Kinematic and dynamic considerations in the forced folding process as studied in the laboratory (experimental models) and in the field (Rattlesnake Mountain, Wyoming)

Typescript (photocopy).The concept of forced folding (Stearns, 1971,1978) forms the basis for the research reported here on Rattlesnake Mountain Anticline. Field observations address fold formation with respect to: causative basement blocks, the role of translations, and the relationship of geometric details to underlying basement configuration. A newly recognized thrust feature transports rocks from the gentle limb of the fold across the steep limb late in the deformation history. Physical models using rock layers and a steel forcing assembly add significantly to the research. A new sequence of fold formation is proposed based largely on the models. Analysis of displacement markers in the models reveals that translation occurs in this system without external "pushing." The issue of line-length balance is clarified by the models in that the assumption or' local balance often used by others for forced-fold arguments is shown to be unnecessary. Based upon both field and laboratory examples, I propose that translation and the late thrust are driven by pressure gradients which are themselves a product of the folding. Many bedding -plane slip horizons are present in the field. Deformation fabrics in the field and in the models suggest that these slip surfaces both distribute strains and limit their magnitudes. The folding process at Rattlesnake Mountain can be described as the relative motion of material response elements of low internal strain which are defined by bedding-plane slip horizons and surfaces which are called fold-segment boundaries. Stearns' (1971, 1978) model appears essentially correct, although this research has added details which improve our understanding of the kinematic and dynamic processes associated with the folding