The Sonoma Range lies at the western edge of the Winnemucca fold and thrust belt of north-central Nevada in which lower Paleozoic rocks are thrust westward over para-autochthonous Triassic shelf rocks that overlie Mesozoic autochthonous lower Paleozoic rocks. Evidence from this range indicates that the Winnemucca thrust sequence developed in overstep, rather than piggyback, fashion.

This assertion is based on fabric elements of the Triassic rocks and on the assumption that the style and attitude of a given fold reflect the relative proximity of thrusts at the time of formation of said fold. The data may be summarized as follows: (1) four generations of Winnemucca-age folds are recognized; all are west verging and show the same sense of asymmetry; (2) in succeeding generations, the apical angle of folds increases and axial planes change from nearly horizontal to nearly vertical; (3) also in succeeding generations, deformation becomes more penetrative, and shortening and hinge thickening decrease. If the Winnemucca thrust system were to have developed in piggyback manner, one would expect subsequent deformations to progress from open to tight folds, upright to recu bent folds, and little to much shortening, and to remain relatively uniform in degrees of penetrativeness and thickening in the hinge.

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