A Geological Tour of Morocco's Atlas Range
As connoisseurs of mountain scenery who enjoy exploring the world’s high places while also immersing ourselves in foreign cultures, we jumped at the chance, while touring southern Spain, to ride a ferry across the famed Strait of Gibraltar to visit mountainous Morocco. The ferry docks in Tangier at the foot of the rugged Rif Mountains, which rise along the convergent Eurasian-African plate boundary precisely where plate tectonic theory teaches us to expect a mountain range. Although we’d heard many stories about the Rif’s wonderful scenery and cultural delights, we chose instead to head inland to explore the Atlas, a much higher and more geologically exotic range.
The Atlas, which rise to 4,167m, 1,700m higher than the Rif’s tallest peak, are a geological puzzle because of their location at the edge of the West African Craton, comparatively far from the active plate boundary, and because their lofty elevation greatly exceeds what is expected for an isostatically compensated range that is supported by a crustal root. By violating the plate tectonics ‘rules’ that neatly explain why most mountain ranges exist where they do, the Atlas epitomise the increasing number of exceptions that geoscientists are recognising. As such, the range is both an alluring geotourism destination and a natural laboratory in which to test new tectonic explanations.
Volcanoes and Roman Ruins in the Atlas Range
We came to Morocco seeking an experience different from that which Europe has to offer. The country delivered this in spades right from our very first stop: the Roman ruins at Volubilis, an impressive UNESCO World Heritage Site located at the foot of an isolated mountain named Jebel Zerhoun. Unlike many other ancient ruins, Volubilis is not a major tourist attraction surrounded by pricey snack bars and kitschy curio shops. It instead rises from a vacant field, to all appearances completely forgotten by history. The serene setting and excellent preservation make it easy to contemplate the city’s heyday as a far-flung outpost on the edge of the once-mighty Roman Empire, as well as the site’s later role as the seat of Idris I, founder of the region’s first Islamic empire.
Just south-east of Volubilis, the town of Moulay Idris, where Idris I is buried, seems to tumble scenically down the flank of Jebel Zerhoun. From here the N13 highway passes through the city of Meknes, one of Morocco’s four former imperial capitals, and then climbs up and over the rolling, 1,200 to 2,000m high upland of the Middle Atlas, the northernmost of the Atlas chain’s several sub-ranges. Near the town of Azrou the road passes through a forest of stately cedar trees. Several intact Quaternary cinder cones and alkalic basalt flows belonging to Morocco’s youngest and most voluminous Cenozoic volcanic field are visible from the highway. These features offer mute testimony to the presence of thin lithosphere, and hence unusually shallow asthenosphere, beneath the Atlas region. Most geoscientists attribute the Atlas’ abnormal height to thermal expansion resulting from the replacement of colder lithosphere with this warmer asthenosphere.
But explanations for why the lithosphere beneath the Atlas is so unusually thin vary. Ideas include a ‘baby’ hotspot plume that taps material from the larger Canary Islands hotspot to the west; lithospheric delamination; a ‘lithospheric drip’ (known more formally as a Rayleigh-Taylor instability); or smallscale convection driven by edge effects at the interface between the West African Craton’s thick lithosphere and the much thinner and younger lithosphere that underlies the Mediterranean Sea. Although research papers on these topics are coming out in droves, no single hypothesis has emerged as the clear leader.
The Todra Gorge: An Inverted Jurassic Rift
We continued south on the N13 road through a small intermontane basin and over the High Atlas, the tallest of the chain’s sub-ranges, via a 1,900m-high pass. The road then descends through the Jurassic limestone walls that line the Ziz River gorge, ushering drivers out of the mountains into the increasingly arid landscape of the Ouarzazate foreland basin, which marks the northern edge of the vast Sahara Desert.
We then turned west, traversing the foreland basin. This basin is unusually shallow, hosting a mere 1,200m of Neogene alluvial and lacustrine fill, likely due to a combination of modest amounts of crustal shortening (estimated at 26 km, or about 24% at the Ziz Gorge) and a relatively strong underlying lithosphere, which limits the basin’s flexural subsidence in response to whatever thrust sheet loading has occurred.
The closer we got to our next destination, the oasis town of Tinerhir, the taller and more dramatic the mountains became. Tinerhir is located at the mouth of the spectacular Todra Gorge. The gorge’s steep limestone walls shelter one of the range’s precious perennial streams, whose life-sustaining waters create a stark juxtaposition of verdant green fields and palm orchards nestled within a barren, brown landscape almost completely devoid of vegetation.
Although most geoscientists agree that the hot mantle beneath the High Atlas has raised the range higher than it would otherwise be, textbook plate tectonics has played an equally important role in the Atlas uplift story. The range was raised primarily by slip along a series of high-angle, basement-involved reverse faults – north-vergent ones on the range’s northern flank and south-vergent ones on its southern side.
These high-angle faults were born as normal faults during the break-up of Pangaea, when the area that is today occupied by the Atlas was a rift that linked a young Atlantic spreading centre to the Tethys Sea. Throughout the Triassic and Jurassic, five to six kilometres of redbeds followed by open marine limestones gradually filled this rift. Dramatic differences in the thickness of these units testify to the activity of these faults, with thicker sections on the hanging walls and thinner ones across the footwalls of the parallel faults. In contrast, the Cretaceous section consists of limestone whose thickness is uniform across the area, suggesting that rifting had ceased by that time. During the Cenozoic, when the region’s stress field switched to one of compression with the advent of the Alpine Orogeny, these normal faults were reactivated as reverse faults to form today’s thick-skinned mountain belt.
Despite the predominance of thick-skinned processes in the modern Atlas, the range’s southern flank displays a classic thin-skinned fold-and-thrust belt whose tight folds in competent Jurassic limestone are magnificently displayed as you wend your way up the 20-km-long Todra Gorge. Triassic evaporites serve as a decollement along which the Mesozoic cover has detached from the basement, in the process thrusting the sedimentary rift fill up and over the foreland’s younger rocks.
Spectacular Canyons of the Atlas Range
About 53 km west of Tinerhir, the Dades, Morocco’s longest river, flows across the High Atlas’ frontal thrust and nto the Ouarzazate foreland basin at the oasis town of Boumalne du Dades. The 5 km Dades Gorge, which begins about 15 km north of Boumalne du Dades, is a memorable sight. It is a true slot canyon that measures more than 200m deep and, in spots, just 20m wide. Because the road has to climb up and around the canyon, it offers breathtaking vistas into the slot both up and downstream. From this high perch, it’s easy to see that the Dades flows down the axis of an asymmetric syncline. The gorge is carved through competent lower Jurassic marine limestone of the Jebl Choucht Formation. Thrusting here has nearly doubled the formation’s thickness, probably playing an important role in forcing the river to carve a deep gorge instead of tracing an easier route through the less competent units that under- and overlie this resistant limestone.
A few kilometres further upstream, both the river and the road squeeze through another narrow canyon, the 150m-deep, 20 to 50m-wide Tarhia n’Dades Gorge. Geomorphologists have deduced that the Tarhia n’Dades marks the spot of a stream capture event that shifted the course of the ancestral Dades River. The gorge used to lie on a tributary that flowed down the limestone face of the syncline limb. The tributary had sufficient stream power to cut down through a comparatively thin portion of the Jebl Chouht limestone, which exposed the much softer, underlying Ouchbis mudstones to rapid erosion, triggering the capture event. The Tarhia n’Dades Gorge provides a dramatic location from which to contemplate the dynamic landscape of the Atlas.
Kasbahs and the Marrakesh Souk
After marvelling at the Dades Gorge, we continued our trek westwards through the Ouarzazate foreland basin to the scenic kasbah (fortress) of Ait Benhaddou. This UNESCO World Heritage Site’s popularity has skyrocketed since it stood in for the fictional cities of ‘Yunkai’ and ‘Pentos’ on the television series Game of Thrones. From our comfortable lodge situated nearby in the Asif Iminni valley, we also explored the much smaller Tiseldei kasbah, whose abandoned remains guard the valley from a perch high on the adjacent slope. Both kasbahs are built on Jurassic redbeds intruded by dolerite sills, further evidence of the Mesozoic rift event.
The road to Marrakesh climbs over the High Atlas at the 2,260m Tise-n-Tichka Pass about 50 km east of Toubkal, the range’s highest peak. Here at the western end of the High Atlas, where the mountains are highest, the amount of crustal shortening is, paradoxically, far less than it is farther east: a mere 13 km (15%) here. Given that crustal shortening is the mechanism that builds the crustal root, the Atlas are clearly not supported solely by such a root. Instead, the lithosphere is thinnest here where the mountains are highest, reinforcing the idea that mantle heat is what supports these high mountains.
The peaks surrounding the pass consist of mildly metamorphosed Palaeozoic and Precambrian basement rocks. For years geologists have used this fact to argue that the Atlas near Marrakesh stood as a palaeo-high in the Mesozoic rift, thereby limiting the thickness of rift fill and thus explaining the basement exposure here. Recent zircon (U-Th)/He thermochronology, however, has shown that the rift fill here was 5–6 km thick, comparable to what is present farther east. So it therefore appears that the exposure of basement rock here in the highest mountains of the Atlas is due not to thinner rift fill, but rather to deeper erosion.
The descent from Tise-n-Tichka Pass is executed in a series of tight switchbacks with sweeping views across the rugged range. The road crosses the northern bounding fault and exits the Atlas a mere 30 km from Marrakesh, a bustling city of a million people that was founded in 1071 as the capital of the Almoravid Empire. Its fascinating old quarter is surrounded by imposing walls of rammed red clay, built as a protective fortification during the 12th century.
But Marrakesh’s main allure for travellers is the chance to wander through its traditional markets, known as ‘souks’. The city’s main souk, which is among the most famous in the Arab world, is a bustling and colourful scene. From watching snake charmers ply their trade in the large public square to getting lost in the warren of narrow alleyways lined with shops selling colourful spices, cloth, jewellery, and kitchenware, a visit to a Marrakesh souk is an unforgettable experience.
For us it was a fitting finale to a trip that delivered all of the exotic culture, equally exotic geology, and rugged mountain and canyon scenery we had come to Morocco to experience.
Further Reading on Oil and Gas Exploration in Morocco
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