We were just on the west coast down at San Diego. A lot of the sandstone cliffs are tilted upward toward the sea. Does that mean that uplift has occurred west of the coast and has been eroded down? I can’t mentally reconstruct the order of events that could have led to that feature unless there had been a large amount of land west of the coast that has since worn away. But I don’t remember learning anything about this. Can someone enlighten me here? thanks. C.
Well, I’m not intimately familiar with the local geology there, but the simplest explanation is that there’s normal faults on either side of you and so the rocks have been tilted up to the west and down to east. This kind of structure would be called a half-graben. For example in cartoon form: http://facweb.bhc.edu/academics/science/harwoodr/geol101/study/Images/HalfGraben.gif
The fairly sparse geologic map that turns up on google does show some fairly tight folds near Mission Bay, so it also could be you were on one of these. The exact geologic history of Southern California is pretty complicated because the big story is transform movement (rocks moving side-to-side), but on the smaller scale the geometry of this leads to little pockets of compression and extension, and plus there’s inherited structures from the more distant past. If you can say where exactly you were, I could try to dig up a better map and try to figure it out exactly what kind of structure you were looking at and maybe speculate why it’s there.
The spot I was talking about was Point Loma, and from what I could find out, it’s a fairly complicated area geologically. However, the tipping of the layers seems to be a reflection of the type of uplift (graben) mentioned. I’m still curious about the rest of the layers that are eroded away. When they formed, they were under water, of course. How far do they extend? Ah - I think I’m asking too much here. Better go read a whole lot more.
This might be useful: Assembling California
In looking for McPhee’s In Suspect Terrain I see that he’s got an even more recent work out called Assembling California. I wish I was more familiar with the history beyond a passing familiarity with accreted terrains and ophiolite suites but in the meantime this would be an excellent read for the layman and professional both.
ETA: sigh As mentioned.
It’s worth noting that California and Baja California roughly west and south of the San Andreas Fault is actually on the Pacific Plate as it is subducted under the North American Plate. (Technically, it’s a terrane striking the NA Plate a glancing blow as it moves north along the coast’; the underlying oceanic crust is what’s being subducted.)
Roadside Geology of Southern California (9990201706). If it’s half as good as my copy of the Washington State version then you are in for a treat.
- subducting oceanic plate makes continental plate buckle at the edges. Old sedimentary deposits become corrugated.
- erosion eats away the edges
- CC visits the coast at a moment (a geologic moment) when erosion is halfway through an upward pointing bulge.
ETA: Assume I changed this so it doesn’t conflict with the facts Polycarp laid out in post #6.
That’s not quite right. There’s no actual subduction going on south of Cape Mendocino, just strike-slip motion. Like I mentioned, there are smaller zones of compression and extension in the form of restraining and releasing bends, but the compressional parts are just manifested in crustal thickening-- nothing is actually “going down the hole” so to speak. This makes sense because there’s no spreading center, so it would be a problem in terms of conserving the total crustal area on the surface of the Earth if crust is being destroyed there and not replaced anywhere. The only place the Pacific Plate is being subducted under the North American plate is up in Alaska-- in the Cascadia subduction zone it’s the Juan de Fuca Plate and in Mexico it’s the Cocos plate, both of which are part of the old Farallon plate.
The (sort of) recent history of this plate boundary is that prior to about 30 million years or so, there was subduction along the whole western margin of North America as the Farallon plate subducted under the North American plate. But at around 30 million the spreading center that separated the Farallon and Pacific Plates (the East Pacific Rise) actually arrived at the subduction zone and was subducted (it’s not a straight feature, so this hasn’t happened in the Pacific Northwest or Mexico yet). Since there was no longer crust being created at this spreading center, there was no need to accomodate its creation with a subduction zone. The only thing that needs to be accomodated where the Pacific Plate is in direct contact with the North American Plate is the different relative motions of the two plates, which are roughly side-to-side, hence the San Andreas fault system. It’s somewhat complicated by the spreading center that is propigating up the Gulf of California and into the Imperial Valley, as well as generalized basin and range extension in the Southwest but that’s the general story as it stands now
USCB has a website that has a bunch of animations of the tectonic history of Southern California in particular that might be of interest (scroll down to section 2): http://emvc.geol.ucsb.edu/downloads.php
Back to the OP, though, I did find an actual USGS geologic quad here: ftp://ftp.consrv.ca.gov/pub/dmg/rgmp/Prelim_geo_pdf/sandiego_map2_ai9.pdf
and the legend: ftp://ftp.consrv.ca.gov/pub/dmg/rgmp/Prelim_geo_pdf/sandiego_cmu2_ai9.pdf
Structurally, that does seem to confirm that Point Loma is all sorts of messy with a bunch of really small poorly definied faults. But interestingly the rocks themselves are actually pretty old-- they’re late Cretaceous, so they’re over 65 million years old and predate the whole tectonic mess mentioned above. Since these rocks have been through so much, I have a feeling that their present orientation won’t actually tell you anything all that meaningful about their greater history. Interestingly in the unit descriptions in the Legend, it says that some of the units seen on Point Loma are corellative to units found in the Santa Ana Mountains.
California was a kit?
This is really interesting, especially the linked animations. I had no idea how [i[mobile* the Pacific plate was on these timescales… I guess I sort of imageined it as a background against which all the other plates moved.
Well, it is all about how you want to look at it. Since those animations are supposed to show the history of North America, they chose to show North America as the stationary reference point. You could just as accurately show the Pacific plate as stationary and the Farallon plate zooming off in the other direction and being gobbled up by North America zooming in. In absolute terms, the spreading centers are actually what’s stationary.
Washington was “assembled” in much the same way. Collision of plates with a subduction zone.