(
2014-09)
Marjanovic, Milena; Carbotte, Suzanne M.; Carton, Helene; Nedimovic, Mladen R.; Mutter, John C.; Canales, J. Pablo
The mid-crust axial magma lens detected at fast and intermediate spreading
mid-ocean ridges is believed to be the primary magma reservoir for formation of
upper oceanic crust. However, the mechanism behind formation of the lower crust is a
subject of ongoing debate. The sheeted sill model proposed from observations of
ophiloites requires the presence of multiple lenses/sills throughout lower crust but
only a single lens is imaged directly beneath the innermost axial zone in prior seismic
studies . Here, high-fidelity seismic data from the East Pacific Rise reveal series of
reflections below the axial magma lens that we interpret as mid-lower crustal lenses.
These deeper lenses are present between 9°20-57′N at variable two-way-travel-times,
up to 4.6 s (~1.5 km beneath the axial magma lens), providing direct support for the
sheeted sill model. From local changes in the amplitude and geometry of the events
beneath a zone of recent volcanic eruption, we infer that melt drained from a lower
lens contributed to the replenishment of the axial magma lens above and, perhaps, the
eruption. The new data indicate that a multi-level sill complex is present beneath the
East Pacific Rise that likely contributes to the formation of both the upper and lower
crust.
