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.

The basic idea for the western U.S. is illustrated by this section created by Hildebrand:

HildebrandCartoon

Cartoon of the creation and initial accretion of Rubia from Hildebrand (2013)

Now Hildebrand’s maps are a bit vague on where the suture between Rubia and North America might be found, but is seems to be a little ways west of the frontal thrusts of the Sevier fold-and-thrust belt.

North America was subducted under Rubia in the late Cretaceous
Typically such sutures are marked by ophiolite fragments, deep water sediments and other remnants of the old ocean floor. High pressure, low temperature metamorphic rocks are typical. No such materials seem to exist anywhere in the region suspected of being the suture between Rubia and North America. The strongest support for this idea came from seismic tomography, where a paper by Sigloch and Mihalynuk claimed high wave speed bodies in the mantle required a west-dipping subduction zone, a claim that has not been widely accepted.
Mesozoic and late Paleozoic magmatism, widespread in Rubia, never extended into “true” North America
By and large this is true, but materials (mostly detrital zircons) derived from that magmatism are present in North American sediments.  Although zircons could be traveling through air as ash, some of the coarser materials seem to require rivers sourced in the arc transport materials onto North America. Additionally, the geochemical evolution of the arc doesn’t seem to reflect the changes in the material being subducted.
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
Certainly many workers have commented on the massive plutonic bodies that make up much of the high country in the Sierra; these were the last things emplaced in the Sierran arc (Mihai Ducea has written on this several times). But estimating the volume of magmatism is tricky: just how deep do these plutons go? And the older plutons were deformed by younger ones, so it isn’t entirely clear that the rate of magmatism in the Late Cretaceous was much greater than some earlier episodes. Similarly, it is unclear if these rates are truly exceptional compared to ignimbrite flareup localities in, for instance, the Puna Plateau region of South America. There is something going on, but it isn’t clear that subducting North America is a solution.
Much of the classic late Precambrian – Paleozoic Cordilleran miogeocline is exotic to North America (i.e., is Rubia)
Basically, this is critical.  Rocks that look North American underlie the exotic Antler orogen in eastern Nevada, but Hildebrand argues these are not North American sedimentary rocks. You would expect the isolation of a continental margin assembly would look different than the temporally equivalent North American shelf, but the class’s examination of the detrital zircon and paleocurrent literature shows no discrepancy. 
Deformation from accretionary events is limited to Rubia.
This is true, and depending on the analogs you pick, this does look unlikely.  The problem is that the analogs differ from the Rubian situation is significant ways. Of course, you have to unwind all the subsequent deformation.
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
Basically Hildebrand argues that if you unwind all the shortening in the thrust belt, rocks most geologists consider to be North American are now far west of the 87Sr/86Sr=0.706 line often taken to mark the edge of North America. There are two issues that make this less than a damning indictment of the status quo model. First, the 0.706 line is a pretty imperfect proxy; other work has argued for attenuated continental crust outboard of this line.  Second, unwinding the surface thrusting ignores the possibility of shortening in the footwall of the thrusts, which would tend to push the 0.706 line to the west. While a real discrepancy might be out there, the data at present don’t demand one.
Magmatism and uplift in the latest Cretaceous and early Tertiary was produced by the oceanic part of the subjected North American plate falling off.
This gets pretty complex, in part because in places Hildebrand expects there was magmatism as the slab broke off but others had no such magmatism. Just exactly what counts as “slab breakoff magmatism” is part of the issue, but Hildebrand also introduces a new variant on the motions of allochthonous terranes to get his breakoff magmatism to be aligned.  Presumably this is decompression melting of rising asthenosphere, which should have positive epsilon-Nd values and low initial Sr ratios, but several of the magmatic bodies fingered don’t seem to have the right chemistry. As for uplift, the problem there is disagreement on interpreting the available proxies, so it isn’t clear at present when or by how much things went up; some papers would make the uplift far later than proposed here. 

So while there are lots of equivocal observations, there is enough that stands in the way of the Rubia hypothesis as currently presented to think it unlikely if not quite disproven. However, the attempt to generate this hypothesis does point out some features that aren’t well explained by current theory (e.g., the big pulse of magmatism near the end of the Cretaceous, the limited or absent deformation of the North American shelf as tectonic accretion events were ongoing). So a dramatic challenge to standard models is helpful is seeing what issues are out there.

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