From Wikipedia: “A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will comprise a fault-aligned valley, a series of fault or fold-aligned hills, a straight coastline or indeed a combination of these features. Fracture zones, shear zones and igneous intrusions such as dykes can also give rise to lineaments.”
I first ran across the term while reading about the New Madrid Seismic Zone. Specifically, the Commerce Lineament, and the Bootheel Lineament. The Commerce Lineament corresponds to the Commerce fault and is in fact, a defacto boundary fault for the Reelfoot Rift. An oddity of it, is that if you extend a line along it’s path to the south, it points right across the planar alignment of quakes that occurred near Guy Arkansas in the last couple of years. That planar alignment of quakes is aligned at approximately 16°, but the faults in the Thebes Gap area tend to be just short of 30° alignment. Connect? Beats me, it could just be an odd coincidence. That ancient mega quakes have occurred outside of the NMSZ is evidence by the sand blow fields that have been explored to the east of there along the floodplain of the Mississippi. But, this isn’t about the NMSZ, so I will leave one of my favorite topics behind.
A long time ago, on the western extent of the North America continent, there was a subduction zone. The other plate involved in this collision was the now extinct Farallon plate. The remnants of that plate are the plate shards off of the Oregon and Washington state coasts, Juan de Fuca, Explorer, and Gorda plates. To the south, the Rivera plate fragment and the Cocos plates still exist. Along the western boundary of the Farallon was a spreading center. Part of that spreading center still exists in the form of the East Pacific Rise and the Juan de Fuca ridge. Located along the Juan de Fuca rise are Cobb Seamount and the Axial Seamount. Both are underwater volcanoes. Both are fed by magma genesis along the ridge, and by the Cobb Hotspot. Yes, you read that right, the same general set-up that feeds Bardarbunga. If you roll back the clock several million years, you have essentially the same geologic set-up that eventually gave birth to Iceland. Where it gets really interesting is when you concider what happened to the subduction zone once the ridge itself was driven under the overlying continental crust. Global Tectonics, 3rd Ed has this to say: “A quadruple junction existed momentarily at about 28 Ma, but this devolved immediately into two triple junctions”. Since these two semi-stable triple junctions only have certain geometries that are stable, the two junctions migrated north and south along the continental crust boundary until they reach a stable geometry. The resulting sites are the Mendocino triple junction and the Rivera triple junction. What was left behind after they migrated is the San Andreas Fault system. As the now subducted spreading ridge passed under the South Western US, it’s high magma production rate initiated crust thinning and the formation of the Basin and Range province. Much of it’s geology is made up of Horst-Graben structures, with many showing up at half Grabens. Poking around in Wikipedia, I get the impression that the brunt of the ancient Farallon plate now lies under the Eastern Seaboard of the US. If this is so, it is likely that what once was the spreading center now lies under Western Texas. I say this because this is the location of the ongoing rift valley formation known as the Rio Grande Rift. It is made up of a chain of basin structures stretching up from Mexico into Colorado. Now, I have no proof that that old spreading center is located there, but the general layout of the suspected remnants of the Farallon point towards that being a pretty good explanation. South of the Basin and Range, is the Sonora Megashear in Northern Mexico. South of the Megashear, the suducting Cocos plate (also a remnant of the Farallon) is undergoing something approximating “flat slab” subduction. This means that it is slipping under at less than 30° dip. This puts Popocapetal far inland near Mexico City. To the east of here, the Cocos plate abruptly stops. A section of what appears to have been part of the Cocos seems to have been detatched and sits at a much steeper angle, plunging down into the Mantle. Initially, it was thought that this was an example of slab rollback and detatchment, but further examination shows that there is another subducted plate segment that seems to have pinched and cut off the now detatched segment of the Cocos. One idea for the owner of that subducted “other” plate section, is the Yucatan block as it moved away from Texas and rotated into its current position. See “Mexico Subduction Experiments” for further info. The only real reason that I bring this up is that it is pretty likely that the Farallon plate north of the Sonora Megashear had a similar shallow angle as it went by. The only real indicator for something different is the existence of the Megashear itself, acting as some sort of boundary marker. This could also be what the Olympic-Wallowa Lineament is up in the Pacific Northwest. It is aligned with the boundary for the Juan de Fuca dn Explorer microplates. Of course, to add to the confusing mix, it’s also aligned with the track of the supposed Newberry Hotspot. And now, for the insane bit. The track of Newberry Hotspot does not correlate with the motion of the North American Plate, like Yellowstone does. Another supposed hotspot is the Raton Hotspot, blamed by some for the Jemez Lineament. The Jemez Lineament correlates to a series of volcanic centers stretching from San Carlos volcanic field through Jemez Mountains.
One website has even stated that the track of the Raton Hotspot is a better record of the North American plate motion than Yellowstone. The strange part is that if they are correct, then the Raton Hotspot should be under the border of Oklahoma and Colorado. I tried tracking down their source material but ran into a paywall. As for the volcanic fields along that region, you can’t rule out that they were caused by eddy currents and turbulence in the athenosphere as the trailing edge of the Farallon went by. In Colorado, there have been a series of earthquakes over the last few years. Despite the alarmism of many people (with vested interests), these seem to be along the eastern boundary of the northern extent of the still forming Rio Grande Rift. In a few million years, when this rift meets the southern end of the ultra ancient Mid Continental Rift, (actually located over in Kansas, bounded on the east by the Nemaha Ridge.) it may or may not reactivate it. Since it tracks down the middle of Lake Superior, it is in a prime position to eventually break up the North American Craton. I won’t be around to see that happens, but the much younger (though still ancient) Reelfoot rift is still making noises over near Tennesee. Speaking of which, while rummaging around for data for this post, I kept getting distracted by info on the Reelfoot rift. At a passing glance, I notices one gob of tex that noted that two sediment beds separated by several thousand years of when they were emplace, were lying within a few inches of each other. That un-nerved me. To do something like that, you literally have to take the sediment beds and throw them up into the air and shake them around. The only time you get that much energy is in MMI XII shaking. You don’t start getting that level of violence until you go above Magnitude 8.2 in earthquake strength. That is where stuff is literally thrown up into the air. We’re talking 770% g acceleration. This was in the vicinity of Thebes Gap along the Mississippi River and was in an article discussing when the Mississippi switched from flowing down the western side of Crowley’s Ridge, to the East where it currently is.
Pilger makes a pretty extensive exploration of various reference frame ideas in “The Bend: Origin and significance” )
Some insightful commentary about the Thebes Gap.:
Harroson and Schultz: Strike-Slip Faulting at Thebes Gap, TectonicsVolume 13, Issue 2, Article first published online: 26 JUL 2010.
And an awesome reference:
Global Tectonics, 3rd Edition
Philip Kearey, Keith A. Klepeis, Frederick J. Vine
January 2009, ©2009, Wiley-Blackwell
This is the “go to” book when it comes to things tectonic.