Microstructures

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Shear Sense Indicators:
False Foliation

Click image to enlarge

87. False foliation and delta grain - The two large feldspar porphyroclasts in this Borrego Springs ultramylonitic granodiorite have asymmetrical 'halos' of fine grained white mica. The mica (001) planes at top right and lower left of the porphyroclasts are facing the maximum incremental shortening direction; sense of shear is left lateral. Note that the right-hand grain is a delta grain. It has recrystallized tails of feldspar that step-up to the left.

FOV 3.2 mm, Nicols Crossed.

88. False foliation - Matrix grain size in this ultramylonitic pegmatite from the Borrego Springs zone is relatively coarse in comparison to the other ultramylonitic rocks in this series, yet the feldspar porphyroclasts do not have any recrystallized feldspar tails. Deformation was at low to middle greenschist facies and involved reaction of feldspar to white mica plus quartz. The feldspar porphyroclast at center of image has a small patch of recrystallized quartz in a pressure shadow area at left side. White mica flakes are oriented with their (001) planes facing the maximum incremental shortening direction and are concentrated at the top right and bottom left of each grain, consistent with left lateral shear.

FOV 3.2 mm, Nicols Crossed.

Shear Sense Indicators: False Foliation	Click image to enlarge
87. False foliation and delta grain - The two large feldspar porphyroclasts in this Borrego Springs ultramylonitic granodiorite have asymmetrical 'halos' of fine grained white mica. The mica (001) planes at top right and lower left of the porphyroclasts are facing the maximum incremental shortening direction; sense of shear is left lateral. Note that the right-hand grain is a delta grain. It has recrystallized tails of feldspar that step-up to the left. FOV 3.2 mm, Nicols Crossed.
88. False foliation - Matrix grain size in this ultramylonitic pegmatite from the Borrego Springs zone is relatively coarse in comparison to the other ultramylonitic rocks in this series, yet the feldspar porphyroclasts do not have any recrystallized feldspar tails. Deformation was at low to middle greenschist facies and involved reaction of feldspar to white mica plus quartz. The feldspar porphyroclast at center of image has a small patch of recrystallized quartz in a pressure shadow area at left side. White mica flakes are oriented with their (001) planes facing the maximum incremental shortening direction and are concentrated at the top right and bottom left of each grain, consistent with left lateral shear. FOV 3.2 mm, Nicols Crossed.