THREE-DIMENSIONAL LARAMIDE DEFORMATION OF THE COLORADO PLATEAU: COMPETING INFLUENCES OF THE SEVIER THRUST BELT AND THE FLAT FARALLON SLAB

Kinematic analysis of six Laramide basement-cored uplifts on the northern Colorado Plateau indicate that four of the uplifts (the Kaibab, Circle Cliffs, Miners Mountain, and Uncompahgre uplifts) were constructed by northeast-directed shortening while the remaining two (that San Rafael Swell and Monument Uplift) were constructed by southeast-directed shortening. Available timing data suggest that all of the uplifts rose at approximately the same time (~72-65 Ma; Lawton, 1983; Goldstrand, 1994), precluding the possibility that they formed sequentially in response to a progressive rotation of regional stresses.

It is proposed here that the opposed simultaneous shortening directions are a predictable result of deformation in a three-dimensional (3D) stress field (i.e. s₁ does not equal s₂ does not equal s₃). Slip vectors in a 3D stress field depend on the orientation of the fault relative to the principal stress axes and are strongly influenced by the magnitude of s₂ relative to the other two principal stresses. Angelier stereonets indicate that all of the interpreted slip vectors are compatible with a northeast-directed s₁ and a southeast-directed s₂ whose magnitude approaches that of s₁. This is corroborated by computer-based forward modeling of the observed faults and slip vectors using an elastic rheology. The best-fit stress state [(s₂-s₃)=0.83(s₁-s₂)] yields modeled fault slip vectors that deviate by less than 5° from the field observations and modeled fault displacement profiles that closely resemble those observed.

Geologically, s₁ is thought to be the result of coupling between the horizontally-subducting Farallon slab and the North American plate. The elevated values of s₂ are attributed to far-field stresses generated by the topographically high Sevier thrust belt which wraps around the western edge of the Colorado Plateau and was coeval with deformation in the Plateau. s₃ is a vertical gravitational stress. It is therefore proposed that Laramide deformation of the Colorado Plateau was driven by both basal traction related to the flat slab and by marginally-applied stress related to the Sevier thrust belt.