Collaborative Research: Subduction Dynamics, Mantle Structure, and Cenozoic Tectonic Evolution of South America

University Of Arizona, Tucson AZ

Investigators

Abstract

The physics behind several fundamental aspects of the South American tectonics remains elusive. For example, various geological records suggest that the Andes Mountains started to shorten significantly since at least 40 million years ago. However, paleo-altimetry proxy data reveal that most of the surface elevation of the Andes remained relatively low until as late as 20 or 15 million years ago, a time frame that is inconsistent with the shortening history. Furthermore, the early Cenozoic arc volcanism along the central Andes suddenly shifted more than 500 km inland around 30 million years ago and formed widespread silicic volcanic activity that continues to the present. Consequently, a clear relationship among these tectonic events is lacking. An important reason is that both the mantle structures beneath South America and their dynamic evolution remain poorly understood. In this proposal, the team plans to carry out a multidisciplinary research project to attempt to improve our knowledge about the observational records of surface tectonics, deep mantle seismic properties, and their geodynamic relationship with the temporal evolution of subduction beneath the continent. By collecting more data related to the tectonic history of South America and to the present-day mantle interior, they will build a sophisticated 4-dimentional geodynamic model using data assimilation, in order to quantitatively reproduce the past subduction history. Ultimately, the team hopes to better understand the Cenozoic evolution of the South American continent. The team plans to achieve this goal through a collaborative research effort that involves seismology (led by PI Beck), geology (led by PI DeCelles), and geodynamics (led by PI Liu). They propose to reproduce the Cenozoic subduction history beneath South America and associated continental deformation using geodynamic models constrained by geophysical and geological observations. This project includes interdisciplinary training of several graduate students at two institutions, and also involves undergraduates at both institutions who will be involved in the research and will all students will participate in a summer field trip to western U.S. Cordillera as an analog for the South American Cordillera. This multi-disciplinary combination will provide a unique opportunity for the students to understand orogenic systems at plate scale. Multiple fundamental questions exist about the Cenozoic tectonic history of South America, including the asynchronous crustal shortening and surface uplift/subsidence history of the Andes, and the enigmatic Central Andean flare-up magmatism occurring during the late Cenozoic. None of the earlier proposed geodynamic models for the Cenozoic evolution of South America could simultaneously explain all these tectonic records and it is unclear whether these different physical processes could co-exist and interact in reality. An important reason for the existence of these alternative models is the uncertain subduction and mantle dynamics due to our imperfect knowledge of deep mantle structures beneath South America. Key observational constraints on the geodynamic and tectonic evolution include an improved present-day mantle seismic structure, especially in the lower mantle, a better identified relationship among structural deformation, surface uplift and magmatic history within western South America, and time-dependent geodynamic models that are consistent with these observational records.