After the Himalayas, the Andes Mountains are the second highest range in the world—but you have to go to the right place. Many parts of the Andes are beautiful but have elevations more similar to other, relatively low-elevation subduction-zone ranges like the Cascades in the Pacific NW (USA). But at 6964 meters (22,841 feet), Alconcagua is the highest peak in the Western Hemisphere and part of an impressively vast range that continues north to the high plateaus of the Puna (Argentina) and the Altiplano (Bolivia) where many peaks reach heights of at least 6,000 meters. Along this distance of more than 20 degrees latitude (~13–33 degrees south) deformation associated with the Andes extends east for a considerable distance. We are traveling along about 10 degrees of this distance (~23–33 degrees south) in the Cordillera Oriental (eastern Andes) which is the part that is being thrust eastward into the non-mountainous part of the country. A future post will explain why this is happening but this post will show evidence for continuing rapid activity—not easy to show, but I’ll try!
Folds, folds everywhere, as shown on this Google Earth image in the Quebrada de Humahuaca. Because of the compression within this part of the Andes, thrust faults are pushing thick sequences of sedimentary layers eastward and causing them to buckle and fold and form spectacular exposures when eroded by the elemental forces of wind and water. But what is the evidence of a young age?
This photo shows sedimentary rocks (pinkish color) that are fairly well lithified (i.e., turned into rock) being thrust over younger alluvial fan deposits (grey color) that are not lithified. [The upper contact is the depositional contact of the fan gravels on top of the older pink sediments; the lower contact is the fault, where the older pink sediments have been thrust over the younger fan deposits.] The fault must be younger than the fan deposits, which are certainly only some thousands of years old—super young in geologic time. My llama helped the photo by pointing his ears in the direction that the fault moved forward.
Another line of evidence is terraces that are former levels of a river that were uplifted by the actions of faults and folds; once the older river surface is at a higher elevation, the river has to carve a new flat surface. In the photo above, there are three obvious terrace levels at the lowest elevations, and there are at least three more at higher elevations that are less obvious because they are older and more eroded.
Alluvial fans are also common in areas of active land uplift—they act to carry sediment from the high mountain ranges to the river in the main valley. With high uplift comes steep slopes and severe amounts of erosion that produces copious amounts of sediments that can have very large sizes. We saw rock pieces in alluvial fan deposits that were as large as cars. Many of these alluvial fans have, in turn, been uplifted and eroded. This photo is looking down on the Quebrada de Humahuaca (view from east to west). The large fan on the left is currently being eroded by the Rio Grande (big river).
The overall feeling in this region is one of raw geology—the steep slopes, large-sized sediments, and deep erosion all leave the viewer with the feeling of a landscape in rapid change.