Sometimes you just feel lucky! In preparation for writing about the geologic history of Patagonia I have been reading papers about the oldest rocks there—aimed at learning how Patagonia was “put together” with the rest of South America during the assembly of the Gondwana super-continent.
I had decided to attend the Geological Society of America’s March meeting of the Cordilleran section, which includes not only the western U.S. states but also the western provinces of Canada and most of Mexico. As chair of the Cordilleran section several years ago, I pushed to more proactively involve Mexico in the society and to have this 2012 meeting there. I have just returned from that meeting, which took place in Querétaro, a beautiful colonial city in central Mexico 2–3 hours north of Mexico City.
Imagine my surprise and delight upon looking at the meeting program several weeks ago and discovering that Victor Ramos was going to give a talk about the assembly of Gondwana! Dr. Ramos is an Argentinian geologist who has been studying his country for decades and probably knows more about the geology of Patagonia than anyone else. In addition to hearing his talk I was able to talk with him about my project—a stroke of luck indeed!
This post is a brief explanation of how Patagonia was added to the South American continent. The map below shows the basic elements of South America. As in most continents, very old rocks are surrounded by progressively younger rocks that were gradually added over millions of years. In South America, the oldest rocks are in Brazil and northeastern Argentina—here called the “South American platform”. The oldest parts are the shields—rocks >2 billion years old that have been eroded to form broad, flat areas. (In North America, the north-central part of Canada is a continental shield.) To the north, west and south of the South American shield, younger crustal pieces were “pasted” on in belts that are now mostly covered by younger sediments (basins) and the modern Cordillera de los Andes.
Patagonia was one of the youngest pieces to be added—in the Paleozoic Era around 300 million years ago. In the reconstruction below (~390 million years ago), you can see Patagonia approaching southern South America, which had already been stitched together with Africa, Antarctica, Australia, and India to form the Gondwana super-continent. The “teeth” between Patagonia and Gondwana indicate a subduction zone. Once all of the ocean crust between was subducted along this zone, Patagonia crashed into South America, where it has stayed since then. This process culminated when North America/Asia collided with Gondwana in the latest Paleozoic (about 250 million years ago) to form Pangea, an even larger grouping that included all of the planet’s continents!
No sooner had the continents all gotten together then they began rifting apart from each other! The separation began in the North Atlantic, as North America rifted away from South America and Africa; by about 115 million years ago (image below), South America and Africa also began to separate to form the South Atlantic Ocean. The rifting process has continued for nearly 200 million years, as the Atlantic Ocean has grown larger and the continents have been dispersed (although you may have seen the bumper sticker—reunite Gondwanaland—that advocates getting the continents back together again!).
Why care about these ancient rocks that are barely visible in today’s landscape? Because they provide an inherent “fabric” to the land that continues to have effects many millions of years later. For example, the wide continental shelf that creates big tides along the Atlantic coast of Patagonia and that holds the continually disputed Maldivian/Falkland Islands is a result of the way the continents broke apart (see geometry of the continental breakup in image above). The massifs of Patagonia (labeled SC and D and outlined in yellow on the topmost map) are rigid blocks that determine where sediments will collect in basins—for example, the economically-important, petroleum-rich basins that have formed between the two rigid massifs.
And how do we know that the continents were once together? You may remember the southern beech trees (Nothofagus) described in several posts that are similar to trees found today in the distant continent of Australia, because they evolved when the continents were together. The figure below shows fossils that are found in belts that make more sense once the continents are reunited. Some mountain belts and other geologic features are also best explained this way.
So although the field guide will be mainly focused on the younger geologic events that are more visible in the landscape, there will be a brief discussion of the older events that were a prelude to today and that continue to exert their influence. Here are several web sites that have excellent animations of the continents dispersing during the past 200 million years (the last one has a collection of animations from many time periods).