STRUCTURAL AND GEOMORPHIC HISTORY OF AN ACTIVELY PROPAGATING DEXTRAL FAULT TIP FROM INTERPRETATION OF ALSM TOPOGRAPHY

Fault segmentation and connectivity control potential earthquake sizes and seismic hazard. Longer fault ruptures produce larger earthquakes, thus it is important to understand the processes by which faults lengthen and how strain is distributed into the crust surrounding fault tips. Some strain in fault-tip damage zones may be accommodated through broad deformation or on structures that are below the resolution of conventional topographic data, motivating study using high-resolution airborne LiDAR. Here, we use newly gathered airborne laser swath mapping topographic data (resolution >6 pts/m²) to investigate the structural and geomorphic development of the northwestern Gravel Hills Fault (GHF) in the central Mojave Desert, California. The ~65 km-long GHF is an actively propagating dextral fault that terminates at its northwestern end without any obvious, mapped connection to other structures. Detailed field mapping and remote studies of the northwest GHF were done using hillshade, contour, and slope maps derived from the point cloud data. Fault activity is determined from offsets in Quaternary stratigraphic units and alluvial fans. Deformation of these deposits and related landforms shows that a set of en-echelon faults and gentle folds define a 3 km-wide deformation zone that extends 10 km along strike of the GHF tip. Previously unidentified active folding within the damage zone was discovered and measured from topographic profiles extracted using 3-D point cloud visualization software. We interpret the faulting and folding of the northwest end of the GHF to infer a spatio-temporal sequence of deformation within a strike-slip fault-tip damage zone. Long-wavelength (5-10 km) warping accumulates ahead of the fault tip, consistent with observed stratigraphic and topographic evidence for subsidence, and the expected elastic strain distribution resulting from termination of earthquake ruptures at the fault tip. This is followed by shorter wavelength (0.25-1 km) folds associated with the leading edge of faulting. Faulting initiates as en-echelon reverse faults, evolving to dextral slip with accumulated displacement and development of linking structures.