June 27, 2010 | David F. Coppedge

Colorado Plateau Stumps Geologists

Many of the West’s greatest parks and scenic areas lie on the Colorado Plateau, a large basin covering parts of Arizona, Utah, New Mexico and Colorado.  Within its rugged acres are the Grand Canyon, Grand Staircase, Bryce Canyon, Zion Canyon, Arches, Canyonlands, Capitol Reef, Natural Bridges, Monument Valley, Mesa Verde, Glen Canyon and Lake Powell, and numerous small parks and scenic byways.  How this vast region rose 2 kilometers high away from plate boundaries, and maintained sedimentary strata miles thick that often lie flat as a pancake for hundreds of miles, is an enigma to geologists – and it underscores the problem historical sciences have with making pronouncements about the unobservable past.
    Rebecca M. Flowers (U of Colorado, Boulder) wrote about “The enigmatic rise of the Colorado Plateau” in the journal Geology this month.1 

How and when the Colorado Plateau attained its current mean elevation of ~2 km has puzzled scientists for nearly 150 yr.  This problem is most dramatically manifest when standing on the rim of the Grand Canyon, viewing the extraordinary 1500-m-deep gorge carved into nearly horizontal sedimentary rocks that were deposited during the 500 m.y. prior to plateau uplift when the region resided near sea level.  What caused the elevation gain of this previously stable cratonic region in Cenozoic time?  Did the source of buoyancy for plateau uplift arise from the crust, lithospheric mantle, or asthenosphere, or through some combination of the three?  Why did this low-relief plateau escape significant upper crustal strain during uplift, in contrast to the Cenozoic surface deformation that is so strikingly apparent in the high-relief landscape of the surrounding Rocky Mountain, Rio Grande Rift, and Basin and Range provinces (Fig. 1)?

The current issue contains two new theories, but Flowers is not convinced of either of them.  Here are a few quotes from the article indicating the degree of doubt and frustration explaining the Colorado Plateau.

  • Although there is a first-order understanding of vertical motions in areas close to plate boundaries, there is comparatively little consensus on the causes of such motions distal from these margins.  The Colorado Plateau exemplifies this problem.
  • Hypothesized mechanisms include partial removal of the lithospheric mantle (e.g., Spencer, 1996), chemical alteration of the lithosphere owing to volatile addition or magma extraction (e.g., Humphreys et al., 2003; Roy et al., 2004), warming of heterogeneous lithosphere (Roy et al., 2009), hot upwelling within the asthenosphere (Parsons and McCarthy, 1995; Moucha et al., 2009), and crustal thickening (McQuarrie and Chase, 2000).  It is clear that there is no shortage of mechanisms that could explain the plateau’s origin.  The core challenge is determining which mechanism, or combination of mechanisms, is indeed the cause.
  • One question arising from these two studies is: are their conclusions compatible?
  • The other obvious question that emerges from these efforts is both more important and far more difficult to answer.  Do the proposed models accurately describe the true origin and evolution of Colorado Plateau elevation?
  • One reason why resolving the cause of plateau uplift is such a tough problem is that deciphering the paleoelevation of continents is extremely difficult, and the plateau’s elevation history is critically important for isolating the correct uplift mechanism.
  • Not surprisingly, contradictory interpretations regarding the uplift history of the Colorado Plateau often arise from the diverse information yielded by the many studies in this region.
  • The two geodynamic studies in this issue of Geology underscore the probable complexity of the plateau’s history.  They especially highlight the unlikelihood of the entire plateau undergoing a single spatially uniform phase of surface uplift, and emphasize the potential for significant geographic and temporal heterogeneity in elevation gain.  Such a history would only exacerbate the challenge of accurately reconstructing the plateau’s evolution from the geological record.

The “perplexing story” is not limited to explaining this one region.  As Flowers said, if we can’t understand this plateau, we can’t explain a lot of other earth formations.  “The answers to these contentious questions are significant for understanding how deep-seated processes control the elevation change and topographic evolution of Earth’s surface.

1.  Rebecca M. Flowers, “The enigmatic rise of the Colorado Plateau” (open access), Geology v. 38 no. 7, p. 671-672, doi: 10.1130/focus072010.1.

They don’t tell you these things on the National Park signs.  The parks make it sound so easy.  A million years here, a few billion years there, and presto: Grand Canyon.  Remember this article next time you travel the Colorado Plateau.  They don’t have a clue after 150 years of thinking about it.  How much more time should we give the clueless before opening the doors to thinking outside the box?
    One of the biggest stumbling blocks for them understanding this region is their insistence on deep time and their denial of the catastrophic power of the Flood.  They should really take some creation geology papers more seriously (06/21/2010) unless they find cluelessness somehow comforting.  Now why would that be?  Job security.

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