Normal-fault and fold development in sedimentary rocks above a pre-existing basement normal fault

Abstract

Comparisons of field, experimental, and theoretical data on the deformation in a sedimentary layer due to movement on an underlying, pre-existing basement fault, provide the basis for a conceptual normal-fault model in such a layer. Initial displacement along a 70(DEGREES) basement normal fault generates arcuate maximum-compressive-stress and shear-fracture trajectories immediately above the basement-fault tip. Their concave surface face the downthrown (hanging-wall) side of the layer. Both a region of low mean stress (compression positive) and high differential stress also overlie the basement-fault tip, providing optimal conditions for failure. Failure occurs by stress release along shear-fracture trajectories, resulting in short, upward-steepening normal faults which may obtain a reverse sense of separation before dieing out toward the downthrown side of the layer. Re-establishment of similar states of stress with continued basement-fault movement produces a series of nested, arcuate fault segments merging to a common surface or surfaces which ultimately comprise the through-going normal-fault system. Once such a system is established, the inactive smaller segments are preserved in the hanging wall. Additional faulting occurs to accommodate compression and extension associated with translation along the nonplanar, through-going fault. Monoclinal folding in the upper portions of the layer accompanies fault development at depth. The fold is slightly asymmetric with a narrower upper hinge and a wider lower hinge. The monocline tends to develop toward the side of the layer over the basement footwall. Narrow, steeper limbed monoclines associated with the upward-propagating fault tip may be superposed on the larger, earlier formed one. Once the through-going fault has fragmented the monocline, the hanging-wall limb may continue to steepen or may develop reverse drag with subsequent fault translation depending on whether the through-going fault steepens or shallows with depth.