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
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…
One of the strange things about the 85 Ma trainwreck (which we have discussed components of here and here and here and here) is the central place that the Mojave Desert seems to play. For the most part we might look to the 2003 paper by Jason Saleeby for elevating the Mojave to star status, tapping it as the entry point of an oceanic plateau (specifically the hypothesized conjugate to the Shatsky Rise now in the northwest Pacific) that he proposed as the main actor in driving the Laramide Orogeny.
So what is really peculiar about the Mojave c. 75-85 Ma? After working through parts of the literature and getting some education at GSA this past week, GG would suggest these are possible candidates:
- Emplacement of subducted sediments against the middle/lower crust after experiencing greenschist grade metamorphism.
- Extension that permitted this juxtaposition to rise to the surface, either shortly after (as in the Salinian Schist) or much later (as the Orocopia Schist)
- An extraordinarily unusual pattern of late Cretaceous magmatism (83-72 Ma) including metaluminous plutons that lack the temporal eastward shift seen elsewhere
There are two pertinent questions for these: (1) how localized are these things really, and (2) do these point towards an impacting plateau and a broader Laramide implication?
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.
Hike the John Muir trail and you find yourself constantly in landscapes formed from the c. 85 Ma intrusions that marked the end of one of the longest stretches of volcanic arc activity in the past several hundred million years. For a long time, this was taken as simply the end of an era; the absence of activity that followed was the big message of something strange going on. But increasingly there are signs that this final pulse was itself out of the ordinary and may well have carried the seeds of its own demise.
Looking beyond the Sierra, chaos quickly reigns. While some papers and textbooks describe an orderly eastward shift of the volcanic arc, the rocks on the ground present a more confusing account. While the arc did shift to the east after 85 Ma in southern Arizona and eventually part of New Mexico, no such orderly shift is evident farther north. Magmatic activity actually approached the trench in the Mojave Desert just before volcanoes popped up in Colorado, skipping much intervening terrain. The few plutonic rocks in Nevada and Utah look like melts derived from the crust, not typical arc rocks.
This crazy quilt of magmatism then takes a toll on geodynamic models. The shutdown of the Sierra has been taken as a sign of the slab flattening under North America–but the clearest evidence of the slab flattening is under the Mojave, where igneous activity continued well after flat slab deformation began nearby. Models that predict that slab flattening should shut off igneous activity have trouble with some of the intrusions in the Colorado Mineral Belt–and their absence to the north and south. And if impinging oceanic plateaus are responsible, just how does that timing work out?
We can try to tear this apart in some ways. Here let’s consider these questions:
- Why was there a flare up of activity in the Sierran arc near 85 Ma?
- What could have shut down the Sierran arc?
- What do we know about the relationship of magmatism to schist emplacement in the Mojave region?
- Are the two-mica (peraluminous) granites really just crustal melts?
- What can we say about the Colorado igneous activity?
[NB. Some additions to the end, Feb 2022]
OK, so GG got distracted from this project, but it is high time to look at the pieces of the late Cretaceous puzzle, and first up are the schists of oceanic and forearc affinity. These are often classed as the Pelona/Orocopia/Rand (or POR) schists after the three more voluminous exposures, but there are many individual schist bodies that have similar lithologies and age constraints that are usually included in any examination of the origins of these bodies.
Generally speaking, these are metasedimentary bodies with occasional metabasalts and far more rarely, pieces of deeper lithosphere. They are broadly aligned today in a northwest-southeast swath more or less along the San Andreas, with the major exceptions being the Catalina Schist on Santa Catalina Island, the Rand Schist well to the east along the Garlock fault, and recently recognized bodies in central Arizona. When Cenozoic deformation is restored, these exposures generally land in a gap between the fairly undeformed Sierra Nevada Batholith and the Peninsular Ranges Batholith. As such, they seem connected to the more chaotic Mesozoic geology of the Mojave Desert, where these rocks underlie in fault contact middle crustal plutonic rocks. As such, these are widely interpreted to represent the product of some kind of very low-angle subduction event.
Dating the emplacement of the schists is important to all the stories of the Late Cretaceous in this part of the world; for a long time ages were hard to come by (the schists were often originally mapped as Precambrian), but the ability to date individual zircons broke open the problem. Basically individual detrital zircons in a schist must predate the metamorphism and creation of the schist. Cooling ages from more traditional geochronology then documents either the emplacement of schists at higher levels in the subduction zone or its cooling down while sitting in a cooling upper plate environment. A good example of this approach is shown below.
There are a number of questions that arise, and we’ll see what we can say about them below (this is apt to get long):
- How robust are these ages?
- What does an emplacement age represent in terms of the broader tectonic environment?
- What does the spatial and temporal variation in emplacement ages telling us?
So in the previous two installments, we reviewed ideas for how the High Plains got so high and some of the observations out there that bear on this question. Beyond satisfying some curiosity, what does this do for earth science? Why pay money to do this?
Let’s consider three outcomes: that the High Plains gained their elevation by the end of the Laramide orogeny (say, 40 Ma), that they gained their elevation after the deposition of the Ogallala Group (say about 5 Ma), and that they were high, went down, and rose again. Read More…
No, not high in that sense…high like “Mile High City”. This still is a problem GG is interested in and so for grins let’s quickly review the main ideas GG has seen with their pros and cons. The candidates are thickening the crust mechanically or by piling on sediment, thinning the mantle lithosphere, dynamic topography, hydrating the mantle or the crust, depleting the lithosphere, and emplacing depleted lithosphere. Whew! GG’s hot takes on these below the fold… Read More…
Occasionally a paper comes along that rattles you out of your present biases; whether the paper is right or not is less important than getting you thinking. A paper in Geology got GG thinking about some things he’d ignored…
Kimberlites are rather famous kinds of igneous intrusions as they host most of the world’s diamonds. These eruptions originate at great depths in the earth but seem to pop up rather erratically and their relationship to subduction zones and the like is somewhere between unclear and non-existent. In North America, they seem to pop up in sort of broad swaths of the continent. One band in particular is of interest to those of us studying the origins of the Cordillera: a collection of Cretaceous kimberlites that seem to have erupted almost under the eastern part of the seaway that ran from the Gulf of Mexico to the Arctic.
Most workers have generally sought to connect these Cretaceous eruptions to the subduction of the Farallon plate under North America. This proposal generally seems to work by adding fluids to the deep continental lithosphere, which would then generate the melts that rise forcefully to the surface to emplace the kimberlites (e.g., Currie and Beaumont, 2011).
In this view, the easterly positions of the kimberlites in the Cretaceous reflects a fairly low-angle subduction regime that would have had to be established by 112 Ma (the oldest intrusion in Kansas) and continued to about 85-90 Ma in the U.S. and into the Tertiary in Canada.
The alternative in a recent issue of Geology by Zhang and others looks at this in a very different direction, namely with westward subduction of North America under the western Cordillera, an idea put forward in some lengthy publications by Robert Hildebrand. Read More…
Spent many hours in November sitting in on sessions and perusing posters at the Geological Society of America annual meeting; one goal was to see what’s up with the evolution of elevation of the U.S. Cordillera.
First a quick recap. There are two camps, more or less, on each side of the Cordillera. The old mountains camp on both sides points mainly to oxygen and hydrogen isotope variations in proxies for precipitation. There are also attempts to retrodeform the lithosphere resulting in thick crust and high elevations. The dominant counterargument is that the paleometeorology used to interpret the isotopic values is flawed. On the young mountain side, classical geologic observations are invoked, including apparent tilting of river channels and the recent incision events in many places. The counterargument to this is that the appearance of a tilted channel may be biased by the depositional environment and that changes in climate can drive incision as easily as uplift. In between in some ways are geophysical observations of the lithosphere; recent changes in the lithosphere seem likely in much of the region, supporting younger mountains, but seem older east of the Southern Rockies.
Well, a meeting in Indianapolis isn’t one to bring out all the western geologists (next year’s meeting in Phoenix is a whole different matter), but a couple of things popped up. Did anything look to change the landscape, either by opening up new vistas or overturning old results? Not that GG discerned. Below are some notes probably only of interest to the most interested….