Trishear kinematic modeling of structures, with examples from the Neuquén Basin, Argentina

Abstract

We have expanded previous trishear fault-propagation-fold forward models by allowing additional structural complexity in the form of multiple ramps and flats, variable propagation-to-slip ratio (P/S) and trishear angle, as well as multiple faults in a single section. The resulting forward models simulate characteristics of real structures very well. Our six-parameter grid search overcomes a longstanding obstacle to the application of trishear by providing a scientifically objective way of choosing the correct parameters to apply to real structures. By grid searching forward models with known initial parameters, we investigate the sensitivity of trishear to various parameters. These experiments highlight the importance of P/S in determining fold shape: A change in P/S of just 0.3 produces a change in fold shape that is equivalent to that produced by a 15-20° change in trishear angle. Application of the trishear model to contractional and extensional structures of the Neuquén Basin highlights its utility for predicting (1) strain and strain path, (2) fracture orientation and distribution, (3) fault-slip magnitude, and (4) fault nucleation point/dêcollement depth. Our study of large basement-cored producing anticlines such as Filo Morado-Pampa Tril emphasizes an important point: Trishear and parallel kink-fold geometries can be compatible when applied at different scales. Trishear provides a bulk description of the deforming zone on the thickened, triangular eastern flank of the fold but makes no explicit prediction about how the strain is accommodated. In these structures, the strain is variably accommodated by tight folding, duplexing, and flow of evaporites and depends significantly on the thickness of the "competent" units in the section.