STRUCTURE OF THE NORTHERN SYMMETRICAL SEGMENT OF THE JUAN-DE-FUCA RIDGE

A seismic refraction profile was shot along the axis of the Northern S ymmetrical Segment of the Juan de Fuca Ridge system. Three models of t he along-axis crustal structure fit the observed data equally well. On e model includes a low-velocity zone, the top of which is at a depth b elow the seafloor of approximately 3 km, that is continuous along-axis for at least 30 km. A second model includes a low-Q layer, the top of which is also at a depth of approximately 3 km below the seafloor and is continuous along-axis for at least 30 km. Both the low-Q layer and low-velocity zone can be explained geologically by a region of elevat ed temperatures. The third model is characterized by a homogeneous sei smic layer 3. All models contain an approximately 1 km s-1 discontinui ty at the seismic layer 2/3 boundary; a wide-angle reflection from thi s boundary is seen on all record sections. Kappel and Ryan (1986) had previously proposed that the Northern Symmetrical Segment was in a sta ge of volcanic inactivity, and this theory is supported by the seismic observations. Two-dimensional modelling of travel times to ocean bott om hydrophone instruments shows that the amplitude variations in the a long-axis depth to intracrustal seismic layers (a few hundred meters) is on the order of the lateral changes in topographic relief. It is su ggested that the crustal emplacement processes reflect the deeper styl e of 3-D mantle upwelling beneath the ridge.