Over the past decade or so, a fairly common event has been the publication of a paper taking on a piece of evidence for large magnitude extension in the Basin and Range. For the most part, this has been going after the various individual constraints used to make Wernicke et al.’s (1988) estimate of 247 ± 56 km of WNW-oriented extension. For instance, there are papers attacking the displacement on (or existence of) the Mormon Peak detachment, on the reconstructions of the fold-and-thrust belt across the Death Valley region, and on the age and amount of extension across Panamint Valley. The net result some would infer is that extension in the Basin and Range here was actually a lot less than 247 km (yes, GG has been told “a lot less”).
Rather than wade into these multiple controversies, GG would prefer to step back and ask, are there other ways of coming at this? There are a couple of approaches. For instance, one can try to improve the somewhat circular logic of Coney and Harms (1984) and try to use the modern and estimates of the pre-extensional thickness of the crust to get at total extension. A problem is that the crust has been an open system and constraining the amount of magmatic additions limits this approach.
The other approach comes from a rather unexpected quarter: plate tectonics. Or more precisely, plate tectonic reconstructions, and is has maybe been overshadowed by these other arguments. It is rather clever, but to see its power, we have to take a moment to understand what is going on.
Last fall GG’s Western U.S. Tectonics class took on trying to evaluate the status quo challenging hypothesis of Robert Hildebrand that the western part of the U.S. (west of central Utah, roughly) was a separate ribbon continent, Rubia, prior to colliding with North America in the early Tertiary, creating the Rocky Mountains. (That status quo holds that the far west was gradually assembled from the latest Paleozoic going on to the Miocene, with an arc being present on the edge of North America from the Permian to the late Cretaceous and again in much of the Tertiary). As Hildebrand’s argument was wide ranging and published as two lengthy GSA Special Papers (457 and 495), it isn’t a casual affair to consider the question of whether Hildebrand has caught western geologists in a huge misinterpretation or not. Many workers, content with their personal knowledge, have not peered into this abyss, so the class set out to take a swing at this. Basically, has Hildebrand identified observations inconsistent with our current interpretation of the geology? And are observables more consistent with Rubia than the standard model? A “yes” to the first might show that Hildebrand has put his finger on a problem even if the answer to the second is a “no”.
The class broke the hypothesis into these elements:
- North America was subducted under Rubia in the late Cretaceous
- Mesozoic and late Paleozoic magmatism, widespread in Rubia, never extended into “true” North America
- The magmatic volumes at the end of the Cretaceous in the western arcs are far too voluminous to have been produced by subduction of oceanic lithosphere
- Much of the classic late Precambrian – Paleozoic Cordilleran miogeocline is exotic to North America (i.e., is Rubia)
- Deformation from accretionary events is limited to Rubia.
- Mesozoic thin-skinned thrusts contain too much shortening to be limited to North America and are far greater than found in backarcs of typical continental arcs
- Magmatism and uplift in the latest Cretaceous and early Tertiary was produced by the oceanic part of the subjected North American plate falling off.
You can go and read the individual assessments made by class members to particular parts of this analysis, but a summary is below.
Traditionally, the Laramide Orogeny starts around 75 million years ago. Probably most geoscientists would agree with the overall analysis of Dickinson et al. (1988), which is mainly based on sedimentary rocks preserved from that time. So their criteria were that marine sedimentation (diagonal hatch) had ended prior to the Laramide, individual basins shifted from sharing facies with adjacent areas (black square) to having distinctly thicker deposits (circle) and coarse clastic detritus derived from nearby uplifts (black triangle) as the Laramide started:
It would be hard to argue that the Laramide Orogeny started later than the kinds of dates that Dickinson et al. proposed–but could it be earlier? If you had shallow sea floor covered in muds and parts started to rise up, might the muds simply get entrained in the existing current systems and be scoured down, creating a lacuna that, later on, would erased by even deeper erosion? In other words, is it possible that there was early deformation that wasn’t vigorous enough to overcome the broad subsidence of the region and so failed to produce positive topography? And if so, would subsurface loads have started to create local depocenters that perhaps have escaped recognition?
Back in the 1980s, Peter Bird considered how the Rockies might have been formed through a pair of papers. The first laid out the physical pieces of how a subducting plate could affect the overriding continent if it was in contact; the second combined all those pieces into a numerical model to see what would happen if enough stress was transmitted into the crust to create the Rockies in Colorado and Wyoming. One clear answer at the time was that the mantle lithosphere in the western U.S. would basically go away; the very clear response from those studying volcanic rocks sourced in the mantle was no, that won’t work. Despite this, the flat-slab hypothesis remains the front runner with most geoscientists.
A new paper by Copeland et al. in Geology [paywalled] seems to return to the basic hypothesis Bird envisioned:
Following the hypothesis that Laramide shortening was a consequence of the traction between the base of the North America (NA) plate and the top of the Farallon plate (e.g., Yonkee and Weil, 2015; Heller and Liu, 2016), we suggest that the southwestward migration of the inboard deformational edge (Fig. 1B) was a consequence of a narrowing of the zone of FA–NA lithospheric interaction by progressive rollback of the Farallon plate from northeast to southwest beginning at ca. 55 Ma and continuing into the Oligocene.
Now a lot of the paper is dealing with that rollback, which is actually an investigation of the post-Laramide landscape, but it is some of the material dealing with the start of the Laramide that caught GG’s eye. So we’re going to unpack in detail one figure in order to see if the data is what it seems to be–and to see if this is different that what GG outlined in a 2011 paper. (And hopefully today GG won’t anger yet another unsuspecting author who never expected their work to be examined in public). So hang on if you are coming for the ride….
Well, time to catch up on the evolution of the Sierra Nevada. Although a large collection of paleoaltimetry papers has bolstered a case for the elevations in the Sierra having been created by the Eocene (most based on Rayleigh distillation of precipitation), a couple of other recent works, one geodetic and the other geomorphic, seem to indicate that Sierran topography has grown over the last few million years.
First up is an update on vertical GPS velocities in California and Nevada by Hammond et al. in the Journal of Geophysical Research. They find “…the Sierra Nevada is the most rapid and extensive uplift feature in the western United States, rising up to 2 mm/yr along most of the range….Uplift patterns are consistent with groundwater extraction and concomitant elastic bedrock uplift, plus slower background tectonic uplift.” This in some ways is trimming the sails a bit on the earlier Amos et al. paper in Nature; as we previously discussed this wasn’t entirely unexpected. Their money figure would be this:
The red blob in most of eastern California is the Sierra Nevada. For most of the range, the pink colors correspond to uplift rates of 0.5-1.0 mm/yr. The presence of the pink/red colors in the central to northern Sierra, where there are no blue colors to the west, would indicate uplift is not being caused by groundwater withdrawal to the west (which is the cause of most of the dark blue south of 38°N and was the focus of the Amos et al. paper). Given the these rates would produce the modern mean elevation of the Sierra in under 6 million years, this would seem to strongly support the young Sierran story and be broadly consistent with the geologic story of a young uplift caused by removal of a dense root.
But, hmm, let’s look more closely…
UPDATE 2 11/22: GNS has assembled quite a lot of information, and the puzzlement deepens. It appears from the satellite and ground analysis that the bulk of the motion–up to 11 m of slip–was more nearly strike-slip and not the thrusting that appears in the focal mechanism (below). But the uplift of some areas of the coast by 6 meters (!) seems to suggest there is something more.
UPDATE 11/18: A considerable amount of information was put in an article on stuff.co.nz. This includes a map from GNS showing where the faults are that ruptured, a good deal of geodetic information.
Yesterday’s M7.5/7.8 Kaikoura earthquake in New Zealand is one of the more bizarre large earthquakes we have seen in some time. On the face of it, this appears to mostly be rupture of a subduction zone under northeasternmost part of the South Island of New Zealand. But there is a lot of other stuff going on….
First, the main focal mechanism as reported by the USGS:
Now this beachball would suggest a fault dipping to the NW while paralleling the coast. But the appearance that a toddler was not coloring in the lines tells you that there is something more here.
Some of that became apparent when the New Zealand’s GNS Science group went looking to see if there was any slip on earthquake faults. This is what they found:
Rapid field reconnaissance indicates that multiple faults have ruptured:
- Kekerengu Fault at the coast – appears to have had up to 10m of slip
- Newly identified fault at Waipapa Bay
- Hope Fault – seaward segment – minor movement
- Hundalee Fault
I’ve tried to sketch these out from my copies of geologic maps of New Zealand:
(The base map is from Google).
This is where the other shoe drops. The Hope and Kekerengu faults are mapped as strike-slip. Now minor slip on the Hope Fault might not mean much, but 10m on the Kekerengu means there was a lot of slip (I’ve assumed above it is strike-slip, but perhaps there is a thrust component). Plus, the epicenter of the quake–where it started–is somewhere between Cheviot and Rotherham, well to the south (this is why initially this was called the Cheviot earthquake). Toss in a very odd slip history (the moment release was low for a minute and then things really broke) and you get the impression that a relatively small earthquake on an unnamed fault southeast of Rotherham started tripping things off to the north, which eventually tripped off a big rupture.
That big rupture probably is not on the map. It is likely offshore, in the very southern end of the Hikurangi Trench (which is in part responsible for the whale watching that is so popular at Kaikoura). This is the northeast trending thrust fault that the focal mechanism captured and is responsible for the large slip amounts found on the finite-fault map the USGS shares. This is probably also the reason for the ~1m uplift of the seashore at Kaikoura, which led to many photos of paua and crawfish out of the ocean (though uplift at the southwest end of the big strike-slip fault is also possible).
Presumably the large strike-slip faulting on the Hope and Kekerengu faults is what has contaminated the focal mechanism, making it a composite of complex motions instead of the clean double-couple. (Pure strike-slip faulting is seen in many aftershocks.) As such, it seems this earthquake might well have captured both major thrust motion on the subduction zone and strike-slip on the upper plate faults, a form of slip-partitioning in a single event that is quite striking.
It will be interesting to see how the seismological and geological analysis continues; the main seismological slip appears north of these faults and so there could well be more to be found. But rain is in the forecast, which tends to ruin the easiest of signals to see.