EVALUATING UNCERTAINTY IN BALANCED CROSS-SECTIONS: A CRITICAL STEP FOR RELATING THRUST-BELTS TO PLATEAU UPLIFT
Balanced cross sections are essentially forecasts of subsurface geometry and should be treated as models which are best fit to the available data. As such, the uncertainty of the fit is of critical importance. For a limited class of fold-fault kinematic models, the confidence limits on the estimated model parameters (i.e., those which produce the shortening estimate) can be obtained by Monte Carlo simulation, as demonstrated by the trishear model of individual structures. For cross sections of entire regions or orogens, such rigorous treatment is not yet possible. Nonetheless, computer-aided section construction now makes it possible for an individual structural geologist to explore qualitatively the complete range of viable models which fit the available data and their attendant uncertainties. This should be regarded as a necessary step before when evaluating the contribution of horizontal shortening to plateau uplift. For whole orogen crustal-scale balanced sections, the uncertainties may be so great as to render the entire exercise meaningless. Where rates are important, as in the comparison of thrust shortening to GPS velocities, uncertainties in timing and duration must also be taken into account. In the few well documented cases in the Central Andes, uncertainty in duration of deformation can be as much as a factor or 4 between minimum and maximum estimates whereas uncertainty in shortening magnitude is seldom more than a factor of 2.