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In the Leppy Hills, a series of 11.6 Ma volcanic flows is separated from the underlying Permian limestone by an angular unconformity of about 15°, indicating the Paleozoic sedimentary rocks dipped gently and the area had low topographic relief as little as 11.6 Ma. Faulting ended by the Pleistocene, for Quaternary deposits overlap the faults.
Most of the rocks in the Leppy Range dip 10°-60° west. Normal faults are common and generally trend north. During extension, the limestone beds deformed brittlely and faulted while shaly beds were stretched ductilely. The fault surfaces range from distinct planar zones with narrow gouge zones to large brecciated zones. Commonly, breccia fragments are covered with concentric bands of fibrous calcite that indicate a cavity filling. Calcite-filled extension fractures increase in abundance toward fault zones. Locally, up to 25% of the rock volume is veined. Some faults up to 100 m (330 ft) long curve to become subparallel to bedding, producing a spoon-shaped geometry, with the bowl of the spoon facing upward. Multiple generations of faulting have subsequently rotated faults and eds. Space problems along curved faults on the tens of meters scale were accommodated by: (a) intense brecciation and formation of calcite-filled veins or voids, (b) cataclastic flowage of limestone or ductile flowage of shale into the space, or (c) antithetic faulting or "passive listric folding" of the hanging wall.
The extension in the Leppy Hills, calculated along the simplest cross section, is at least 70%. In summary, the complicated faulting history resulted from crustal extension since the Miocene.
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