Testing Hildebrand's Rubia Hypothesis


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.


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.


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).

    Summary   Evaluation
H: North America was subducted under Rubia

Are there modern analogs of subduction of a continent under something like Rubia?

  Best possibilities Taiwan, Banda Sea, Himalaya   Very different than Rubia, so unclear.

Is the architecture of the late Mesozoic arc system in Rubia consistent with west-dipping subduction?

  -   -

Is there evidence from seismic tomography for west-dipping subduction?

  Yes, in some interpretations of multi-plate scale seismic tomography, but others interpret the same features differently   Presently equivocal with respect to the main question

Are there blueschist or other subduction-related metamorphic rocks associated with the westward subduction under Rubia?

  Basically, no.   Tends to refute Rubia

Does the composition of the arc evolve appropriately for subduction of a continental margin?

  For the most part, no.   Tends to refute Rubia
H: Magmatism never extends into North America but does affect all of Rubia

Are there any magmatic products on Hildebrand’s North American margin?

  Appears not   Concurs with Rubia hypothesis

Are there early Mesozoic detrital zircons on Hildebrand’s North America sourced from Rubia?

  Possibly   Tends to refute Rubia

Are these magmatic bodies an island arc?  Continental arc? Back arc?

  All three types are present    

Do these plutons uniformly have low 87Sr/86Sr (below 0.706)?

  No   Inconsistent with an oceanic affinity Rubia.
H: The ~120-80 Ma magmatism along the Mesozoic arc is far more voluminous than would be expected for a typical arc

 Are these volumes higher than earlier in the Mesozoic?

  Maybe; data pretty uncertain   Equivocal

How do these volumes compare with modern analogs?

  Generally appears higher   Supports Hildebrand's evaluation of the literature, though unclear if this contradicts classic interpretation.

Is Hildebrand’s model substantially different from the underthrusting of continental sediments envisioned by others?

  Yes, but each has problems   Equivocal

Does the arc migrate sensibly given the physical controls on the position of an arc relative to a subduction zone?

  -   -
H: Much of the classic late pC-Paleozoic “Cordilleran miogeocline” is exotic to North America

Are there 1.5 Ga zircons (considered a characteristic of Rubia) present in Paleozoic-early Mesozoic sediments on Hildebrand’s North America?

  Possibly in Ellis Group   Refutes Rubia if robust, but weak at present

Is there a break in the detrital zircons populations going west from stable North America out to the “Antler platform” thought to be part of Rubia?

  Not much variation   Tends to refute Rubia

Is there a change in the paleontology in the Paleozoic going from stable North America to the “Antler platform”?

  -   -

Do paleocurrents, clastic source regions and facies belts in Rubia make sense with Rubia’s isolation from North America?

  No, appears a continuation of North American patterns   Refutes Rubia

Is there a North American signal in the form of detrital zircons, bulk sediment initial Sr or other materials in Rubia?

  Appears so.   Tends to refute Rubia

Are there variations in depositional environment so abrupt on the classic North American shelf/slope that a large distance probably separated these deposits originally?

  -   -
H: Accretion of exotic terranes in the late Paleozoic and early Mesozoic did not produce any changes in Hildebrand’s North American shelf

Is there really nothing changing in the shelf at these times?

  Very little change   Tends to support Rubia

What kinds of signals are present in modern analogs like the Apennines/Adriatic Sea and parts of Indonesia?

  Do see deformation, though scales might vary   Tends to support Rubia
H: Palinspastic restoration of the thrust belt places classically defined North American rocks west of the edge of North America as defined by 0.706 87Sr/86Sr line

Is the 0.706 line an appropriate marker for the continental edge?

  No; continental material often extends beyond it.   tends to weaken Hildebrand argument that restoring thrusts places classical NAM material beyond North America

Is the 0.706 line correctly or robustly defined?

  squishy in Canada, OK in U.S. except Washington   equivocal

How well do we know the geometry of the Precambrian margin of North America, and how does that affect this reconstruction?

  -   -

Is there evidence of shortening of the lower plate that would push the 0.706 line farther west?

  -   -

Are there alternate reconstructions that do not place classical North American sediments off the west side of the continent?

  -   -
H: The magnitude of crustal shortening/thickening in the Sevier hinterland is inconsistent with a backarc origin

Are there modern backarcs with such thick crust?

  -   -

What is the origin of the thickness estimate (i.e., is this a robust number)?

  Kind of complex, but Hildebrand might be misrepresenting this data   Unclear

What are the observational constraints on the early Tertiary thickness of the hinterland crust?

  Structural, geobarometric, magmatic   Unclear
H: Widespread asthenosphere-sourced magmatism in western Canada reflects asthenosphere rising as the west-dipping slab breaks off from North America, while roughly coeval removal of the slab produces widespread uplift of western U.S., driving Laramide deformation

Do analogs for slab breakoff (e.g., Taiwan, the Himalaya) produce this kind and volume of magmatic activity?

  Maybe in Tibet, not so much elsewhere   Equivocal to tending to refute Rubia

Do isotopic variations in the Canadian igneous rocks support such widespread asthenospheric origins?

  Possibly   Equivocal

What evidence is there on the evolution of the elevation of the western U.S. over this time period?

  Complex and disputed   Unclear
The southern end of the Sevier belt and shelf deposits and their juxtaposition with profoundly different histories requires the presence of a late Cretaceous Phoenix Fault connecting the Sevier fold-and-thrust belt with the stylistically similar belt in northeastern Mexico.

What is the deformational history of southern Arizona in the latest Mesozoic?