High-frequency seismic reflections from the slab-mantle interface beneath Costa
Rica indicate fluid release beneath the forearc

Harm Van Avendonk 
UT Austin

The oceanic lithosphere and sediments subducting beneath the volcanic arc of
Central America experience high-pressure and high-temperature metamorphism
during which they gradually release aqueous fluids and hydrous melts. To
understand the transfer of fluids from the subducting lithosphere to the mantle
wedge, it is important to study the fine structure of the very top of the
subducting oceanic crust. We present S-P and S-S seismic reflections from the
Cocos plate slab-interface beneath Costa Rica that were recorded during the
2005 TICOCAVA explosion seismology experiment. These seismic arrivals from a
shot point that was located in the backarc, were recorded by an array of 740
Reftek Texans. Instruments located in the forearc recorded both P and S waves
that must have reflected from the top of the slab beneath the mantle wedge at
depths between 35 km and 58 km. The slab reflections are unique in two aspects:
We clearly observe S-P and S-S reflection over a wide range of incidence angles
at the slab-interface (13 to 53 degrees). In addition, we find that the
frequency of these slab reflections is between 15 and 20 Hz, which is much
higher than the dominant frequency (10 Hz) of turning waves recorded from the
same explosion. Modeling of these slab reflections shows that the slab-mantle
interface may be a ~0.5 km thick zone with roughly 10 layers of just ~5 meters
thickness, inside which the shear velocity is reduced to 1.0-1.5 km/s. The
compressional velocity in these thin layers may not be much lower than in the
overlying mantle wedge or in the underlying subducted oceanic crust. The
presence of pressurized fluids at the slab interface can explain the unusual
seismic structure. Thermal models of the Central American subduction zone
predict that large amounts of water are released from the subducting plate into
the upper portion of the mantle wedge. The presence of a continuous, reflecting
boundary at the top of the slab suggests that these slab fluids travel along
the slab interface before they are absorbed into the overlying mantle. This
form of lubrication of the shear zone may be essential for maintaining
subduction of oceanic lithosphere into the deeper mantle over many millions of
years.