Post-2.6 Ma tectonic and topographic evolution of the northeastern Sierra Nevada; the record in the Reno and Verdi Basins

• Published in: Geosphere (Boulder, Colo.) Colo.), 2012-10, Vol.8 (5), p.972-990
• Main Authors: Cashman, Patricia H, Trexler, Jr, Widmer, Michael C, Queen, S. June
• Format: Article
• Language: English
• Subjects: applied (geophysical surveys & methods); Bouguer anomalies; Cenozoic; clasts; depositional environment; faults; geophysical methods; geophysical surveys; Geophysics; gravity anomalies; gravity methods; igneous rocks; lithostratigraphy; Neogene; Nevada; northeastern Sierra Nevada; northwestern Nevada; paleochannels; Pleistocene; provenance; Quaternary; Reno Basin; Reno Nevada; sedimentary rocks; sediments; Sierra Nevada; Structural geology; surveys; tectonics; Tertiary; tilt; United States; Verdi Basin; Washoe County Nevada
• ISSN: 1553-040X; 1553-040X
• DOI: 10.1130/GES00764.1

A coarse conglomerate, known as "the Gravel of Reno", fills a deep channel incised into a 2.6 Ma sedimentary section a few km west of Reno, Nevada. The canyon and its conglomerate fill record an abrupt shift in both provenance and paleocurrent direction compared with the underlying lake-marginal Neogene strata. Notably, the intermediate volcanic provenance of the Neogene section is supplemented in the overlying conglomerate by large plutonic clasts derived from the Sierran batholith. The syntectonic Gravel of Reno signals the initiation of Pleistocene faulting along the eastern edge of the Sierra Nevada near latitude 40°N. Structures within the Neogene and Quaternary rocks reveal the progressive deformation of the Sierra Nevada's eastern margin. There is no discordance between the basal Gravel of Reno conglomerate and the underlying Neogene sedimentary section, and both are presently tilted 23° east. Therefore, significant tilting did not occur until after channel incision and deposition of the basal conglomerate. The dip within the Gravel of Reno decreases with stratigraphic height, documenting ongoing tilting during deposition. Several pervasive fault sets cut the Neogene rocks; one set of normal faults probably predates much of the tilting, but strike-slip faults appear to have been active synchronously with it. Fault sets include early west- and northwest-dipping normal faults, and two mutually cross-cutting sets of strike-slip faults: northwest-striking dextral faults and northeast-striking sinistral faults. The most continuous mappable fault surfaces, with probably much of the most recent movement, are north-striking faults with normal or oblique-slip motion. Overall, these faults accommodate east-west extension. In summary, the structural style at the northern termination of the Carson Range is characterized by distributed slip along many minor faults, and faulting was synchronous with tilting of the sedimentary section. Gravity studies constrain the location and geometry of the main structures as they project eastward under the Reno basin. A negative anomaly extends eastward from the east-tilted Gravel of Reno. This gravity low (and the Gravel of Reno it represents) terminates eastward against a steep, north-striking gravity gradient under central Reno; we interpret this to mark a west-dipping normal fault, the "Virginia Street fault," which was active throughout deposition of the coarse clastic section. A more localized and pronounced gravity low