It would be interesting if anyone had geologically identified the apparent flagstone paving rock type? Whether it is hardened sedimentary clay or volcanic ash /pumice type , or other types of tuff?
I think that the type of stone varied. Sources indicate the use of volcanic stone (basalt) and marble (maybe for pedestrian areas). The images provided by Thanos5150 in this thread show that these paving stones are subject to compression by loads. All I know is that the ones on which I have trod are difficult to walk on as they are irregular in shape, size and surface and slippery when wet and look to me, probably incorrectly, like a type of river stone. I've always wondered why the Romans didn't use regular stones as in the piazzas more often.
As for Azores and Mount Pico. I wonder whether in your research you came across this statement of the age of the volcano:
Mount Pico is part of the Madalena Volcanic Complex, one of three volcanological units that comprise the island of Pico, associated with three historic eruptions in 1562, 1718 and 1720. Current morphology suggests an age dating to the Holocene age, confirmed by radiocarbon dates younger than 6000 years.
No, I hadn't but that is a slam dunk against the 12 000 000 year team as far as the Azores are concerned.
However some of these tracks (particularly limestone ones ) are wide so maybe at the time of cart travel, it was a slush, requiring wide wooden wheels to restrict sinking?
Yes. I think that you may be right. And also for the weight. It is my impression that many of these carts were big and heavy and had large wheels. Not chariots or personal carriages. The Romans, for one, transported heavy items. The wagonwheel tracks on the Oregon Trail being an exception, but proving that these ruts do not take centuries to develop.