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.
(CAUGHT), was a multi-disciplinary effort to understand how high plateaus form. We focused on the northern Altiplano Plateau, the second largest plateau in the world. Recent research looking at evidence for the timing of the uplift of this plateau has produced conflicting versions of how this uplift occurred. One version suggests uplift occurred rapidly between 10 and 7 million years ago. Another suggests that the plateau has formed gradually over the course of the past 22 million years. My contribution to the project was to image the structures underneath the plateau that could serve to "hold up" or "pull down" the mountains. In particular, we were looking to see if the lower part of the tectonic plate, the "mantle lithosphere" was still present. If the mantle lithosphere has been completely removed, then it is possible that it dropped off catastrophically ~10 Ma, resulting in a sudden uplift. If, however, the mantle lithosphere is still present, this explanation becomes much less plausible. We can determine if mantle lithosphere is present by looking at the speed with which seismic waves travel through this material. Original mantle lithosphere would be characterized by very fast seismic wavespeeds, but the material that would have replaced a delaminated mantle lithosphere ("asthenosphere") would be seismically slow. We can create 3D images of seismic wavespeeds using methodologies that are akin to medical CAT scans. To collect the data for these images, we deployed 50 broadband seismometers (provided by the Incorporated Research Institutions for Seismology-Program for Array Seismic Studies of the Continental Lithosphere (IRIS-PASSCAL)) across the northern Altiplano. These stations recorded ground motions continuously at 40 samples per second for over two years. The data we recorded were databased at the Incorporated Research Institutions for Seismology Data Management Center (IRIS-DMC) and are now freely available to the public. My research group used these data, in conjunction with data collected from other nearby stations, to determine the structure of both the subducting Nazca plate, and the lithosphere of the South American continent under the Altiplano. Data sets such as the one collected for this project are a treasure trove of information, so research using these data are still ongoing. However, our preliminary results have been very informative. As is so often the case in nature, the answer to the question of "Did the Altiplano form because of a catastrophic mantle delamination event or because of a gradual uplift" seems to be a complex combination of both/neither. While perhaps at first this may seem disappointing, it is important to remember that learning that the problem needs to be entirely rephrased is often a bigger scientific step forward than just answering a binary either – or. Or primary observations thus far are as follows. 1) By determining extremely accurate locations of earthquakes happening in the subducting plate, we have determined the precise geometry of the Nazca Slab. This geometry becomes particularly complicated in the very northernmost portion of the CAUGHT study area, because the slab goes from very steeply dipping to nearly horizontal. While the approximate geometry has previously been surmised, data from this study gave us an unprecedented level of detail on exactly where this slab was seismically active, and crucially, determined for certain that the slab itself was not torn. This is important because a torn plate would affect mantle flow patterns, which in turn would affect delamination patterns. 2) The local body wave tomographic image of the velocity structure indicates that pieces of the mantle lithosphere are still present under the eastern Altiplano, but other portions seem to be missing. Results from inversions of surface wave phase velocities see similar structures, but can also resolve structures that look like these pieces may be in the process of ongoing delamination. This suggests that the evolution of the Altiplano Plateau is still very much a work in progress. Further detailed analyses of our data will help us develop a more detailed picture of how these structures evolved to their current state. What is clear is that it is not a simple story that has already run its course. 3) By combining the data from this project with those of projects that were deployed at the same time in adjacent regions, we have been able to develop a better understanding of why the slab geometry is so complicated to the north, and how that geometry evolved over time. The results of these combined study are currently under review at Nature. In summary, while the work on these data is still ongoing, we have already learned a great deal about the Altiplano plateau, most crucially that the processes responsible for its formation are not in the past, but rather are still ongoing.
