NEOGENE TECTONIC HISTORY OF THE SIERRA NEVADA – BASIN AND RANGE TRANSITION ZONE AT THE LATITUDE OF CARSON CITY, NEVADA: GEOLOGICAL AND GEOPHYSICAL EVIDENCE

Neogene sedimentary rocks of the Gardnerville basin were deposited between 7 and < 2 Ma, and record the tectonic history along part of the Sierra Nevada – Basin and Range transition zone during this time interval. These rocks lie west of the Walker Lane and directly east of the Sierran frontal fault system. The rocks dip west and range from locally-derived fluvial conglomerate and sandstone to lacustrine diatomite. Mammal fossils and tephras provide age control. Several local basement highs punctuate the sedimentary basin; one is directly overlain by 5 Ma sedimentary rocks and another by 2 Ma rocks. Normal faults locally offset the Neogene sedimentary rocks, but no strike-slip faults have been observed. Geophysical data supplement the geology and resolve questions about the age, internal structure and faulting history of the Neogene Gardnerville basin. A seismic reflection line documents depth to basement; it also shows fanning of bedding dips, indicating on-going slip along a western basin-bounding fault during deposition. In addition, two prominent reflections within the section appear to represent changes from fluvial to lacustrine sedimentation, and may correlate with uplift of the intra-basin highs farther east. A compilation of new and existing gravity data shows one low associated with the western – thickest – part of the sedimentary section, and another in a sub-basin east of a mid-basin basement high. An east-west belt of higher gravity across the eastern half of the basin coincides with an accommodation zone between southward-terminating intra-basin normal faults to the north and northward-terminating intra-basin faults to the south. Thinner, coarser-grained sediments, anomalous dips suggesting local unconformities, and apparent funneling of paleoflow between the basement highs indicate that the structural accommodation zone exerted an active control on sedimentation. The combination of the gravity and seismic data suggests a thickness of 1.5 – 2.0 km for the Neogene and Quaternary rocks along the west edge of the Carson Valley.

The Neogene Gardnerville basin constrains the evolution of the transition zone at this latitude. The Sierran frontal fault system has been active here since >7 Ma, changing slip rate estimates. Intrabasin faulting occurred at least twice, around 5 and 2 Ma.