Determining the origin of Haxby lineaments using magnetotelluric and bathymetric data

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An unsolved mystery of the deep ocean seafloor is the presence of regular, wave-like fluctuations in the strength of Earth’s gravity above the seafloor. These features repeat over regular intervals and can be clearly detected by satellites orbiting the Earth. There are three theories for the processes forming the gravity fluctuations: seafloor cracking as ocean crust moves away from mid-ocean ridges and cools, mantle convection cells, or melt channels beneath the crust. This project will find out which of these theories is correct - or perhaps discover the answer is something altogether unexpected - by acquiring geophysical data at a region in the Pacific Ocean where the gravity features are particularly prominent. Regardless of what is causing the gravity fluctuations, the answer will reveal more about the dynamics of our planet beneath the oceans and how plate tectonics works.The objective of this study is to determine the origin of Haxby lineaments which are small, periodic undulations in the gravity field above oceanic lithosphere that, despite decades of study, remain mysterious. This project will deploy 40 ocean-bottom broadband magnetotelluric instruments to measure variations in mantle conductivity in the lithosphere and asthenosphere, and will map the seafloor topography and sonar backscatter, in a 200 km by 600 km area northeast of the Marquesas Islands in the Pacific Ocean basin. By correlating conductivity, bathymetry, and gravity, the project will be able to distinguish between multiple competing models attempting to explain the Haxby lineaments. The proposed experiment would effectively discriminate between three hypotheses: thermal contraction of the lithosphere, small-scale convection in the asthenosphere, and asthenospheric return flow channels. The thermal contraction model predicts that bathymetry should be positively correlated with the gravity rolls without an asthenospheric component. The small-scale convection and return flow channels models suggest an asthenospheric source of the gravity lineaments - hot, low-density mantle detectable by magnetotelluric data - but differ in how the conductivity anomalies correlate to topography and gravity. The results will help determine the nature of lithosphere-asthenosphere dynamics far from ridges or subduction zones, including whether there is partial melt beneath older lithosphere (42 Ma).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.