On origin of near-axis volcanism and faulting at fast spreading mid-ocean ridges Yuri Fialko * Seismological Laboratory, California Institute of Technology, Pasadena, CA, USA Received 5 January 2001; received in revised form 30 April 2001; accepted 14 May 2001 Abstract 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 wide) neovolcanic zone within which the bulk of the upper oceanic crust is created. However, detailed near-bottom observations indicate that the volcanic construction may occasionally persist several kilometers off of the ridge 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 migration of magmatic sources in the crust and upper mantle. We demonstrate that off-axis eruptions may be a natural consequence of variations in magma supply rate even if the ridge axis is stationary in space, and the magma delivery is perfectly focussed at the ridge axis. Theoretical modeling and field observations indicate that off-axis volcanism may result from magma emplacement in sills that propagate toward the surface after their characteristic horizontal size exceeds their emplacement depth. Volcanic construction and faulting due to sill intrusions may contribute to the formation of abyssal hills, arguably the most abundant relief form on Earth. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: mid-ocean ridges; magmas; sills; models; abyssal hills 1. Introduction At the fast spreading East Paci¢c Rise (EPR), most of the upper oceanic crust is accreted in a relatively narrow (several tens to several hundreds of meters wide) neovolcanic zone, as inferred from the sea£oor morphology [1,2], hydrothermal activity [3,4], and lava geochemistry [5,6]. Geo- morphic expression of the neovolcanic zone ap- proximately coincides with the lateral extent of a shallow magma lens beneath the ridge axis. Seis- mic imaging of the sub-axial magma lens suggests that the latter resides at depth of V1^2 km below the sea£oor, and is about 1^2 km wide, and a few tens to a few hundreds of meters thick [7^9]. The shallow magma lens is believed to be a primary source of basalts comprising the upper oceanic crust at the fast spreading mid-ocean ridges (e.g., [10]). While it is generally accepted that the axial neo- volcanic zone is the main locus of magmatic con- struction at the EPR, detailed ocean £oor surveys including visual observations from submersibles and rock sampling along the northern EPR reveal 0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII:S0012-821X(01)00376-4 * Present address: Institute of Geophysics and Planetary Physics, Scripps Institute of Oceanography, University of California San Diego, La Jolla, CA, USA. Tel.: +1-858-822-5028; E-mail: ¢alko@radar.ucsd.edu Earth and Planetary Science Letters 190 (2001) 31^39 www.elsevier.com/locate/epsl