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Transcript
Focused melt supply at the Cobb hot spot / Juan de Fuca ridge intersection West, Michael1, William Menke and Maya Tolstoy Lamont-Doherty Earth Obervatory of Columbia University, Palisades, NY 10964 1Now at New Mexico State University, Physics Department, Las Cruces, NM 88005 Interaction between the Juan de Fuca ridge and the nearby Cobb hot spot manifests itself at Axial Volcano, a discrete volcanic edifice with 50 km rift zones parallel to the ridge. The surface expression of this interaction is complex both structurally and geochemically. It has not been clear how, or if, the two closely spaced melt supplies affect each other. Results from an active source seismic experiment show an abrupt increase in crustal thickness near the summit of Axial Volcano. Wide-angle seismic reflections are used to contour the surface of the Moho across the region. Under the caldera, the crust is 11 km, thinning rapidly to 8 km under the flanks of the volcano. Gradual thinning over a 75 km radius returns the crust to a normal ~ 6km thickness. This 11 km root is radially symmetric and no more than 25 km in diameter. It does not extend along the volcano's rift zones to the north and south, nor does it extend toward the previous seamounts created by the Cobb hot spot. The relief on the Moho is steeper and larger than the topside topography of the crust. Steep Moho, despite zero-age crust and the large heat flux associated with the volcano, suggests the root is maintained dynamically. That is, the crustal thickening is a proxy for the delivery rate of magma to the crust. Melt is being supplied to Axial faster than it can be accommodated by ridge spreading. In addition to the thick crust, evidence for the high flux of melt and heat includes the previously-observed mid-crust magma body which is larger and shallower than predicted for an intermediate rate spreading ridge. Yet the small extent of the thickened crust demonstrates that magma is delivered to Axial through a focused channel as narrow as 25 km. The channeling of magma through a narrow corridor suggests that the Cobb hot spot remains distinct from the ridge despite being co-located. Assuming the hot spot provides the melt necessary to construct the additional crust beneath Axial, we estimate local magma flux rates at least as high as under fast spreading ridges. The high flux of melt is accommodated by distributing it along the rift zones through diking events and surface eruptions. The diminished supply of melt to the distal ends of the rift zones is reflected in the progressively thinner crust away from the volcano. This is corroborated by petrologic studies that have found a degree of mixing between the ridge and volcano magmas along the rifts.