The structural evolution of the Aghouhan-Ressas Structural Belt, NE Tunisia

Morgan, Mark Andrew (1994) The structural evolution of the Aghouhan-Ressas Structural Belt, NE Tunisia. (PhD thesis), Kingston University, .


Structural analysis of the Zaghouan-Ressas Structural Belt (ZRSB) of NE Tunisia, based upon detailed field mapping and Landsat TM image interpretation, has shown there to be a definite link between uplift across the area and the development of a mid Miocene aged, thin-skinned Tunisian Atlas thrust belt. The construction of balanced cross-sections through the ZRSB has proved that it is possible to generate the major structures of this belt by invoking a combination of an early fault-propagation fold, a foreland propagating duplex structure and a later out-of-sequence thrust that propagates in the hanging-wall of the earlier fold. Balancing of sections across the ZRSB, however, was complicated by the presence of several, previously noted (along and across strike) lateral facies variations, particularly in the Mesozoic sequence. This thesis has confirmed the presence of these facies variations and has shown that their distribution has had a direct bearing on thrust-belt evolution. In order to understand the effects of the main lateral thickness and lithostratigraphic variations on the mid Miocene structural evolution of the ZRSB, a modern interpretation of their origins has been made, based upon an investigation of the pre-thrusting tectono-stratigraphic evolution of the region. It has been established that, from the mid-Jurassic to the Late Cretaceous, the ZRSB was the site of a northeast/southwest-trending basin margin with a major depocentre to the northwest and a long standing palaeohigh to the east. This palaeogeographic setting is thought to have resulted in thick accumulations of Lower Cretaceous sediments to the northwest of the line of the ZRSB while non-deposition and/or condensed sedimentation took place to the east of this line. The presence of unconformity surfaces, and the fact that large parts of the Lower Cretaceous sequence are missing to the east of the ZRSB, has confirmed this hypothesis. Two pre-thrust stratigraphic templates were constructed that took account of Mesozoic lateral facies variations across the ZRSB. These templates have been used as a constraint on the restoration of balanced cross-sections, thereby proving a relationship between the locations of Mesozoic basin margin structures and the characteristics and evolution of the ZRSB. All thrust-related structures in the Zaghouan area have been dissected by a series of steep fractures. At least three distinct post-thrust fault trends have been established. The movement histories associated with these faults are extremely complex and long-lived, and no simple explanation exists for their evolution. A model has been devised that invokes alternation of extension in two, perpendicular directions, as the result of independent, but competing, deformational processes: one as the result of orogenic collapse and the other related to transtension in the Sicily channel. By noting the presence of pre-existing fault trends and by allowing for an overlap in time between the two tensional stresses acting alternately, the orientations and kinematic histories of the post-thrust faulting can be reproduced. The interpretations made in this thesis are summarised through their use as a constraint upon a new plate tectonic model for the evolution of the central and western Mediterranean region. This model shows that the complex Mesozoic structural and stratigraphic evolution of northern Tunisia was not dominated simply by crustal stretching on the south Tethyan margin, as previously thought, but rather has been controlled by an interaction of Tethyan stretching, leading to the development of north/south-east/west-trending structures, and intraplate rifting between Europe and Africa, giving rise to northeast/southwest - northwest/southeast-trending structures. Subsequent structural and stratigraphic development in northern Tunisia is shown to be intimately related to the effects that the overall stress regime between the major plates has had through the reactivation of these pre-existing trends. Miocene to recent evolution of the region, however, is shown to have been largely controlled by the development and movements of microplates in the western Mediterranean.

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