This project aims to determine the tectonic and paleogeographic development of the central Basin and Range in the Lake Mead area near Las Vegas, Nevada, through a high-resolution analysis of syn-extensional basins. This region includes low-angle detachment faults, normal faults, transtensional fault systems, areas of complex three-dimensional strain with extensional and contractional structures, and locally voluminous magmatism. This study will focus on three major questions that are important for extensional tectonics globally: (1) Is there a predictable series of processes in the evolution of a major extensional episode in wide, magmatic rifts built on thick crust such as in the Basin and Range? (2) Are extensional processes in the internal part of wide continental plate boundaries controlled by far-field or internal forces? (3) How does climate change and evolving topography affect sedimentation in a major extensional orogen? The main hypotheses the research team is testing are that: (1) the Lake Mead domain developed from east to west in discrete stages from detachment faulting to transtensional faulting, to tectonic escape accompanied by shortening; (2) extension, detachment faulting, and exhumation were initially driven by over thickened crust, but the temporal evolution was controlled by far-field plate boundary changes; (3) changes in patterns and rates of faulting exert a first-order control on basin geometry and stratal thickness, but climate controls significant details of the stratal architecture that have heretofore been attributed to tectonic processes. Basin analysis techniques - including characterization of stratigraphic architecture, facies analysis, geochronology, structural mapping, and stable isotope geochemistry - are being utilized to address these hypotheses.
This research will examine Miocene age basins in Lake Mead to determine the detailed development of faulting and the changing landscape between 6 and 10 million years ago during an episode of crustal extension. This is the geologic episode that fundamentally formed the low landscape of the region and made the Basin and Range a distinct province from the adjacent Colorado Plateau. Despite decades of research, there remain fundamental unanswered questions about the mechanism and evolution of deformation in the central Basin and Range that this project aims to address. The research has wide application because extensional regions are common hosts of oil and gas globally and the Basin and Range is considered a well exposed example of these processes.
Intellectual Merit The area that lies just north of Lake Mead in southern Nevada is geologically dynamic. Geoscientists historically developed critical ideas about how the crust pulls apart (extends) by plate tectonics in this region. Rocks that underlie mountains just east of Las Vegas formed in lake and river basins some 60 km east and were moved along a complex system of faults to their present-day location. These sediments hold key information about how this region evolved. Erosion and downcutting by the relatively young Colorado River has exposed them; this is rare. We investigated these rock units to develop a clearer understanding of plate tectonic processes during extension of the crust. We were able to precisely date many volcanic ashes within these rocks, which provides a very high resolution time record of the geologic history of the area. Our major conclusions include: 1) Prior to this work, it was thought that from 24 Ma (million years ago) to about 17 Ma, extension played little to no role in the development of the Lake Mead region’s landscape. Instead, rivers that once drained north and northwest toward the Colorado Plateau were dammed by volcanism and uplifts north and east of the South Virgin Mountains, forming the Rainbow Gardens basin. Topography was mainly controlled by ancient structures remaining from the Laramide Orogeny, an earlier compressional mountain-building event. Our data, however, suggest that the area south of the Rainbow Gardens basin, on the southwest margin of the modern Colorado Plateau region, began to slowly uplift as early as 18.5 to 19 Ma. This may have implications for the debate of the timing of formation of ancient rivers related to the Grand Canyon. 2) Beginning at about 17 Ma and continuing until about 14.5 Ma, extension ‘turned on’ in the region. In the vicinity of Gold Butte, low angle faults accommodated considerable stretching of the crust and the early Rainbow Gardens basin broke up along small faults to form smaller lake basins connected by river systems. 3) From 14.5 to 13 Ma crustal stretching occurred along a combination of strike-slip (San Andreas-like) and smaller faults. This led to the formation of a widespread lake that existed for nearly 600,000 years. This was in turn broken up into smaller lake basins once again with a variety of depositional environments. These deposits were folded, uplifted and exposed due to further deformation in the area from 13-10 Ma. 4) From the period 16.5 to 10 Ma the Lake Mead region’s geography was a complex patchwork of saline lakes and sandy rivers. These lakes were very inhospitable places for life, with microbes the only living thing in them. This time frame coincides with a period of known global warming called the Mid Miocene Climatic Optimum and suggests that, under these conditions of global warmth (much warmer than present), perennial alkaline lakes like the Dead or Aral Seas were a primary feature of the landscape. Broader impact Petroleum exploration and development. Over the past five years new oil and gas finds from offshore Brazil and Nigeria are housed in lake deposits similar to those we examined in detail as part of this project. These basins formed during early break up of Africa away from S. America. Understanding these subsurface deposits is crucial, yet it is difficult to find analogues of them on the Earth’s surface. Petroleum geologists use outcrop analogues to deepen their understanding of deposits that they cannot see directly. The lake deposits in our region have already benefitted petroleum geologists via field excursions and conference participation by the project investigators. Undergraduate STEM education This project has involved over 90 University of St. Thomas undergraduates, representing both geology majors and non-majors. We implemented a unique model whereby we performed reconnaissance level research on this project as part of our January-term field course. Students interacted with the principal investigators on problems that were relevant to the project, but that did not have "canned" answers. They brainstormed with us, saw us wrestle with difficult questions, witnessed heated scientific arguments around the campfire, developed and tested their own hypotheses about the tectonics of the area, and actively participated in "doing" science on this project. Furthermore, many geology majors worked on this project as part of an independent research project, thereby training future geoscientists and critical thinkers.
