The global range of lithosphere-asthenosphere properties

Karen Fischer

Brown University

Date & Time
Online-only seminar via Microsoft Teams

The paradigm of plate tectonics is fundamental to our understanding of the Earth, yet the question of what makes the lithosphere “plate-like” remains unanswered. As Earth’s outer thermal boundary layer, the lithosphere derives its high viscosity largely from its cold temperatures, relative to the warmer asthenosphere. However, the roles of partial melt and volatiles in further reducing asthenospheric viscosity are still debated. Seismic wave conversions generated by a velocity gradient within the lithosphere-asthenosphere transition, combined with surface wave tomography, provide key constraints on these questions.

Beneath broad regions of young continental lithosphere, such as the western U.S. and much of Alaska, surface wave constraints on lithospheric thickness are compatible with the depths of lithosphere-asthenosphere velocity gradients implied by converted body waves. However, typical steady-state conductive models consistent with continental heat flow produce temperature and velocity gradients that are too gradual in depth to produce observed converted and scattered body waves. The presence of an additional factor, such as partial melt at the base of the thermal lithosphere, is needed to sharpen lithosphere-asthenosphere velocity gradients in many young continental regions. Anatolia provides an example of an asthenosphere that is particularly warm and rich in partial melt. In contrast, beneath cratons, lithosphere-asthenosphere velocity gradients are often gradual enough in depth to be consistent with a purely thermal origin.

In the oceans, many observations of scattered and reflected body waves indicate velocity contrasts whose depths follow an age-dependent trend. Modeling of fundamental mode Rayleigh waves from the Pacific ocean indicates that cooling plate models with asymptotic plate thicknesses of 85-95 km provide the best overall fits to phase velocities at periods of 25 s to 250 s. These thermal models are broadly consistent with the depths of scattered and reflected body wave observations, and with oceanic heat flow data. However, the lithosphere-asthenosphere velocity gradients for 85-95 km asymptotic plate thicknesses are too gradual to generate observable Sp phases, both at ages less than 30 Ma and at ages of 80 Ma or more. To jointly explain Rayleigh wave, scattered and reflected body waves and heat flow data, oceanic lithosphere can be characterized as a thermal boundary layer with an asymptotic thickness of 85-95 km that also contains other features, such as zones of partial melt from hydrated or carbonated asthenosphere, that enhance the lithosphere-asthenosphere velocity gradient.

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