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

Abstract

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 datasets, 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 dataset, 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 in σ most likely results from sealing of fractures, which is supported by observations of a significant decrease in porosity with depth (from ∼15 to < 2 per cent) through the dyke sequence in Ocean Drilling Program borehole 504B. High Poisson's ratio (>0.31) is observed throughout the crust of the north flank of the CRR axis and, whilst this falls within the ‘serpentinite’ classification of lithological proxies, morphological evidence of pervasive surface magmatism and limited tectonism suggests, instead, that the cause is porosity in the form of pervasive fracturing and, thus, that this is the dominant control on seismic velocity in the newly formed CRR crust. South of the CRR, the values of Poisson's ratio are representative of more typical oceanic crust, and decrease with increasing distance from the spreading centre, most likely as a result of mineralisation and increased fracture infill. This is supported by borehole observations and modelled 3-D seismic anisotropy. Crustal segments formed during periods of particularly low half-spreading rate (<35 mm yr−1) demonstrate high Poisson's ratio relative to the background, indicating the likely retention of increased porosity and fracturing associated with the greater degrees of tectonism at the time of their formation. Across the south flank of the CRR, we find that the average Poisson's ratio in the upper 1 km of the crust decreases with age by ∼0.0084 Myr−1 prior to the thermal sealing of the crust, suggesting that, to at least ∼7 Myr, advective hydrothermal processes dominate early CRR-generated oceanic crustal evolution, consistent with heat flow measurements.

Application of a seismic network to baleen whale call detection and localization in the Panama basin–a Bryde's whale example

“…This study uses data recorded by OBSs and a vertical array of hydrophones provided by the UK's OBIF (Ocean-Bottom Instrumentation Facility -see Acknowledgements) and deployed during RRS James Cook cruise JC114 of the OSCAR research project (Hobbs & Peirce, 2015). The primary objectives of this deployment were to image the oceanic crust around the CRR and study fluid interactions between crust and the ocean through the acquisition of multi-channel seismic reflection data and wide-angle seismic profiles (e.g., Gregory, 2018;Wilson et al, 2019;Robinson et al, 2020;Funnell et al, 2021;Peirce et al, 2023). The acquired data, thus, has periods with and without active-source seismic operations.…”

“…To contribute to the study of whales in the Eastern Tropical Pacific Ocean region, we take advantage of the deployment of oceanbottom seismographs and hydrophones close to the CRR in the Panama basin (Hobbs & Peirce, 2015) (Figure 1). These instruments were originally deployed to study the lithospheric structure of the CRR and its surroundings using mainly seismic data (e.g., Wilson et al, 2019;Robinson et al, 2020;Funnell et al, 2021;Peirce et al, 2023). More specifically, we here focus on the analysis of whale calls observed in the data of the North Grid (NG) where 25 closely-spaced OBSs and a vertical array (VA) of 12 hydrophones were deployed in January-February 2015 (Figure 1).…”