3 Research approach
To assess the kinematics of rupture and slip partitioning on the Fairweather fault, we used multiple approaches including interpretation of recently-acquired lidar topography, digital and field-based geomorphic mapping, high-resolution multibeam bathymetry and multichannel seismic reflection surveys, and both radiocarbon accelerator mass spectrometry (AMS) and infrared stimulated luminescence (IRSL) techniques to estimate the ages of Holocene landforms.
Lidar data acquired in 2015 (Witter et al., 2017a) provided 1-m-per-pixel bare-earth digital elevation models that inform our interpretations of tectonic geomorphology and coastal and glacial landforms. Using a helicopter-mounted lidar system, the U. S. Geological Survey and collaborators (Acknowledgments) collected lidar data in three ice-free sections of the Fairweather fault within 33 km northwest of Icy Point (Witter et al., 2017b). Here, we focus on the southern lidar map, a 9 by 5 km area that includes the Icy Point peninsula and extends north-northwest to where the Fairweather fault crosses the southern-most Holocene moraines of the Finger Glacier. Elevations have ±0.10 m accuracy and are reported as NAVD 88 orthometric heights.
Geomorphic maps (Figure 4) developed from lidar topography and 2014–2015 satellite imagery guided our field investigations in May-June 2017. Prior to the fieldwork, we mapped uplifted terraces and candidate fault scarps on lidar and satellite imagery. Fieldwork involved establishing ground-based geodetic surveys to field check lidar DEMs, collecting Miocene-to-Pleistocene rock samples for a fault-perpendicular thermochronometry transect (Lease et al., 2021), verifying fault traces of the Fairweather fault, coring or excavating exposures in uplifted beach deposits on Terraces A and B, describing degree of soil development on Terraces A and B, describing fluvial and tidal-slough deposits that inform RSL changes, and collecting samples for radiocarbon and luminescence age analyses.
Between 2015 and 2017 the USGS led a series of marine geophysical surveys offshore and to the south of Icy Point to map the seafloor morphology of the Queen Charlotte fault and image the stratigraphy and structure along the entire continental shelf and slope to the U.S.-Canada international border. Multibeam bathymetry data (gridded to 10-m resolution DEM) (Dartnell et al., 2022)and a dense grid of high-resolution multichannel seismic profiles (Balster-Gee et al., 2022a, b) were collected across the Queen Charlotte fault aboard the R/V Solstice in 2015 in the region between Icy Point and Cross Sound, and then again in 2017 aboard the R/V Ocean Starr (Brothers et al., 2020). The multichannel seismic profiles were processed using the commercial software Shearwater Reveal ; faults, folds and seismic stratigraphy were imaged to sub-bottom depths of several hundred meters depending on the substrate geology and water depth.
We used a combination of infrared-stimulated luminescence (IRSL) and AMS radiocarbon age analyses to estimate the Holocene ages of coastal landforms and terrace deposits at Icy Point (Witter and Bender, 2021). The terrace deposits lacked wood or charcoal, so we employed IRSL analyses of feldspar grains to estimate ages of sandy beach and aeolian deposits. IRSL-derived ages indicate the last time that the deposit was exposed to sunlight (Aitken, 1998). We sampled according to the procedures as outlined in Gray et al. (2015). IRSL analyses (Table 1) were performed by the Utah State University Luminescence Laboratory using the single-aliquot regenerative-dose procedure of Wallinga et al. (2000) on 2mm small-aliquots of feldspar sand at 50°C. The IRSL age on each aliquot is corrected for fading following the method of Auclair et al. (2003) and using the correction model of Huntley and Lamothe (2001). The equivalent dose (DE) and IRSL age are calculated using the Central Age Model (CAM) or Minimum Age Model (MAM) of Galbraith and Roberts (2012).
Radiocarbon (14C) AMS analyses (Witter and Bender, 2021) were used to estimate the ages of fluvial and tidal-slough deposits. Dated material included individual rings of in-growth-position tree stumps, conifer needles and cones, detrital wood, and herbaceous material. We use OxCal (version 4.4.2, Bronk Ramsey, 2009; 2023) and the IntCal20 atmospheric 14C curve (Reimer et al., 2020) to calibrate 14C dates reported in this study as well the 14C dates published by Rubin and Alexander (1958) and Mann (1986), which constrain ages of marine terraces. Calibration of a radiocarbon date of a marine bivalve shell from Terrace A beach deposits uses the Marine20 calibration curve (Heaton et al., 2020) and incorporates regional ΔR values (Reimer and Reimer, 2001; http://calib.org/marine/). We report calibrated14C ages in years before 1950 Common Era (CE) (Table 2), which result from correcting lab-reported 14C ages to account for variations in atmospheric 14C concentrations over time.