No spreading across the southern Juan de Fuca Ridge axial cleft during 1994-1996

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 degrees 40' N and 130 degrees 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 (similar to 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.