First up, and right up front, I don’t know. There is no answer waiting at the bottom.
This is more “admiring the problem” than having an answer (with a bit of ‘what does it mean to us’ along the way).
So, looking at the Sea Floor, we notice the spreading zones. That is where new sea floor is made. Most of the sea floor is under 150 Million years (out of 4.5 Billion years) so this is a relatively fast process in geologic terms. Looking at the age of the rocks, we see “stripes” that get older the further you are from the spreading zone.
But, look just a bit more closely, and we notice that the stripes move in long strips that are of different offsets. The little stripes of rock of the same age in any one strip are not aligned with their neighbor… One has to wonder “Why?”…
The wiki is not particularly helpful, just noting that the thesis is that the cold ends are subducting under their own weight (and one wonders why not subduct 1/2 way from California to Japan… it ought to be cooled off an is just as heavy then?)
The new oceanic crust is quite hot relative to old oceanic crust, so the new oceanic basin is shallower than older oceanic basins. If the diameter of the earth remains relatively constant despite the production of new crust, a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust is forced under either continental crust or oceanic crust.
So it takes 150 Million Years for the crust to cool and thicken as it slowly moves from California to Japan? Really? Something just doesn’t sound right… I’d be more inclined to believe a massive upwelling of magma in the middle, lateral transport on the spreading plume, and then decent of the other side of the large magmatic convection cell, with the crust just “along for the ride” (and while it might thicken some, not being the major transport system).
Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge. Only a small portion of the oceanic crust produced in the Atlantic is subducted.
So they don’t cool down enough to sink in, well, in “forever”? … Again, something not quite right. Looks to me more like the crust subducts where the viscous drag on the plate from the sideward moving magma is greater than the force needed to shove it under a continent and when one place becomes ‘the weak link’ the other places don’t need to subduct but just push their continent along.
However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Ring of Fire of the Pacific Ocean. The Pacific is also home to one of the world’s most active spreading centres (the East Pacific Rise (EPR)) with spreading rates of up to 13 cm/yr. The Mid-Atlantic Ridge is a “textbook” slow spreading centre while the EPR is used as an example of fast spreading. The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced.
Notice that the East Pacific Rise comes ashore under the Gulf of California next to Baja and runs under the Salton Sea on up part of California, then mysteriously reappears Off Shore near Eureka… It gets displaced “sideways” somehow from the Salton Sea out to sea…
Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world’s ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean.
Hmmm…. So plate tectonics changes the depth of the oceans and when actively spreading the sea level rises / sea volume decreases… Wonder if the “CO2 Climate Drives Ocean Depths” guys know that…
OK, enough general observation, back to those strips of stripes.
Look closely at the area off the coast of Baja California and one up toward Oregon. Notice that one “block” is offset from the neighbors on each side? Off the coast of Canada, things are moving fast. Very fast. Near California, not so much. But down around Baja it picks up again. (Then further south, slows again).
Notice the ‘neat edges’ between this strips? Each “block” has a distinct “edge” that lasts for hundreds of miles away from the spreading zone. Each strip is a unique entity and moves on its own schedule. (Do they always have the same differential? Or to they ‘take turns’ and play ‘catch up’ some times? One can only wonder…)
If we look at a close up of the fault system in California, we notice something interesting:
Most of the fault lines run up / down along the length of the state. But just south of The Great Valley there is one big one that runs side to side. The Garlock Fault.
The Garlock Fault is believed to have developed to accommodate the strain differential between the extensional tectonics of the Great Basin crust and the right lateral strike-slip faulting of the Mojave Desert crust.
Unlike most of the other faults in California, slip on the Garlock Fault is left-lateral; that is, the land on the other side of the fault moves to the left from the perspective of someone facing the fault. In the case of the Garlock Fault, this means that the terrain north of the fault is moving westward relative to the terrain south of the fault, which is moving relatively eastward.
But we know that the “Great Basin” area is having “extensional tectonics” as the East Pacific Rise has been subducted under the continent… and this says that the middle area of California is moving West faster than the southern side… IF you click on that spreading zone image above to get a larger view, you will see that the place where the Garlock Fault is located is just about the same place as the line between the two “strips” or blocks of ocean floor come ashore…. and in that case, the very long term view is that the California strip has been lagging behind as the southern strip kept on moving…
So which is it? Is the middle of California moving faster, or slower, than each side? Is the Garlock Fault just an extension of that inter-strip boundary on land? (I think so). Did the middle of California “fall behind” as the EPR got subducted under California as an “easy slip under” only to now get “stuck” on the Rockies and start the hard work of splitting the Basin and Range area off of the rest of the continent? Finally warming and softening the land above enough to start playing “catch up” with the strips on each side? Or are we just seeing a momentary “move” of the middle while most of the time it’s NOT moving and the spreading zones on each side have faster runs? (Them not being encumbered by a continent to stretch out..)
Does the fact that the volcanoes on each side of that strip (in Washington and Mexico) are more active mean anything? Or will the ones near Mono Lake and down toward Death Valley wake up again as soon as this episode of faster spreading ‘pauses’? Is the Garlock due for a major movement any time soon?
The last significant ruptures on the Garlock were thought to be in the years 1050 A.D. and 1500 A.D.. Research has pinned the interval between significant ruptures on the Garlock as being anywhere between 200 and 3000 years depending on the segment of the fault.
So it has had two about 500 years apart, and we’re now at 1500+500= 2000 AD… yet they state the ‘period’ is variable from a couple of hundred to a couple of thousand years, so don’t plan on anything.
You can see it running from the lower right to the upper left at the base of the mountains in this image.
At the end of the day, I just don’t know.
It looks to me like the “easy under” of California let the other parts of the spreading zone ‘get ahead’ and now the spreading zone is starting to ‘catch up’ with the middle moving at normal rates again (and a bit faster than the neighbor to the south). An interesting “Dig Here!” would be to find the matching place up north where that side of the strip joint comes ashore and see which way it is moving, and how fast. If that IS what is happening, then we ought to see more quakes on the Garlock in the future and we ought to see action “up north”. If we get volcanoes where this is happening most, then we ought to see volcanoes waking up again along that Dead Sea to Tahoe line. And I can’t help but wonder if the “offset” line runs right under the Sutter Buttes where a volcano popped up 6 million years ago “for no good reason”… and if we’re moving again, when might it have some more slip, weakness, and bit of a rumble? Or has the movement “moved on” to under Lassen and Shasta, where Lassen had a burp in about 1914…
I do note that in the following diagram, the “spreading zone” on the ocean floor next to the Juan de Fuca plate comes ashore with a trajectory that sends it more or less under the Sutter Buttes and on toward that Mono Lake, Owens Valley, Death Valley, Salton Sea arc of spreading zone. So is THAT where the “catch up” ought to be happening next? Or will the block just sort of bend as things think about it for a couple of million more years?
While we don’t see much in the way of “sideways” faults on the California map in that area, the history written in the rocks at sea says “something moves”…