On origin of near-axis volcanism and faulting at fast spreading mid-ocean ridges

At fast and superfast spreading mid-ocean ridges, such as the East Pacific Rise, a plate boundary is defined by a narrow (tens to hundreds of meters w ide) neovolcanic zone within which the bulk of the upper oceanic crust is c reated. However, detailed near-bottom observations indicate that the volcan ic construction may occasionally persist several kilometers off of the ridg e axis. It has been proposed that off-axis volcanism manifests tapping of a wide melting region that supplies magma to the ridge axis, or spatial migr ation of magmatic sources in the crust and upper mantle. We demonstrate tha t off-axis eruptions may be a natural consequence of variations in magma su pply rate even if the ridge axis is stationary in space, and the magma deli very is perfectly focussed at the ridge axis. Theoretical modeling and fiel d observations indicate that off-axis volcanisin may result from magma empl acement in sills that propagate toward the surface after their characterist ic horizontal size exceeds their emplacement depth. Volcanic construction a nd faulting due to sill intrusions may contribute to the formation of abyss al hills, arguably the most abundant relief form on Earth. (C) 2001 Elsevie r Science B.V. All rights reserved.