Development of collapse dolines in Al Jabal Al Akhdar, NE Libya: geomorphic analysis and numerical modeling-based approach

Abstract

Al Jabal Al Akhdar anticlinorium, NE Libya, has the most extensive and diversified karstic features. Collapse dolines are among the most prominent karst phenomena with many of them developed during the Holocene. This research uses field observations and numerical modelling to understand the geometry and formation mechanisms of these dolines. Detailed measurements of 66 collapse dolines including locations, elevations and dimensions (diameters and depths) are documented herein. The dolines have diameters ranging from several meters to tens of meters, while their depths vary from few meters to over a hundred meters. Some giant dolines can be classified as "Tiankengs". The dolines have average diameter of 55 m and height/depth of 60 m. There is a strong linear correlation between dolines areas and volumes with a determination coefficient (R 2 ) of 0.88. When 65 collapse dolines in China are added to the dataset for comparison, the correlation becomes R 2 = 0.83. This geometric similarity may suggest an analogous mechanism of their development. This research reveals that these dolines developed from the enlargement of fractures and bedding planes, forming various sizes of cavities in the sub-surface through chemical weathering. The subsequent collapse of the roof of these enlarged voids was most likely influenced by local stress fields around them, leading to extensive extension and shear fractures. Numerical models suggest that the local stress field favoured rupture and fracture propagation, once tensile and shear stresses exceed the tensile and shear strengths of the host rocks, thus causing the breakdown and the formation of collapsed dolines.