No spreading across the Southern Juan de Fuca Ridge axial cleft during 1994–1996

Abstract

Direct‐path acoustic measurements between seafloor transponders observed no significant extension (−10±14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44°40′ N and 130°20′ W). The acoustic path for the measurement is a 691‐m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full‐spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (∼ 1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft‐parallel 714‐m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5±7 mm/yr) between the 1994 and 1996 surveys.