This is an ambitious project that has the potential to fill in important gaps in the overall picture of orogenesis in the central Andes, and of convergent-margin tectonism in general. The project is constructed around a well defined basic-science question, did the Andes rise in a rapid pulse, or did they rise gradually? Producing elevations and crustal thicknesses of the magnitude found in this study area remains a key problem in continental tectonics.
This question provides a foundation from which the PIs develop a variety of linked projects, including: 3-D structural analysis of fold-thrust belt shortening in the Andes, testing of new methods of paleo-elevation analysis, use of seismic studies to characterize the roots of the range (both in the deep crust and in the underlying mantle), creative use of petrologic and isotopic data to constrain thickened crust at times in the past. The project has the potential to address 3-D mass balance issues during orogeny, as well as the impact of a rising mountain belt on continent-scale weather systems. Of note, to put the analysis of orographic weather studies in context, the PIs will also undertake a broader paleo-climate study. All of the questions to be studied are current and important, and are of interest across traditional disciplinary boundaries and, the research strategy as outlined has a high potential to answer the questions that it poses.
We studied young (less than 4 Ma) volcanic rocks from the northern part of the Altiplano plateau in order to decipher processes that take place at depth during the formation of the plateau. In addition, in some of the volcanic centers, we discovered xenoliths, solid fragments from the lower crust of the Altiplano. They represent unusually rare fragments that provide direct evidence for the composition and physical conditions of the sub Altiplano crust at 20-70 km below the surface. The most important scientific discoveries are as follows: (1) We determined that magmatism is an important contributor to the thickening of the crust beneath the plateau despite the small volume of surface volcanism; less that 3% of sub plateau recent magmatism penetrates to the surface at volcanoes. (2) Magmatism is long-lived and fundamentally driven by melting of a thick lower crust; we argue this is typical for all thick plateaus in the geologic record. (3) We discovered xenoliths of sedimentary origin, which were at the surface during the Triassic but are at >35 km deep today; these rocks constrain the amount of shortening beneath the plateau, an important and hard to resolve parameter in plateau tectonics. (4) We determined through geochronology that the formation of the northern part of the plateau commenced during the Eocene (40 Ma), much earlier than typically thought (<15 Ma). (5) Volcanic rocks and xenoliths from the Lake Titicaca region (one of the fields examined in this study) suggest that a dense root exist locally under that region and may be responsible for the topographic low represented by the lake. This project was carried out in collaboration with American and Peruvian geologists and trained a MS student, a postdoc and two undergraduate students at the University of Arizona.
