Gravity Rolls

Overview The objective of this study is to determine the origin of Haxby lineaments (Haxby & Weissel, 1986), which are small, periodic undulations in the gravity field above oceanic lithosphere that, despite decades of study, remain mysterious. The results of this study will advance our understanding of the dynamics of the oceanic lithosphere-asthenosphere system between mid-ocean ridges and sub- duction zones. To determine the origin of Haxby lineaments, we propose to deploy 40 oceanbottom broadband magnetotelluric instruments to measure variations in mantle conductivity in the litho- sphere and asthenosphere, and to 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 the measured conductivity and bathymetry with the gravity anomalies, we will be able to distinguish between multiple competing models attempting to explain the Haxby lineaments. Intellectual Merit Despite being discovered over three decades ago, the origin of Haxby lineaments is still debated. Our proposed experiment would e↵ectively discriminate between three of them: thermal contrac- tion of the lithosphere, small-scale convection in the asthenosphere, and asthenospheric return flow channels. The thermal contraction model predicts that bathymetry should be positively corre- lated with the gravity rolls without an asthenospheric component. The small-scale convection and return flow channels models, on the other hand, suggest an asthenospheric source of the gravity lineaments—hot, low-density mantle detectable by magnetotelluric data—but di↵er in how the conductivity anomalies correlate to topography and gravity. This means that a combination of conductivity and bathymetry measurements compared to the gravity observations should be suf- ficient to distinguish between these three models. The answer 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). Broader Impacts The area we proposed to survey is one of the most remote and poorly surveyed in the oceans. The data will be contributed to global seafloor maps being assembled by the SeaBed 2030 project. In addition, a postdoc and several students will participate in the cruise and geophysical data anal- ysis which is important training for the next generation of seagoing scientists. In addition, these research e↵orts will be accompanied by an educational outreach plan, implemented through the UCSD’s CREATE teaching center. This plan will include training K-12 educators and demon- strating geodynamics in action for K-12 students and their teachers, as well as participation in the Teacher Professional Development program hosted by CREATE.