Print this pageTesting 3D Numerical Models of Normal Faults and Associated Fractures Using Outcrop Analogs and Seismic Data
POLLARD, DAVID D., ATILLA AYDIN, JULIET G. CRIDER, SIMON KATTENHORN, and LAURENT MAERTEN
Numerical models of rock deformation based on continuum mechanics provide one important tool for the interpretation of geologic structures in the context of hydrocarbon exploration and production. We use the boundary element method to approximate major normal faults as three dimensional surfaces of displacement discontinuity in an elastic material. Computed stress fields predict the locations and orientations of sub-seismic faults and fractures.
Given the tip-line shape and slip distribution of normal faults from seismic data and the loading history from tectonic analysis, one can determine the local stress perturbations which control the mechanical interaction of fault and the development of sub-seismic scale fractures. We show how mechanical interaction leads to significant strike-slip motion and characteristic distributions of structure contours near V- and Y- shaped normal faults.
Zigzag normal fault traces on maps commonly are composed of longer traces with a dominant strike and shorter traces at a subordinate strike possibly formed from a breached relay structure. For echelon fault segment 3D rendering of the stress field indicates new fractures would strike 35 to 50 degrees from the trend of the segments. These geometric relationships correspond to well-exposed normal-fault system in south-central Oregon, suggesting that the zigzag pattern may be, in part, the result of breached relay-zones.
The classic theory of normal faulting predicts joints parallel to the fault strike. However, outcrops at Arches National Park, Utah, display a joint set nearly perpendicular to normal faults. As a fault grows, the surrounding stress field is perturbed, so joints can form in a variety of orientations. Numerical models predict joint growth at high angles to faults near their tiplines.