NEOGENE BASINS NEAR RENO, NEVADA RECORD TECTONIC CHANGES ALONG THE SIERRA NEVADA – BASIN AND RANGE TRANSITION ZONE SINCE MID-MIOCENE TIME

Neogene sedimentary basins record the tectonic history along the Sierra Nevada – Basin and Range transition zone since mid-Miocene time, bridging an important gap between contemporary kinematics and the older geologic record. The basins straddle the topographic and geologic eastern boundary of the Sierra. They were active depositional centers between 10 Ma and the present, the time period during which many of the tectonic features in the transition zone formed. The internal stratigraphy of each basin records paleogeographic changes during deposition; subsequent deformation is expressed as tilting, folding and faulting of the sedimentary rocks These Neogene basins occupy several different structural positions, and record distinctive tectonic histories; they can be grouped based on style of post-depositional deformation. The Gardnerville and Long Valley basins, west-dipping half-grabens bounded by faults of the Sierran frontal fault system, record extensional deformation as old as late Miocene. They document extensional faulting west of the Walker Lane and record sporadic normal faulting throughout their histories. In contrast, although the Verdi and Boca basins are also tilted by normal faults, the tilting was followed by distributed strike-slip and oblique-slip faulting along several fault sets. The Verdi basin occupies a modern structural reentrant in the Sierran range front, while the Boca basin, to the west, records deformation within the Sierran “block”. The Neogene sediments of the Honey Lake basin, within the Walker Lane, are locally folded and uplifted by strike-slip faulting. All of the basins mentioned here were still lacustrine environments at 3 Ma (with the possible exception of Long Valley, which is poorly dated in the upper part of the section), and parts of all have been exhumed since that time. Basin sediments as young as 2.6Ma, or maybe even 1.9Ma, are now exhumed in the Verdi and Gardnerville basins, demonstrating how quickly local structural controls have changed.