| Abstrakt: |
The ocean lithosphere is classically described by the thermal half‐space cooling (HSC) or the plate models, both characterized by a gradual transition to the asthenosphere beneath. Scattered waves find sharp seismic discontinuities beneath the oceans, possibly from the base of the plate. Active source studies suggest sharp discontinuities from a melt channel. We calculate synthetic S‐to‐Preceiver functions and SSprecursors for the HSC and plate models and also for channels. We find that the HSC and plate model velocity gradients are too gradual to create interpretable scattered waves from the base of the plate. Subtle phases are predicted to follow a similar trend as observations, flattening at older ages. Therefore, the seismic discontinuities are probably caused by a thermally controlled process that can also explain their amplitude, such as melting. Melt may coalesce in channels, although channels >10 km thick should be resolvable by scattered wave imaging. The ocean plates form in ocean basins at mid‐ocean ridges, where two plates diverge. At the mid‐ocean ridge, the tectonic plates are hot and buoyant. As they move away from the ridge and age, they become cooler, denser, and subside. Conductive cooling models explain many observations like bathymetry of the seafloor and heat flow. These thermal models predict a gradual transition from the ocean plate to the weaker mantle below. However, scattered seismic energy images the plate at depth, finding sharp discontinuities related to the base of the plate. Here we quantitatively demonstrate that the thermal models predict scattered phases that are too weak to explain observations. Interestingly, the age‐depth trends of the predicted weak scattered phases match those from observations. Therefore, the seismic discontinuities are caused by a thermally controlled process that also explains their amplitude, such as melt. Further, some active source seismic studies have suggested that melt may coalesce in channels beneath the plate. Here we show that global seismic studies should be able to detect these features but do not. This suggests a greater degree of complexity than accounted for in our models and/or that melt channels are transient features. Weak scattered phases are predicted from half‐space and plate models, increasing in depth with age, and flattening to 50–80 kmA match with observed scattered wave depths, but not amplitudes, suggests a thermally controlled process, e.g., melt defines the plateTeleseismic scattered phases should resolve an LAB melt channel imaged by active source studies, suggesting greater complexity |
Plný text