@article {saikia_upper_2021,
	title = {Upper Mantle Anisotropic Shear Velocity Structure at the Equatorial Mid-Atlantic Ridge Constrained by Rayleigh Wave Group Velocity Analysis From the PI-LAB Experiment},
	journal = {Geochemistry, Geophysics, Geosystems},
	volume = {22},
	number = {3},
	year = {2021},
	note = {_eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020GC009495},
	month = {02/2021},
	pages = {e2020GC009495},
	abstract = {The evolution of ocean lithosphere and asthenosphere are fundamental to plate tectonics, yet high resolution imaging is rare. We present shear wave velocity and azimuthal anisotropy models for the upper mantle from Rayleigh wave group velocities from local earthquake and ambient noise at 15{\textendash}40-s period recorded by the Passive Imaging of the Lithosphere Asthenosphere Boundary experiment at the equatorial Mid-Atlantic Ridge covering 0{\textendash}80-Myr-old seafloor. We find slow velocities along the ridge, with faster velocities beneath older seafloor. We image a fast lid (25{\textendash}30-km thick) beneath the ridge that increases to 50{\textendash}60 km beneath older seafloor. Punctuated, \~{}100 km wide low velocity anomalies exist off-axis. There are multiple layers of azimuthal anisotropy, including (i) a lithosphere (20{\textendash}40 km depth) characterized by strong anisotropy (4.0\%{\textendash}7.0 \%) with fast-axes that rotate from ridge subparallel toward the absolute plate motion/spreading direction at distances >60 km from the ridge, and (ii) weak anisotropy (1.0\%{\textendash}2.0\%) at >40 km depth. Our results are consistent with conductive cooling of lithosphere, although with some complexities. Thickened lithosphere beneath the ridge supports lateral conductive cooling beneath slow-spreading centers. Undulations in lithospheric thickness and slow asthenospheric velocities are consistent with small scale convection and/or partial melt. Lithospheric anisotropy can be explained by vertical flow and a contribution from either fluid or mineral filled cracks organized melt beneath the ridge and plate motion induced strain off axis. Deep azimuthal anisotropy is suggestive of upwelling beneath the ridge and three-dimensional flow possibly caused by small scale convection off-axis.},
	keywords = {asthenosphere, azimuthal anisotropy, Lithosphere, Mid-Atlantic Ridge, shear wave velocity},
	issn = {1525-2027},
	doi = {10.1029/2020GC009495},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1029/2020GC009495},
	author = {Saikia, Utpal and Rychert, Catherine and Harmon, Nicholas and Kendall, J. M.}
}
@article {harmon_evolution_2020,
	title = {Evolution of the Oceanic Lithosphere in the Equatorial Atlantic From Rayleigh Wave Tomography, Evidence for Small-Scale Convection From the PI-LAB Experiment},
	journal = {Geochemistry, Geophysics, Geosystems},
	volume = {21},
	number = {9},
	year = {2020},
	note = {_eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020GC009174},
	month = {08/2020},
	pages = {e2020GC009174},
	abstract = {The oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphere-asthenosphere system from 0 to 80 My at the equatorial Mid-Atlantic Ridge using data from the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PI-LAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20{\textendash}30 km near the ridge axis to 70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV < 4.1 km/s) in the asthenosphere between 50 and 150 km depth, not necessarily focused beneath the ridge axis. Some of the slow velocity regions are located within 100 km of the ridge axis, but other slow velocity regions are observed at distances > 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80{\textendash}130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of off-axis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and off-axis character of the asthenospheric anomalies and lithospheric drip suggests small-scale convection is active at a range of seafloor ages. Small-scale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated low-viscosities and enhanced buoyancies.},
	keywords = {asthenosphere, Atlantic Ocean, oceanic lithosphere, PI-LAB, shear velocity, surface wave tomography},
	issn = {1525-2027},
	doi = {10.1029/2020GC009174},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1029/2020GC009174},
	author = {Harmon, Nicholas and Rychert, Catherine A. and Kendall, J. Michael and Agius, Matthew and Bogiatzis, Petros and Tharimena, Saikiran}
}
