FROM EXTENSIONAL TO TRANSTENSIONAL FAULTING IN THE LAKE MEAD REGION: AN EXAMPLE OF PAUL UMHOEFER’S PASSION FOR BASIN ANALYSIS, FIELDWORK, TECTONIC PUZZLES AND COLLABORATION

Paul Umhoefer loved to tackle continent-scale tectonic puzzles almost as much as he loved to bring people together to address these questions. The Lake Mead region was both a classroom and a major research focus for him for twenty years. Ernie Anderson (1971, 1973) first recognized the magnitude of faults in the area and Bohannon (1984) and Sue Beard (1996) established key stratigraphic and structural relations. Ernie and Sue, however, knew the tectonic drivers and structural evolution were poorly understood and encouraged us to heed the Lake Mead call. So Paul, my co-authors and I, with many students, embarked on a major basin analysis study, from Frenchman Mountain to the South Virgin Mountains. Paul championed the area for studying wide-rift extensional processes and hypothesized that faulting occurred in two major stages (Umhoefer et al., 2010).

To capture the well-exposed stratigraphic and structural complexity and test Paul’s hypothesis, we completed 1:5000-scale mapping, measured stratigraphic sections and created a detailed chronostratigraphy. Bill McIntosh and Nelia Dunbar analyzed tuff samples for tephrochronology and geochronology. We determined the timing, paleogeography and evolution of the Rainbow Gardens Formation (Lamb et al., 2015, 2018). We established the varied facies and ages of strata within the members of the Horse Spring Formation (HSF) and informally-named red sandstone unit. These data allow reconstruction of numerous basins and faulting across the region, e.g. Lamb et al. (2022). In 2021, Paul led the charge as we developed a fault model with five stages, in part focused on the Bitter Spring Valley, White Basin and Longwell Ridges. Stage I encompasses the main extension from 17 to 14.5 Ma, deposition of the HSF Thumb Member and high sedimentation rates. During Stage II, 14.5 to 13.86 Ma, faulting jumps westward, sedimentation rates drop and a long-lived stable lake produces the HSF Bitter Ridge Limestone Member. Subsequently, sedimentation rates and faulting increase from 13.86 to 13.46 Ma, Stage 3, before dropping again in Stage 4, from 13.46 to 11.5 Ma. The location and style of faulting also change during Stages 3 and 4. Most major faults strike northeast while medium and smaller faults trend northeast, north-south, and northwest. During Stage 5, 11.5 Ma to present, faulting slows further.