Using High-resolution Basin Analysis to Unravel Complex Fault Kinematics, Understand Tectonic Events and Address Climate Change in the Central Basin and Range

Melissa Lamb, University of St. Thomas
Sue Beard, USGS
Hickson Thomas, University of St. Thomas
Paul Umhoefer, Northern Arizona University

The Lake Mead Area is a key area to study extensional processes (e.g., Anderson (1971; Wernicke and Axen, 1988) as well as the evolution of the Colorado River and Grand Canyon (e.g., Karlstrom et al., 2014). Much work in this area focused on structural analysis and thermochronolgy (e.g., Reiners et al., 2000; Dubendorfer, 1988) but Beard (1996) recognized the need to study the complex stratigraphy as well in order to answer many remaining questions. The Horse Spring Formation (HSF) records the fill of basins that formed prior to and during the main phase of extension, ~17 and ~8 Ma. Ranging from >25 to ~12 Ma, the HSF consists of a variable mix of carbonates, siliciclastics, tuffs, and evaporites that formed in a series of large and small basins. The deposits have been subsequently beautifully exposed due to down cutting of Colorado River tributaries, possibly due to establishment of the Colorado River in the area at 5-6 Ma. We set out to define and characterize these basins in detail to reconstruct the complex faulting and tectonic evolution of the area.

Because the HSF stratigraphy is so variable in terms of lateral and vertical facies, it can be hard to map across faults but this complexity is actually advantageous: separate basins and subbasins have their own unique characteristics that can be used to unravel the deformation. Highly variable lithologies allowed us to better define these basins and understand depositional environments. Numerous tuffs allowed us to create a detailed chronostratigraphy: we probed 216 tuff samples for geochemical fingerprinting and dated 22 samples using 40Ar/39Ar geochronology. We measured numerous detailed sections and, in the highly variable areas, walked out beds to document lateral changes. We mapped at 1:5000 and 1:10,000 scales and conducted paleocurrent and provenance analyses in key areas. Finally, we ran 715 carbonate samples for stable O and C isotope analyses.

With this huge integrated dataset, we defined several large and small basins and determined many of the faults that created them and when faulting occurred. We identified growth faulting at two different times within HSF deposition and began to distinguish tectonic and climatic signals in the stratigraphic record. We are creating a step-by-step tectonic reconstruction of the entire area from 18 Ma to present. Our structural analyses have benefitted greatly from the multidisciplinary, detailed approach. It has also been a very successful tool for engaging students in research at all scales.