NOVEMBER 12th MEETING INFORMATION
SPEAKER: Vince Matthews, Retired Colorado State Geologist and Director of the Colorado Geological Survey
TITLE: The Middle and Southern Rocky Mountains are NOT Laramide in age— they are tens of millions of years younger
DATE: Tuesday, November 12th
LOCATION: Fort Lewis College, Student Union Building, Vallecito room.
TIME: 5:30-6:30 dinner & drinks, 6:30-7:30 society business and presentation.
COST: $25pp / Sponsored students free (talk to Dr. Gonzales: Gonzales_d@fortlewis.edu)
REGISTER: https://form.jotform.com/240926885399072
ABSTRACT:
In 1971, I arrived in Colorado to teach at the University of Northern Colorado and was quickly introduced to the widely-held dogma that the Colorado Rocky Mountains were formed during the Laramide orogeny. Only two years later, at a Geological Society of America symposium in Boulder, I learned that was not the case. The presentations in that symposium were
documented two years later in Geological Society of America Memoir 144. Following is an excerpt from the Preface to that memoir. “Taken together, these articles demonstrate that Cenozoic orogeny in the Southern Rocky Mountains was by no means composed of just a few scattered goings-on associated with decaying Laramide deformation, as many geologists have tended to picture it. Instead, the end of the Laramide orogenic phase about mid-Eocene time can be discerned rather clearly along with a following quiescent interval that lasted for somewhere around 5 m.y. Following the quiescent interval, the rock records show renewed tectonism, but of a different style which involved extensive volcanism and faulting of basin-range type that created some displacements of at least several kilometers. The Laramide and the later Cenozoic tectonic phases probably were related through deep internal earth processes, but the evidence here shows that the later Cenozoic activity was, by itself, of impressive stature and quite deserving of recognition.” — Professor Bruce Curtis (University of Colorado).
Yet half a century later, I still find all too many claims to the effect that “the Colorado Rockies formed during the Laramide orogeny.” Don’t get me wrong, I love the Laramide and its magnificent structures. Indeed, I put together a Geological Society of America memoir on those very structures across the Colorado Plateau and Wyoming Province of the Rocky Mountain Foreland. But, there is a plethora of evidence that demonstrate 1) that the present-day, high elevations formed in the last 20 million years, 2) are a result of a tectonic extensional event that began about 30 million years ago, and 3) that event continues today. The mountains that exceed 13,000 feet in elevation in the Southern and Middle Rocky Mountains are part of an organized tectonic framework around the Colorado Plateau Microplate (CPM). The rotational rifting of the CPM is part of the larger extensional province that was triggered by, and is driven by, lithospheric plate interactions along the west coast of North America. The resulting mountain chains in Utah and Colorado are characterized by abnormally high elevation, normal faults with thousands of feet of displacement, grabens with thick synorogenic deposits, extensive young magmatism, and a regionally organized framework of Neogene/Quaternary stress and strain.