POST 5.5 MA DEVELOPMENT OF LAS VEGAS BASIN AND THE CENTRAL BASIN AND RANGE

The deformation histories between ~18 – 5.5 Ma and after 5.5 Ma differ in the Central Basin & Range province (CBR). The earlier time is marked by formation of large sedimentary basins, low-angle normal faulting, and prominent strike-slip faulting. About or after 5.5 Ma, extension rate slowed and new basins formed. These basins are controlled by mostly high-angle normal faults. We suggest that the difference results from changes in the plate boundary to the W and a localization of deformation onto large segmented normal faults to the E.

Las Vegas basin (LVB), in the central CBR, exemplifies the changes in tectonism through time. LVB contains up to ~5 km of Neogene sediments based on data from ~1150 wells <1 km deep and 6 deep wells The ~18 – 5.5 Ma basin was formed by three main faults. (1) The Las Vegas Valley shear zone (LVVSZ) has ~50 km of right slip, ~2.5 km of down-on-the-S dip slip and forms the northern basin margin. (2) The FMF, along the E side of the basin, has ~6 km of down-on-the-W dip slip, but some slip is post-5.5 Ma. (3) A fault under the basin that generates major paleorelief. 25 wells record the sub-Pliocene unconformity and show that ~2/3 of the basin-fill is Miocene. The <5.5 Ma basin has less dramatic paleorelief along its base, and is broader and more symmetric than the ~18 – 5.5 Ma basin. The deepest part of the younger basin lies W of that of the older basin. The geometry of the younger basin also is created by the FMF on the E and the inactive, but topographically significant LVVSZ on the N. The shape difference and shift in the depocenter was caused by three E-dipping faults, with scarps, exposed near the basin center. Thus, LVB is a composite basin with structural controls that change through time.

Comparison of available timing data on these <5.5 Ma faults with other young faults in the region suggest that they were active synchronously with (1) strike-slip faults in the southern Walker Lane and the Eastern California shear zone to the W and (2) high-angle normal faults along the eastern edge of the CBR. Those strike-slip faults accommodate the step-over of some plate boundary slip from the San Andreas to the east side of the Sierra Nevada. The normal faults along the eastern side of the CBR (e.g., Hurricane and Sevier faults) linked to form long, discrete structures during this time. Thus, the <5.5 Ma faults in LVB appear to accommodate a change in shape of the volume of rock between these strike-slip and normal faults.