Earth’s mountains have long been recognized as one of the primary observable features that show plate tectonic modification of the planetary surface. From early on, geologists have observed that the planet’s highest mountains are also where the crust is unusually thick. At the simplest level, mountain range elevation is comparable to an iceberg floating at sea. Based on Archimedes’ principle of buoyancy, the height of an iceberg above water is related to the weight of water it displaces below the water line. The thicker the ice is, the higher the iceberg will appear to be. In the case of a mountain range, the “ice” is the mountain range crust floating on a “sea” of hot, semi-fluid rocks that make up the earth’s mantle. The thicker the crust is, the higher the mountain range will be. The Himalayas and the Andes both have peak elevations greater than 6 km surrounding low-relief plateaus perched at ~3.5–5 km above sea level. In both places, the collision of tectonic plates has gradually squeezed and compressed the crust over millions of years, resulting in the extreme crustal thickness and high elevations we see today.

In the Andes, shortening of the crust has been going on for at least 50 million years. The classical view of mountain building suggests that the elevations in the Andes would have been gradually increasing as the crust got thicker with time. However, new evidence from 10 -20 million year-old lake bed and soil sediments found on the Altiplano in the center of the Andean plateau suggest that the Andes may have stayed low even after the crust was thickened. This paleoelevation data paints a radically different picture of mountain belt development where mountain elevations would have been low during compression and then suddenly increased to their present day height in a few millions of years. It is possible that the data from the plateau sediments are the finger-print of processes deep in the crust, with dense material at the base of the crust acting like an anchor, keeping mountain belt elevations low. After a certain point, the anchor is lost as the dense material peels of the bottom of the crust and sinks into the mantle.  We can independently evaluate the locations and timing of rapid elevation gain and removal of lower crust proposed by paleoelevation records. By creating a deformational model of the central Andes based on age, location and magnitude of faults, we can track how the crust has thickened over the last 50 million years. By comparing the model to what we know about the modern crustal thickness, we can see where material may have been removed.

Modeling the 3-D evolution of crustal thickness in the central Andes requires unraveling deformation at the bend in the Andes, referred to as the Bolivian orocline. We created a new high-resolution geologic map, balanced cross section and acquired new timing constraints from thermochronology. Field mapping showed significant strike-slip faulting parallel to the mountain belt trend that may have focused crustal thickening near the orocline center, facilitating the development of the plateau. These data were combined into a series of map view reconstructions of the central Andes from 50 Ma to present in 5 million year increments. These reconstructions showed that rocks in the Andes may have rotated as much as ~13° to create the bend we see today. By including rotation and strike-slip faulting, the predicted map-view shortening estimates were up to 90 km (30%) greater than previously estimated from balanced cross sections. This is partially due to the fact that balanced cross sections do not resolve motion parallel to the trend of the mountains, however this parallel component is necessary in allowing a mountain to bend.  The crustal thickening history from these map-view reconstructions indicate that modern crustal thickne...

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