George Davis | Trishear Development of Colorado Plateau Monoclines

Forward and Inverse Trishear Modeling of the San Rafael and Waterpocket Monoclines, Colorado Plateau, Utah

Basement-cored uplifts of the Laramide orogen are bounded by crustal-scale reverse faults.  In Wyoming and Colorado, extensive petroleum exploration has revealed much of the geometry of these faults.  By contrast, the subsurface geometry of Colorado Plateau uplifts is relatively unknown.  We present 1) results of inverse and forward modeling of the San Rafael and Waterpocket monoclines using the Trishear algorithm; and 2) a comparison of the strains observed in the field with t hose predicted by the Trishear model.

Inverse modeling search using dip and contact data from a horizontal transect across the fold and regional stratigraphic depths indicates that both folds are underlain by low-angle faults.  Forward modeling offers the opportunity to improve on inverse models by varying the propagation to slip ratio (p/s) during folding in accordance with the mechanical stratigraphy cut by the fault tip.  For the Waterpocket monocline, our best fit is:  ramp angle 30?; trishear angle 100?; initial fault tip 2.3km below top basement.  The fault propagates to the basement-cover contact with a p/s ratio of 6.0 at which point the p/s ratio is dropped to 2.1 for the remainder of the total 3.5km fault displacement.   For the San Rafael monocline the best fit is:  initial fault tip 0.6km below top basement; trishear angle 100?; ramp angle 30?; p/s ratio 4.0.  The fault propagates to the basement-cover contact, at which point the p/s ratio is dropped to 2.1 until the base of Paradox salt.  The p/s ratio is reduced again to 1.4 until the top of the Paradox where p/s is finally increased to 1.9 for the remainder of the 2.8km of total slip.  The fact that both faults appear to have begun their Laramide growth well below the basement-cover contact suggests that they were reactivated only in their lower portions and formed new paths through the uppermost basement.  This resembles the footwall shortcuts commonly found in inverted extensional systems and may support the idea that many Laramide faults are reactivated Precambrian normal faults.

Strain predicted by the Trishear model is compared with field measurements based on mapping of deformation band arrays exposed in canyon walls.  Measured strains are uniformly less than 1.10 whereas predicted strains are generally 1.50-1.80.  We find that much of the difference is attributable to flexural slip between sedimentary units, which is not allowed by the Trishear model.

          [Bump, A., and Davis, G.H., 2002, Forward and inverse trishear modeling of the
San Rafael and Waterpocket monoclines, Colorado Plateau, Utah:  Geological
Society of America Abstracts with Programs, v. 34, no. 6, p. 40.]