Normal faulting and mass movement during ridge subduction inferred from porosity transition and zeolitization in the C osta R ica subduction zone

Subduction of the buoyant Cocos Ridge offshore the Osa Peninsula, Costa Rica substantially affects the upper plate structure through a variety of processes, including outer forearc uplift, erosion, and focused fluid flow. To investigate the nature of a major seismic reflector (MSR) developed between slope sediments (late Pliocene‐late Pleistocene silty clay) and underlying higher velocity upper plate materials (late Pliocene‐early Pleistocene clayey siltstone), we infer possible mechanisms of sediment removal by examining the consolidation state, microstructure, and zeolite assemblages of sediments recovered from Integrated Ocean Drilling Program Expedition 344 Site U1380. Formation of Ca‐type zeolites, laumontite and heulandite, inferred to form in the presence of Ca‐rich fluids, has caused porosity reduction. We adjust measured porosity values for these pore‐filling zeolites and evaluated the new porosity profile to estimate how much material was removed at the MSR. Based on the composite porosity‐depth curve, we infer the past burial depth of the sediments directly below the MSR. The corrected and uncorrected porosity‐depth curves yield values of 800 ± 70 m and 900 ± 70 m, respectively. We argue that deposition and removal of this entire estimated thickness in 0.49 Ma would require unrealistically large sedimentation rates and suggest that normal faulting at the MSR must contribute. The porosity offset could be explained with maximum 250 ± 70 m of normal fault throw, or 350 ± 70 m if the porosity were not corrected. The porosity correction significantly reduces the amount of sediment removal needed for the combination of mass movement and normal faulting that characterize the slope in this margin.