EOCENE TOPOGRAPHY IN NORTHEASTERN NEVADA: IMPLICATIONS FOR EARLY CENOZOIC EXTENSION IN THE NORTHERN BASIN AND RANGE

The amount of early Cenozoic extension in the northern Basin and Range is actively debated. Geobarometric and thermochronologic data from the Ruby Mts core complex indicate as much as 30 km of exhumation between ~80 and 22 Ma. The distribution of ash-flow tuffs in paleovalleys, which drained eastward across northeastern NV in the Eocene, and a small angular unconformity between ~40 and 38 Ma rocks support only a minor episode of extension at 40 Ma. Further examination of Eocene topography indicates exhumation through 40 Ma was not accompanied by measureable upper crustal extension. The distribution of Eocene tuffs and clastic rocks suggests that paleovalleys crossed what is now the Ruby Mts – one from the northern Adobe Range to the Clover Hill area in the northern East Humboldt Range and another from the Elko Fm in the Elko Hills to Eocene rocks in the southern East Humboldt Range.

Examination of the Eocene unconformity in a broad transect at 41° further indicates that the rocks exposed beneath the unconformity are dominantly a result of Mesozoic folding with little or no evidence for extension. Based on published geologic maps and my examination, Eocene rocks were everywhere deposited on uppermost crustal rocks. In the western part of this transect, in the Independence and Tuscarora Mts and Pinon and Adobe Ranges, Eocene rocks were deposited on upper plate rocks of the Roberts Mountain thrust. In the central and eastern part, Eocene rocks sit on unmetamorphosed upper Paleozoic to Triassic rocks, mostly Pennsylvanian-Permian. Triassic rocks underlie Eocene rocks in the eastern Windermere Hills and at the south end of the East Humboldt Range. The stratigraphically lowest sub-unconformity rocks are Mississippian, for example at the bottom of the Nanny Creek paleovalley in the Pequop Mts and above an anticline in the Adobe Range. A few basal Eocene contacts were mapped as faults, but my examination suggests they probably are depositional. All stratigraphic-structural relief can be attributed to Mesozoic folds and to as much as ~1 km of erosion in paleovalleys. The paleovalley and unconformity data preclude major extensional uplift and exposure of deeper rocks at least through the Eocene. The exposed metamorphic gradient eastward through the Ruby Mts, Wood Hills, and Pequop Mts must reflect purely deep seated and/or later events.