TIMING OF MEAD SLOPE FAULT RUPTURES, LAKE MEAD AREA, ARIZONA

The Mead Slope fault (MSF) has been considered an active late Quaternary fault for several decades; however, until this study, there have been weak constraints on slip rates, and the age and size of surface-rupturing earthquakes. We used high-resolution DEMs, cosmogenic dating of alluvial fans, OSL dating of faulted sediments, and detailed examination of fault exposures to better constrain the earthquake histories for the main fault strand. Detailed fault mapping was accomplished using DEMs generated from multiple drone flights and ground-control points. We determined that the fault zone consists of two main strands, both offsetting Quaternary alluvial fan remnants. The northwestern strand offsets late to latest Pleistocene fan deposits, as well as relatively young tributary gravel deposits exposed below a wave-cut bench associated with past high levels of Lake Mead. Examination of this exposure revealed 2 or 3 identifiable surface ruptures that occurred within the past ~60,000 yrs. We collected 21 surface rock samples from various Quaternary landforms displaced by the fault for exposure dating via cosmogenic ³He. Preliminary exposure dates from several alluvial fans provide age constraints for a long-term slip rate as well as a slip rate for the past 150 ky. Three samples were taken from large boulders exposed on the surface of a Qo alluvial fan resulting in preliminary exposure ages that range from 620 to 880 ka. Given the Qo landform has been left-laterally offset by at least 80m, the long-term slip rates for the northwestern most strand range from 0.09 to 0.13mm/yr. Three samples collected from large boulders on late Pleistocene fan units (Qi3-4) yielded preliminary ³He exposure ages ranging from 80 to 147 ka. Left-lateral offsets on the Qi3-4 surfaces are approximately 5-6 m, with less than 2 m of vertical offset. This indicates a lateral slip rate of 0.03 to 0.06mm/yr since 150 ka. Given the fault exposure in the tributary sediments reveals at least 2 events since 60 ka and at least one older earthquake, it is likely that they are represented in the 5-6 m of offset of the Qi3-4 fan surfaces.