How a fault zone looks like

alternative: tectonics in the Santa María Basin

Although the fieldschool is mainly focussing on the development of the Geology and Stratigraphy of the intermontane basins in NW Argentina, I had several moments of happiness when we could observe some fault zones. Fault zones are planes along which earthquakes take place to release accumulated stresses. Big earthquakes occur mainly along the plate boundaries which are very big fault zones. But as explained in my first post, also far away from the plate boundaries we can have fault zones because the accumulated stresses due to plate tectonics influence regions on a much greater scales and bring mountains up in the air. The movement of rock masses and mountains is always related to a deformation of them. For example you can take a piece of plain paper and when you push both sides of the paper together you will end up with a deformation of the paper as it will become a convex bridge. But with respect to the table where the paper lays on, the paper will scratch and move along the table surface. This small area is then our fault zone when you apply this example in rocks.

The first example we found is east of Cafayate and you can see the fault in the images above. With greater distance you see a sharp contrast of white (on the left) and red rocks (on the right). The fine line between both formations is the fault zone. It is only a very thin line and slightly darker red as the material on the right hand side. This happens due to earthquake activity along this fault because during a seismic event both blocks of rock can be moved several meters (also called slip), but with a velocity of a few kilometers per second. Due to friction of the rocks, very high temperatures can be reached for a very short time. So the rock in the fault zone gets baked and therefore change its physical and sometimes chemical attributes. It’s the same when you try to move your hands along each other with maximum speed and you will feel the heat on your skin. It could be observed (not here) that the fault zone material is completely melted, that means an active fault zone can reach up to 1000°C. These rocks are then called pseudotachylit. But not only the rocks within the fault zone are influenced by the seismic activity. Up to one meter from the fault the rock is disturbed and fractured. The degree of fracturing increases gradually towards the fault. These kind of rocks are then called fault-breccia. It is quite common to observe the brittle deformation close or in the fault zone.

A second example of the fracturing of material into breccias can be seen above. We went to this outcrop to see lake sediments, but the lake existed 30,000 years ago and was produced by a landlside which dammed the Calchaqui river. Typically, lake sediments are very fine, clay rich material with a horizontal layering with different colors due to seasonality of sediment influx. Anyway, these sediments where folded to an anticline whithin 30,000 years only. That is extremely fast. The folding process is very similar to the paper folding experiment described before. You can expand this experiment and try to fold several book pages; you will observe a relative movement of the pages to each other. The pages of the book are our lake sediment layers and we see in the images some very deformed layers and layers who moved in respect to each others. Above and below the movement zone the layers are completely intact. In the fault zone different kind of movement can be observed with reddish layers of breccias. In the pictures we see (over-)thrusting due to compressional forces and also normal faults due to extensional forces. The overall deformation is quite chaotic, but we could see some rotational movement of some parts, too. That’s makes it difficult to explain the stresses of this formation. We discussed also the possibility of having slumps in the material, that means gravitational movements in the sediment during the sedimentation process. These weakening makes it then much easier for an earthquake to break through the rock.

The last site I want to describe shortly was intended to discuss landslides, which is still a topic of my personal interest because I’ve spend the last 1.5 years of research on the seismological imprint of landslides. Anyway, directly on top of a rotational slump (a special description of this landslide event) we had a nice fault, two actually. In a conglgomeratic rock, short to say former deposits from rivers, two straight bright lines represent two fault lines with a small angle between them. Nice features of faults in conglomerates are drag faults. The clasts close to a fault are oriented parallel to the fault. It looks like they were dragged towards the fault and can show the movement direction of both blocks of a fault. Due to the two close faults the clasts are oriented like a “S” between them.

In the end, faults are the contact of two blocks in a tectonic active mountain which are under deformation. They are mainly quite small but long; their effect can be much bigger and more impressive like in the two pictures above, which were taken along the “Ruta 40” (also the title of a documentary). The forces of plate tectonics and mountain building (orogenesis) can tilt rocks over a much greater area and rotate parts of mountains from horizontal into nearly vertical. The examples above are textbook quality.

EDIT

On our last day of the field school we had an additional very nice example of a fault zone, which I have to show in here. It’s a nice example of a thrust fault in the Humahuaca-valley close to Tilcara. Here, a reddish sandstone is pushed over a younger conglomerate in grey. Close to the fault zone the subhorizontal conglomerates are again dragged towards the nearly vertical fault. The sandstone is completely sheared and looks a lot like a schist. But the most interesting thing are the small brown points in the shear zone. These could be pseudotachylites as I explained above or some heterogenics in the old sandstones. I asked our mineralogy Professor who joined the field-school, but he was not shure what it could be. Hence, we will analyse this piece of rock in the lab when we are back in Potsdam. I will give additional information as soon as we now more about it.

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