INCORPORATING STRAIN AND MICROSTRUCTURAL DATA INTO BALANCED CROSS-SECTIONS: IMPLICATIONS FOR VIABILITY OF RESTORED SECTIONS

Balanced cross-sections must satisfy the conditions of admissibility and viability. For most fold-thrust belt (FTB) settings admissible cross-sections are routinely drawn using known field relations and standard geometric rules. Geometric viability is generally achieved by following standard rules for constructing line-length balanced and area-balanced restorations that satisfy standard configurations of pre-deformational basin geometry. Such two-step balancing is prone to errors, particularly in the internal portions of FTBs, where penetrative strains are significant and deformation history is complex. While an ideal restoration (under these settings) would involve undoing the total displacement field along a known deformation path, such an approach may not be practicable. However, geometric viability of cross-sections can be significantly improved by incorporating detailed strain data, measured over a large region, into the restorations. But, even such restorations are not kinematically viable unless appropriate strain increments are removed at the proper stages of step-wise retrodeformation, and the restoration satisfies geometric viability at every stage. Detailed field observations and microstructural overprinting relationships provide useful tools for deciphering successive phases of deformation so that a reasonable retrodeformation path can be established. Successful retrodeformation along such a path adds significantly to the kinematic viability of a restoration. The interpreted deformation path together with microstructural data can also be used to constrain large-scale tectonic evolution, and tested against regional structural, sedimentological and geophysical data. For 3-D restorations that involve significant vertical axis rotations microstructural relationships and incremental strain data may provide the only tools available for step-wise retrodeformation along curved paths. Examples from the internal portions of the Sevier FTB illustrate the use of these techniques and suggest that they may be useful in interpreting the tectonic evolution of FTBs in other orogens.