Download Focused melt supply at the Cobb hot spot / Juan de Fuca ridge

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.