Fold style inversion: Placing probabilistic constraints on the predicted shape of blind thrust faults

We develop a new methodology which compares quantitatively styles of foldin g from seismic reflection data. The goal of the "fold style inversion" (FSI ) method is to provide an objective choice of the most appropriate model us ed when solving for the shape of an unimaged blind fault from folded layer geometry. FSI is a discretization of the dip isogon fold classification sch eme reformulated as simple vector transformations. A data set's goodness of fit to parallel (class 1c) or similar (class 2) fold geometry is assessed by calculating misfit between the predicted and observed bed geometries thr ough a grid search of the parameter space specific to each transformation; the two fold types correspond to the constant bed length and arbitrarily in clined simple shear (AISS) fault solution routines, respectively. For seism ic reflection data, confidence estimates may be placed on the preference of fold style and its corresponding fault solution by Monte Carlo simulations of depth correlative, spatially limited depth conversion errors. For synth etic geometric examples FSI determines fold style preference and parameters exactly. At low fold limb dips (<similar to 15 degrees) the actual geometr ic difference between parallel and similar folds is very small, and the dif ference between fold styles cannot be resolved, highlighting a general diff iculty in the analyses of young blind thrust structures. For a synthetic se ismic line of an AISS fault-related fold the method chooses the correct fol ding style and leads to the correct fault geometry at depth. In examples of real data from the Barrancas/Lunlunta-Carrizal anticlinal complex in Mendo za, Argentina, FSI analysis determines 71% and 54% probability of similar p reference for two seismic lines on separate structures. The corresponding f ault solutions for the first example are well constrained, whereas for the second example the solutions are widely variant. This analysis helps to qua ntify the relationship between the predicted sub surface fault trajectories and hypocenter and aftershock data of the 1985 M(w)5.9 Mendoza earthquake, showing that the earthquake and the fault causing the Barrancas/Lunlunta-C arrizal anticlinorium are most likely unrelated.