Testing Hildebrand's Rubia Hypothesis

Overview

In 2009, the Geological Society of America published Special Paper 457 entitled "Did Westward Subduction Cause Cretaceous-Tertiary Orogeny in the North American Cordillera?" by Robert Hildebrand.This was followed up in 2013 with a second Special Paper by Hildebrand, SP 495, "Mesozoic Assembly of the North American Cordillera". These two papers present an alternative view of the creation of the North American Cordillera that, if correct, means that decades of geological work in the region has been misinterpreted. As these are sizeable works and their implications would overturn the work of dozens of geologists, many in the field probably have dismissed the implications of thie out of hand. So the 2016 Western U.S. Tectonics class at the University of Colorado tried to evaluate this hypothesis, considering both observations Hildebrand had suggested were not well explained by current theory as well as observations that should differ between this hypothesis and the current paradigm for the creation of the western U.S. The resulting materials are presented below.

Background

The commonly accepted framework for the western U.S. is that the western edge of Laurentia and North America was first defined by rifting in the late Precambrian. In the western U.S., this produced a passive margin with a hingeline roughly extending from near the modern San Bernardino Mountains northeast through the Lake Mead area and along the Wasatch Front and then northward across Idaho and westernmost Montana. West of that line the crust and lithosphere had been thinned and subsided, accumulating great thicknesses of carbonate and clastic sediment on the shelf and slope at the edge of the continent. Farther west was either extremely attenuated continental crust or newly formed oceanic crust that accumulated deep water cherts and related sediments at some distance from North America. Starting in the Mississippian the passive margin became an active margin, first with the thrusting of the Roberts Mountain allochthon onto the edge of North America, then the emplacement of another allocthonous terrane, the Golconda allocthon, in the Permian. A part of the southwesternmost part of the U.S. was probably translated to the southeast, most likely in the Pennsylvanian, though the timing is disputed. Starting the Permian, a volcanic arc was created as ocean floor to the west subducted under the western U.S. This arc would wax and wane through the entire Mesozoic, accompanied by varying amounts of strike-slip and compressional faulting, only to shut down in the Sierra Nevada about 85 million years ago and migrate well to the east from southernmost California across Arizona into New Mexico into the early Tertiary. During the Jurassic in particular, some series of tectonic events deformed parts of the Sierra Nevada, events termed by some the Nevadan orogeny, which might reflect the collision of a second arc with North America, or the creation of marginal back arc seas that were then sutured on to North America. Regardless of the details, by the late Jurassic, all of the western U.S. was part of North America save younger materials in the Coast Ranges of California, Oregon and Washington and material that filled the Columbia Embayment in Washington and Oregon (Silitizia).

Hildebrand's alternative retains the late Precambrian history but instead of assembling all of the western U.S. on the edge of the continent through the Paleozoic into the Mesozoic, he proposes that these orogenic episodes created a ribbon continent well offshore of North America that he terms Rubia. Thus the volcanic arc from the late Paleozoic into the Cretaceous is built on Rubia, the Antler and Sonoman orogenies are assembling Rubia, and it is only at the end of the Mesozoic that subduction of the old ocean floor west of North America under Rubia leads to the accretion of the Rubian continent onto North America, driving the Laramide orogeny. Rubia in this scenario rides over North American lithosphere as subduction reaches its climax.

Questions

To try and identify key differences between Hildebrand's hypothesis and the current paradigm, we crafted several topics and more specific questions that we then used to focus an examination of the existing literature, trying to see if older interpretations better fit the Rubia concept and if there were observations not fitting the standard model well that did work within the Rubian framework. These are listed below with links on questions examined to detailed summaries prepared by class members on individual elements. The grayed-out questions here were not examined but are included for completeness (or an exercise for the reader).

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