Ductile FoldingThis beautiful outcrop was photographed on the island of Crete by Dr. Marli Miller. T

Ductile FoldingThis beautiful outcrop was photographed on the island of Crete by Dr. Marli Miller. The rocks are part of the Plattenkalk series of sediments that form the backbone of that island; they include fine-grained limestones deposited in the ancient Tethys seaway alternating with more siliceous layers that include terrestrial sediments and sometimes buried sponges. These rocks have been uplifted and clearly folded as part of the collision between Africa and Eurasia that is uplifting islands in the Mediterranean.These rocks have been folded into a pair of folds – one anticline, where the rocks bend downwards, and then a syncline where the rocks are bent back to a more normal angle. Finding these 2 folds together is common as they start off as flat-lying layers and this pattern brings them back to flat-lying, just with a kink in-between that shortens the layers to accommodate the strain.There are several important details to see in this image. First of all, on the left hand side of this image, we see normal stratigraphy, with the oldest layers at the bottom. The same is observed at the right – if you look at the lowermost layer and move upwards, you progressively go through younger units. However, if you start at the bottom of the kink and move upwards, you will find that you actually move through older units. These units have been folded so intensely at this point that these layers have been overturned; older layers are found on top of younger units. The relationship is clear in this case because we can see the full fold, but overturned units are often difficult to characterize out in the field since the order of layers we get used to gets reversed.Next, look at the thickness of a single layer to the left, outside of the fold, and then again at the axis of the fold and inside the kinked band. You can clearly see that in this case, the layer thicknesses have changed. Sometimes it is possible for rock layers to just bend, but if the layers are weak enough and the rocks are folded intensely, the layers themselves can be squished, causing their thicknesses to change. Geologists would call this “ductile” behavior since the layers aren’t just bending, the sediments in them are actually flowing in response to the bending strain.Some of the white layers are a little more resistant to this ductile flow, and you can see them being stretched and pinched into little sausage-like shapes that geologists call boudins.-JBBImage credit: Dr. Marli Miller (CC Licensed)https://geologypics.com/ -- source link

More pics about