You may have seen some of the spectacular images of theearth in southern Algeria, curves and colors like some Picasso in the oppositeof his cubist period. If you haven’t, check out the one from NASA, below. 

The ovals and swirls, with their concentric bands, areimmediately obvious to a geologist as patterns of folds, but not just linearfolds like many anticlines and synclines form. These closed ovals representdomes and basins – imagine a large scale warping, both up and down, in a thicksuccession of diverse sedimentary rocks, like sets of nested bowls, some ofthem right-side up and some inverted, then all sliced off halfway through.

But “obvious to a geologist” has plenty of limitations in aspace image. Without knowing more information, it’s difficult to be sure if anoval is a basin or a dome. And you can speculate, but without some groundtruth, 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 HoggarMountains 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 athick sequence of marine sandstones, shales, and limestones, spanning a hugerange of ages, from at least the Ordovician to the early Carboniferous – 150million years or more, a great chunk of the Paleozoic era.

The core of the Hoggar Mountains is an old Precambrianblock, not as big as the cratons and shields that form the hearts of most ofthe continents, but otherwise similar. It might have been something like amicrocontinent that became amalgamated into the growing supercontinent ofGondwana about 600 million years ago. After that amalgamation, seas came andwent much like they did in western North America throughout much of thePaleozoic era, laying down the sediments that became the rocks we see today inthe northern Hoggar Mountains.

That’s all well and good – but here’s the next question, howdid the rocks get deformed into these oval domes and basins? If you imagine thekinds of collisions that are typical on earth, you think of linear orcurvilinear things – island arcs, edges of continents and such – that when theycollide, are likely to make linear belts of deformation. This is why so manymountain ranges are long, linear features, and the folds and faults that makethem up also tend to be linear. Domes and basins happen, but that seems to bealmost all we have here in these mountains.

We have to look for a deformational event that is later thanthe youngest rocks deformed. So if some of these rocks are as young as earlyCarboniferous, about 340 million years old, the mountain-building event thatfills the bill is the Hercynian Orogeny, where ‘orogeny’ just meansmountain-building.

The Hercynian, at about 350 to 280 million years ago,represents the complex collision between Gondwana and the combined NorthAmerica and Europe, which were already more or less attached to each other. Theleading edge of Gondwana that collided was in what is now North and WestAfrica, and the collision produced mountain ranges all over – the Allegheniesin the central Appalachians in North America, and a complex swath of mountainsacross central Europe, from Spain, across France to northern Germany and intoPoland, as well as elsewhere. In Africa, the most intense squeezing was at theleading edge, in what is now Morocco and Mauritania, colliding with NorthAmerica, and northern Algeria, impacting Iberia.

The basins and domes of southern Algeria that we’re tryingto understand are 1500 kilometers or more from that leading edge of continentalcollision. So I think – and full disclosure, I’ve never really researched thisarea in detail – that what must have happened is that that distant hinterlandwasn’t pushed into tight, linear belts like those we find along the lines ofcollision, but the force was enough to warp the sediments into these relativelysmall domes and basins. Alternatively, it might be possible that the brittlePrecambrian rocks beneath the sedimentary layers broke from the force of thecollision, so that the sedimentary layers draped over the deeper brittlesurface like a carpet lying over a jumble of toy building blocks – some high,some low.

The latter idea, that the brittle basement rocks were brokenand pushed upward with the sedimentary layers draped over them is supported byresearch published in the journal Terra Nova in 2001. Hamid Haddoum andcolleagues studied the orientations of folds and faults in this area, trying tofigure out the orientations of the stresses that caused them. Their data show ashortening direction – which means compression, or squeezing – during earlyPermian time oriented about northeast-southwest. That is consistent with thecollision that was happening at that same time between what is now Senegal andMauritania, in westernmost Africa, and the Virginia-Carolinas region of what isnow the United States. Haddoum and his colleagues show cross-sections withbasement upthrusts, basically high-angle reverse faults where older rocks aresqueezed so much that they are pushed up and over younger rocks. This is quitesimilar to the Laramide Orogeny in the western United States about 80 to 50million years ago, but this compression was happening about 280 million yearsago as the supercontinent of Pangaea was assembled during the early PermianPeriod. Both represent deformation at relatively great distances from the linesof continental collision. In the case of the Laramide in western United States,one idea for transmitting the stress so far from the collision is that thesubducting slab of oceanic crust began to go down at a relatively gentle angle,even close to horizontal, creating friction and stress further away from thesubduction zone than normal. Whether that’s the case here in southern Algeriaisn’t clear for this Hercynian collision.

I wouldn’t think of this area as high mountains, such asthose that must have formed along the lines of Hercynian collision. Maybe morelike warped, uplifted plateaus – but whatever they were, they were certainlysubject to erosion. Erosion probably wore the domes and basins down to a commonlevel, so that the nested bowls were exposed in horizontal cross-section –which for geologists is the equivalent of a geologic map. And that’s what thebeautiful photos reveal.

The area might have been planed off even more by Permian glaciersduring and after the Hercynian mountain-building events. But then, during theMesozoic era, seas returned to the region and all this mess of eroded domes andbasins was buried beneath even more sediments. Sometime relatively recently,during the Cenozoic era, the past 65 million years, everything was uplifted atleast gently, so that the highest parts – including today’s Hoggar Mountains,were stripped of the younger Mesozoic sedimentary rocks, revealing the mucholder Paleozoic rocks in the domes and basins.

—Richard I. Gibson

Link: Haddoum, H., Guiraud, R. and Moussine-Pouchkine, A. (2001),Hercynian compressional deformations of the Ahnet–Mouydir Basin, AlgerianSaharan Platform: far-field stress effects of the Late Palaeozoic orogeny.Terra Nova, 13: 220–226. doi:10.1046/j.1365-3121.2001.00344.x