NEOGENE DEFORMATION OF THE WHEELER FAULT SYSTEM: APPLICATION OF HIGH-PRECISION GPS AND PHOTOGRAMMETRIC MAPPING TECHNIQUES

The Lake Mead region has long served as a natural laboratory for developing and evaluating models of crustal extension. Much work has focused on large-magnitude, pre-Middle Miocene extension. We focus here instead on Late Miocene to recent structures to gain insights into active tectonics, evolution of the Colorado River system, and processes of extension. In the eastern Lake Mead region, the Wheeler fault system cuts and offsets the Late Miocene lacustrine Hualapai limestone. In the study area the fault system is composed of two major strands, the Wheeler and Lost Basin Range faults. A newly identified exposure of the Lost Basin Range fault with a 50 cm² slickensided fault surface has an average strike of 202 degrees and dip of 53 degrees west. The slickensides rake at 76 degrees southwest. Beds of the Hualapai limestone provide a stratigraphic marker with which to quantify the relative vertical component of deformation associated with slip on the Wheeler fault system and thus gain insight into fault geometry and kinematic evolution. GPS surveys with centimeter scale precision and photogrammetric structure mapping (+/- 5 m accuracy) reveal a doubly-plunging syncline with a wavelength of ~1,000 m in the hanging wall of the Lost Basin Range fault near the relay between the Wheeler and Lost Basin Range faults and classic reverse-drag folds both north and south of the relay. Beds of the Hualapai limestone are generally highest in elevation south of the relay, lowest in the relay zone itself, and of intermediate elevation north of the relay. Combined with age constraints of others, our work may lead to accurate slip rate estimates for the Wheeler fault system as it steps between the Wheeler and Lost Basin Range faults. Furthermore, the kilometer scale syncline and reverse-drag folds accommodate the majority of deformation in the study area, leaving little net vertical motion. This observation indicates that while slip on the Wheeler fault system has little effect on regional river base level, it perhaps influences local gradient. Lastly, the occurrence of the long-wavelength syncline with the relay zone indicates a relationship between extensional propagation fold wavelength and fault properties along strike and down dip, including depth to detachment, magnitude of fault interaction, and depth of the propagating upper fault tip.