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?
Moho is generally taken to be about 28 km under the Mojave Desert (e.g., Godfrey et al., 2002). How much of an average thickness of the stuff above the schists is quite unclear; an older aeromagnetic study argued that POR schists should be below 8 km (Langenheim, 1999) but others have put schists near the surface over much of the western Mojave (e.g., Yan et al., 2005). So… let’s say that we need an average of 20 km under the area where schists are thought to be pretty widespread. The Rand Schist area covers roughly 15,000 square kilometers (km2). The Pelona and Orocopia schist area is a bit of 40,000 km2. So together they would require something like 1.5 x 106 km3 of protolith. As impressive as this is, it is maybe 13% of the largest fan complex in the world in the Bay of Bengal. So we are not in hopelessly surreal territory.
What can we compare our estimate to? The protolith for the schists is some combination of forearc sediments like those currently in the Central Valley and eastern Coast Ranges to the north and the Franciscan Complex, making up much of the rest of the Coast Ranges. If we look at the volume per unit length along the trench for these two sources, the fill in the Central Valley amounts to about 750 km2 per unit length and the Franciscan to maybe 1000 km2. With about 330 km of along-strike length of the two main schist areas, we might expect there to have been about 0.6 x 106 km3 of protolith, a bit under half of what we need.
This is a crude calculation, but two things should be noted. One is that the estimate of forearc materials includes a lot of Cenozoic material. Of course some is recycled older forearc material, but some significant amount is new material. The other is that we left 8 km of non-schist crust across the whole region and didn’t try to put schist in under the swath of land between the two main schist exposures, nor has it been extended very far inland. While we’re kind of in the ballpark, this would have to be a very efficient process that wasn’t losing material to greater depth or recycling. So it seems likely there is something else–maybe a lot of something else.
One possibility is the missing lower crust that the schists have replaced. But for them to still be hanging out in the Mojave, the simple cartoons of a slab scraping off the bottom of North America and then replacing it with schist needs major surgery. And frankly, once that door is open, lots of possibilities emerge. Is there a way to simply shuffle the schists into the crustal section such that there are lots of older lower crust still hanging out? Or do you buy the idea of “relamination” as blobs of subducted schist protolith rise up? Inquiring minds want to know…
Geophysics to the rescue?
Before we walk away from this much more, the thing you’d really like is observational evidence of what the middle and lower crust really is today.
The earliest seismic study that seems to bring the POR schists to the forefront was the COCORP line across the Mojave (Cheadle et al., 1986). Although the reflections allowed for a large number of possibilities, the apparent continuation of the Rand Thrust (at the top of the Rand Schist; this is often now referred to as the Rand Fault) as a fairly gentle structure extending some distance into the Mojave provided support to the notion that the Rand Schist lay under a large region. The interpretation that the Garlock fault was a fairly shallow upper crustal feature also lent support to a large detachment that separated the Mesozoic and older rocks above from the schists below. That interpretation, however, was challenged as misinterpreting a side reflection (Serpa and Dokka, 1992)
The prominence of the schists was increased by a CalCrust profile interpreted by Peter Malin’s group (Malin et al., 1995). A seismic profile from the Mojave across the Tehachapis yielded reflections that, along with fairly low seismic P-wave velocities (6.2-6.5 km/s) was interpreted to reveal Rand Schist below 5-10 km along the whole section. However, they also had interpreted a much higher seismic velocity under about 18 km as magmatic underplating from mantle rising into a gap produced by a steepening slab; this part of the interpretation is not seen in later publications, e.g., Saleeby, 2003. This was followed by the two LARSE profiles; LARSE I just nicked the Mojave a bit (e.g., Godfrey et al. 2002), but LARSE II led to the Yan et al. (2005) interpretation of a very thin veneer of Mesozoic material over widespread schists.
Since then there have been lots of seismological studies of Southern California–too many to fully address here at present. One relatively recent one that addressed the schists in the Mojave is a bit of a compilation and so might serve as a decent representation of what is out there. This paper, by Eymold and Jordan (2019), attempts to regionalize seismic velocity structures from the SCEC CVM‐S4.26 model in a manner that should reduce the uncertainties on parameters so long as similar seismic structures are indeed being combined. This is also notable for having P- and S-wave structures and so an estimate of Poisson’s ratio. They specifically address the schist question in section 5.5, concluding that the fairly normal Poisson’s ratio for the Mojave limits the volume of schist in the crust. There could be higher amounts in the Tehachapis and westernmost Mojave, where a more prominent mid-crustal low-velocity zone is present.
What lies beneath…
Another constraint might be where mantle lithosphere continues to hang out. Probably the most direct attack on this was made by Allison et al. (2013), who found that mantle lithosphere is present under much of the Mojave, with the mantle lithosphere possibly absent only under an area roughly around exposures of the Rand Schist and extending southwest into roughly the Central Mojave Extensional belt/Waterman Hills detachment system (e.g., Walker et al., 1990). And as Allison et al. point out, mantle lithosphere removal could be ongoing (they use its removal as a driver for low-volume Quaternary to Holocene volcanic eruptions), so the absence of modern lithosphere might not have been true during creation of the POR schists. On the other hand, there is nothing in the geophysics that requires this mantle lithosphere to be continental.
The argument for continental lithosphere comes more from volcanic rocks. Most convincing are the xenoliths in the Dish Hill locality, where many of the mantle xenoliths have a geochemistry expected of continental mantle (Luffi et al.., 2009). Of course, this is a single locality well away from surface exposures of POR schists.
The composition of the volcanic rocks themselves might contain important clues. A 2018 paper by J. Chapman et al. argues that a change in the isotopic chemistry of volcanic rocks west of about 114°W reflects melts originating since ~40 Ma in more primitive mantle, more like asthenosphere or oceanic lithosphere than the Precambrian signals seen earlier and to the east. Their interpretation is that continental mantle lithosphere was removed from the region and replaced with oceanic affinity material. There are other explanations for the variations seen (similar signals were interpreted as gradual erosion of mantle lithosphere during the Miocene associated with crustal extension, for instance). A full discussion of this paper seems relevant but isn’t possible here now.
On the whole…what might work?
Overall GG’s impression is that most of the crust in the Mojave is not POR schist. So how does this work? Perhaps the exposures of POR schists are quite steep-sided and so much more limited than we’d guess. The prevalence of fairly low-angle structures associated with these exposures makes that seem unlikely, and even if that isn’t the issue, how do you emplace vertical slivers of schists with inverted metamorphic gradients into otherwise continental affinity rock?
Easier to lay out in a general way but far harder to make work kinematically (let alone mechanically) is that the crust has been shuffled. Slabs of POR schist have been interleaved with slabs of Mesozoic arc rocks and some older materials. Triangle wedge structures are the kinds of things you might imagine could work kinematically, though the physics of such a process are challenging. Another way to shuffle things is to combine multiple episodes of deformation. Maybe schists were only emplaced in a small area that was subsequently extended in multiple orientations that brought these slivers up over continental material. Easy to say, a lot harder to draw.
Probably the most imaginative explanation was put forward by J. Chapman in 2021. He suggested that POR schists if subducted very far would be extremely buoyant. Such material could rise off the slab as a diapir. Similar structures from salt can look like elongated toadstools, with a broad horizontal extent of a relatively thin layer with a pipe from the source under the center. This would solve the volume problem nicely, but it isn’t clear you could get the inverted metamorphic gradients seen in these schists. It is also somewhat remarkable that the Cemetery Ridge site would bring up fairly high density peridotites as that diapir would have transited the mantle wedge under Arizona.
In essence, we need to know if we are looking at a whole-crustal event that would have started with the emplacement of the Rand Schist (or the San Emigdio Schist) or are seeing a series of nappes, in essence, created by some still-unclear means. As the POR schists are themselves a defining element of the Mojave terrain, knowing just what the represent in terms of damage to the continental lithosphere is pretty essential to unravelling the trainwreck in the Mojave.
Other entries in this series:
- Introduction
- Schists
- Igneous Activity
- Deformation
- Mojave Mojo
- Crustal structure (this page)
- Sedimentation
The Grumpy Geophysicist 
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