Processes of regional Tertiary extension in the western Cordillera: Insights from the metamorphic core complexes

Abstract

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...

Detachment zones of Cordilleran metamorphic core complexes: thermal, fluid and metasomatic regimes

“…The tectonic regime is characterized by extensional mylonite zones, with younger-on-older faults in the allochton, in contrast to the mylonites and faults of the classic older-on-younger thrust and fold belts. Twenty-five such core complexes have been identified to date, discontinuously arrayed in a belt along the axis of the North American Cordillera, from northwest Mexico to British Columbia (DAVIS, 1980;DAVIS et al, 1980;REHRIG, 1986;REHRIG & REYNOLDS, 1980;REYNOLDS & REHRIG, 1980, for an overview see CONEY, 1980). Metamorphic core complexes are interesting from the standpoint of fluid regimes accompanying crustalscale deformation.…”

Fission-track evidence for Cenozoic uplift of the Nelson batholith, southeastern British Columbia

“…Estimates of uplift in the Snake Range of Nevada range from 6 or 7 km (Gans and Miller 1983;Miller et al 1983) to 15 km (Bartley and Wernike 1984) and are compatible with the amount of Eocene uplift suggested for the Valhalla complex (Parrish et al 1988) and suggested here for the Nelson batholith. In contrast with some of the metamorphic core complexes in the southwestern United States (Rehrig 1986), roof rocks of Canadian core complexes, such as the Valhalla and Okanagan complexes, appear to have been affected by significant uplift and thermal disturbance, probably as the result of their formation at deeper crustal levels. Core complexes of the southwestern United States are thought to have formed at shallow crustal levels as a result of high heat flow, shallow intrusion, and fluid-induced strain softening (Rehrig 1986).…”

Volcanism Studies: Final Report for the Yucca Mountain Project

“…The onset of the extensional deformation is emphasized rather than the age of continued faulting. In many areas of the Basin and Range province, extensional faulting may have begun in the Oligocene (Proffett, 1977;Crowe, 1978;Crowe et al, 1979;Rehrig, 1986;Snoke and Miller, 1988;Gans et al, 1989;). …”