Episode 388 Folds in Algeria History of the Earth

You may have seen some of the spectacular images of the earth in southern Algeria, curves and colors like some Picasso in the opposite of his cubist period. If you haven’t, check out the one from NASA, below.  The ovals and swirls, with their concentric bands, are immediately obvious to a geologist as patterns of folds, but not just linear folds like many anticlines and synclines form. These closed ovals represent domes and basins – imagine a large scale warping, both up and down, in a thick succession of diverse sedimentary rocks, like sets of nested bowls, some of them right-side up and some inverted, then all sliced off halfway through. But “obvious to a geologist” has plenty of limitations in a space image. Without knowing more information, it’s difficult to be sure if an oval is a basin or a dome. And you can speculate, but without some ground truth, it’s challenging to be sure what the rock types are. Ahnet-Mouydir, Hoggar Mountains, Algeria. NASA image - source This area, called the Ahnet-Mouydir, on the flank of the Hoggar Mountains close to the middle of the Sahara Desert, is remote, inhospitable, and arid, and called the “land of terror” for a reason. The rocks represent a thick sequence of marine sandstones, shales, and limestones, spanning a huge range of ages, from at least the Ordovician to the early Carboniferous – 150 million years or more, a great chunk of the Paleozoic era. The core of the Hoggar Mountains is an old Precambrian block, not as big as the cratons and shields that form the hearts of most of the continents, but otherwise similar. It might have been something like a microcontinent that became amalgamated into the growing supercontinent of Gondwana about 600 million years ago. After that amalgamation, seas came and went much like they did in western North America throughout much of the Paleozoic era, laying down the sediments that became the rocks we see today in the northern Hoggar Mountains. That’s all well and good – but here’s the next question, how did the rocks get deformed into these oval domes and basins? If you imagine the kinds of collisions that are typical on earth, you think of linear or curvilinear things – island arcs, edges of continents and such – that when they collide, are likely to make linear belts of deformation. This is why so many mountain ranges are long, linear features, and the folds and faults that make them up also tend to be linear. Domes and basins happen, but that seems to be almost all we have here in these mountains. We have to look for a deformational event that is later than the youngest rocks deformed. So if some of these rocks are as young as early Carboniferous, about 340 million years old, the mountain-building event that fills the bill is the Hercynian Orogeny, where ‘orogeny’ just means mountain-building. The Hercynian, at about 350 to 280 million years ago, represents the complex collision between Gondwana and the combined North America and Europe, which were already more or less attached to each other. The leading edge of Gondwana that collided was in what is now North and West Africa, and the collision produced mountain ranges all over – the Alleghenies in the central Appalachians in North America, and a complex swath of mountains across central Europe, from Spain, across France to northern Germany and into Poland, as well as elsewhere. In Africa, the most intense squeezing was at the leading edge, in what is now Morocco and Mauritania, colliding with North America, and northern Algeria, impacting Iberia. The basins and domes of southern Algeria that we’re trying to understand are 1500 kilometers or more from that leading edge of continental collision. So I think – and full disclosure, I’ve never really researched this area in detail – that what must have happened is that that distant hinterland wasn’t pushed into tight, linear belts like those we find along the lines of collision, but the force was enough t