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.
Update 7/14/19. Things are steadily quieting down in this area, though there are still a lot of small (M<2.5) quakes just west of the rhyolite domes. This spot and the area near Little Cactus Flat to the north remain the most active areas outside of the original ruptures.
Update 7/11/19. While the number of quakes in this area is declining, there was a M4.3 that also had a large non-double couple mechanism–according to Caltech. The USGS-NEIC also estimated a solution and got something much more like regular fault slip. Which indicates that getting mechanisms for very shallow M4s can be tweaky. While more action is now farther north, those events look more fault like–though those mechanisms are also from NEIC, so could be NEIC’s procedures tend towards double-couple solutions more than CIT’s. And as an aside, it is a bit surprising how little activity has been at Mammoth–it is an area that has had seismicity triggered by surface waves in the past, but has remained fairly quiet this go round.
Original post: One thing GG has kind of been looking for is whether the M7.1 Ridgecrest event is triggering things near the Coso volcanic field. And it seems there is something worth being concerned about going on.
Seismicity in this area is traditionally shallow, meaning above 5 km depth (Monastero et al., 2005). The tight cluster of orange dots include 2 M4+ earthquakes. This area is at the west edge of a seismic discontinuity at about 5 km depth inferred to represent the top of a magma chamber (Wilson et al., 2003). While there has certainly been seismicity in this region before, given the proximity to fairly recent volcanic activity, one has to wonder if there is magma on the move. Supporting that are the focal mechanisms for the two M4 earthquakes, both of which have substantial non-double couple components (indeed, the mechanism for one looks very much like a diking event). Given that all these events are being located in the top 2 km (probably relative to sea level, so top 3 km of crust), this could get pretty interesting pretty fast.
As background, the central core of the Coso volcanic field are silica-rich rhyolites that appear as blister-like bodies in the image above. Surrounding this core area that overlies the seismically inferred magma body are basaltic eruptions (like Red Cone, in lower left corner). The troubling seismicity is directly on the road into the geothermal area from Coso Junction to the west.
An overview of the M7.1 with the first InSAR image of the 7.1 rupture is at Temblor.com. This also discusses seismicity in this area, but with less consideration of volcanic activity.
All the attention on the Ridgecrest earthquakes has returned the Eastern California Shear Zone and its rather more obscure relative, the Walker Lane. Among the news articles out there is a pointer back to an article on the notion that the plate boundary will shift in to these fault systems sometime in the geologic future. This then results in western Nevada eventually facing an ocean–or, more extravagantly, Salt Lake City as a coastal town. So let’s clarify our terms, understand when and why these faults have emerged, and then set out to consider what it takes to move a chunk of continent onto an oceanic plate.
First off, there are two named parts to any potential new plate boundary (well, actually, they are already taking up about a quarter of the plate motion): the Eastern California Shear Zone and the Walker Lane. The Eastern California Shear Zone is a well-defined feature in the Mojave Desert south of the Garlock fault and named recently in 1990 by Dokka and Travis. The Walker Lane, in contrast, has had multiple incarnations: it was originally defined by Locke et al. (1940) on the basis of more confused topography along a swath of ground running from Las Vegas through the Walker Lake area to north of Reno. At times it was thought to be as old as Jurassic. John Stewart redefined it in a 1988 book chapter, and subsequent workers have now generally drawn the Walker Lane as kind of hugging the east side of the Sierra out about 150-200 km into the Basin and Range, removing the Las Vegas Valley Shear Zone from being within the Walker Lane and, generally, dating the strike-slip motion within the last 9 million years, with ~4 Ma seemingly a good guess (Andrew and Walker, 2009).
The northern edge is more of a mystery and the names for it more inconsistent. The Central Nevada Seismic Zone takes off from the Walker Lane to the northeast as a more dominantly normal fault system. Strike-slip faulting cuts into the Sierra on a Northern California Fault Zone (Wesnousky, 2005) and might connect to the coast (e.g., Unruh et al., 2003) or towards the Cascades arc (e.g., Waldien et al., 2019), and some have carried deformation northward through central and eastern Oregon and Washington (e.g., Pezzopane and Weldon, 1993). If we really are making a new transform boundary, its northern extent sure isn’t obvious.
So why is strike-slip faulting a relatively late arrival in this area where deformation extends back 15 or 25 million years?
Update 7/5/19 ~ 9:30-10:30 pm PDT. Well, looks like the rest of the Airport Lake fault zone ruptured this evening (that is the alignment of the northwest trending limb of seismicity from the earlier sequence). This probably is the farthest edge of this fault zone. It might well load the Little Lake fault, which is kind of the next right-lateral system to the north. So if there is a further progression, rupturing the Little Lake fault would be the next logical domino to fall. But that isn’t terribly likely. No doubt the naval weapons center has a bit of a mess as the rupture tracked right past their main facility. What will be interesting will be the focal mechanisms at the northwest end; the NW-SE trending folds of the White Hills anticline might produce some thrust mechanisms, or the oblique-normal Coso Wash fault might produce normal faulting mechanisms. [The M5.5 aftershock in the northwest corner is an oblique normal mechanism on–probably–a north-south trending fault, which looks likely to be the Coso Wash fault].
The southeast end isn’t home free either; the Blackwater fault on the south side of the Garlock probably has seen some increase in stress from these events. It is less clear if that is true of the Garlock fault. But given the complexity at each end of the rupture of this M7.1, stress transfer is probably complex.
And listening to the media is a bit disheartening. “People in LA are used to this.” Um, well, they are 150 miles from the event and any directivity with this event (it appears to have been a bidirectional rupture from the aftershocks) would affect places like Barstow or perhaps Porterville far more than LA. The long rolling motion at those distances is generally not a problem (it does produce sloshing pools). Sleep in a house or not? Certainly depends on the house: if the gas is off and the house is on its foundation and there are no structural issues, you could stay in the house, but if you aren’t certain of all those, staying out would be for the best. As Lucy Jones [not related to GG] is fond of relating, there is about a 5% chance there could be a larger quake, so some caution is warranted. They were saying “there is not a continuous stream of ambulances”–but there are not too many in Ridgecrest to start with, so the absence of a stream is hardly a surprise even if there are numerous injuries. Asking a resident of Ridgecrest “did you get an alert”? Simple answer is no: the town was too close for the alert system to give a useful warning. But a good question is whether there was a warning at other communities–it sounded like the system didn’t trigger on the M6.4–shaking from the 7.1 could be of concern in some areas.
And now we have speculation on triggering distant events. This is where things get a little shaky. Lucy Jones, when interviewed, downplayed remote triggering, instead emphasizing that triggered large aftershocks are usually pretty close–but the Little Skull Mountain earthquake in southern Nevada looked a lot like it was triggered by the Landers earthquake, so this is a bit less certain. Generally triggered seismicity occurs in areas with magma or geothermal systems–Mammoth Lakes area and Yellowstone have had earthquakes (generally pretty small ones) triggered by the shear and surface waves of distant events, so it isn’t impossible to trigger something at a distance. Right now not much such seismicity is showing up, but then the systems right now are pretty heavily hit by aftershocks near Ridgecrest.
Original post: So SoCal finally got an earthquake above M6. GG suspects the residents of Ridgecrest are tired of hearing in news reports about how they live in a remote rural area. (If your vision of “rural” are widely separated farm houses with crops or cattle in between, this ain’t it–the main business is the China Lake Naval Weapons Center and, to a far lesser degree, tourists on their way somewhere else). But let’s take a brief look at the seismicity associated with this event, for this is a curious area.
First off, this is hardly a new thing in this area. In the mid-1990s there were several earthquakes very near this spot--a northwest trending band to the NW and a southwest-trending band to the SW. This sequence appears to illuminate both, with the southwest trending band appearing to be a left-lateral strike-slip fault and the northwest trending band a right-lateral strike-slip fault. The seismicity extends to a mapped Quaternary fault; it seems likely that the “crack” reported and filled by Caltrans had a couple inches of left-lateral offset (there are some photos on Twitter showing the white line on the edge of the road having been offset). There is no mapped fault on the northwest trending leg, but the existing mapping in the area wasn’t focused on identifying such minor faults. We’ll learn shortly whether there was surface rupture in that direction.
So why is this curious? Generally the swarms in the area have been on one system or the other; this includes both, and for that reason is a great reminder of the peculiar tectonics of this region.
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…