Abstract

Three metamorphic core complexes, Ruby Mountains-East Humboldt Range (R-EH), Albion Mountains-Raft River Mountains-Grouse Creek Mountains (A-RR-GC), and Snake Range (SR) are exposed in the northeastern Great Basin (Nevada, Utah, and Idaho). Their structural, magmatic, and metamorphic histories are synthesized and compared to evaluate fundamental tectonic processes in this area in which large-magnitude crustal contraction was followed by large-magnitude crustal extension. Throughout the region, contraction and magmatism began in the Late Jurassic, and in the R-EH and SR areas culminated at ∼80–90 Ma. The A-RR-GC core complex experienced multiple episodes of Late Cretaceous tectonic denudation beginning at ∼105 Ma, and it records no Mesozoic magmatism, apparently due to an infertile lower crust. The R-EH core complex also underwent tectonic denudation in latest Cretaceous/early Tertiary time. This illustrates the importance of syncontractional adjustments to thickened orogenic wedges and that these adjustments need not occur simultaneously throughout an orogen. Syncontractional extension and denudation in the A-RR-GC and R-EH areas also limited the amount, extent, and distribution of subsequent Tertiary extension.

Eocene magmatism was immediately followed by an intense pulse of crustal extension in all three areas. However, late Oligocene and early Miocene extension throughout the region was diachronous and was not always coincident with a widespread mid-Oligocene magmatic event, suggesting that Oligocene magmatism and extension are not entirely linked. The magnitude of tectonic denudation in the R-EH and SR core complexes varies along the strike of the core complex. In the R-EH area, this variation may be due to the restriction of Late Cretaceous(?) and middle Eocene episodes of extension to the northern part of the core complex. In the SR core complex this variation may be the result of a lateral ramp in the structural surface of the Snake Range décollement. These intra-core complex variations in tectonic denudation appear to be partly accommodated by solid-state flow of the lower crust into regions of high mid- to upper-crustal extension, and they require a complex, three-dimensional pattern of lower-crustal redistribution and magmatism. Oppositely vergent extensional fault systems in the A-RR-GC metamorphic core complex may have developed because the rigid Archean lower crust in this area could not flow into highly extended domains.

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