Forearc Uplift in Northern Chile

Cornell University, Ithaca NY

Investigators

Abstract

The places where one tectonic plate descends beneath the margin of another are the sites of Earth's largest earthquakes and most explosive volcanic activity. In about half of all such convergent plate margins, rocks that originally formed the lower part of the overriding plate are removed and dragged deep into the earth. Even though one manifestation of this action of tectonic erosion is clearly visible as the progressive elimination of the plate margin, the processes by which the rocks are removed are poorly understood. The processes thought to be involved include some that break rock and some that generate a flow of broken rock from its original site to a much greater depth, which seemingly requires lubrication of a channel of flowing rock. Few direct observations are possible because the processes occur tens of miles beneath Earth?s surface. Yet much can be learned by study in active systems of secondary consequences of tectonic erosion. The coast of northern Chile is an excellent natural laboratory, where the plate underlying the Pacific Ocean descends below the South American plate. There, secondary consequences of tectonic erosion are found in patterns of stress, spatial variations in the gravity field, and the topography of the continental surface. This study focuses on the latter, topographic changes, because the continental surface would change in altitude through time as subsurface material is added or removed. One major step of this project is to gather data that illuminate the changes in altitude of what is now the coastal mountain belt. The researchers are documenting the changes of the topographic form during the last 10 million years through examination of the modern orientations of what were originally horizontal surfaces. In addition, using a new method based on isotopes in salts that occur in ancient soils and in ancient salt pans, they are determining which locations stood above or below an ancient altitude of about 3000 feet above sea level. The researchers are testing the hypotheses that the topography rose by over 2000 feet to form the spectacular cliffs of the coastline since 10 million years ago, and that the study area tilted down to the west during the last 3 million years. Part of the project work involves gathering the appropriate data and samples in the field study area. Simultaneously, the research team uses numerical models of the processes and materials that interact at a plate boundary to predict the topographic outcomes for a set of scenarios. Two principal scenarios are modeled. The first scenario is that the physical properties of a channel in which broken rock mass would move, or the amount of materials entering the channel, varied through the time span of interest, because the climate varied or because the plate movements varied. A second scenario explores the outcomes in the near-coastal region of the processes that occurred near the volcanic arc, because those distant processes might obscure the signals of tectonic erosion. The predictions of the scenarios are compared to the topographic history, to determine which processes led to the observed landforms. The models also predict patterns of stress and of the gravity field, which are compared to existing measurements. An improved understanding of these plate boundary physical processes may enable better understanding of the processes that generate major earthquakes at convergent plate margins. The findings are shared via peer-reviewed reports in journals. While working on this project, a graduate student is learning to combine understanding of surface processes with inner earth processes, to use a variety of approaches, to think critically, and also is developing "soft" skills as a collaborating member of a diverse research team. The project team includes geoscience collaborators from Chile and Europe. Results that are directly applicable to commerce and policy, such as regional hydrological history, industrial mineral resource distribution, and insights into controls on seismic activity, are shared with water managers, companies who mine brines and evaporite minerals, and government agency staff, respectively. Displays at PRI?s Museum of the Earth present to the public the general results.