Tectonic plates at the end of the world
As we have headed south, we have gotten closer to active plate boundaries – closer to the convergent plate boundary (subduction zone) along the western coast of Chile and closer to Argentina’s only plate boundary – the transform boundary between the South American plate and the Scotia plate. Below is a map of the plates in this region.
You may notice that the Scotia plate is sliding to the east, relative to the South American plate. That makes the transform plate boundary (Magellan fault on the map) a left-lateral strike-slip fault – the opposite sense of our own San Andreas fault, which has a right-lateral sense of motion between the Pacific and North American plates.
Yesterday we rented a car to travel across the plate boundary, which in most places is covered by water of Lake Fagnano. Like all of the lakes and fjords in this part of the world, this lake was recently occupied by a glacier and there are abundant fluvio-glacial deposits along the edges of the lake, where streams carried sediments away from the melting glaciers. The photo below is looking west along the length of Lago Fagnano and the plate boundary. The red-roofed buildings are a hosteria that would be an amazing place to stay, with views of the lake and the cordillera (the Andes) to the west and south.
On the map above, you will notice that the Patagonian Andes are oriented north–south, like the rest of the Andes farther north. However, the cordillera bends eastward at the south end of South America, and the Fuegian Andes are here oriented east–west. Like the rest of the Andes, the core is located toward the Pacific coast, and the rocks are progressively younger and less deformed eastward (here northward) toward the Atlantic coast.
We continued northeast from the plate boundary to the Atlantic Coast at Cabo San Pablo – a headland from which there are stellar coastal vistas. As we saw along the Atlantic coast of Patagonia, there is uplift of the land that has created terraces along the coast; there are vast quantities of gravel on the beaches that were carried by rivers from the Andes; and there is a large tidal range that results from amplification of the tides by the broad continental shelf in this region. We were fortunate to arrive at low tide, when a very broad wave-cut platform was exposed. In the photo below you can see the platform and a ship on the platform that wrecked, probably because it ran aground, not realizing the water was so shallow so far from the land (it’s very rusty and has clearly been there for many years). In the foreground, you may also be able to see a small fault in the marine sedimentary rock that was eroded to make the wave-cut platform.
On the next photo you can see an immense amount of gravel on a beach berm on the north side of the headland. Note the variety of rock types, including white rocks that are vein quartz – more on that later.
It was totally awesome to stand on the coast, where there are the younger Cenozoic sedimentary deposits that are barely deformed, and to look southwest to see the cordillera (Fuegian Andes) that are older, highly deformed, and the source of gravel found on the beaches – at least the rock types that are durable enough to withstand the trip to the sea via rivers. On the photo of the coast (above the gravel berm photo), you may be able to see the cordillera just barely poking up above the horizon in the distance.
We did not see any interesting marine critters on this part of the coast but we did see lots of guanacos as we traveled back from the coast. Here is a group of guanacos with some babies, one of which is having its lunch (photo courtesy of Jay).
Traveling back south to Ushuaia, we stopped at the Garibaldi pass, which is the highest point on the road before descending to the Beagle Channel. This is a good place to get a sense of the contrast from north to south. The first photo is looking north across the Lago Fagnano / plate boundary that I’m showing with my finger (the lake in the foreground is oriented perpendicular to the plate boundary). Photo is courtesy of Jay – I had to include it to show t-shirt advertising the student club in our Geosciences Department!
The next photo is looking south at the pass, at a road cut into the rocks at the core of the cordillera. Even for geologists, these are ugly-looking rocks! They are highly metamorphosed and deformed and we decided we would really not like to have to decipher their history!
On Friday, we saw more of the old deformed rocks in the national park – Parque Nacional Tierra del Fuego. These rocks were more attractive though, certainly aided by their scenic location along the Beagle Channel. There are green schists that erode to make unusual green-sand beaches; also note the abundant white vein quartz that is the source of the white pebbles we found on the Atlantic Coast beach.
Here’s a close up for those who want to see the deformation in more detail! (Those are size 11 shoes for scale. Some of the quartz veins are green from the marine algae.)
As in most national parks, this one is now a refuge for plants that can no longer be logged. So there are large trees of nothofagus, the southern beech tree that evolved >100 million years ago, when the continents were together as the super-continent Gondwana. As the continents moved apart, the trees remained and the same species is also found today in Australia and New Zealand – another piece of evidence that these locations were once adjacent to each other. Below is a photo of the lenga tree, the largest of three types of nothofagus in this region.
Finally, a note about the weather. Our guide at the penguin colony the other day said that Ushuaia has only about 10 days/year that are sunny all day. We had one of those Friday, when we enjoyed a cloudless, warm day hiking in the national park. Yesterday was about as good and we’ve really had amazingly weather for our five days here – even the wind has not been so fierce. We feel as if Charles Darwin is smiling down upon us, as we sate our curiosity about the natural environment as he did here nearly two hundred years ago. So that friends and family can see that we’re doing well, here we are (self-timer shot) with the Beagle Channel and Darwin Range in the background. We really do wish you were here.
