Three Dimensional Analysis of Fault Interaction along a Simple Pull-Apart Basin

Abstract:

Pull-apart basins reflect basic interaction between two faults and have been studied in detail during the last decades. However, the true three-dimensional geometry of pull-apart basins is seldom known. We therefore present a new study focused on the architecture of a pull-apart basin and the associated stresses. We analyze the structure of a ~250 m long pull-apart basin developed in carbonate rocks at the Galilee heights, Israel. The reconstruction of the basin geometry is based on detailed mapping and LiDAR measurements of fault scarps. The architecture of faults is then used as a boundary condition for calculating the stress pattern in the vicinity of the basin, using a dislocation model. The basin is found to be an asymmetrical structure bordered by two longitudinal oblique right lateral strike-slip faults. The strike of one of the faults is bent at the eastern edge of the basin, generating a transverse boundary fault which joins the second boundary fault orthogonally. The overall lateral displacement is smaller than the basin length and no transverse or diagonal fault is observed in the western end of the basin. The deformation around the basin is mostly displayed by fractures. Yet, folds and fault branches are observed near the tips of the boundary faults and near kink points of fault segments.

We found that the interaction of the two subparallel strike-slip segments observed in the outcrop is expressed by a V-shaped basin structure. Therefore, a subsurface bending in the main fault is expressed by step-over and by subsidence close to the upper surface. The geometry of the basin deviates from the typical rhomb shape even in map view, as the basin is not bordered by transverse or diagonal faults on its western edge. The lateral displacement along the basin is localized on oblique bordering strike-slip faults with less slip partitioning than observed in larger pull-apart basins. 3D modeling of the stress field around the basin demonstrated that the pull-apart structure affects the stress pattern and that the stress tensor is rotated locally to a variety of orientations near the faults. These calculations explain the abrupt variations of fracture orientations near the borders of the basin and the intensive deformation near the tips of the faults.