Geopic of the week: Reverse fault or normal fault?

Fault for geopic

Depicted above is a fault in the Jackfork Formation.  The fault is exposed in a quarry in southern Arkansas in the Ouachita Mountains.  Along the plane that separates the two blocks is about a one meter (3 ft) zone of highly fractured rock called fault gouge that formed due to the friction between the blocks as they moved.  The block on the left has moved upward relative to the one on the right; I could determine that because I was able to trace beds from one block to the other and see the sense of offset between them.

The beds on the left block are bent upward near the fault plane (center of photo); this implies that they have moved down relative to the block on the right – the opposite of what I said in the last paragraph.   Most likely this fault has experienced multiple episodes of movement, first thrusting upward due to compressional stress, and later sliding back down as the compression relaxed.  This kind of complex structure is typical of the greatly deformed rocks of the Ouachita Mountains.

To see more views of this fault click here

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