Structural Trend Analysis by Axial Surface Mapping

We present a new method of structural trend analysis in which axial surfaces (fold hinges) are mapped through grids of seismic reflection profiles and balanced forward models. Axial surface maps (1) define fold trends, lateral terminations, and closure in structure, (2) constrain underlying fault geometry and slip, and (3) highlight and connect regions that can be interpreted by a series of balanced models or cross sections. Maps of folds in southern California and northeastern Pennsylvania are compared with forward model solutions and used to construct balanced three-dimensional structural interpretations using fault-bend fold theory. Axial surface maps generated from balanced fault-bend fold models define theoretical relationships between map patterns and structural structural geometry and fault slip. Kink-band widths between paired axial surfaces in map view are related to underlying fault slip. Lateral variations in fault slip, which can be identified on axial surface maps, may cause fold plunge and provide lateral closure.Lateral changes in subsurface fault geometry, which include lateral and oblique ramps, are also reflected by terminations, bends, or offsets of axial surfaces in map view. Based on these relationships between folding and faulting, we develop map-view interpretations and balanced cross sections across structures that are consistent with the geology along trend.