Following on from Anne’s pretty photo last weekend, let’s nip over to southwest Spain to see a place where art meets tectonics:
We’re looking at a section throughout the Carboneras Fault zone. Africa and Spain are still very slowly colliding with each other, and although it’s very hard to draw a neat plate boundary in the collision zone, this fault and others in the region (including one that ruptured earlier this year) are accommodating this motion in a variety of ways. The Carboneras Fault is a left-lateral strike-slip fault that is cutting through a number of different units – metamorphic rocks that were formed and brought to the surface in a collision event, a range of younger sediments (mainly limestones and sandstones, deposited in the last 10 million years) deposited in small extensional basins, and some volcanic rocks of a similar age. One quite common feature of strike-slip motion is the formation of multiple parallel strands, or splays. In the area this photo was taken, as you walk along the valley that cuts through the fault zone, you are walking over several such splays, each containing slices of these very different rocks juxtaposed against each other. Because fault motion tends to grind things up, the exposed rocks weather quite easily, and because all the different units are so different lithologically, they also weather quite differently, producing a colourful striped appearance. The lack of vegetation in southern Spain (it’s practically a desert) enhances the effect rather nicely.
Some more details on the Carboneras Fault can be found at Paleoseismicity.