Abutting faults: A case study of the evolution of strain at Courthouse branch point, Moab Fault, Utah

Abstract

Slip planes and slip directions of subsequent generations of faults were measured and analysed in the interaction damage zone of two abutting faults in porous sandstones in order to understand the palaeostress/palaeostrain evolution. The Courthouse branch point of the Moab Fault in SE Utah (USA) is a much-studied, spectacular outcrop of two abutting faults, located in the footwall block of the main fault and in the hanging wall block of the abutting fault. The abutting fault is synthetic to the main fault. The outcrop shows a wide range of deformation structures and fault-related diagenesis such as striated slip planes, deformation bands, veins, Liesegang bands and copper-rich mineralization. By combining our own measurements with published data on the relative age of these structures, we classified the data in four sets. Using a Numeric Dynamic Analysis (NDA) to calculate the orientation of the kinematic axes we found three different palaeo-extension directions in the four sets, recording the evolution of stress/strain axes during the abutting process. The first phase of deformation is regional extension in the NE-SW direction. As the second fault approached the main fault from its footwall side and the two faults started to become kinematically linked, the extension direction changed so that the overall extension became perpendicular to the approaching fault (NW-SE). Finally, the extension direction changed back to being perpendicular to the first segment (NE-SW) when the two faults became geometrically linked and regional extension became dominant again.