Structural context of meso-scale and cartographical duplex structure genesis in thrusting tectonics: role of tear faults. Zaghouan thrust belt. Case of Bir M'Cherga-Ain Asker area

Abstract

Bir M'Cherga-Ain Asker area, situated in the hinterland of Zaghouan thrust (Tunisian dorsal), was the land example treated with metric and cartographic scale in order to identify duplex genesis criteria and to include thrusting tectonics associated with tear faults, which are in fact the directory response generating duplex structures identified in outcrop for the first time through Tunisia in this case. Given its geological location between the "dome" and the "dorsal" zone of Tunisia, this area was the most exposed to a highly paleostress history expressed by a huge fault system remobilization and reactivation through several tectonic events from NE–SW middle Cretaceous distension to a NW–SE and NNW–SSE Paleogene compression. Regarding fault planes generated analysis, they show numerous streak generation of normal, strike-slip, and reverse faults that go with geodynamic and paleostress evolution of the studied area; we note that each streak generation is perfectly matching with one of the tectonic event (mentioned before) affecting the area. In this paper, we analyze duplex structure elaboration scenarios to assess the involved kinematics and their geometrical recognition criterious. We propose to discuss the causes of duplex structures installation in a thrust belt system and the predictable geometrical styles after its installation on foreland or backland. Using the geometrical criteria acquired through this analysis, we will show that such, however, exceptionable structures exist on the land, and that they record the mechanisms of their genesis linked to the tear faults acting in this case. We describe "tear faults" as the sliding breaks which disunite two compartments during deformation, allowing them to undergo different independent deformations in their drawing and their width (for example more or less stretched folds). These types of faults differ from that of the true stick-slip faults, which slice and shift preformed structures (it's even this shift which makes it possible to highlight them); here, there are no shift but dissimilarity of the structures on both sides of the fault; therefore, deformations are the direct results of displacements; they are expressed in thrust belts by ramp folds, intense internal deformations, and even by complex duplex structures. A duplex feature that is not mainly studied is made up by tilted imbricate sedimentary sequences (or horses), separated by link thrusts and underlined respectively at their tops and bottoms by roof and floor thrusts. Imbrications cause a shortening, a thickening, or a thinning of stratigraphic columns and even its crushing and inverting. In thrust belts, duplexes are usually set up following two or even more deformational events; those structures start typically with decollement and imbricate sedimentary unit ones which are made cover by a roof thrust sometimes visible at outcrop. Through this paper, we suggest to discuss geometrical duplex criteria, and we will try a zooming through different scales, from regional to local one in order to show how the shape (expression) of the deformation differs.