The structural evolution of the Zaghouan-Ressas Structural Belt, northern Tunisia

Abstract

Abstract From Middle Jurassic to Late Cretaceous time the African-European Rift Zone (AERZ), a seaway connecting the western part of the Tethys Ocean to the embryonic Atlantic Ocean, was characterized by sinistral transtension. On the African margin of the AERZ this caused break-up of Tethyan Triassic and Lower Jurassic evaporite and carbonate platform sequences by linked, strike-slip and normal-slip fault displacements that delineated a system of sedimentary basins separated by horsts. The Zaghouan-Ressas Structural Belt (ZRSB) in northern Tunisia was initiated as a north-tapering horst in this system, bounded to the northwest by a pelagic basin (the Tunisian Trough) in which thick Lower Cretaceous sequences were deposited and to the east by a north-south trending system of reactivated Tethyan margin faults. Thickness variations in syn-rift stratigraphy led to lateral flow in underlying Triassic evaporitic sequences and the initiation of pillows and, perhaps, piercement diapirs. Mid- to latest Cretaceous post-rift sequences onlapped syn-rift fault blocks, but the post-rift period is complicated by a reversal in the displacement sense across the AERZ leading to dextral transpression and local fault inversion. In Paleocene and Eocene time, northern Tunisia was characterized by northeast-southwest extension accommodated by displacements on linked systems of reactivated AERZ-related and Tethyan margin faults at the southern margin of Mesogea. This was associated with drift of the Apulian microplate into eventual collision with the European margin to form the Western Alps. Further west, convergence of Africa relative to Europe was initially taken up by subduction of oceanic lithosphere in the remnant AERZ. The Oligo-Miocene evolution of the western Mediterranean reflects the destruction of this oceanic lithosphere and its successor oceanic basin, the Proto-Mediterranean. The Atlassic orogeny in northern Tunisia began in Oligocene time as a result of collision of microplates rifted off the European margin with the North African margin, and coincided with the progressive elimination of Proto-Mediterranean lithosphere from west to east along the African margin. Evidence of the contractional deformation is the development of an Oligocene-Miocene foreland basin in northern Tunisia and its deformation in the Atlas fold-thrust belt of mid- to Upper Miocene age. Within this tectonic framework, two models for the structural evolution of the ZRSB during the Atlassic orogeny are evaluated. The first recognizes the importance of facies variation in controlling thrust geometry, but is essentially a thin-skinned model in which detachment on incompetent Triassic strata forms the main control of structural style. The second model emphazises reactivation of AERZ-related basin margin faults during contraction and accounts for the major folds in the ZRSB at Djebel Zaghouan and Djebel Ressas as forced folds formed by fault inversion. Anticlines at Hamman Zriba, and east of Grombalia, are also interpreted as fault-inversion folds formed in normal sequence on the external side of the ZRSB. Flow of Triassic strata into the cores of these folds may have been assisted by tectonic loading during fault inversion along the ZRSB. Subsequently, structures in the ZRSB were dissected by northeast-southwest-trending faults that propagated through the post-rift sequence during post-Miocene reactivation of syn-rift extensional faults. These faults accommodated dextral oblique-slip displacement and were linked to extension in northwest-southeast-trending graben that cut the ZRSB and the Intermediate Atlas Zone.