This project will install 40 seismometers in southern Peru in order to study the causes and consequences of flat-slab subduction. Unlike most subduction zones where one plate descends beneath another plate at a relatively constant dip angle, flat-slab subduction zones are characterized by a descending plate that reaches some depth (in this case ~100 km) and then flattens, traveling horizontally for hundreds of kilometers before resuming its descent into the mantle. It is this type of unusual subduction geometry that may have caused the formation of large mountains far from tectonic margins such as the Rocky Mountains in the western United States or the Sierras Pampeanas in central Argentina.
However, very little is known about how this type of subduction zone forms, or what processes link this geometry to the formation of large inland mountain ranges. Existing theories generally involve subducting oceanic plateaus whose thick crust makes them neutrally buoyant at some depth until the crustal material undergoes a phase change that increases its density. While horizontal, the flat-lying slab releases its water, but does not form volcanism as is usually observed in subduction zones because the temperatures are too low. The released water would instead accumulate between the two plates until flat-slab subduction ended, at which time the water would interact with inflowing hot mantle material to create a large flare up of volcanism at the surface.
Today there are only two flat-slab subduction zones in the world. One is in central Chile and Argentina, and has been associated with the Sierras Pampeanas. Several seismic studies have now been performed in this area, and have found surprising results. There does not appear to be any evidence for water above the horizontal plate in Chile and Argentina, but there is evidence that silica has been added. This may help to explain the formation of early continents which required significant silica enrichment to maintain their buoyancy.
The flat slab in Chile/Argentina is, however, much narrower than the one that has been postulated as the cause of the Rocky Mountains in the western United States. The only other flat slab in existence today, beneath southern Peru, is much broader than it?s counterpart in Chile, and is therefore a better analogue. We propose to investigate the structures in the mantle along the southern half of this broad flat-slab subduction region to see if we find similar structures to those found in Chile and Argentina, and to better understand whether such a subduction geometry could be responsible for the formation of the Rocky Mountains.
This project will use broadband seismology to image the crust, mantle lithosphere, downgoing plate, and sub-slab mantle beneath south-central Peru in order to improve our understanding of flat slab subduction. Flat slab subduction has become a popular concept used to explain a wide host of geological observations including the cessation of arc volcanism, thick-skinned deformation far removed from tectonic plate margins, and the formation of high plateaus. Its usefulness as an explanation, however, stems in part from the paucity of details available on both the requirements for its genesis and the consequences of its existence. Perhaps the best-known invocation of a paleo-flat-slab is that of the Farallon plate during the Laramide, which was purportedly responsible for the formation of the Rocky Mountains and associated ignimbrite flare-up. Some have also attributed the formation of the Bolivian Altiplano to flat slab subduction, citing its width and volcanic history. However, to make these theories truly testable hypotheses, better constraints on two key questions are required: 1) How do flat slabs form? and 2) What effects do they have on the continental lithosphere?
Today, both of the truly "flat" slabs lie along the South American margin: one below central Chile and western Argentina at ~30° S, and one beneath most of Peru between ~3° S and 15° S. These slabs vary greatly in size; however, neither is believed to be as wide (along strike) or as broad (perpendicular to strike) as the suggested Farallon flat slab must have been in order to have caused all of the associated Laramide-age tectonic features. It is therefore vital to understand what factors contribute to the formation of a flat slab and how flat slabs are dynamically supported if we are to comprehend whether these factors can reasonably be scaled upwards and be applied to the Laramide flat slab. In order to understand the effect of the flat slab on continental lithosphere, we need to understand the nature of the "filling" between the horizontal portion of the downgoing slab and the base of the overriding crust. Tight constraints on the nature of this material (including its composition, stress state, and evolution over time) are key to understanding any coupling between flat slab subduction and inland crustal deformation. Results from the CHARGE deployment, which studied the central Chilean flat slab, contradicted some previously held assumptions and raised many additional questions about the nature of flat slab subduction. Peru represents the widest flat slab currently in existence, and as such could be argued to be the best location to study how a possible Laramide age flat slab could have formed, and what kinds of geologic observables we might expect to be able to find today as a result.
In order to provide answers to these fundamental questions about the structure and dynamics of flat slabs, we propose to deploy 40 broadband seismometers above the Peruvian flat slab in three roughly linear transects. The instruments will be deployed for approximately two years and the data set thus obtained will provide an unprecedented look into the workings of a large, broad flat slab segment. We propose to carry out a variety of analyses on the data, including body wave and surface wave tomography, receiver function analysis, shear wave splitting analysis, and a variety of other tools. These analyses are tied tightly to the investigation of the two fundamental questions outlined above and will provide tight constraints on the isotropic and anisotropic structure of the crust, mantle lithosphere, slab, and sub-slab mantle. In turn, these structural constraints will provide information about mantle dynamics, the transmission of stress through the crust and mantle lithosphere, and the processes which have modified the continental lithosphere.
This project is supported by the Geophysics Program and the Americas Program of the Office of International Science and Engineering
