MAPPING MIOCENE EXTENSION IN THE WESTERN BITTER SPRING VALLEY TO BETTER UNDERSTAND THE FAULT HISTORY OF THE LAKE MEAD REGION, SOUTHERN NEVADA

The area around Lake Mead, Nevada, underwent large-scale extension that is recorded in the Miocene deposits and exposed faulting. Discovering more details about the paleogeographic and fault evolution of this area is important to better understand processes of continental extension. Ongoing work by St. Thomas students and their collaborators led to a hypothesis that faulting in this region occurred in stages with different sets of faults active at different times from 17 to ~11 Ma (Lamb et al., 2022). We focused on an area that has not been studied before in detail: the Horse Spring Formation (HSF) within the western Bitter Spring Valley. To the north of our study area, previous works records a mix of major northwest- and northeast-trending faults. We set out to understand if one or both sets continued into our area and if so, how and when they were active. Our study is important because it adds to research surrounding Miocene basin fill throughout the Lake Mead region with a more detailed look at the Thumb Member of the HSF. Within the three subsections of this member there are conglomerates, several volcanic tuffs, limestones, and sandstones. We mapped and collected structural data in the upper half of the Thumb Member using Strabospot. Next, we used ArcGIS Pro to create a detailed geological map of the Thumb Member bedding and faults, and Faultkin to analyze kinematic indicators of the faults.

In our mapping area, most Thumb Member beds shallowly dip to the northwest and have been offset by many faults. The faults are variable in size from small centimeter-scale lengths and offsets to large regional faults with tens of meters of offset. Most faults are transtensional with some pure normal and pure strike-slip faults. Faults tend to strike northwest with moderate to steep dips. Most of the kinematic analyses show an east-west extension axis, similar to previous studies. A minor set shows a north-south extension axis and faults that have steep dips. This set has strike-slip faults that likely formed as secondary faults as the region was extended. Shallowly dipping northeast- and northwest-trending faults with no visible lineation were found in the field as well. Given that the majority of and largest faults are northwest trending, we hypothesize that these faults were active during the last few stages of extension in the Lake Mead area.