Tectonic signatures in centimeter‐scale velocity‐porosity relationships of Costa Rica convergent margin sediments

Abstract

The Costa Rica convergent margin, investigated during Ocean Drilling Program Leg 170, offers a unique opportunity to compare the tectonic effects of rapid subduction on incoming oceanic sediments to their laterally equivalent underthrust counterparts and to terrestrially derived wedge materials. Elevated pressure laboratory measurements of ultrasonic compressional and shear wave velocity, and porosity, are used to examine the importance of tectonic, lithologic, and diagenetic controls on physical and elastic properties of sediments in these three key tectonic domains. Depositional and stress path histories of the three domains, for example, can be distinguished by (1) trends of in situ velocity‐porosity (and derived measurements) correspondence and (2) the mechanical response of representative materials to isotropic consolidation. A compressional wave velocity‐porosity model, critical for the application of seismic imaging to margin‐wide physical property and mass balance estimates, is developed from the laboratory measurements and shown to be consistent with the information derived from LWD bulk density and migrated seismic reflection data. This consistency of the velocity‐porosity model over the large range of both frequency and length measurement scales is a key result, supporting the assertion that core and borehole physical property measurements can be extrapolated to larger domains. Finally, dewatering and overpressure effects, critical factors in subduction zone and fault process dynamics and increasingly common multiphase/converted wave imaging targets, are discussed in terms of laboratory‐estimated in situ compressional and shear wave velocity relationships.