LATE CRETACEOUS EXTENSIONAL COLLAPSE OF THE SEVIER HINTERLAND: STRUCTURAL AND STRATIGRAPHIC EVIDENCE FROM THE SHEEP PASS FORMATION, EAST-CENTRAL NEVADA

Previous studies have suggested that the latest Cretaceous to Eocene Sheep Pass Formation (SPF) of east-central Nevada was deposited within an extensional setting, though a clear relationship between the alluvial/fluvial and lacustrine strata of the SPF and basin forming faults has been lacking. Our new 1:12,000 scale mapping within the southern Egan Range suggests that the SPF was deposited in response to movement along a (presently) low-angle normal fault placing complexly deformed upper Paleozoic units against highly sheared lower Paleozoic units. Deformation of the SPF by comparison is limited to a series of syn-depositional normal faults, post-depositional normal faults, high-angle reverse faults and open, east plunging folds. Syndepositional normal faults cause pronounced thickness changes and fanning of dips in the lower SPF members but are overlapped by the upper members. In contrast, reverse faults and folds appear to post-date the SPF and may be the result of fault propagation during later extension. An extensional basin setting for the SPF best explains the fanning of dips, megabreccia deposits and the presence of intra-basinal syndepositional normal faults. Renewed faulting during Middle(?) Eocene times resulted in the deposition of the Stinking Springs conglomerate which overlies the SPF with 10-15 degrees of dip discordance. Preliminary LA-ICP-MS detrital zircon dating of the Stinking Springs conglomerate indicates that this younger episode of extension began prior to ca. 40 Ma. Older extension is loosely bracketed between the mid-Cretaceous (ca. 100 Ma) detrital zircon population of the basal SPF, and the Maastrichtian (ca. 70-65 Ma) fossil assemblage of the lacustrine limestone member. Lack of detrital zircons of Late Cretaceous to Paleogene age in the SPF indicate local magmatic quiesence, with Early and mid-Cretaceous zircons derived from recycling of local volcaniclastic deposits such as the Newark Canyon Formation. Late Cretaceous extension in the SPF temporally overlaps with mid-crustal extension within core complexes of the Sevier hinterland, as well as continued contraction within the Sevier foreland to the east. These data provide new insight into the tectonic evolution of the Sevier Orogen and the role of syn-contractional extension within broad contractional orogens.