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Clue: This mylonite was strongly deformed in a fault zone separating moving
blocks of rock. The two prominent muscovite flakes were originally rectangular.
Can you tell which way the rock at the right was moving relative to that at the
left?
Hint: Imagine modeling clay containing flakes of aluminum foil being
squeezed between two blocks moving in opposite directions. The clay near
each block moves with the block. Foil flakes are bent and dragged by the
movement. The mylonite is like the clay and the muscovite is like the foil.
Answer: The rock at the right was moving up relative to the rock at the left.
Clues like this one help to determine which way rocks moved along ancient
faults.
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