EARLY CRETACEOUS CONSTRUCTION AND PRE-LATE EOCENE EXTENSIONAL COLLAPSE OF A STRUCTURAL CULMINATION, EUREKA, NEVADA: IMPLICATIONS FOR OUT-OF-SEQUENCE DEFORMATION IN THE SEVIER HINTERLAND

Assessing temporal relationships between foreland and hinterland deformation in fold-thrust belts is critical to understanding the dynamics of orogenic systems. Geologic mapping and balanced cross-sections through the Fish Creek and Diamond ranges near Eureka, Nevada, along with U-Pb detrital zircon and ⁴⁰Ar/³⁹Ar volcanic rock ages, demonstrate the probable Early Cretaceous construction of the Eureka culmination, a 20 km-wide, 4.5 km-tall anticline with limb dips of 25-35°. A Cambrian over Silurian relationship defined in drill holes under the culmination crest is interpreted as a blind thrust fault and the culmination is modeled as a fault-bend fold that formed from ~9 km of eastward displacement on this thrust over a buried footwall ramp.

The Early Cretaceous Newark Canyon Formation (NCF) was deposited on Mississippian-Permian rocks in a piggyback basin that developed on the eastern limb of the culmination as it grew, which is consistent with published paleocurrent and provenance data. After deposition, the NCF continued to be folded during late-stage growth of the culmination. Early Cretaceous deformation near Eureka post-dated migration of the Sevier thrust front into Utah by ~25-30 Myr, and therefore defines out-of-sequence hinterland deformation, coeval with emplacement of the Canyon Range thrust sheet in the type-Sevier thrust belt in western Utah. This out-of-sequence deformation is interpreted as internal shortening of the Canyon Range thrust sheet, which acted to decrease basal frictional strength, and therefore to decrease the critical taper angle, to promote eastward translation.

After its construction, the Eureka culmination underwent 7-8 km (40-45%) of extension in two phases that pre-dated the late Eocene (~37 Ma) sub-volcanic unconformity. The earliest extension was accommodated by oppositely verging normal fault systems developed in each limb, with 0-20° cutoff angles to bedding and offset magnitudes between 2-5 km. The second episode was accommodated by high-angle (60-70°), down-to-the-west normal faults with offset magnitudes between 2-4 km, accompanied by ~20° eastward tilting. Faults of this set cut an ~84 Ma contact aureole and tilt a conglomerate with a ~72 Ma maximum deposition age, and therefore can be bracketed between Late Cretaceous and late Eocene.