PROJECT SUMMARY

High-resolution basin analysis of a large-offset extensional system, Lake Mead domain, east-central Basin and Range Province

We propose to conduct a high-resolution basin analysis of the syn-extensional basins in the Lake Mead area. This study will allow us to determine the tectonic and paleogeographic development of the domain at the 100 – 200 ky time scale. Large-scale continental extension is a fundamental tectonic process and extensional areas are prime targets for oil and gas exploration; basins in these settings provide basic data on paleoclimate, water supply, mineral resources, and the evolution of high- and low-angle normal faults. The central Basin and Range—Colorado Plateau to Sierra Nevada—is a corridor in which low-angle normal faulting was first recognized, lower crustal flow accompanying extension was first proposed, the rolling hinge hypothesis was proposed, and along which large-scale extension was quantified 20 years ago. This region includes low-angle detachment faults, normal faults, transtensional fault systems, areas of complex 3-D strain with extensional and contractional structures, and locally voluminous magmatism. Despite decades of research, there remain fundamental questions on the mechanism and evolution of deformation in the central Basin and Range. Miocene sedimentary basin fills in the Lake Mead domain (east of Las Vegas, Nevada) hold critical clues to address some of these questions and in most areas lack detailed study.

Processes of extension are recorded in both the lower and upper plate, yet in no extensional system in the Basin and Range has the upper plate been studied in detail for many tens of kilometers away from the detachment fault, mainly because of limits of exposure. The Lake Mead domain is ideal for understanding extensional processes because (1) the footwall of the South Virgin Mountains–White Hills detachment fault has been studied in detail; (2) there is widespread exposure of syn-extensional sedimentary and volcanic rocks that stretch 50 km in front of the detachment fault and 100 km north of it; (3) there are numerous tuffs; (4) the area is mapped at 1:100,000 scale, and most is mapped at 1:62000 or 1:24000 scale; (5) the basic stratigraphy and structure framework are known; and (6) there is a mix of clastic and carbonate rocks that, when combined with a detailed chronostratigraphic framework, will allow us to begin to separate the influences of tectonics and climate.

This study will focus on three major questions that are important for extensional tectonics globally and for which we can make major progress in the Lake Mead domain.

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 we are testing are that (i) the Lake Mead domain developed from east to west in discrete stages from detachment faulting to transtensional faulting, to tectonic escape accompanied by shortening; (ii) extension, detachment faulting, and exhumation were initially driven by overthickened crust, but the temporal evolution was controlled by far-field plate boundary changes; (iii) 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.

The PI’s and colleagues form a new team that spans the requisite diverse expertise: basin analysis, sedimentology, stratigraphy, structure, paleoclimatology, and geochronology and petrology of volcanic rocks. The team has worked for 5-6 years in Lake Mead; our USGS and NAU colleagues have worked for 20 – 40 years in the area. This collaboration represents professors and students from PhD+MS, MS-only, and BA-only departments that will give a broad spectrum of research opportunities to students in the field and lab. We also are planning strong synergistic relationships with the Lake Mead National Recreation Area and the BLM that will increase the societal impact of this study considerably.