sense to the other grains in the image. Note, however, that its tails of biotite and chlorite are consistent with those on other grains, except that they are growing from the broad side of the grain rather than from its narrow ends. Deformation was at lower greenschist facies The explanation for this anomalous grain is that it has rotated backwards with respect to the left lateral shear, whereas its tails have behaved like the matrix and like the other grains which have rotated forwards, with the shear sense (see alsoimage # 94). Back-rotation indicates that the shear zone did not undergo simple shear only, but also had a component of pure shear acting across it. In pure shear only, half of a population of rigid grains would rotate one way and half the other way. Where there is general shear (pure plus simple shear), as in this case, most of the grains rotate with the simple shear component of strain, but a few grains in appropriate orientations will rotate backward, in response to the pure shear component. FOV 2.9 mm, Plane Light.
76. Shear bands - This shear band in mylonitic Orocopia Schist is from the Vincent-Orocopia fault zone in southeastern California. Deformation was at uppermost greenschist facies; sense of shear right lateral. Foliation planes, dipping to the left, are picked out by the bright interference colors of the muscovite. Where they are deflected into the right-lateral shear band, which runs from top left to right center of image, muscovite grains are in extinction and difficult to see. The term 'shear band' is used here, rather than 'S-C mylonite', because the timing of formation of the foliation and of the shear band that cuts it are not known. FOV 6.4 mm, Nicols Crossed.
77. S-C mylonite - S-C mylonite from deformed granodiorite of the Borrego Springs mylonite zone, shows the deflection of the S planes (dipping to right) into a left-lateral C plane (horizontal). Deformation was at middle greenschist facies. Quartz ribbon grains in the S planes (light colored on right of image) become narrower and undergo additional grain boundary recrystallization as they are deflected into the zone of the C plane. Note that all minerals, including mica and feldspar, are reduced in grain size as they turn into the C plane. FOV 1.5 mm, Nicols Crossed.