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Why is the geology of the Atlas Mountains complex?

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Map showing the Atlas Mountains across Morocco, Algeria, and Tunisia in North Africa, with color shading to indicate elevation changes. Major mountain ranges, cities, rivers, and physical features such as the High Atlas, Anti-Atlas, Saharan Atlas, and surrounding deserts are labeled. An inset locator map highlights the region's position in northern Africa. — Atlas Mountains The Atlas Mountains.

The geology of the Atlas Mountains is considered complex because the system is composed of two distinct structural regions with different origins and developmental histories.1

The Tell Atlas

The Tell Atlas is a young, actively forming mountain range. It originated from a sediment-filled basin that underwent significant uplift during the Paleogene and Neogene periods (approximately 65 to 2.6 million years ago). Its formation involved the folding of the Earth's crust and the displacement of flysch—deposits of sandstone and clay—which were pushed southward over a marginal rim. The ongoing nature of this process is evidenced by the region's continued seismic activity.12

The Saharan Atlas

In contrast, the Saharan Atlas is part of the ancient African continental base rock. Its structure is defined by:
* Sedimentary cover: Large areas are covered by deposits from shallow seas and alluvial activity.1
* Tectonic folding: The range resulted from the intense folding of the Earth's crust during the Jurassic Period and Cretaceous Period.1
* Base rock uplift: In areas like the Moroccan High Atlas, the geology features horsts, which are uplifted blocks of the Earth's crust.1