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?
…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?
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.
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…
Hollywood, it seems, has been in an exceptionally strong recycling mode, with reboots or re-envisionings or extensions of hits (and not-so-hits) past (think Star Wars, Star Trek, live-action versions of classic Disney animations, etc.). So maybe it isn’t a surprise that we are seeing the same thing to some degree in tectonics.
There are two problems manifest in the late Mesozoic to early Cenozoic geology of western North America that face those of us interested in understanding how everything was made: the creation of the southern Rocky Mountains in the Laramide orogeny, and the assembly of exotic terranes into western Canada. That these two events overlap in time inspires many to look for a common cause. So let’s do a quick review and update and see where we stand. (“Quick” by the ways, does not mean short–sorry).