AGE VARIATIONS OF OCEANIC-CRUST POISSONS RATIO - INVERSION AND A POROSITY EVOLUTION MODEL

Porosity in the oceanic crust is one of the most important factors inf luencing measured seismic velocities. Porosity is particularly importa nt in the uppermost young crust, where rapid variations in velocities with depth and crustal age are observed. Knowledge of the concentratio n and aspect ratios of inferred crack populations can be improved cons iderably if estimates of Poisson's ratio are available from observatio ns of compressional and shear seismic velocities upsilon(p) and upsilo n(s). In this paper I present a joint seismic waveform inversion for u psilon(p) and upsilon(s); velocities are found while maximizing or min imizing Poisson's ratio using a hypothesis-testing mechanism. I apply this method to ocean bottom hydrophone data in 140 Ma Atlantic crust; the resulting solution corridor agrees with laboratory measurements wi thout the low Poisson's ratio anomalies at depths of 0.8-1.5 km found by Spudich and Orcutt (1980) and Au and Clowes (1984) on younger (< 15 Ma) Pacific crust. Compiling other published upsilon(p) and upsilon(s ) solutions, an age-dependent pattern emerges: none of the solutions f or crust older than 60 Ma display the Poisson's ratio anomaly. I propo se a simple crustal evolution model, using thin and thick cracks, to e xplain these observations: thin cracks preferentially close at shallow depths in the crust, producing the localized Poisson's ratio anomaly. Sealing of all cracks by hydrothermal deposits as the crust ages rest ores the seismic velocities to consistency with laboratory measurement s. This model is consistent with similar models of crack populations a nd their evolution from shallow measurements.