Normal faults play a dominant role in the morphology and internal stru
cture of crust formed at slow-spreading mid-ocean ridges, The surface
expression of these faults has been studied well, but few direct contr
ols are available on their shape at depth. Multichannel seismic reflec
tion profiles of old oceanic lithosphere potentially contain a wealth
of structural information, but the origin of many of the features obse
rved in these profiles remains controversial, Sedimented rifts provide
a rare opportunity to study present-day tectonics of mid-ocean ridges
with conventional seismic reflection profiles, We have used two such
profiles and a recently developed inverse method to infer, from the de
formation of sediments in the hanging mall, the geometry of a normal f
ault bounding the axis of the Juan de Fuca Ridge at Middle Valley, The
fault is listric in cross section and can be traced about 5 km below
the sea bed, Its shape is similar to whole-crust reflectors imaged in
old oceanic lithosphere.