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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, chloritic brecciation overlying the mylonites. The MCC were uplifted by isostatic response to upper-plate denudation, lower-plate attenuation, and magmatic upwelling from below.

In terms of regional or plate tectonic setting, the extreme extension of the Tertiary orogeny is attributed to the incursion of hot asthenosphere into the Cordilleran crust above a segmented and sinking subduction slab. During the preceding Laramide orogeny, this oceanic slab had been driven shallowly under the North American crust with essentially no intervening mantle wedge. After about 40 Ma, dehydration fluids from the descending slab triggered magmatism throughout the Cordillera when the lower crust was contacted by hot asthenosphere. Mantle diapirism, converting upward and laterally, became the fundamental mechanism for crustal softening and extension. The metamorphic core complexes may therefore represent local sites where mafic subcrustal material penetrated highest in the crust, causing the most visible effects of attenuation. As such, the MCC can be visualized as small-scale analogs of the extended Cordillera. It is likely that flat, stacked, en echelon mylonitic zones exist at deeper levels throughout much of the western United States.

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