UNRAVELING THE DETAILED STRUCTURAL EVOLUTION OF BASIN AND RANGE EXTENSION NORTH OF LAKE MEAD: AN EXAMPLE OF BUILDING ON THE LEGACY OF ERNIE ANDERSON

Ernie Anderson inspired and encouraged researchers to tackle big unanswered structural and tectonic questions in the central Basin and Range area. Although he first recognized the magnitude of normal and strike-slip faults in the area and published key papers and maps (Anderson, 1971, 1973, 2003), he knew that the tectonic drivers and the structural evolution were poorly understood, and his mapping was ‘just a start’. Starting in the 1990’s, Paul Umhoefer, my co-authors, and I, along with many graduate and undergraduate students, undertook both collaborative and individual mapping and detailed basin analyses from Frenchman Mountain to the South Virgin Mountains (SVM). Building on Ernie and other earlier researchers’ work, Umhoefer et al. (2010) summarized the state of knowledge on Lake Mead geology. Our subsequent studies build on that as we work to create a detailed structural evolution of this corridor. As an example, in the Longwell Ridges (LR) area, we completed 1:2000 to 1:5000-scale mapping and a detailed chronostratigraphic framework. Kinematic data from fifty measurements were analyzed using the programs of Marrett and Allmendinger (1990). Most major faults strike northeast while medium and smaller faults are northeast, north-south, and northwest trending. Northeast-striking, dip-slip faults yield an extension axis of 272°, almost identical to a kinematic analysis to the east in the SVM (Beard, 2010). East-dipping Miocene Horse Spring Formation strata in the Echo Wash area east of LR correlate to the SVM and we suggest both underwent deformation and tilting concurrently, around 14.5 to 13.9 Ma, as the Thumb basin was dismembered by faults related to the Lake Mead Fault System. Other faults in the LR are likely related to and concurrent with younger stages of faulting on the White Basin and Muddy Peak Faults between 13.9 to 11.5 Ma. Additionally, one major fault may be younger still and related to the development of the Overton Arm Pull-apart Basin.