Bjarne, that sentence of mine that you quoted so selectively came immediately before a short description of the evidence. I will say it again here: not only can tectonic motion itself be observed directly, but subducting slabs can be imaged within the mantle by seismic tomography and earthquake focal mechanism studies.
Allow me to expand on that. Seismic tomography allows one to image the deep Earth by analysis of seismic wave speeds. Slabs of lithosphere which have been subducted into the mantle are revealed by this method, since they have different seismic properties to their surroundings. A
quick search on Google Scholar will be a good place to start if you're interested.
Another method by which subducting lithospheric slabs can be imaged is in analysis of the earthquakes which occur within them. The depth of subduction-related earthquakes, and their distance from the surface trace of the subduction zone, plot the course of the sinking slab. Furthermore, the variation of earthquake focal mechanisms throughout the slab is just as one would expect in such a situation.
You seem to have misunderstood me. I did not claim that subduction zones are the only source of seismicity. I do not know much about the tectonics of Denmark, but I would guess that earthquakes there might be caused by isostatic readjustment of the crust since the removal of the ice sheets.
Bjarne, these are incredibly unscholarly references to cite in support of your argument. In the video that focuses on India and the Himalayas, for example, the narrator asks "how can an island uproot itself from the ocean floor [and] seemingly float across the ocean?" This is such an elementary misunderstanding that I can scarcely believe it's not a deliberate misrepresentation; I don't know which paints the maker of the video in a worse light. Is this the quality of evidence that you really intended to bring to this discussion?
In the same video it is claimed that the plain of India to the south of the Himalayas was stretched. This is a claim that is falsified by the reverse faulting which characterises the southern edge of the Himalayan arc. If mountain ranges were built by the adjustment of continental plates to a greater radius of curvature, then compressional structures at the surface would be mirrored by extensional structures at the base of the lithosphere, and the thickness across the plates would be conserved. This is the opposite of what is revealed to us by seismic and gravimetric studies. Furthermore, if the expansion of the Earth is accommodated at the surface by creation of new oceanic crust around relatively undeformed continents, reverse faulting in the flexing continents would be expected to occur at their centres and not their margins. Can the theory you support explain crustal shortening in the Andes?
One of the videos shows the deformation of North America, with Hudson Bay appearing to grow like a rifting ocean basin. This situation, if it were reality, would be easily provable by the detection of extensional faults and active rifting perpendicular to the axis of extension (appearing to be east-west in the video). Do I need to tell you that this idea conflicts with reality?
What relationship do you think Gravity Probe B has to plate tectonics? I was not aware of a link.
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but then ....