DETERMINING THE SIGNIFICANCE AND SENSE OF OFFSET OF THE FRENCHMAN MOUNTAIN FAULT, LAS VEGAS, NEVADA: A PAIRED STRUCTURAL AND BASIN ANALYSIS

The Frenchman Mountain fault (FMF) has historically been considered to be one of the key faults that translated the Frenchman Mountain block (FMB) within the Lake Mead Region of the Central Basin and Range. Published tectonic models of southern Nevada ascribe different senses of fault motion to the FMF; some researchers classify fault motion as solely normal and others maintain the fault has experienced a combination of strike-slip and normal motion. The FMB is an important structural block within the region and its translation has been a key element in virtually every Lake Mead reconstruction over the last 30 years. These models were established in order to estimate magnitude and orientation of extension and understanding the sense of offset and the faults significance within the region is critical to understanding how the region has evolved structurally. Offset of the FMF has directly impacted how the basin adjacent to it has filled, thus documentation of provenance and evolution of basin fill as well as kinematic analysis and detailed geologic mapping allows for extrapolation of fault offset sense, magnitude, and timing. Our observations of stratigraphic, structural and kinematic relationships in the field reveal normal fault motion indicated by: 1) the presence of vertical and sub-vertical slickenlines on fault surfaces; 2) relatively little lateral stratigraphic variation within the basin indicating basin-fill was linked to the same source areas through time, supported by paleocurrent and clast count data; and 3) a lack of evidence suggesting strike-slip motion. Based on fieldwork and subsequent analysis we concluded that what is currently referred to as the FMF is actually the result of three synchronously active orthorhombic normal fault sets as opposed to being one major structure. Landslide blocks with Gold Butte affinity are encased within the Miocene Thumb Member and are located within both the hanging wall and the footwall of the FMF which requires that both the hanging wall and the footwall were translated from near Gold Butte. Therefore the FMF cannot be the FMB-bounding fault and we hypothesize that the FMB-bounding fault is located further to the southwest where it is buried beneath younger sediments, which is consistent with some previous models.