GEOMORPHIC CONSTRAINTS ON ACTIVE FOLDING ABOVE A HISTORICALLY PERSISTENT CLUSTER OF SHALLOW SMALL EARTHQUAKES IN CENTRAL ALASKA

Thousands of small earthquakes occur annually in the Kantishna region of central Alaska, where recent research predicts at least 3 m/k.y. of north-directed far-field plate boundary shortening. Absent constraint on related deformation rate and style, however, the Kantishna earthquake cluster’s neotectonic significance remains speculative. Here, analysis of 5 m/pixel radar-based digital topography reveals late Quaternary anticlinal surface folding above the northern Kantishna earthquake cluster. The surface folding occurs on a single, near-symmetric half-wavelength of ~15 km, and plunges west into the adjacent basin from a maximum amplitude of ~200 m over ~20 km along-strike. Several trunk rivers that drain the north flank of the Alaska Range flow around and cut across the anticline, providing fluvial records of westward fold propagation and vertical growth. Where the McKinley River incises a bedrock gorge perpendicular to strike across the anticline, field and remote observations show that the modern channel narrows by a factor of nearly 10 across the fold crest, indicating fluvial adjustment to rock uplift via increased unit stream power. Three warped strath terrace levels flank the gorge and record progressive fold growth by limb rotation (≤1˚), which we attribute to simple detachment folding. Optically stimulated luminescence and ¹⁰Be depth profile ages of deposits on the three terrace levels (~22, ~18, and 9-14 ka) broadly correlate to independently dated regional glacial advances. We calculate the uplifted rock area beneath the folded terraces to quantify shortening above a detachment-like sub-horizontal band of well-located earthquake hypocenters at 10±2 km depth (M_L≤4, n=1380, 1988-2018), and use the terrace ages to estimate rates of rock uplift (0.5-1.0 m/k.y.) and shortening (0.5-1.3 m/k.y.) over the last ~22 ka. Shortening across the Kantishna anticline occurs at a rate well below the 3 m/k.y. predicted across the region, implying the presence of (a) additional active structures south of the fold that account for the apparent shortening deficit, or (b) a regional east-west decrease in shortening rate, consistent with observed geomorphic evidence for westward fold propagation.