AVA analysis reveals in situ sediment diagenesis at the Costa Rican décollement

Complete sediment subduction at the Costa Rica subduction zone makes this convergent margin an ideal place to investigate the effects of tectonic deformation in situ. We present a seismic reflection study along a line located 3 km landward of the Middle American Trench and oriented parallel to the strike of the décollement. The Ocean Bottom Hydrophone (OBH) seismic data include large offsets and incidence angles at the reflectors. We derive the P- and S-waves velocity distribution below the décollement using a P-wave analysis of amplitude with reflection angle. The investigation shows that there are unexpected large lateral velocity variations at a scale of only a few 100 m. The shear wave velocity in the uppermost subducted sediment varies between 300 and 700 m/s, while the variation of the compressional wave velocity is in a range of 1700 to 2000 m/s. The variation of the vP–vS ratio between 2.8 and 5.2 can only be explained by variations of the pore fluid pressure. The modelled velocities correspond to a normalised pore fluid pressure ratio λ* in the range between 0.02 and 0.93. The most reasonable explanation for these observations is the localised presence of fluids, which are released during diagenesis by smectite to illite transformation. During this process, which takes place in three discrete steps, the interlayer water of the smectite is added to the pore fluid and the permeability of the sediment is decreased. Both effects lead to the formation of small, overpressured cells.