Late Quaternary offsets along the Fairweather fault and crustal plate interactions in southern Alaska

Abstract

The northwest-trending Fairweather fault has undergone Cenozoic strike-slip displacement, with the most recent pulse of movement occurring in late Quaternary time. During the Lituya Bay earthquake (Ms = 7.9) of July 10, 1958, movement occurred probably along the entire 280 km onshore length of the Fairweather fault, with maximum measured displacements of 6.5 m dextral slip and 1 m dip slip near Crillon Lake. Three streams at Crillon Lake that flow on glacial till and bedrock have dextral displacements with maximum offset of roughly 55 m, and a lateral moraine of Finger Glacier southeast of Crillon Lake appears to have been offset at least 50 m in the same sense. Radiometric dating of wood from moraines near Crillon Lake and at Finger Glacier indicates that the offset stream drainages are probably not older than 940 ± 200 years BP and that the offset lateral moraine is about 1300 ± 200 years old. Data from the offset streams indicate that the minimum average displacement rate along this part of the Fairweather fault for the last millennium was at least 4.8 cm year−1 and was probably closer to 5.8 cm year−1.The deduced average offset rate is approximately equal to the full relative displacement of about 5.4 cm year−1 between the Pacific and North American plates in the Gulf of Alaska region that has been deduced from deep sea paleomagnetic data. These data indicate that the Fairweather fault is presently a transform boundary along which most, if not all, of the relative motion between the Pacific and North American plates is currently being taken up. Major valleys crossing the Fairweather fault that are probably older than Sangamon are systematically offset in a dextral sense an average of 5.5 km. These data suggest that the present high displacement rate on the Fairweather fault could not have begun more than 100 000 years ago and, together with data from other onshore faults, imply that before that time the relative motion between the plates was mainly on one or more submarine faults.