En Echelon Volcanic Ridges Along Seamount Chains Result from Episodic Changes In Stress Orientations That Open Cracks to the Asthenosphere and Permit Magma Ascent: They do not Require Plumes

E.L. Winterer & M.A. Lynch

Scripps Institution of Oceanography, 9500 Gilman Dr., La Jolla, CA 92093-0220


An alternative to the plume/hotspot hypothesis for seamount chains is formation via cracks through the lithosphere. Many Pacific Cenozoic seamount chains comprise intermittently spaced volcanic ridges aligned en echelon to the overall trend of the chain, a pattern that reflects tensional stresses in the lithospheric plate at angles to the trend. The overall trend is a line of incipient cracking close to the average direction of plate motion in the fixed-Antarctica reference frame. The ridges mark episodic, relatively local deviations in the orientation of the stress field, permitting the incipient tensional cracks to open through the lithosphere to the asthenosphere. The upper parts of the asthenosphere are at the solidus temperature, as manifested in lower seismic velocities indicating the presence of small fractions of melt, such that through-going cracks allow magmas to form and ascend toward the surface where they erupt to form the volcanoes and en echelon ridges. High fertility of the source region favors increased volumes of magma. Cracks typically break though in the younger parts of the lithosphere, which is thinner and weaker than older lithosphere, but cracking is possible anywhere along the volcanic trend where the lithosphere is thin or weak. Cracking is common, for example, along the thinned lithosphere of the boudinage-like structures imaged on regional gravity maps, as in the Pukapuka chain, which follows one of the regional gravity lows. There, the time sequence of volcanic ridges is not progressive. The markedly different orientation of the youngest parts of the Hawaiian chain, compared to the long-term average trend of the chain, may record a change in regional stress orientations in the Pacific plate beginning about 3-4 Ma, reflected also in the Marquesas and younger parts of the Society Islands. The en echelon crack mechanism requires no excessive "hotspot" temperatures and no plumes.