Relationship between fractures patterns and fold kinematics; the case study of Jebel Sehib, a typical fault-propagation fold of southern Tunisia

摘要

Fracture patterns in folds are resulting from complex tectonic history. Indeed, in thrust and fold belts fractures occurred because of three different stress tensors: (1) Regional field stress, (2) Tensile stress in hinge area or (3) simple shear in limbs. Therefore, the same regional stress field could create too many different fracture and joint types. Detailed works have characterized fractures observed in nature, but only few studies have linked the fractures to the fold kinematics. On the other hand, we believe that fold kinematics have a key role in fracture amplitude and distribution. For this reason, we studied the fracture network affecting the Jebel Sehib, a typical fault-propagation fold of the Gafsa basin, southern Tunisia. On the same bed, and all over fold parts, we measured the fracture net density. The results showed clear difference in fracture density spread with highest density in the forelimb, medium in the backlimb and lowest in the hinge area. The, we compared the results with the theoretical kinematics of related fold/fracture models. We concluded that the fracture net is mainly anterior to the folding process. In fact, at the beginning of compressive stress, layer parallel shortening (LPS) occurred and generated homogenous fracture net in a pure shear regime attested by conjugated shear joints. During the growth of the fault-propagation fold, the existing fracture net is locally reactivated and amplified by limb simple shear as prospected in the mechanical behaviour of the fault-propagation fold. This leads to higher fracture density in the steeper flank.This hypothesis can be generalised to at least all fault-related folds in which limb simple shear is present. In the Jebel Sehib case study, we observed the flexural flow (internal deformation of the rock) because of the chosen massive limestone reference bed. In other natural examples, most of the beds accommodate simple shear by a flexural slip and rarely by flexural flow. This work have direct consequences on fractured reservoirs exploration in fault-related folds.