STRUCTURAL TREND ANALYSIS BY AXIAL SURFACE MAPPING

We present a new method of structural trend analysis in which axial su rfaces (fold hinges) are mapped through grids of seismic reflection pr ofiles 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 reg ions that can be interpreted by a series of balanced models or cross s ections. Maps of folds in southern California and northeastern Pennsyl vania 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 fo ld models define theoretical relationships between map patterns and st ructural geometry and fault slip. Kink-band widths between paired axia l surfaces in map view are related to underlying fault slip. Lateral v ariations in fault slip, which can be identified on axial surface maps , may cause fold plunge and provide lateral closure. Lateral changes i n subsurface fault geometry, which include lateral and oblique ramps, are also reflected by terminations, bends, or offsets of axial surface s in map view. Based on these relationships between folding and faulti ng, we develop map-view interpretations and balanced cross sections ac ross structures that are consistent with the geology along trend.