CAMILLERI, PHYLLIS A., University of Wyoming, Laramie, WY

ABSTRACT: Evidence for Large-Magnitude Cretaceous or Early Tertiary Extension in the Pequop Mountains, Nevada: Implications for the Early Tertiary Unconformity in Northeast Nevada

Mesozoic and Cenozoic tectonic reconstructions of the hinterland of the Sevier thrust belt in the northeastern Great Basin rely on inferences of Mesozoic to early Tertiary paleogeography drawn from geologic relationships beneath an early Tertiary unconformity. Because middle Tertiary volcanic rocks tend to depositionally overlie upper Paleozoic or Mesozoic rocks with little angular discordance, it is widely assumed that little structural or erosional relief occurs across the unconformity. Moreover, it is

commonly assumed that large-magnitude extension involving surface-breaking faults postdates the unconformity.

The northern Pequop Mountains expose a Cretaceous or early Tertiary top-to-the-west-northwest low-angle normal fault (Pequop fault) that juxtaposes two different Paleozoic sections previously duplicated by thrust faulting. Upper Paleozoic strata in the footwall and hanging wall of the Pequop fault are depositionally overlapped by a middle Tertiary (Eocene) volcanic sequence with little angular discordance. Based on reconstruction, the amount of structural relief on the unconformity due to slip on the Pequop fault is approximately 10 km. These data emphasize (1) that although the middle Tertiary volcanic sequence commonly lies depositionally on upper Paleozoic rocks, significant upper crustal structural relief existed prior to the development of the early Tertiary unconformity in this egion, (2) that there exists a preunconformity extensional event, involving a surface-breaking fault or faults, comparable in magnitude to postunconformity extension, and (3) a need to reevaluate northeastern Great Basin Mesozoic and early Tertiary tectonic models that require no major surface-breaking normal faults.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.