The Ancestral Rockies

The first signal – the first hint – that the crust that has supported Northern New Mexico for the last 1 billion years or so might not be as stable as it should be came about 315 million years ago. For all those millennia beforehand, New Mexico sat as flat and dull and stable as Iowa – flatter, actually, and much closer to sea level, and not nearly as green. A section of old rigid continental crust and mantle – the stuff that forms the geologically uneventful interiors of continents – is ¬†sometimes called a craton, derived from the Greek work kratos, or ‘strength’. And the portion of North America, extending from Minnesota to Northern New Mexico, was about as cratonic as they come, forming a continental backbone that stood above the ocean through many cycles of rising and falling sea level that flooded other parts of the continent, stable or otherwise.

Around 345 million years ago, during the Mississippian Period, the sea did creep over the ridgepole to leave a thin veneer of sand and tropical limestone, only to retreat and see much of its work stripped away.

WNA_340_M_PZ_Tect-sm

And then, about 30 million years later, near the beginning of the Pennsylvanian Period, the craton ruptured. The sea began to move in. A set of uplifts now called the Ancestral Rocky Mountains formed, centered in Colorado but reaching southeastward to join similar uplifts in Oklahoma and North Texas:

WNA_300_Late Penn_Tect-sm

A beautiful paleogeographic reconstruction by Ron Blakey dramatizes the change:

WNA_300_PPvir-sm

The above three images were captured from Blakey’s remarkable website which I urge you to visit.

The full set of causes for this failure is disputed, but it does coincide with the assembly of Pangea, the last supercontinent. In the second tectonic map above you can see South America attaching itself to North America along a boundary marked “Ouachita-Marathon orogen”.

Now an orogen is the complement of a craton. It refers to the “mobile” belts of deformed rock that are frequently found on the perimeter of stable cratons. In fact the two terms were introduced by the same German geologist early in the 1920′s. In more familiar language, an orogen is a belt where mountains are formed, with all the folding, faulting, terrane accretion, and volcanism we associate with mountain building. These associated processes are collectively referred to as orogeny. (Oro is the Greek word for ‘mountain’. Something tells me students back then got a better grounding in the classical languages than I did)

As an orogeny the formation of the Ancestral Rocky Mountains is a puzzle, occurring well inboard of the continental margins on old, cold crust. The sea flooded in rather than being driven out. There was no associated volcanism to speak of. Nevertheless, this episode of crustal disturbance completely altered the face of New Mexico and set the course of events here for the next 70 million  years.