I'm no geologist but doesn't this explain mountains well enough?
Understanding the Formation of the Rocky Mountains
I hope you would agree that the more information one has the closer they are to knowing how -things-actually-work, not just the topic of this thread.
In the paragraph below it is saying BC and Alberta came at each other from different directions. That was a result of the North American plate riding over the Pacific plate, if the Pacific plate is moving northwards at the same time then there should be a definite mark where that begins to happen. In California it is marked by a rift that shows not only northward movement but also movement in opposite directions.
Google Earth 5 shows the floors of the oceans. The floor of the Atlantic has a spreading Zone, right. It goes in a jagged line that is mostly north/south. At about 24 degrees N it takes a jog to east/west. That should mean the spreading is also in that direction rather than it being a sheering-only motion. If that crack is expanding then all the other cracks (and there are many) that are similar would also be expanding.
The north/south rift is expanding east/west at (say) 5cm/yr. If there are 1,000 cracks in a east/west direction that results in movement to the north/south from all 1,000 cracks then all cracks would have a combined movement that was equal to the 5cm/yr.
In the Pacific things work quite differently. From a height of 4,000 miles you can see 3 formations that run east/west that are expansion rifts. The lowest one runs on a line going from Hawaii to the bottom of Baja. The other two are equally spaced between there and Alaska. Rather than a main rift like the Atlantic the Pacific has a spider web of them. When the Rockies were formed it was from a sudden cracking of the crust and an outflow from the mantle. The main fault for that goes from California to the western tip of Alaska. That rift was originally on the coast-line of North America, in the 180 million years it has been spreading it has created about 400 miles of new land between Vancouver BC, on the other side of the rift about 5,000 miles of new land has been created in that same time. The North American plate is the reason so little new land has been formed there. At first BC was right above the rift so there was only uplifting, all outflow was was to the west. The land masses that surround the Atlantic also provide resistance to expansion, that is why it is so much smaller than the Pacific.
Once you wind back the clock to where there were no modern oceans and all the current land masses fit tightly together and remained just like that for eons. The water that makes up what we have today could have been in any combination of solid, liquid, and gas. A large shallow sea rather than the deep oceans of today and high land masses. Enter a icy comet the size of Hudson's Bay at such a high rate of sped that it enters deep into the mantle.
Yowzer!! Thar she blows!!! Would be like Mr Creosote in the Meaning of Life.
This is where Cliffy's idea comes into play. If the crust was much thinner the ice would make it to the mantle. That sudden change from ice (smallest size) to super-heated gas (largest size 600km cube of ice) would be an explosive type of event. The 1st crack would have been mantle outflows that mark Canada/Russia today. Greenland and the Canadian Islands broke off later because they first moved towards the Canadian/US border and then changed direction (floating means you seek the lowest altitude), the rest of the crust experienced a tearing motion because the force of the expansion of that water-vapor was enough to lift North America. The drop (of liquid mantle) that is ejected at fist impact and then returns was about 300km in dia. Yellowstone would have been a burp before the beginning of the Rockies being built . The continents would have looked like blankets being shook (slowly) as the first pressure wave went all through the upper mantle.
The above is an instance where very little added mass (600km round comet of ice) can cause much more damage than an iron one of the same weight as one that size if iron might be a moon-maker.
Depending at what angle the comet entered, nearly straight in would cause the most damage.
"As the Pacific Plate moved north, the crust over which it moved was forced down by the North American Plate, back towards the Earth's core. However, as the plate closed in on the 1st Terrane, this land mass was too buoyant to be forced downward and so it was added onto the edge of the continent. This is where much of British Columbia joined North America. Along with this collision came intense forces compressing the already existing land mass. This brought on the first orogeny, known as the Columbia (it formed the Columbia Mountains made up of the Caribous, Selkirks, Purcells and the Monashees)"
"One hypothesis for the information of the Rocky Mountain structures in late Cretaceous through Eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the North American lithosphere. "
Basically that is saying the Pacific plate is moving eastward (or the North American plate is moving westward) and is going under the North American plate. In what year was that theory proposed because when they did survey ot the age of the ocean floors the youngest crust in the Pacific region was right next to the Rockies. That is a rift that has out-flow from below the crust.
How many of those 13 or so vids did you watch? It wouldn't appear that you watched them all.
The mountains in the eastern regions of North America are said to be old and weathered, those are an example of the sort of 'hill-building' that will occur when the crust has to flatten out some to fit the new larger diameter. That is as close to subduction as you will ever get in the earth's crust. Notice the material in collision goes up and away from the mantle simply because the mantle is twice the density of granite.