The La Plata Mountains as seen from above the author’s

Durango Bill's

Paleogeography (Historical Geology) Research

Appendix to the Evolution of the Colorado River and its Tributaries (Part 10)

A Theory for the Numerous Local Uplifts Starting with the Oligocene

Bill Butler

   The following theory is offered as a possible cause for the multiple areas of local mountain building since the Oligocene. It is speculative in nature and should be regarded as something for further study as opposed to the final authority as to what is happening.

   One of the recurring themes that we have seen is there have been multiple local uplifts of various mountain ranges and plateaus starting with the late Oligocene and continuing to the present. One method that can produce mountain ranges is plate convergence and subduction. However, convergence and subduction ended over the southwestern U. S. during the Eocene. Hence, this is not the source of the activity. A second possibility would be plate rifting. Plate rifting tends to build “fault block” mountains. These ranges usually have axes at right angles to the direction of stretching. Plate stretching has produced the Wasatch Ranges and other ranges further to the west, but at first glance does not seem to be responsible for activity east of the Wasatch Ranges.

  A clue as to what may be happening is provided by the “Yellowstone Hot Spot”. Over the last ten million years, a series of violent volcanic eruptions has traced a path from Idaho’s Snake River Plain east-northeast to the present location in Yellowstone National Park. The apparent movement across the surface of the earth is actually caused by the earth’s crust sliding west southwestward over a stationary hot plume in the mantle. The movement may be due to gravity pulling the crust and underlying Lithosphere down a slope produced by a bulge in the mantle. The important part is that the crust is sliding over the mantle. (It is also possible the crust is stationary and the mantle has local motion. The significant part is there is relative motion for the two layers.)

   Since there are no strike-slip faults (similar to the San Andreas) or subduction zones between Yellowstone Park and the rest of the southwestern U.S., we can assume this same crustal sliding pattern is present over most of the Colorado River basin. If the crust and its underlying downward displacement into the mantle formed a smooth surface, then this sliding could take place without additional complications. For example, the Hawaiian Islands indicate the Pacific Plate is sliding west northwestward, but the ocean floor is smooth here except for the islands themselves.

   The southwestern U. S. is not smooth. It has many big mountain ranges that in turn have much bigger displacements downward into the mantle. Thus, we have a series of large obstacles moving through a fluid. This is somewhat like stationary rocks in a moving river. We just have to turn everything upside down.

   If a log floats down a river and then encounters a stationary submerged obstruction (a large rock), the river may try to push the log over the top of the rock. Gravity will try to prevent the log from going up high enough to get over the rock. Frequently, this will result in the log becoming stuck on top of the rock. If more logs come down the river, they may be added to the growing pile and produce a logjam. In northern rivers a similar phenomenon may produce ice jams during the spring melt-off.

     We think of the earth’s mantle as a simple homogeneous object, but it is actually a hodgepodge of various components. Over hundreds of millions of years, it has accumulated a mixture of various hot plumes and plate subductions. Plate subductions in turn contain various kinds of rock. Thus, the mantle is actually a fruitcake of components that have varying densities. If the mantle were a mixture of liquids that have low viscosities (such as a mixture of oil and water), these fluids would quickly separate into distinct layers. However, the mantle (and particularly its upper portion) is extremely viscous and does not mix or separate readily. Thus, any density and viscosity variations will tend to persist for hundreds of millions of years.

   In the river example, objects that are lighter than water but heavier than the overlying air formed a logjam when they encountered the stationary rock. Let’s turn everything upside down and see what the interaction is between the crust and the various density components of the mantle.

   In the earth, the displacement under an existing mountain range plays the part of the stationary rock. The only difference is everything is upside down. Instead of the rock sticking up toward the log/water motion, the underlying displacement of the mountain range forms an obstruction that reaches tens of miles downward into the mantle. As the displacement plows through the mantle, it will try to force the fluid portion of the mantle downward under the displacement. If the mantle had a consistent density, this would not cause any more of a problem than water steadily flowing around and over the rock.

   When a “chunk” (as in several or more miles across) of lower density (less weight) material is encountered, a problem arises. The flow will try to pull the lower density material down under the displacement. Since the lower density material is less subject to gravity, it will resist being pulled down. Thus, it may get stuck under the mountain range. Also, the downward displacement under the mountain range may cause an “eddy” or a standing wave just as a rock will cause ripples and eddies in a moving stream. This could also act to cause the lower density material to become stuck. If a series of low density “chunks” are encountered, then a “logjam” could accumulate under the mountain range. Every time a new chunk of lower density material is added to the underground logjam, it will thicken the total column of lighter density crustal material. Just as the barge rose when we added lightweight material under it, a mountain range or plateau could undergo renewed periods of uplift at irregular intervals.

    Finally, if crustal sliding has any irregularities of motion, it would tend to produce crumples at right angles to the direction of motion. This would be similar to the crumples in a sliding rug. The large upside-down displacements under mountain ranges would accentuate irregularities in crustal motion as the crust tries to slide over the mantle. If a chunk of crust (e.g. several thousand sq. miles or so) slid faster than the surrounding area of crust, it would compress and thicken the crust and lithosphere in front of it. This would cause a “crumple” that would be oriented at right angles to the direction of motion. The “Yellowstone hot plume” indicates the crust is sliding west southwestward. Thus, the axes of recently uplifted mountain ranges should favor a north-northwest to south-southeast direction. Recently uplifted ranges such as the Wind River Mountains, the Gore Range, the Sangre de Cristo Mountains, as well as the Kaibab Plateau may be a result of this differential movement.

Return to the Kaibab Plateau (Part 9)

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