INTERPRETATION OF PERICLINES USING 2D OBSERVATION METHODS

The use of stereographic projections in determining the overall shape of a geologic structure is widely adopted amongst geologists. However, the use of a stereonet or tangent plot alone in discerning the orientation of a 3D structure may be misleading. For example, a pericline (doubly plunging anticline) will plot differently on a stereonet based on the location of the measurements. This study aims to investigate the insufficiencies of using a two-dimensional view to analyze a three-dimensional structure. Such a problem is encountered commonly in field outcrops of fold systems that are partially exposed in a cross-sectional view (e.g. in roadcuts). In this study, using the finite element pre-processing software package Altair HyperMesh™, nodes from discretized periclinal structures are used to calculate the attitude of the elements within the structure. Different simulated roadcuts are used to extract multiple sets of orientation data. Calculated orientations are plotted on stereonets and tangent plots for further analysis. Based on the specific set of elements used, the resulting stereonet reflects different geologic structures which may be observed on roadcuts oriented in differing directions. For example, a vertical elliptical cone could be interpreted from a periclinal cut. These alternative interpretations are found from analyzing few and specific datasets of the original structure. The results based on 3D periclinal structures indicate several different structures can be interpreted through stereographic reconstructions of various cross-sectional views. For example, periclinal structures may show up as cones but do not follow the stereonet pattern of a cone. Therefore, it is necessary for three-dimensional structures to be based on a dataset that covers a significant portion of the structure to accurately describe the geometry and orientation for analysis.