A 36-km-wide corridor of Seabeam bathymetry was collected on the Juan de Fuca Ridge, north of 48°00′N, in May 1983. These data, merged with previous Seabeam bathymetric compilations for the Cobb Offset, as well as with Sea MARC I and Sea MARC II side-scan sonar, seismic-reflection, magnetometer, and deep-tow photography data, have been used to identify morphological variations along the spreading center of the Endeavour Segment of the Juan de Fuca Ridge (between the Cobb Offset and Sovanco Fracture Zone). The Endeavour Segment consists of two ridge sections, separated by the 13-km-wide Endeavour Offset, which formed when spreading jumped from Middle Valley to West Valley < 200,000 yr ago. The locus of spreading occurs along the entire Endeavour Segment as a narrow (1–2 km wide), shallow (10–30 m deep) inner rift, which is superimposed upon larger structures that range from broad (5–10 km wide), deep (as much as 3,000 m) median valleys to narrow (5 km wide), shallow (2,100 m) volcanic ridges. An abrupt discontinuity in the tectonic fabric occurs at about 48°05′N. South of this latitude, spreading occurs within an axial high (Endeavour Ridge), which gradually deepens to the south (South Endeavour Valley) as it approaches the zone of overlap with the propagating rift at the Cobb Offset. North of 48°05′N, the axis is characterized by broad, deep, median valleys (North Endeavour Valley and West Valley).

These large-scale morphological variations reflect a complex interplay of the spreading center with seamount volcanism, waning magma supply at the distal ends of ridge axes, thermal contrasts across ridge offsets, propagating rifts, and an unstable triple junction. The pronounced axial deep of West Valley is interpreted as reflecting the youth of the structure (< 200,000 yr) and may be primarily due to collapse, with little extension and magmatism. The rift axis jump to West Valley fortuitously isolated Endeavour Seamount, the youngest edifice in the Heck Seamount chain, from the Pacific plate and trapped it between a pair of overlapping spreading centers at the newly formed Endeavour Offset. Development of a remarkably simple overlapping conjugate rift pair at Endeavour Offset has resulted, in spite of the complex and highly variable crustal thickness, magma supply, and pre-existing structural grain. Rifting in South West Valley has caused pre-existing topography generated at the Endeavour Ridge to be destroyed by subsidence and burial, thereby creating the apparent discontinuity in morphology north of 48°05′N.

A folded insert (Fig. 2B) accompanies this article.

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