Dating Off-axis Volcanism Along the 8°20’N Seamount Chain Using Sediment Thickness Proxy from Near-bottom Chirp Images

Abstract:

The 8°20’N seamount chain extends 200 km west from the fast-spreading East Pacific Rise (EPR), parallel to the Siqueiros fracture zone, and provides a unique opportunity to study off-axis volcanism and melt source variability. While dating seamount volcanism is challenging using geochemical methods, the relative age of magmatic constructions can be obtained by measuring the thickness of sediment draped over the basaltic crust. We used near-bottom chirp, bathymetric, and sidescan sonar data collected during 15 dives of autonomous underwater vehicle Sentry to test the hypothesis that seamount volcanism is age-progressive along the 8°20’N seamount chain. We combined these geophysical data with imagery from the submersible Alvin, to evaluate sediment character and thickness. Assuming a constant sedimentation rate from the water column, and the absence of ocean-bottom currents, we find that sediment thicknesses observed on the seamounts do not increase linearly with the surrounding seafloor age. We find ~0.5–1 m thick sediment layers nearest to the EPR, ~2 m thick sediment on top of the oldest seamount (Ivy; 3.6 Ma), and up to ~7 m-thick sediment on the intermediate age Otto Ridge (~1.2 Ma). These results suggest that seamount volcanism likely persists off-axis over periods of millions of years, consistent with observed variations in radiogenic isotope ratios from coincident lavas sampled along the chain. Our findings show that the sediment thickness proxy may be a useful tool for relative age dating, with important implications for melt composition and distribution under fast-spreading conditions.