ABSTRACT

Stratigraphic separation on the western boundary fault of the Franklin Mountains is about 10,000 ft (3,000 m) at the south end of the range and possibly 13,000 ft (4,000 m) in the central part of the range. Exposures of three major fault surfaces in the western boundary fault zone at one place show that two faults dip vertically with major drag and little breccia, and one dips 45°E with little drag and much breccia. In the other exposure of the boundary fault with stratigraphic separation of about 13,000 ft (4,000 m) dip-slip slickensides on the west side of the breccia zone plunge 45°E.

The vertical fault surfaces formed early in the faulting; the east-dipping reverse fault occurred late. At least two geometric interpretations can be made: (1) the major fault zone dips east and is reverse; (2) the major fault zone is an upthrust like those that bound many major uplifts in the central and southern Rocky Mountains.

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