EXTENSIVELY DEVELOPED NETWORK OF NON-TECTONIC SYNCLINES IN EOCENE LIMESTONE OF THE WESTERN DESERT, EGYPT

High resolution satellite imagery of the Western Desert of Egypt reveals an extensive network of hundreds of long narrow synclines developed in Early Eocene limestone over an area of >20,000 km² west and east of the Nile. These structures are unusual and have characteristics not typical of tectonic fold structures. The terrain is dominated by long narrow synclines (100-200 m across) with shallow limb dips, porpoising hinges with shallow plunges, and multiple basin closures along their lengths. Synclines are similar in scale across the region, with no parasitic folds and no larger structures. Although two dominant orientations are common (NNW-SSE and WNW-ESE), synclines from the two trends branch, merge, and curve into one another, forming a network. Over large parts of the area, narrow synclines are the only fold structures present and form isolated downwarps 1-3 km apart in otherwise flat-lying limestone. Where synclines are locally more closely spaced, inter-syncline areas are broadly anticlinal, but geometries suggest that they are “accidental anticlines” formed by the proximity of two nearby syncline limbs rather than by an active anticline-forming process. Syncline limbs are commonly cut by faults striking parallel to bedding but dipping more steeply than bedding toward syncline cores. The syncline network and related faults developed in a narrow time window between Early Eocene deposition of the limestones and formation of cross-cutting faults associated with Red Sea rifting.

The characteristics of the syncline network are consistent with sag of limestone layers caused either by volume reduction at depth (e.g., by hypogenic karst processes or localized silica diagenesis) or, more likely, by mobilization of underlying shale sequences. Industry seismic reflection data show variable thickness in the underlying Esna Shale, as well as extensive faulting. Both would be consistent with sag of a limestone carapace into structures developed in a mobilized underlying sequence. A Late Eocene tectonic event uplifted portions of northernmost Egypt and may have triggered translation of subsurface mobile units to the south or southwest. Broad structural domes 10-50 km across in the southern part of the Sinn El Kaddab Plateau may reflect subsurface thickening associated with south to southwest mobilization.