Structural and basin evolution of the mud hills, lake mead miocene extensional domain, Nevada

This study details the stratigraphic and deformational history of the Mud Hills area and the eastern Lime Ridge fault located in the south Virgin Mountains of the eastern Lake Mead domain in southeastern Nevada. The Virgin Mountains record the earliest Miocene extension in the Lake Mead domain and this deformation is not heavily overprinted by younger deformation, unlike the western Lake Mead domain. In the Virgin Mountains, the latest Oligocene to Miocene Horse Spring Formation was deposited prior, and during, extension between ca. 26 Ma and ca. 14 Ma. This study focuses on the northeastern terminus of the Lime Ridge fault and southern tip of the Whitney Ridge fault and the structures between them. To better understand the stratigraphy and deformation of the area, a detailed map (1:10,000 scale) was produced, data were collected from all structures in the area, and ash-fall tuffs were dated and correlated through 40Ar/39Ar geochronology and tephrachronology. The Rainbow Gardens Member of the Horse Spring Formation was deposited in a slowly subsiding, low-relief sag basin between ca. 26 Ma and ca. 17.0 Ma, and records an overall fining upward sequence. An unconformity developed within the basin at ca. 17.0 Ma at the base of the Thumb Member, which records an overall coarsening upward sequence. The Thumb Member lake at the base of the unit was disrupted between 17.0 and 16.3 Ma by east-side-down normal faults, which represent the earliest deformation internal to the Virgin Mountains. These normal faults developed in the stepover between the Piedmont fault to the north and the Lakeside Mine fault to the south. By 15.4 Ma, the deformation style changed such that the majority of east-west extension was accommodated along the left-lateral Lime Ridge and Pakoon Ridge faults. Motion along these left-lateral faults reached a peak at ca. 15.0 Ma, which corresponds to peak uplift in the footwalls of the Piedmont and Lakeside Mine faults. The Lime Ridge fault and the Whitney Ridge fault were kinematically linked through a diffuse zone of minor faults and folds near the end their respective tips. By 14.0 Ma faulting had ended in the Virgin Mountains and the upper Thumb Member conglomerate prograded across the low-lying areas. Extensional strain was transferred to the north and west to the Bitter Ridge and Hen Springs faults of the Lake Mead fault system and deformation continued in the western Lake Mead domain until ca. 8 Ma. The western Lake Mead domain experienced the same pattern of normal faulting followed by transtensional faulting approximately one million years after the change in deformational style occurred in the Virgin Mountains.