Analogue modeling of the role of salt in the structuration of thin-skinned pull-apart basins: The case study of El Hamma basin, Central Tunisia

摘要

Pull-apart basins are structurally complex features that typically form in between two laterally offset strike-slip zones. The vast majority of analogue experiments on the formation of this type of basins have been conducted using basal discontinuities that transmitted from the bottom up strike-slip motion into the overlying sedimentary cover. Here, we present a series of models whose aim is to demonstrate that pull-apart basins above a viscous layer can be initiated not from the base up, but by the sole effect of lateral forces that allow two blocks to move laterally one past the other. This occurs independently from the basement, from which the brittle cover is fully decoupled by the viscous layer. This work was inspired by a natural example, the El Hamma Basin, located in Central Tunisia West of the city of Gabes and is bounded by two N100-110 trending major fault corridors, the Oum Ali-Fejij corridor in the North, and the Oglet Merteba-Matmata in the South. The area underwent multiple tectonic episodes of both extension and compression from Late Liassic to the Plio-Quaternary times. The region was subjected to regional NE-SW extension during the upper-cretaceous and major NW-SE compression during upper Miocene (Tortonian) and early quaternary (Early Pleistocene). The latter stage resulted in right-lateral strike slip along both corridors with faults forming a pull-apart basin that comprises two lateral mega-grabens separated by a central horst. Although one sub-circular diapir in the area (the El Melah diapir) crops out along the northern wrench zone, whether or not the mobile Triassic series extend farther South of it, beneath the pull-apart basin, is still debated. Through a series of analogue experiments using an innovative technique that generates two narrow, parallel strike-slip zones located within the sedimentary cover, rather than in the basement. Experimental results show remarkably similar geometry and evolution with those of the El Hamma basin. Moreover, from a regional perspective, our results suggest that the pinch out of the Triassic evaporites extends further South than previously thought and underlies at least part of the El Hamma basin.