Evolution and properties of young oceanic crust: constraints from Poisson's ratio

• Published in: Geophysical journal international 2021-06, Vol.225 (3), p.1874-1896
• Main Authors: Funnell, M J, Robinson, A H, Hobbs, R W, Peirce, C
• Format: Article
• Language: English
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1093/gji/ggab062

SUMMARY The seismic velocity of the oceanic crust is a function of its physical properties that include its lithology, degree of alteration and porosity. Variations in these properties are particularly significant in young crust, but also occur with age as it evolves through hydrothermal circulation and is progressively covered with sediment. While such variation may be investigated through P-wave velocity alone, joint analysis with S-wave velocity allows the determination of Poisson's ratio, which provides a more robust insight into the nature of change in these properties. Here we describe the independent modelling of P- and S-wave seismic data sets, acquired along an ∼330-km-long profile traversing new to ∼8 Myr-old oceanic crust formed at the intermediate-spreading Costa Rica Rift (CRR). Despite S-wave data coverage being almost four-times lower than that of the P-wave data set, both velocity models demonstrate correlations in local variability and a long-wavelength increase in velocity with distance, and thus age, from the ridge axis of up to 0.8 and 0.6 km s−1, respectively. Using the Vp and Vs models to calculate Poisson's ratio (σ), it reveals a typical structure for young oceanic crust, with generally high values in the uppermost crust that decrease to a minimum of 0.24 by 1.0–1.5 km sub-basement, before increasing again throughout the lower crust. The observed upper crustal decrease inσ most likely results from sealing of fractures, which is supported by observations of a significant decrease in porosity with depth (from ∼15 to