PALEOSEISMICITY AND STRAIN PARTITIONING IN THE FOREARC FOLD AND THRUST BELT OF THE KODIAK SEGMENT, EASTERN ALEUTIAN SUBDUCTION ZONE

The Kodiak segment of the eastern Aleutian subduction zone is characterized by a high convergence rate (~57 mm/yr), wide (~ 100 km) low angle (7-10 degree) locked zone and small left oblique component of relative plate motion (~8 mm/yr). The architecture of the upper plate includes a well developed fold and thrust belt in the outer part of the forearc above the locked zone (Albatross Banks - Kodiak Shelf fault zones), a mid-forearc high above the transition zone (Kodiak Archipelago), and a large forearc basin adjacent to the arc (Shelikof Strait). Late Quaternary uplift of the Kodiak Islands has resulted in the formation of a widespread raised coastal terrace interpreted to have been cut during oxygen isotope stage 5e (133-120 ka). The elevation of the terrace increases gradually from about 15 m on the northwest side to about 40 m on the southeast side of the islands, reflecting long term uplift rates of about 0.15 mm/yr (west side) to 0.3 mm/yr (east side). However, at Narrow Cape, located near the zero isobase of the 1964 earthquake, the elevation of the terrace increases abruptly to ~280 m. Narrow Cape is traversed by at least four active faults (post-glacial, ~14 ka) including the Narrow Cape fault, one of principal structures in the arc-side margin of the fold and thrust belt. At the Cape the late Pleistocene coastal terrace is displaced by the Narrow Cape fault vertically ~180 m (vertical slip rate ~ 1.5 mm/yr) and post-glacial fluvial channels crossing the fault are offset left laterally as much as 30 m (horizontal slip rate ~2 mm/yr). Trenches across post-glacial scarps on the Claudia, Bison, and Bear Paw faults at the Cape show at least four post-glacial episodes of high angle left lateral and left normal oblique slip, the most recent less than about 3 ka. Holocene slip rates for these faults derived from offset tephras and soils exposed in the trenches range from ~0.03 to ~ 0.1 mm/yr. We interpret the faulting at the Cape to reflect deformation at a large releasing stepover on the Narrow Cape fault. The Narrow Cape fault and nearby Kodiak Island fault are major active predominately strike slip structures in the arc-side margin of the fold and thrust belt. These faults appear to accommodate much of the lateral component of subduction and constitute sources of large strike-slip earthquakes in the forearc.