Observations of Anomalous Subcrustal Reflections Along the East Pacific Rise: Possible Detection of a Melt Permeability Barrier
Abstract
Crustal accretion at mid-ocean ridges primarily occurs within the narrow neovolcanic zone at the spreading axis, with supplementary lower crustal accumulation thought to originate from the crystallization of magma bodies at the base of the crust. The narrowness of the neovolcanic zone requires melt focusing - a process that has been proposed to arise from the presence of melt impermeable boundaries, or permeability barriers, within the thermal boundary layer near the base of the lithosphere that inhibit the upward migration of melt, effectively focusing it laterally to the ridge axis. Numerical simulations, as well as structural and petrological characteristics of the Oman ophiolite, suggest the existence of such melt impermeable boundaries. A recent analysis of seismic data from the East Pacific Rise (EPR) between the Siqueiros and Clipperton transform faults (8°15'N-10°20'N) reveals anomalous subcrustal reflections ~20 km east of the rise axis and ~20-50 km south of the Clipperton transform. The reflections are characterized by large amplitudes, high frequency content on the order of 20-30 Hz, and a travel time curve that is parabolic with arrival times increasing rapidly at ranges <20 km from the receiver. The approximate depth, slope, and geographical extent of the reflector are estimated by back projecting the onset times of the anomalous reflections into a predefined velocity model. This method reveals that the reflector dips both away from the ridge axis and northward toward the Clipperton transform with a minimum depth below seafloor of ~7.2 km (0.7 km below the Moho) nearest to the ridge. Further off-axis and roughly 20 km to the north, closest to the Clipperton transform, the depth of the reflector increases to ~10.6 km (4 km below the Moho). The slope of the observed reflector thus conforms to the base of the thermal boundary layer (i.e. the 1200-1300° C isotherms) in thermal models adjacent to oceanic transform faults (Roland et al., 2010). The 1240° isotherm is thought to represent the temperature of the plagioclase multiple saturation point that is associated with permeability barriers. We therefore infer the anomalous subcrustal reflections originate from a melt impermeable boundary at the base of the lithosphere. In addition to subcrustal reflectors, we also observe exceptionally bright PmP arrivals at ranges near 80 km that we interpret to stem from melt-sills within the Moho transition zone (MTZ). For melt-filled sills, the reflection coefficient is ~0.1 between incidence angles of 40° to 80° and approaches unity at greater angles of incidence, consistent with our observations. The anomalous PmP arrivals are characterized by a long coda, possibly indicative of reverberations within the medium. Our seismic results are the first possible detection of a melt permeability barrier beneath a spreading center and also indicate a heterogeneous MTZ underlying the EPR. In concert with previous studies, we suggest permeability barriers facilitate melt focusing toward the ridge axis and that crystallization of magma bodies at the base of the crust contribute to crustal accretion.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMOS43A1883A
- Keywords:
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- 3035 MARINE GEOLOGY AND GEOPHYSICS Midocean ridge processes;
- 7218 SEISMOLOGY Lithosphere;
- 7245 SEISMOLOGY Mid-ocean ridges