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The foreland region of the American Rocky Mountains was deformed during the Laramide Orogeny to produce a number of basement uplifts and sedimentary basins. One well-studied example is the Wind River Mountains and Wind River basin of southwestern Wyoming. The uplifts and basins are thought to result from the tilting of large, fault-bounded blocks, but there is a disagreement as to whether the tilting is driven by vertical forces or horizontal compressive forces. Little work has been done to quantitatively evaluate either of the proposed driving mechanisms. In this study a mathematical model is used to investigate block tilting under horizontal compressive forces.

A two-dimensional elastic plate, representing the upper brittle portion of the lithosphere, is cut by two planar faults. Fixed horizontal displacements applied to the ends of the plate cause tilting of the central block and flexure of the lithosphere. Buoyancy effects of the underlying ductile lithosphere and the loading effects of sedimentation and erosion are included. The finite element method is used to obtain the displacements and stresses of the elastic plate.

Conclusions from the models are:

1. Horizontal forces of reasonable magnitude (from a plate tectonic viewpoint) can produce the observed basins and uplifts.
2. Fault dips do not significantly affect the geometry of basins and uplifts, but do affect the required driving stresses.
3. It is unlikely that all faults in the region are high-angle, but a combination of low angle thrusts dipping 30^o-40^o and high angle reverse faults >45^o is possible.
4. Gravity anomalies suggest that the Moho is not presently offset by faults.
5. Minor thrust faults and thrust splays near the major thrusts could have resulted directly from tilting and flexure of the lithospheric blocks.
6. Stresses resulting from movement on faults which do not break the surface may cause the faults to propagate to the surface with different dips than the original faults.

Keywords:
Pages: 266
Supervisor: Chris Beaumont