Off- and Near-Axis Crustal Melt Lenses Accreting the Oceanic Crust

J.P. Canales¹*, M.R. Nedimović², S.M. Carbotte³, G.M. Kent⁴, R.S. Detrick¹, J. Mutter³, and H. Carton³

Corresponding author: jpcanales@whoi.edu
¹Woods Hole Oceanographic Institution, Woods Hole, MA
²Dalhousie University, Halifax, Canada
³Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY
⁴Scripps Institution of Oceanography, La Jolla, CA

Abstract:
Seismic reflection experiments conducted in recent years at the Juan de Fuca Ridge (JdFR) and East Pacific Rise (EPR) have produced the first seismic images of magma lenses accumulating at different crustal levels outside of what has been traditionally thought to be the accretionary zone of intermediate- to fast-spreading ridges. At the Cleft segment of the JdFR, seismic data reveal the presence of a ~2.4-km-long, 5-6-km-deep bright reflector located 1.4-3.2 km to the east of the spreading axis. The amplitude variation with offset of this reflector observed in CMP gathers indicates that it corresponds to an off-axis melt sill within the lower crust, ~850-900 m above the seismic Moho. Preliminary inspection of 3D seismic reflection data collected in 2008 at the EPR near 9°50’N also indicates the presence of at least three regions with prominent off-axis crustal reflectors up to ~7 km off the spreading axis and raging in depth between ~1.8 and 3.1 km. Although the nature of these reflectors is not yet established, their characteristics suggest they are related to crustal melt.

Contributions to Integration and Synthesis:
Geological observations at the Cleft segment near the location of the seismically imaged near-axis melt lens have reported active low-temperature venting ~3.3 km off the spreading axis and young-looking lava flows and small seamounts. Although similar features have yet to be found off the EPR in the region of off-axis crustal reflectivity, the current observations at the JdFR and EPR indicate that it is likely that near- and off-axis crustal melt lenses contribute to accretion of the oceanic crust, and possibly to seafloor eruptions, and may provide heat for off-axis hydrothermal circulation. These new observations should be considered in synthesis models of crustal formation and its relation to hydrothermal circulation.

Figures:
Figure 1. Three-dimensional perspective of crustal structure and seafloor topography at the Cleft segment of the JdFR. a. Southern and eastern parts of seismic profiles 40 and 75, respectively, are shown hanging from the seafloor topography. Moho and lower crustal melt lens (LCML) events are emphasized with colour markers. White dashed line locates the spreading axis. SM is a young-looking, near-axis seamount with chemistry distinct from the axial lava flows. Red triangle locates low-temperature hydrothermal vents observed on the seafloor. Solid lines locate profiles shown in b. b. Profile 80, northern part of profile 40, and central section of profile 75, all shown hanging from the seafloor topography. Moho, LCML, and AMC are emphasized with colour markers. White dashed line locates the spreading axis, which is coincident with profile 80. Figure from Canales et al., (2009) Nature, 460, 89-93. Canales_fig1.jpg