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In the western United States, a major Tertiary extensional orogeny is distinguished from the more recent and subdued Basin and Range disturbance. This orogeny, characterized by dynamic horizontal translations (i.e., >100% extension in the Great Basin) occurred during the Eocene (-55-40 Ma) north of the Snake River Plain and during Oligocene-Miocene time (-35-16 Ma) farther south. Tectonic processes at shallow crustal levels included widespread listric or rotational faulting, decoupling along flat detachment faults, pervasive tensional fracturing, and calc-alkaline magmatism that yielded copious volumes of volcanic rocks and systematically oriented (north-northwest to north-northeast striking) dike swarms, veins, and elongate plutons. At deeper levels, the extensional deformation produced more dikes, intrusions, and the gently dipping mylonitic rocks exposed in Cordilleran metamorphic core complexes (MCC). Radiometric age criteria and ductile, kinematic strain indicators in the deeper rocks coincide with equivalent features in the brittly extended rocks above. The MCC are found in a regional setting of long antiformal axes of north to north-northwest trends, and transform discontinuities striking northeast to west-northwest, roughly parallel and normal, respectively, to the elongation of the MCC. Tertiary deformation in the complexes is commonly overprinted on earlier, gently dipping, compressional shear or metamorphic fabrics of Mesozoic to Paleocene age. Many MCC exhibit positive gravity anomalies and contain a preponderance of mafic dikes.Deep-seated, mylonitic, normal fault zones have recently been cited to explain the Cordilleran MCC. This simple-shear explanation differs from the crustal stretching or boudinage model. Although attractive conceptually, the crustal shear-zone model in its present form has difficulty in sufficiently explaining described deformational fabrics and unique upper-plate lithologies restricted to the mylonitic complexes. Major displacements required of the model appear to be precluded by certain geometric constraints. Strain analysis has also been confused by superimposed compressional and extensional fabrics. New data from the Picacho MCC in southern Arizona support relatively shallow, in situ, Miocene mylonitization and detachment, and document the importance of pre-Oligocene low-angle deformation.Various extensional mechanisms are proposed to explain flat detachment faulting. Mylonitic rocks exposed in MCC are derived from a setting of high heat flow and intrusion; preestablished, flat, crustal anisotropism and fluid-induced strain softening. Textural, isotopic, and geochemical evidence suggests that deuteric fluids were locally derived from the lower plate as a result of intense intergranular strain. These fluids, which concentrated or ponded at the detachment interface, may have enhanced upward mylonite development and are believed to be the principal cause for hydrothermal, 97 on March 28, 2016 specialpapers.gsapubs.org Downloaded from IV. A. Rehrig chloritic brecciation overlyi...

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. on July 7, 2015 memoirs.gsapubs.org Downloaded from

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