Microstructures

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Shear Sense Indicators:
Microfolds and Ultramylonites

Click image to enlarge

78. Microfolds in ultramylonite - Use of the vergence of microfolds as a shear sense indicator can be problematic unless the complete fold structure is visible. In this right-lateral, ultramylonitic granodiorite from the Borrego Springs mylonite zone, folds at top left and lower left of image show an overall vergence to the right which is consistent with the sense of shear from sigma grains (the left side of a large sigma feldspar porphyroclast and its tail are visible at lower right). The upper part of the section is ultramylonitic - that is, extremely fine-grain size and almost total recrystallization and/or reaction of all original igneous minerals. Deformation was at middle greenschist facies.

FOV 3.2 mm, Plane Light.

79. Microfolds in ultramylonite - In this left-lateral, ultramylonitic granodiorite sample from the Borrego Springs mylonite zone, the folds at base of image are asymmetric, with vergence to the left, but they are more upright than is usual for microfolds in ultramylonite zones. This is because a large, rigid feldspar porphyroclast to the left and off the image caused a disruption in the flow. Note that the small, isolated fold hinges at left center of image are not useful as shear sense indicators because the vergence of a fold with such short limbs is ambiguous.

FOV 2.0 mm, Plane Light.

80. Heterogeneous deformation - Sense of shear is very difficult to determine in this heterogeneously strained granodiorite from the Borrego Springs mylonite zone. The few porphyroclasts in the dark, fine-grained, biotite-rich ultramylonite at top of image do not have any discernible tails, and there is no asymmetry of texture within the matrix. Even in the mica-poor, light-colored lower part of the image, mutual interference between feldspar porphyroclasts makes them difficult to use for sense of shear determination. There are, however, two or three right-lateral shear bands in the lower part of the image. Deformation was at middle greenschist facies.

FOV 2.0 mm, Plane Light.

81. Sigma grains in ultramylonite - This sample of ultramylonite, also from the Borrego Springs mylonite zone, shows several sigma and one delta grain (at extreme left of image) within the ultramylonite zone (top two-thirds of image), all of which indicate a left lateral movement. The lower part of image is deliberately over-exposed in order to see the detail in the very dark, ultra-fine grained ultramylonite. Deformation was at low to middle greenschist facies.

FOV 3.2 mm, Nicols Crossed.

82. Delta and tail-less grains in ultramylonite - At first glance, this upper greenschist facies, ultramylonitic diorite from the Borrego Springs mylonite zone looks very like a cataclasite. The ultra-fine grained matrix is dark and structureless. Porphyroclasts of amphibole are angular (bottom left of image) to subrounded (central zone of image). However, all feldspar porphyroclasts are rounded and although many have no tails (right center of image), several have very thin, almost invisible tails (upper center of image) that indicate they are delta grains (see image # 86 for a good example of a delta grain).

FOV 1.5 mm, Plane Light.

83. Feldspar grains in ultramylonite - Same specimen as image #82, here shows extremely narrow mantles of recrystallized new grains around feldspar porphyroclasts, which indicate a right-lateral shear sense.

FOV 1.5 mm, Nicols Crossed + Gypsum Plate.

Shear Sense Indicators: Microfolds and Ultramylonites	Click image to enlarge
78. Microfolds in ultramylonite - Use of the vergence of microfolds as a shear sense indicator can be problematic unless the complete fold structure is visible. In this right-lateral, ultramylonitic granodiorite from the Borrego Springs mylonite zone, folds at top left and lower left of image show an overall vergence to the right which is consistent with the sense of shear from sigma grains (the left side of a large sigma feldspar porphyroclast and its tail are visible at lower right). The upper part of the section is ultramylonitic - that is, extremely fine-grain size and almost total recrystallization and/or reaction of all original igneous minerals. Deformation was at middle greenschist facies. FOV 3.2 mm, Plane Light.
79. Microfolds in ultramylonite - In this left-lateral, ultramylonitic granodiorite sample from the Borrego Springs mylonite zone, the folds at base of image are asymmetric, with vergence to the left, but they are more upright than is usual for microfolds in ultramylonite zones. This is because a large, rigid feldspar porphyroclast to the left and off the image caused a disruption in the flow. Note that the small, isolated fold hinges at left center of image are not useful as shear sense indicators because the vergence of a fold with such short limbs is ambiguous. FOV 2.0 mm, Plane Light.
80. Heterogeneous deformation - Sense of shear is very difficult to determine in this heterogeneously strained granodiorite from the Borrego Springs mylonite zone. The few porphyroclasts in the dark, fine-grained, biotite-rich ultramylonite at top of image do not have any discernible tails, and there is no asymmetry of texture within the matrix. Even in the mica-poor, light-colored lower part of the image, mutual interference between feldspar porphyroclasts makes them difficult to use for sense of shear determination. There are, however, two or three right-lateral shear bands in the lower part of the image. Deformation was at middle greenschist facies. FOV 2.0 mm, Plane Light.
81. Sigma grains in ultramylonite - This sample of ultramylonite, also from the Borrego Springs mylonite zone, shows several sigma and one delta grain (at extreme left of image) within the ultramylonite zone (top two-thirds of image), all of which indicate a left lateral movement. The lower part of image is deliberately over-exposed in order to see the detail in the very dark, ultra-fine grained ultramylonite. Deformation was at low to middle greenschist facies. FOV 3.2 mm, Nicols Crossed.
82. Delta and tail-less grains in ultramylonite - At first glance, this upper greenschist facies, ultramylonitic diorite from the Borrego Springs mylonite zone looks very like a cataclasite. The ultra-fine grained matrix is dark and structureless. Porphyroclasts of amphibole are angular (bottom left of image) to subrounded (central zone of image). However, all feldspar porphyroclasts are rounded and although many have no tails (right center of image), several have very thin, almost invisible tails (upper center of image) that indicate they are delta grains (see image # 86 for a good example of a delta grain). FOV 1.5 mm, Plane Light.
83. Feldspar grains in ultramylonite - Same specimen as image #82, here shows extremely narrow mantles of recrystallized new grains around feldspar porphyroclasts, which indicate a right-lateral shear sense. FOV 1.5 mm, Nicols Crossed + Gypsum Plate.