Extensional collapses in the overpressured frictional upper crust based on limit analysis

Overview & Synopsis

This manuscript develops a 2D kinematic approach of Limit Analysis to examine the extensionalfailures in the brittle, upper crust resulting from fluid overpressures and normal faulting. There aremany interesting topics related to the extensional deformation such as (1) the roles of fluid pressure,topographic process, material and fault properties on the stability of extensional structures; (2) theformation of low-angle and listric normal fault; (3) the deformation pattern due to slip on a low-anglefault; and (4) the influence of fault softening and sedimentation processes on this deformation pattern.This mechanical approach applied to wedge prototypes is validated by the critical Coulomb wedge(CCW) theory, and it generalizes the CCW theory to investigate the complex topography on theMejillones peninsula, Northern Chile. Additionally, this approach is also applied to investigate gravityinstability of Niger Delta by linking down-slope compressional to up-slope extensional failures througha deep detachment. We predict much higher fluid overpressures than that of the CCW theory. Finally,this Limit Analysis methodology is applied to investigate the shape of normal fault linking a lowdetachment to the surface. The application to Niger Delta implies that the formation of very low-angleand strongly listric faults results from a shallow fluid-retention depth. The sequential version of LimitAnalysis opens new ways to envision the structural evolution through time resulting from normalfaulting. The simulations show that the normal fault rotates during extension, forming a region of Footto-Hanging Wall (FHW) where the material in the footwall is sheared upon entering the hanging wall.The creation of the FHW region is illustrated by sandbox experiments and field examples.