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A (nearly) forgotten constraint

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.

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Geohero or geochump?

A comment at a meeting GG was at got him to thinking about the popular view of scientists.  The comment was that scientists in the 19th century were heroes for Americans because they helped open up the West, while in the 20th century they were more thorns in the sides of growth.  Of course, this is so oversimplified it collapses quickly: John Wesley Powell, a hero for his explorations of the Colorado River, was viewed with great disdain when he closed claims for public lands. And post-WWII America fell in love with science in many ways. But still, when are scientists lauded and when are they scorned? An interesting pair of cases in the late 1860s and 1870s may shed light on this.

In both cases a scientist running a geological survey became aware of claims of major mineral finds within the area of his survey.  In both cases, the scientist claimed that these finds were incorrect. In both cases, the finds were not economic. Yet in one case, the scientist in question, Clarence King, was lauded, became first director of the USGS, and was viewed as one of the best and brightest America had to offer.  The other, Josiah Whitney, lost his survey and spent years grousing about the outcome. Why the difference?

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Rubia Slippers

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.

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Laramide Times

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:

DickinsonLaramide

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?

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Mountains that Remade America

For those who come hoping to see material related to the Grumpy Geophysicist’s upcoming trade book on the Sierra Nevada, The Mountains that Remade America, here are a few quick pointers. Read More…

A Range of Opinion

…or, perhaps, when engineers and geologists collide. (This is one of those little episodes GG had pulled up for possible inclusion in his book The Mountains that Remade America only to find it something of an orphan).

In 1863, the Big Four responsible for construction of the Central Pacific Railroad were in desperate need of funds. The 1862 Pacific Railroad bill allowed $48,000/mile to be loaned once the railroad entered the Sierra Nevada but only $16,000/mile before then. Up to 150 miles of mountainous terrain could be claimed.  Eager to get the most money possible, Charles Crocker got the state geologist, Josiah Whitney, to go on a buggy ride to decide where the western edge of the range might be. Whitney allowed that the Sacramento River might be the most appropriate spot, but both he and Crocker could see the terrain was quite flat.  So they rode off some 6 or 7 miles east to Arcade Creek where some reddish sedimentary rock was exposed, and Whitney allowed that this was an appropriate spot to claim the edge of the range. But when Theodore Judah, who was director of the line and was responsible for identifying the route and who had shepherded the 1862 bill through Congress heard, he was appalled. This was 21 miles west of where the line would encounter its first real grades.

This episode is widely derided as Whitney engaging in deception to aid the railroad.  Was this another example of Josiah being a total ninny, as when he accepted the Calaveras Man claims, or when he said there had never been glaciers in Yosemite Valley?

SacramentoGeol

Detail of Wagner et al., 1981, Geologic map of the Sacramento quadrangle, California, 1:250,000.

On the map above, the Sacramento River is at the left and the edges of crystalline rock of the Sierra is at the right. The Central Pacific runs under the “r” in the Arcade Creek label. The orange patterned unit on the map is the Turlock Lake Formation.  According to Unruh (1991), the 0.6 Ma Turlock Lake is tilted 0.19° to the west. For a geologist, this western edge of tilted and eroded Tertiary and Quaternary rock is indeed the most likely spot to mark the edge of the range. (Geologists today would not go along with the Sacramento River as the edge as its floodplain extends well east of Sacramento).

Whitney had, in fact, chosen a perfectly appropriate spot even though it was one that Stanford and Crocker had decided upon by measuring backwards 150 miles from the Truckee Meadows on the east side of the range. That this was not the essence of “mountains” as envisioned by an engineer like Judah was not Whitney’s fault; indeed, had the bill been written to assure that only truly mountainous terrain be included, Judah should have inserted minimum railroad grades or some other direct measure.

Whitney’s written acceptance of the Arcade Creek edge to the Sierra was seconded by two government surveyors in California and sent on to Washington, where Abraham Lincoln agreed that this was the appropriate point for the mountains to start. The Central Pacific got their higher loans earlier, but more through the ambiguity of the original legislation than deliberate misrepresentation on the part of the state geologist.

 

Is Bird ’88 Flying Again?

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….

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