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Rocks in the South Mountains of central Arizona are representative of rocks found in metamorphic core complexes elsewhere in Arizona. These core-complex terranes are in part characterized by low-angle mylonitic foliation that contains penetrative northeast-trending mineral lineations and pervasive smearing out of mineral grains. In the South Mountains, mylonitic rocks form a doubly plunging, northeast-trending foliation arch and have been derived from Precambrian amphibolite gneiss and a composite mid-Tertiary pluton. The pluton is undeformed in the core of the arch, but shows a progressive increase in pervasiveness of mylonitic fabric up structural section. Mylonitic plutonic rocks are exposed as a carapace overlying their less-deformed equivalents. Mylonitically foliated Precambrian amphibolite gneiss is restricted to a zone underlain and overlain by nonmylonitic (crystalloblastic) gneisses that are lithologically identical and largely retentive of their Precambrian foliation.

Fabrics in mylonitic rocks indicate extension parallel to the east-northeast–trending lineation and flattening perpendicular to the gently dipping foliation. The fabric is well dated as late Oligocene to early Miocene (25 to 20 m.y. B.P.). Mylonitic deformation was followed by more brittle deformation which produced a chloritic breccia that overlies mylonitic plutonic rocks in the northeast half of the arch. The chloritic breccia is probably related to normal faulting along a low-angle dislocation surface.

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