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?
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…