VOLUME 42 NUMBER 1

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Comparative anatomy of core-complex development in the northeastern Great Basin, U.S.A.

Walter A. Sullivan^* and Arthur W. Snoke

Department of Geology and Geophysics, Dept. 3006, 1000 East University Avenue, University of Wyoming, Laramie, WY 82071, U.S.A.

 

*Correspondence should be addressed to: wasulliv@uwyo.edu

 

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 8090 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 dcollement. 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.

Key Words: metamorphic core complex • crustal contraction and extension • hinterland • Sevier orogenic belt • Albion Mountains • East Humboldt Range • Grouse Creek Mountains • Raft River Mountains • Ruby Mountains • Northern Snake Range • Southern Snake Range • Great Basin • Basin and Range province

Field rheology and structural evolution of the Homestake shear zone, Colorado

Colin A. Shaw^1,* and Joseph L. Allen²

¹ Department of Earth Sciences, Montana State University, Bozeman, MT 59717, U.S.A.
² Department of Geology and Physical Sciences, Concord University, Athens, WV 24712, U.S.A.

^* Correspondence should be addressed to: colin.shaw1@montana.edu

Proterozoic tectonites within the Homestake shear zone (HSZ) in the northern Sawatch Range, Colorado, record two major cycles of progressive deformation under contrasting PT conditions. Field relations and microstructures in these tectonites can be used to constrain the geometry, kinematics, and rheology of rocks in the HSZ during these events. The first deformation cycle occurred 1.7 Ga at temperatures near the granite solidus and produced structures indicative of pervasive viscous flow at low differential stress levels. The second deformation cycle at 1.4 Ga resulted in localized plastic and brittle failure concentrated in relatively narrow shear zones and seismogenic faults. This second deformation occurred at temperatures of 350450C and produced mylonite, ultramylonite, and pseudotachylyte. Overprinting relationships between brittle-frictional faults and plastic shear zones suggest that both modes of deformation were active at about the same time. Thus, the latter deformation cycle probably occurred near the brittle-plastic transition. Simple calculations using standard rheologic relationships show that tectonic loads of 300400 MPasufficient to drive brittle failurecould have been transmitted by ductile ultramylonite zones deforming at relatively high strain rates (on the order of 10⁹10¹¹). Thus, plastic shear zones and brittle-frictional faults probably acted together as a dynamically coupled deformation system.

Key Words: Homestake shear zone • Colorado mineral belt • brittle-plastic transition • structural geology • rheology • faults • mylonite • ultramylonite • pseudotachylyte

W. H. Bradley, premier paleolimnologist

M. Dane Picard

Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, U.S.A.

^*Correspondence should be addressed to: picard@earth.utah.edu

Key Words: Biography  • Colorado •  Earth history •  Green River Formation •  history of geology •  sedimentary geology  • Utah  • Wyoming