STRUCTURAL CONTROL ON THE INCEPTION AND EVOLUTION OF EAST SAHARAN MEGA-DEPRESSIONS

The East Saharan Desert in North Africa houses several mega-structures sculpted in the Libyan Plateau, among the most extensive and best-exposed on Earth. However, to this day, the origin of these prominent features is still controversial. The present study addresses the origin and evolution of these mega-scale structures using integrated field, remote sensing, structural, and geochemical observations acquired over the Eastern Farafra plateau, Egypt. The surface of the Plateau features thousands of parallel and connected depressions of varying sizes coupled with a network of strike-slip faults, offering a unique opportunity to study the inception and evolution of mega-depressions. We propose depressions initiated as small, subcircular to elongated depressions along faults that grew and merged with time, driven by the enhanced discharge of pressurized groundwater in previous wet climatic periods by sapping processes. We observed five geomorphic settings that portray the inception and growth of topographic depressions. (1) Inception of depressions along the strike-slip faults network as a pull-apart basin, en echelon grabens, or as trough along normal faults. These faults are highly fractured deformation zones with complex subsidiary structures. (2) Preferential growth and elongation of depressions along individual faults and at their intersections. (3) Coalescing of small (area: exceeds 0.5 km²) and neighboring depressions into larger depressions (area: up to 5 Km²; depth 15-30m) that propagate along domains affected by fault and fracture systems. (4) Development of Theater-Headed Valleys (THV) along the escarpments of coalesced depressions and removal of weathering and dissolution products by eolian or fluvial processes. (5) Erosion of topographic ridges separating the coalesced depressions, giving way to even more significant depressions. The advocated model is further supported by the isotopic composition of travertine samples from the depressions (δ¹⁸O: -10.78 to -12.58‰), which is consistent with deposition from depleted Nubian Sandstone Aquifer groundwater (δ¹⁸O: -8.00 to -12.8‰). Our study provides a new vision to understand the long-term landform evolution in hyperarid regions on Earth and Mars, and sheds light on the role of geological structures in mega-depressions' development.