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Sedimentary rocks of the Neogene Gardnerville Basin record a complex normal faulting history from mid-Miocene to the present; this record bridges an important gap between contemporary tectonics and the older geologic record. The upper Neogene sediments are preserved in a west-dipping half graben, and fanning of dips within the section shows that the basin-bounding Carson Range frontal fault system to the west has been active since at least 7 Ma. In addition, the sedimentary history clearly shows that several north-striking normal faults within the basin have been active at different times during deposition. Gravity data enable us to extend the faulting history back beyond what is exposed at the surface and reveal mid-Miocene(?) normal faults that are no longer active below the western part of the basin. Gravity modeling suggests that the underlying fault-bounded basin is structurally symmetric. These faults have accommodated extension within Sierran crystalline rocks west of the Walker Lane, in the eastern part of the Sierra Nevada microplate.

The Neogene Gardnerville Basin documents the tectonic evolution of a distinctive part of the Sierra Nevada–Basin and Range transition zone. It lies west of the Walker Lane at this latitude, and, during its history from >7 Ma to the present, it shows no evidence of the distributed dextral slip that characterizes that zone. The field relation-ships, combined with sedimentology of the Neogene strata, document a multistage intrabasin faulting history during deposition; several intrabasin normal faults have acted in concert with the Carson Range frontal fault system to accommodate extension. This could be an analog for other normal fault systems in the vicinity, e.g., the Lake Tahoe Basin, immediately west of our study area.

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