Tectonic structures on the Juan de Fuca Ridge

Abstract

The northern Juan de Fuca Ridge is an actively spreading part of the East Pacific Rise system that is flooded with young turbidite sediments from the nearby continental margin. A detailed geophysical survey was completed at the intersection of the ridge with the Sovanco Fracture Zone. North-south and east-west seismic reflection profiles, spaced 10 km apart on a grid 110 km on a side, were taken with a medium-frequency (200 Hz) electromagnetic source. Profiles give as much as 1.5 sec of penetration, yet show very detailed structure in the turbidite layering. The ridge-crest area is typified by its two axial valleys and associated normal-fault blocks rotated away from the crest. These fault blocks, made strikingly visible by the once-horizontal turbidites, range in horizontal dimension from less than 1 km to 8 km, and they are rotated as much as 8°. Normal faults are clearly visible in the sediments of the axial valley (Middle Valley) as discontinuities along acoustic reflectors. These faults dip toward the center of the valley at angles from about 45° to nearly vertical (>80°). Of the two axial valleys, only one (West Valley) is currently active. Profiles and bottom photographs show pillow basalt cropping out over most of the West Valley floor, but some rifted sediments exist as well. The second axial valley (Middle Valley) appears to be currently inactive. It is completely sediment filled (sediment thickness as much as 3.0 km), and although clearly visible normal faults extend through the full section seen in the profiles, extensional rates calculated on these faults are only about 0.2 cm/yr. Over the rest of its length, the Juan de Fuca Ridge crest has no significant axial valley. Only on its northern end near its intersection with the Sovanco Fracture Zone does it develop an axial valley. Similar observations have been made on the Explorer Ridge and on the East Pacific Rise, Gulf of California, although the mechanism for this is not yet understood. Other observations of axial valleys in general — namely, the nonisostatic relief of the valleys, their dependence on spreading rate for generation, and the formation and rotation of normal-fault blocks away from the valley floors — all seem best explained by a passive mechanism where complete crust and upper mantle accretion occurs by passive intrusion over a zone wider than the width of the initial formation of a rigid surface layer.