Strain analysis of Late Ordovician tectonic events in the In-Tahouite and Tamadjert Formations (Tassili-n-Ajjers area, Algeria)

The Tassili-n-Ajjers area in southern Algeria is located in the south-east of the Saharan platform. It is bounded to the east by the Tihemboka N–S trending structural system, the Hoggar shield to the south and to the north by the Illizi Basin. During the Palaeozoic, the Saharan Platform was part of the northern passive margin of the Gondwana supercontinent and the tectono-stratigraphic evolution of sedimentary systems in the Algerian Sahara was typical of the Gondwana stable cratonic platform. During the Late Ashgill (Hirnantian), the North African platform was located near the South Pole and an icecap developed over much of Africa and South America. Field observations, in the study area, together with integrated strain analysis and previous studies provide evidence for a number of Late Ordovician deformation phases. An extensional event of probably Late Caradoc–Early Ashgill age (?) was associated with the development of planar domino-style normal faults and listric faults that occurred prior to the infill of glacial palaeovalleys. This is observed in the pre-glacial In-Tahouite Formation. Glaciotectonic over the Tassilli-n-Ajjers is related to the Late Ashgillian (Hirnantian stage) Taconic event. Probably, the Taconic unconformity is a combination of traditional tectonic movements and glaciotectonics. Many soft-sediment deformation structures are attributed to glaciotectonic process. A systematic description of these deformation structures and strain analysis show that there are two types of deformation, pure shear at the top of the Hirnantian syn-glacial Tamadjert Formation and simple shear in the basal part and there are two types of glaciotectonic deformation. The first type at the base is the subglacial deformation and is characterized by simple shear strain and extensional tectonics. The second type at the top is characterized by pure shear strain and compressional tectonics and consists on proglacial deformation. The glacial palaeovalleys show a gravity-sliding mechanism affecting the unconsolidated deposits.

Research highlights
► The strain analysis provides evidence of Late Ordovician deformation phases.
► An extensional event of probably Late Caradoc-Early Ashgill.
► The soft-sediment deformation structures are attributed to glaciotectonic process.
► Glaciotectonic is related to the Late Ashgillian (Hirnantian stage) Taconic event.
► The strain analysis shows two types of deformation in the synglacial Formation.