A Wrench Fault/Thrust-Fold Clay Model Study and Comparison with the Bonanza-Zeisman Area of the Eastern Big Horn Basin

The kinernatics and geometry of some of the basement-involved structures observed in the central Rocky Mountain foreland province have been investigated by means of a series of simple clay models. In these models, a relatively rigid Precambrian crystalline basement is simulated with plasticine oil-base clay (OBC) and the overlying, relatively ductile sedimentary section IS simulated with colored layers of a salt, cornstarch, and water mixture (SCW). In the model described here, a vertical cut and two perpendicular cuts dipping 30" were made in a block of OBC and lubricated. Horizontal layers of uncut SCW were laid over the cut block of OBC Horizontal compression was applied parallel to the vertical cut so that shortening along one side was greater than along the other side. Shortening was manifest in the differential development of perpendicular thrust-folds which accompanied left-lateral s11p along the buried vertical fault plane. In the overlying, unfaulted SCW layers, a plunge was created on both fold trends, and the anticlinal hinges above OBC thrust locations were deflected sinistrally across the buried wrench fault zone. There are some basic similantries, but also important differences, in the geometry of the deformed clay model and the structure observed in the Bonanza-Zeisman area of the eastern Big Horn basin of Wyoming. A significant component of left slip along a postulated northeast-trending wrench fault zone buried in the basement of the Bonanza-Zeisinan area 1s inferred from the northeastern alignment of sinistral deflections and abrupt terminations of thrust-fold structures which trend perpendicular to the zone Although the numbers of thrust-fold structures developed along either side of the postulated Bonanza-Zeisman wrench fault zone are not equal as in the clay model, the total horizontal shortening measured along either side of the fault line approximately balances. The inferred left-lateral slip on the postulated wrench fault zone 1s thought to be the result of Laramide reactivation of a pre-existing Pre-Cambrian fracture or shear zone