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New Constraints on the Factors That Control the Lithosphere-Asthenosphere Transition and the Driving Forces of Plate Motions From the Pi-Lab Experiment

Description: 

The transition from the lithosphere to the asthenosphere is an important part of plate tectonics. The exact depth and the factor(s) that control it are widely debated. The PI-LAB (Passive Imaging of the Lithosphere-Asthenosphere Boundary) included 39 ocean bottom seismometers and 39 ocean bottom magntotelluric instruments near the equatorial Mid-Atlantic Ridge. We find seismic attenuation quality factors of 175 ± 16 at lithospheric depths (< 50 km ) and 90 ± 15 at asthenospheric depths (> 60 km). These values fall within the range of other global results and do not necessarily require melt pervasively in the asthenosphere (Takeuchi et al., 2017; Saikia et al., 2021). SKS splitting measurements reveal predominantly weak anisotropy (< 1 s) with fast directions that are mostly consistent with strain caused by plate motions, again potentially consistent with a thermal model. However, near the ridge axis SKS splitting is strong (1.7 – 3.7 s) with predominantly ridge parallel fast directions. This is much different than observations from other ridge systems and likely requires the presence of aligned melt beneath the ridge. This melt organisation may be facilitated by the existence of a thick, sub-ridge lithosphere, as predicted by models for slow-spread lithosphere (Parmentier and Morgan, 1990; Holtzman and Kendall, 2010). Our new body wave tomography model includes a 10 – 20 km thick low velocity channel in the southeast and a strong punctuated anomalies in the north. I will discuss a range of causes for these observations. However, the striking agreement between seismic and magnetoteulluric imaging and very strong anisotropy near the ridge likely require a small amount of partial melt in discrete regions, given the independent and complementary sensitivities of the methodologies. Therefore, while temperature plays a first-order role in dictating the lithosphere-asthenosphere transition, melt can also affect the location and sharpness of this interface. This melt likely plays an important and dominant role in driving the plates and facilitating their motions.

Session: Advances in Marine Seismoacoustics

Type: Oral

Date: 4/18/2023

Presentation Time: 03:00 PM (local time)

Presenting Author: Catherine Rychert

Student Presenter: No

Invited Presentation: Yes

Authors