Tectonic geomorphology and late Quaternary deformation on the Ragged Mountain fault, Yakutat microplate, south coastal Alaska

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The 33 km-long Ragged Mountain fault (RMF) forms the northwestern corner of the Yakutat Terrane, which is colliding with the North American plate in south coastal Alaska at ∼5.5cm/yr. The fault zone contains three types of scarps in a zone up to 175m wide: (1) antislope scarps on the lower range front, (2) a sinuous thrust scarp at the toe of the range front, and (3) a swarm of flexural-slip scarps on the footwall. Trenches across the first two scarp types reveal evidence for two Holocene surface ruptures, plus several late Pleistocene ruptures. In the antislope scarp trench, ruptures occurred at 0.5–3.9 ka; slightly younger than 8.3 ka; and at 18.1–21.8 ka (recurrence intervals 4.4–8 kyr and 9.8–13.3 kyr). Displacements per event ranged from 15 to 40cm. In the thrust trench ruptures are dated at 2.8–5.9 ka; 5.9–17.2 ka, and 17.2–44.9 ka (mean recurrence intervals 7.2 kyr and 19.5 kyr). Displacements per event ranged from 26 to 77cm. We interpret the thrust fault as the primary seismogenic structure, and its largest trench displacement (77cm) equates to the average displacement expected for a 33 km-long reverse rupture. The flexural-slip scarp, in contrast, was rapidly formed ca. 4 ka but its sag pond sediments have continued to slowly fold up to present. The southern third of the fault is dominated by large gravitational failures of the range front (as large as 2.5km wide, 0.6-0.7km long, and 200–250m thick), which head in a linear, 40 m-deep range-crest trough filled with lakes, a classic expression of deep-seated gravitational slope deformation.