Tracking stress and hydrothermal activity along the Eastern Lau Spreading Center using seismic anisotropy

Abstract

Using P-wave refraction data from the L-SCAN controlled-source seismic experiment, the three-dimensional anisotropy structure of the upper crust is determined along the Eastern Lau Spreading Center (ELSC). The tomographic image of anisotropy magnitude and orientation is constructed from ∼197 000 travel time measurements of P-wave arrivals recorded on 83 ocean bottom seismometers, providing the most detailed and extensive view of upper-crustal spreading center anisotropy to date. P-wave speeds vary with the azimuth of the seismic ray path, a type of anisotropy produced by stress-aligned lithospheric cracks and microcracks. Across the study area, the fast axes of anisotropy are generally oriented parallel to the trend of the spreading center, as expected for ridge-parallel cracks that form in association with seafloor spreading. The seismic study encompassed four individual spreading segments of the ELSC separated by three overlapping spreading centers (OSC). The two larger OSCs exhibit high anisotropy that penetrates deep into the upper crust. Surrounding these OSCs, and in the wake of the largest, are regions where the fast axes of anisotropy are misaligned relative to the general trend. In these areas, the observations agree with numerical models and seafloor morphology that suggest tensile stress concentrations and a high degree of brittle crack formation. The tomographic images indicate that the location and dynamics of persistent ridge offsets can be tracked back through time via their anomalous anisotropy. Along the spreading centers, and away from ridge tips, the anisotropy is greater where the melt supply is inferred to be greater, and smaller where the melt supply is inferred to be smaller. This is opposite to the trend expected if simple tectonic stress models govern anisotropy. Alternatively, hydrothermal activity is expected to increase with magma supply and can explain higher anisotropy via hydrofracturing. This study provides the first evidence that seismic anisotropy tracks variations in hydrologic activity along the crests of oceanic spreading centers.