Early Cretaceous construction of a structural culmination, Eureka, Nevada, U.S.A.: Implications for out-of-sequence deformation in the Sevier hinterland

Abstract

Assessing temporal relationships between foreland and hinterland deformation in fold-thrust belts is critical to understanding the dynamics of orogenic systems. In the western U.S. Cordillera, the central Nevada thrust belt (CNTB) has been interpreted as a hinterland component of the Sevier fold-thrust belt in Utah. However, imprecise timing constraints on CNTB deformation have hindered evaluation of space-time patterns of strain partitioning between these two thrust systems. To address this problem, new 1:24,000-scale geologic mapping and balanced cross sections are presented through the CNTB near Eureka, Nevada, in conjunction with industry drill-hole data, conodont age determinations, and 40 Ar/ 39 Ar and U-Pb ages from volcanic, intrusive, and sedimentary rocks. Our mapping redefines the first-order structures and deformation geometry of the CNTB at the latitude of Eureka. Contractional structures include two north-striking, east-vergent thrust faults, the Prospect Mountain thrust and Moritz-Nager thrust, which are connected as the same fault in cross section, several north-striking map-scale folds, and a Cambrian over Silurian relationship observed in multiple drill holes, corresponding to repetition of ∼2–2.5 km of stratigraphy, that defines the blind Ratto Canyon thrust. Two distinct sets of normal faults cut the contractional structures, and are overlapped by a regional late Eocene (ca. 37 Ma) subvolcanic unconformity. Retrodeformation of both sets of normal faults reveals the existence of the Eureka culmination, a 20-km-wide, 4.5-km-tall anticline with limb dips of 25°–35°, that can be traced for ∼100 km north-south on the basis of Paleogene erosion levels. The culmination is interpreted as a fault-bend fold that formed from ∼9 km of eastward displacement of the Ratto Canyon thrust sheet over a buried footwall ramp. The type exposure of the Early Cretaceous (Aptian) Newark Canyon Formation (NCF) is preserved on top of Mississippian, Pennsylvanian, and Permian rocks on the eastern limb of the Eureka culmination. We propose that the NCF was deposited in a piggyback basin on the eastern limb of the culmination as it grew, which is consistent with published east-directed paleocurrents and provenance data suggesting derivation from proximal late Paleozoic subcrop units. Syncontractional deposition of the NCF is used to define the probable Aptian construction of the Eureka culmination and associated slip on the Ratto Canyon thrust at depth. After deposition, the NCF continued to be folded during late-stage growth of the culmination. Aptian deformation in the CNTB at Eureka postdated migration of the Sevier thrust front into Utah by at least ∼10 m.y. and possibly as much as ∼30 m.y., and therefore represents out-of-sequence hinterland deformation. CNTB deformation was coeval with emplacement of the Canyon Range thrust sheet in the type-Sevier thrust belt in western Utah, and may represent internal shortening of this orogen-scale thrust sheet that acted to promote further eastward translation.