GEOMORPHOLOGY, HIGH-RESOLUTION LIDAR, AND AERIAL PHOTOGRAPHY DATA - EXTRACTING GEOLOGIC INSIGHTS FROM 3D MODELING - RAGGED MOUNTAIN FAULT SOUTHERN ALASKA, USA

The application of high-resolution LiDAR and aerial photography datasets are widely used to explore and evaluate surface manifestations of active tectonics. The ~30 km-long Ragged Mountain fault is located in southern Alaska in the western edge of the St. Elias Orogen where the Yakutat microplate is colliding into Alaska. The study area contains hundreds of geomorphic surface features indicative of active deformation such as fault scarps, ponds, and off-set streams, but there has been a challenge in separating tectonic structures from gravitational collapse features. The 15 cm ground resolution LiDAR DEM together with the 0.5 m resolution aerial photography is used to create an accurate geomorphologic map of the study area along the length of the east flank of Ragged Mountain, which contains the Ragged Mountain fault scarp, uphill facing fault scarps, flexural-slip fault scarps, talus deposits, landslides, an alluvial fan, and stream channel patterns. The geometry of talus slopes revealed by 36 extended profiles across the Ragged Mountain Fault varies from concave to convex immediately below extensional scarps. Trenching at one locale revealed reverse faulting in Quaternary deposits beneath a convex surface, with extension upslope possibly caused by flexure of the hanging wall. However, an extensive system of extensional scarps extends along the length of the fault, through areas with no discernable evidence for contraction, or reverse faulting. 3D modeling, geomorphic mapping of the surface ruptures of the Ragged Mountain fault system, together with elevation profiles across it, show a pattern of a consistent array of uphill-facing extensional fault scarps in the hanging-wall of the fault, increasing scarp heights from north to south, and distinctive convex upward slopes on both sides of the extensional scarps.