THE USE OF HIGH-RESOLUTION 3-D LASER SCANNING TO UNRAVEL THE COMPLEX RUPTURE PATTERN OF A FLOWER STRUCTURE, REX HILLS, NEVADA, USA

Fault scarps are the most obvious surface expression of tectonic activity. Scarp-morphology studies constrain fault kinematics and scarp degradation by using geomorphic dating techniques. Data acquisition and interpretation is facilitated where scarps are exposed in areas of simple topography, but it is challenging in complex, dissected topography. The latter is commonly observed in flower structures which are important to evaluate the evolution and linking of strike-slip fault systems, and seem to be scale-independent. We combined field mapping and high-resolution digital elevation model (DEM) analysis to evaluate the structure and surface expression of one flank of the Rex Hills pressure ridge. Based on terrestrial laser scanning (Riegl LMS-Z420i) we derived a detailed DEM with cm-scale resolution and extracted high-resolution topographic cross-sections. This enabled us to identify fault scarps and determine their relative ages and geometry.

The Rex Hills are located on the transpressional bend between the Pahrump and Amargosa segments of the dextral Stateline fault system, ~100 km NW of Las Vegas. The topography of the southern Rex Hills slope is characterized by alternating valleys and ridges (~100 m long, ~30-75 m wide). It exhibits three WNW-ESE trending fault scarps related to three reverse fault branches: the basal scarp is the most continuous, and it is composed of five segments; the upper two scarps are less continuous. Fault scarps exposed on ridge crests are more numerous (up to 5) and smaller (~5 m high); valleys often exhibit single large (>10 m high), smooth scarps. We analyzed the height and slope angle of the scarps to detect differences. Our results indicate that scarp shape is dominated by degradation processes yielding large scatter and overlap in scarp-height – slope-angle space, and that scarp degradation is stronger in the valleys. Hence, preservation potential of small, single scarps is greater on ridge crests. A comparison of our data with calibrated fault-scarp data yielded an age of ~2 ka, and we suggest that scarp shape mainly reflects degradation since the most recent surface rupture. Lastly, we propose that medium-range laser scanners with measurement distances of up to hundreds of m are best suited to efficiently analyze closely-spaced fault scarps across a broad range of spatial scales.