Well I think the confusion is because the locations when the motion between the Pacific plate and the NA plate changes depends on latitude.
There was a migrating triple junction that started down south MEXICO way and migrated NORTH. So...for example the triple junction was located near VOLCANO COLIMA, about 15 Million years ago and migrated North... so the ANCESTRAL SAN ANDREAS (if you want to call it that) is indeed more than 15 million years old... going back to the Miocene.
Now the same triple junction is off the San Juan Islands WAY UP NORTH Cascadian way!!!!
So you can see that the motion has been time transgressive going back ~ 20-30 million years... So its really an issue of where you are on the plate boundary.
There also are some big fracture zones, across which the age of the Pacific plate changes...so that is another complicating factor.I hope this makes some sense.
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