✦ The twin Magellan telescopes (the Baade and Clay telescopes) of the Carnegie Institution for Science's Las Campanas Observatory (LCO) with the Astronomy Service Building in the foreground. (Photo: Matias del Campo, used with permission).

Summary

The following is a talk (scroll down past this summary!) delivered at the Las Campanas Observatory (LCO) on February 17, 2018 by Steven Shirey, Carnegie Institution for Science Senior Staff Geologist to explain to astronomers and non-scientists about the special geologic setting of the LCO.

A photographic review of the long-term association between the Department of Terrestrial Magnetism (DTM) and the countries of Peru, Bolivia, Argentina, and Chile shows the wide array of geophysical work starting in 1905 and continuing to the present. This work includes magnetic surveys, active and passive source seismology, volcanology, radio and optical astronomy.

There are some basic aspects of Earth to consider:

• Earth has major internal divisions (inner and outer core, upper and lower mantle, and crust)
• Earth has tectonic plates with LCO location on the South American Plate
• the tectonic plates have age structure with LCO sitting on a young continental arc added to the west of the much older stable crust in Brazil
• young continental arcs are a the major continental growth mechanism through Earth's geologic history.

In general, continental volcanic arcs of the type where LCO is located have important geologic features:

• as a locus of volcanic activity and active magmatism
• as the site of copius ore deposit production and hence economic significance
• as the site of the world's greatest earthquakes
• as the place of Earth's greatest crustal thickening this leads to great elevation (the latter being critically important to the LCO site success) by the process known as isostasy.

Specific aspects of the continental arc under LCO are important:

• the subducting oceanic lithosphere here, known as a 'slab', is dipping off to the east at a shallow angle
• the dip is so flat that this region is known as the 'Pampean flat slab' to geologists
• the dip is also so flat that volcanic activity in this region has been blocked by the cold temperatures of the slab
• one reason the LCO site is perfect for astronomy is that the flat slab keeps the area free of ash, steam, and volcanic gases.

The nature of the flat slab and why it exists can be learned from mutli-investigator, multi-institutional studies:

• volcanology research --involving dating the eruption age of volcanoes, their position, and their composition-- establishes how and when the flat slab moved upward to block volcanic activity
• in principle, like medical tomography, campaign-style array seismology leads to a three dimensional picture of the slab at 100-250 km depths and especially its surface shape
• the flat slab appears to be caused by the subduction of a large sub-sea oceanic ridge, the Jan Fernandez Ridge, made up of 800 m (or greater!) thick, relatively buoyant oceanic crust
• like a rat moving through a snake, this ridge pushes the slab upward and flattens it.

The current work of DTM seismologist Lara Wagner and volcanologist Diana Roman with their DTM Geochemistry Group colleagues (Rick Carlson, Erik Hauri, and Steve Shirey) with postdoctoral fellows and students, focuses on the specific details of plate tectonics along the continental arc of North and South America. Studies such as these tell us exactly how and why the Las Campanas Observatory site is so special.