Shear Sense Indicators:
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Click image to enlarge |
84. Delta-type porphyroclasts in ultramylonite
- Upper greenschist facies, ultramylonitic grano-diorite from the Borrego Springs
zone contains several d-type porphyroclasts in addition
to the more numerous tail-less grains. Two d-type grains
at right of center are shown in close-up in image #85. Sense of
shear is left-lateral.
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85. Delta-type porphyroclasts in ultramylonite
- Close-up of d-type porphyroclasts in image
# 84. On both of the lower grains there are incipient tails that start to leave
the grains at their upper right and lower left edges. Unlike s-type
tails, these d-tails do not extend from the top of the
grain parallel to foliation, but instead they sweep down from the top of the grain
to lower right, and from the base of grain to upper left, consistent with left lateral
shear.
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86. Delta grain in ultramylonite - Sense of shear is right lateral in this mylonitic granite from the Pinaleño
Mountains, Arizona; deformation was at middle greenschist facies. Tails of recrystallized
feldspar leave the feldspar porphyroclast at top right and lower left edges, but
do not immediately extend parallel to foliation, as would sigma tails. The delta
tails sweep from top left to lower left, and from lower right to upper right, and
in each case the tails cross an imaginary median line drawn through the center of
the porphyroclast, parallel to foliation. Thus the tails step-up to the right once
they are away from the porphyroclast's influence, consistent with dextral shear. Click here to view Flash animation in a new window. |
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