Reading A Section
or, “you point to this section a lot, but I don’t think it means what you say it means.”
The section in question today is a north-south profile drawn by Jason Saleeby in his paper arguing that the Shatsky conjugate came through the Mojave Desert…
What does this show? Saleeby wrote that “geophysical data indicate that the base of the batholithic crust and its underlying mantle lithosphere have been tectonically removed and replaced by the Rand schist.” Many people have given GG the impression that this diagram shows that the lower crust has been removed tectonically and therefore requires the collision of an oceanic plateau.
It shows no such thing.
Read More…The 85 Ma Trainwreck: Sedimentation
It’s been a long time, but GG needs to try to wrap all this up and do…something with it. So let’s plow a bit farther and look at what sedimentary rocks might tell us.
In previous entries, we’ve looked at the emplacement of oceanic-affinity schists, the evolution of magmatism and the changes in deformation in the broader Mojave region as well as considered which peculiarities of the Mojave are truly special. Here we will consider what significant patterns exist in the sediments that accumulated in and around this time.
Overall there are two foci of astonishment present. One is that there is very little in the way of sediment preserved in the greater Mojave area; this is surprising because of widespread evidence of major extension. Where did those extensional basins go? The other is far to the northeast, in the foreland, where sedimentation rapidly increased well away from the Sevier thrust front but died off farther to the south. These changes cannot be from loading effects at the surface but must reflect some degree of subsurface loading. A third, and seemingly underappreciated, aspect of accumulation of sedimentary material is that the Franciscan complex covers a large amount of ground north of the Mojave but virtually none south of the Mojave. Maybe this is hiding in the continental margin offshore, but the difference seems noteworthy.
For now, we’ll leave the more distant foreland deposits (which were largely discussed in Jones et al., 2011) and instead focus on the forearc and the immediate foreland.
The forearc is surprisingly well preserved, all things considered, but there are suggestions of hiatuses and discontinuities in sediment source that are likely clues to unusual deformational events. GG here is largely relying on an analysis by Sharman et al. (2015).
So what do the sediments say? In the Great Valley of California, the detrital zircon population of the forearc looks awfully close to the age population of Sierra rocks in the latest Cretaceous into the Eocene. The same thing is true in the south by the Peninsular Ranges. What goes on in the middle? As Sharman et al. point out, after resembling the areas to the north and south prior to ~75 Ma, you get an awful lot of Precambrian grains, and that population in the Maastrichtian and Paleocene looks a lot like the eastern Mojave and Mogollon highlands. When did this happen? The plots in Sharman et al. are a bit hard to use to really look at this in detail, but digging into the supplementary data, we can make this plot, where % grains > 300 Ma is a proxy for the pC grains coming through:
Now a lot of these points represent one or two grains, but that only adds clutter near the bottom. While there is a real hint of something going on as long ago as 85 Ma, clearly things change dramatically about 72 Ma. As this has to follow the creation of an elevated area, we have to have some highland by 72 Ma east of the late Cretaceous arc (which, in the Mojave, still seemed to be active).
What does this represent? Given the distribution of the Precambrian grains as examined by Sharman et al., the source almost has to be the eastern Mojave and/or the Mogollon highland of western Arizona. But how did those come to be present? Well, it could be incision through arc cover rocks into local Precambrian bedrock. It could be expansion of the drainage area into the backarc where these rocks are found. It could represent exposure of footwall basement rocks in some large normal faults. Regardless, this is profoundly different than what was going on to the north and south. So certainly by 72 Ma and quite probably earlier (c. 85 Ma) the regular order of structure along the arc was broken. What is more, this happened in a place where magmatic activity was continuing; by this time the Sierra was dead and the magmatism to the south had already started migrating well inland.
The other point worth noting is that the Salinian forearc rocks are missing in the 100-86 Ma timeframe; subsequent deposition is described by these authors as on to deeply eroded arc rocks. Just exactly what this unconformity represents (and exactly where this occurred relative to North America) isn’t entirely clear.
Let’s look to the other side of the orogen. For a long time we’ve known of erosion from the Mogollon highland from the Eocene Rim Gravels and the Music Mountain formation, but most of that is younger than our interests here. Clues that this was a longer lived activity started to show up, and Davis et al. (2010) noted that the Paleogene detrital zircons found at the south edge of the Uinta Basin in Utah looked a lot like the zircons in the Late Cretaceous Maria Basin of southeastern California. This led them to propose a “California River” that ran from southeastern California to the north-northeast, revising ideas on the sediment dispersal in the Laramide orogen.
But that is still younger than our interests here, but there has been followup. Two recent papers push the California River back into the Cretaceous: St. Pierre and Johnson (2022) and Wersen and Johnson (2023). This work more directly addresses how far transverse drainage were coming off the Sevier belt and added the use of lead isotopes in feldspar. At this time in the Cretaceous the Western Interior Seaway reached far to the west, so these sediments in this earlier California River didn’t make it to the Uinta Basin but instead were dropped in the Straight Cliffs Formation. They point back to the same source area near the McCoy Mountains, but now back in the 80-90 Ma range.
While there is some controversy on this California River (a PhD thesis by Winn (2020) argues that these sediments look more like material in the San Juan Basin than the Uinta Basin), new work in the more proximal area makes it clearer that the uplifted area in the eastern Mojave was generating sediment in the Late Cretaceous. The 2016 paper by Hill et al. dates a limestone within the Music Mountain Formation, which fills some rugged relief in the western Grand Canyon area, to be 64 Ma. This overturned longstanding biostrat interpretation of the unit to be Paleocene. This requires the incision of that topography to be Late Cretaceous or even earlier, consistent with the notion that sediment derived from the Mogollon highlands was heading north in the Cretaceous. The coarse sediments within the Music Mountain formation clearly are from the south to southwest.
This area (described by Dickinson et al., 2012, as a “paleotopographical syntaxis”) also provided sediment to the east. Lawton et al. (2009) placed the headwaters of rivers traversing northern Mexico as in this same area as means of getting 1.4 Ga and 1.8 Ga zircons into the sediments:
So the last place to look is underneath: how do the POR schists compare to these foredeep rocks? Although Sharman et al. updated this somewhat, the main presentation is in Jacobson et al., 2011. given our attention to the >300 Ma zircons above, it is interesting that virtually all of the POR sites have a fair amount of old zircon:
What is a bit striking here are the values from the Rand Schist. As that supposedly was emplaced by 75 Ma (middle Campanian), this feels like a bit of a timeline problem, for not only are the deposits as young as only 80 Ma, you also need time to stuff these underneath and begin to cool them. So either there is a different source of zircon for these protoliths or there is a timing issue. This is far less severe for the other schists as we had a lot of pC before their creation…except we also got a ton of Jurassic zircon. The proportions are not the same.
But when Jacobson et al. plot up all their forearc samples of specified age ranges (similar to the dataset in the 2015 paper above), they seem to get a decent match between the schists of different ages and the zircon populations:
Comparisons with other late K-early T sections farther inland do not look remotely like these distributions. While there are clearly issues here, the schists look an awful lot like the forearc sediments still out there.
So what is the big finding here? The thing striking GG is that region where the Sevier belt had died was high enough for canyons to form in the western Colorado Plateau and for sediment to head out from this area in all directions from sometime in the late Cretaceous into the Paleocene. That is quite a trick. Either we are selling short some other exposures of Precambrian basement or this area stuck up high enough to be getting eroded pretty seriously for a long time. The question is why…
Previous entries in this series:
The 85 Ma Trainwreck: The Sevier Waistland
No, that isn’t a typo, as we’ll see. In essence, the question to be considered here is, what might the crustal structure of the backarc/foreland looked like c. 85 Ma?
A trick that igneous petrologists sometimes like to use is to look at rare earth element distributions and see if elements that like to go into garnet are relatively rare in a granitic rock. Such an absence is then ascribed to the melt having been generated at a sufficiently high pressure that garnet was stable. Thus the absence of such elements would require a crust thick enough to have the high pressures necessary for the garnet. Related tricks look for plagioclase feldspar crystalizing; this phase is only present at shallower levels.
One such analysis was published by Economos et al. (2021) for some of the younger plutonic rocks in the Mojave. In general, they found that there was not a signal of plagioclase but there was a signal from the rare earths that garnet was stable where melts were originated. For the most part, that puts the origin of the melts below 35 km depth. An empirical calibration suggested by Profeta et al. (2015) was applied by Economos et al to their measurements, yielding Moho depths of ~50-80 km. While the most consistent results were from the younger Cadiz Valley batholith, a consistent range of La/Yb ratios suggested that there was no large change in crustal thickness back to 83 Ma. It seems likely then that the crust at 85 Ma was already very thick.
For the most part, this seems in reasonable agreement with the deformational story we’ve already looked at.
This might seem unexceptional. The Coney and Harms (1984) reconstruction of crustal thicknesses, while deeply flawed (it is a rather circular argument, which we could discuss some other time), does provide some context regionally and crustal thicknesses approaching 60 km were inferred from restoring Cenozoic deformation. More recent work by Long (2019) and Bahadori et al. (2018) tends to confirm pre-extensional crustal thicknesses in the Great Basin to be ~55 km, though these are mainly along the Sevier belt, much as Coney and Harms envisioned. These all tend to have some issues with magmatic additions and flow in the lower crust, so some caution is warranted.
Consider this cartoon of the situation c. 80 Ma:
Here is where the “waistland” business comes in. Something like 200 km of shortening was seen across the Sevier belt in northern Utah prior to 80 Ma (e.g., DeCelles, 2004). With the ending width of the fold and thrust belt being about 400 km (from the map above), that would be a thickening of about 50%, so if the average thickness was 55 km afterwards, it was about 37 km before.
Now there is not much reason to suspect a different shortening in the Mojave (DeCelles and Coogan, 2006, had over 300 km of total shortening in central Utah, and Levy and Christie-Blick, 1989, while having smaller shortening numbers, saw little variation from SE Idaho to southern Nevada). The hinterland in the Mojave can’t get too wide as the Colorado Plateau restores pretty closely to the arc; this is the “waist” of the Sevier belt alluded to before. If this hinterland was maybe 150 km after shortening (and quite plausibly after considerable early Cenozoic extension), then if it started with a 37 km thick crust, it could have reached thicknesses over 80 km with 200 km of shortening, in the ballpark from the Economos et al. analysis.
Farther to the southeast, we encounter an area that was thinned significantly in the Jurassic, with the effects of that still evident into the Cretaceous, when strata broadly correlative with the Mesa Verde group capped Paleozoic rocks along the Mogollon Rim. So while that hinterland could have been very thin, too, the crust was pretty thin to start with.
What this suggests is that the Mojave was a special place before the POR schists were emplaced, and before a putative oceanic plateau was thrust under the area. It was unusual because of the preexisting structure of the North American plate. What is more, this unusually thick crust is a potent driver for a lot of the peculiarities we’ve already been examining.
Is there other evidence that might support this inference of a peculiarly thick crust? Well, maybe; for that, we’ll want to look at the sedimentary history of the region…which hopefully GG will finally get around to finishing up…
85 Ma Trainwreck pages
- Introduction
- Schists
- Igneous Activity
- Deformation
- Mojave Mojo
- Crustal structure
- Sevier Waistland (this page)
The 85 Ma Trainwreck: Crustal Architecture
At some point GG is going to actually have to commit to assembling all these thoughts into a real paper (or admit defeat in trying to understand this area). But for now, time to resume exploring some of the issues in the Mojave.
We’ll look over the details of sedimentation later on. Here the focus is briefly on just what sits under the region where schists are seen in fensters. Consider this map that we’ve used before:
There are two main areas where these schists are exposed: the Rand Schist in the northern swath somewhere in the 77-85 Ma range, and the Pelona/Orocopia swath to the south that is younger. As the boundaries between these schists and overlying rocks are taken to be old megathrust zones (usually reactivated), the common interpretation is that the area under these belts is made up entirely of these POR schists. If you dig around a bit, you will find cross sections drawn to show that nearly the whole crust is made up of POR schists. So let’s ask a few questions: was there enough of this stuff to fill out these regions? And what can we say about what is there now?
Read More…The 85 Ma Trainwreck: Deformation
In previous entries, we’ve examined the emplacement of oceanic/forearc affinity (POR) schists and the igneous activity from a similar timeframe. Here we will consider what was deforming when and how. There are four main pieces to this puzzle: the termination of ongoing thrusting of the Sevier, Eastern Sierra belts, and Maria belts, the emergence of thrusting in southeastern Arizona and New Mexico, the appearance of extensional faulting, and the beginnings of Laramide shortening in the Colorado Plateau and Southern Rockies.
The outline version is that thrusting in the north-south trending Sevier foreland fold-and-thrust belt shutdown by about 90 Ma in southern Nevada but continued for another 30+ million years farther north. Northwest-trending retroarc thrusts probably continued to be active in southeastern California until 80 Ma and possibly 75 Ma. Rock uplift and extensional shear zones between ~75 and ~65 Ma in several localities may reflect extension of the crust in the Sevier hinterland, but some kind of intra-continental convection is hard to rule out (e.g., Wernicke and Getty, 1997). Closer to the coast, right-lateral strike-slip deformation in the dying Sierran arc reflects some obliquity to convergence at the plate margin. As time passes, Laramide-style basement-cored uplifts begin to emerge, perhaps including structures very close to the Sevier thrust front in the Kingman arch and associated uplift of the southwestern Colorado Plateau. Thrusting appears to have accompanied magmatism in expanding eastward across southern Arizona.
There is a lot here and yet GG is confident he’s missed some important papers–feel free to point some out in the comments.
The Grumpy Geophysicist 
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