July 6. The Ancestral Rockies

By Middle Pennsylvanian time, around 310 million years ago, the collision between Gondwana on the south and the combined North America-Eurasia on the north was well underway. It had probably started by late Mississippian time, and if you want to count the early episodes of island arcs and microcontinents colliding, you could say it was underway by the end of the Ordovician. But now we’re talking about the continents themselves. The big battering rams are finally coming together.

The main force of the continent-continent collision was felt in eastern and southern United States, where the Alleghenian and Ouachita Mountains were formed. But well into the continent, in the west, effects of the massive collision were seen too. This is analogous to the tectonic deformation thousands of kilometers into Asia that results from the collision of India and the southern edge of Eurasia.

It’s challenging to reconstruct the stress fields and other factors that led to the uplift of the Ancestral Rocky Mountains, but the rock record does clearly show where the uplifts were located. Mostly in Colorado, with a pair of long, sub-parallel mountain belts extending from southeast to northwest across the state, into parts of Utah and Wyoming. The southeastern ends of the ranges continued into northern New Mexico, and across the Oklahoma and Texas panhandles and on into southwestern Oklahoma.

Generally speaking the Ancestral Rockies were big block uplifts, bounded by faults that broke the interior of the continent as a distant result of the collision of Gondwana. The uplifts and the faults created high mountains with associated thick piles of sediment, and the breaking of the continent created weak zones in the crust that had impacts in the geologic development of this region right up to today.

There are other ideas about how the Ancestral Rockies formed, including the presence of a possible subduction zone in western New Mexico and Utah that might have caused compression to force the mountains up, as well as a possible persistent weak zone that may have run from southern Oklahoma northwest to Idaho and beyond. But most alternative ideas still rely on the Pennsylvanian collision of Gondwana to provide the impetus, the force, that broke what was fundamentally a pretty strong part of the ancient craton of North America.

—Richard I. Gibson

References
Ancestral Rockies  

Kluth & Coney 1981


Map

Drawing after Mallory, W. W., 1958, Pennsylvanian coarse arkosic redbeds and associated mountains in Colorado, in Symposium on Pennsylvanian rocks of Colorado and adjacent areas: Rocky Mountain Assoc. Geol., p. 17-20.