LIDAR-BASED MAPS OF ACTIVE TRACES OF THE FAIRWEATHER FAULT IN GLACIER BAY NATIONAL PARK, ALASKA

The Fairweather Fault accommodates 43–58 mm/yr of plate motion where the Yakutat block collides into North America in southeast Alaska. Fault rupture during the 1958 M_w 7.8 Lituya Bay earthquake extended >260 km from Yakutat Bay to Cross Sound. Ice or water covers >80% of the 1958 rupture length limiting land-based observations of surface deformation to post-earthquake surveys along <40 km of the fault.

Here, we use 1 m/pixel lidar-based digital topography to map late Quaternary tectonic geomorphology, surficial deposits, and active fault traces along the southern Fairweather Fault. Mapped features reflect the geomorphic expression of landforms illuminated by lidar, which are bolstered by rupture mapping completed soon after the 1958 earthquake and our field observations in 2015–2017. Fault-related geomorphic features include east-facing, 25-m-high scarps, linear troughs, slope breaks, shutter ridges, ponded alluvium, and dextrally offset landforms that include 11 stream channels offset 12–128 m, and 2 moraines offset 35–115 m. Other mapped features include ridge-top grabens (sackungen), landslides, stream terraces, marine shorelines, and glacial landforms.

We map the fault in three sections: Crillon Lake to La Perouse Glacier (north), La Perouse Glacier to Finger Glacier (middle), and Finger Glacier to Icy Point (south). In the north, two uphill- (east) facing scarps dextrally offset glacial and stream channel deposits by 66–128 m at the north end of Crillon Lake. Southeast of the lake, multiple fault traces follow a ~3-km-long linear valley where alluvium ponds against a fault-shattered, axial ridge. In the middle section, prominent uphill- (west) facing scarps along a shutter ridge delineate a single active trace along the western flank of a linear valley where the fault dextrally offsets Finger Glacier’s lateral moraine by 115 m. In the south, the fault vertically displaces stream and marine terraces by >25 m along a series of fluvially modified east-facing and right-stepping scarps that project into Palma Bay at Icy Point. Reverse faults along the coast also offset marine terraces. Our lidar-based mapping depicts up-to-the-west transpressional deformation along the southern Fairweather Fault and reveals the structural complexity of the Yakutat-North America plate boundary.