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3-D Structural Modelling of the East Kaibab Monocline in Northern Arizona


The East Kaibab monocline forms the eastern margin of the ca 200 km-long Kaibab Uplift, extending from northern Arizona into southern Utah. The structure is one of a series of folds attributed to the reactivation of steeply dipping basement faults during Laramide compression across the Colorado Plateau. Whilst the geometry and timing of these structures are well constrained, the kinematics associated with the formation of the East Kaibab monocline has continued to be the focus of debate. On the one hand, slip-data suggests that the monocline developed due to dextral-reverse oblique reactivation of an underlying fault, consistent with broadly ENE-oriented Late Cretaceous/early Cenozoic shortening. However, on the other hand, the orientation of conjugate deformation bands associated with the formation of the fold has been presented as evidence of purely dip-slip, reverse reactivation of the fault at depth. Although contradictory, both scenarios have been supported by 2D and 3D kinematic modelling. Availability of exposure has focussed almost all recent analysis of the monocline on a ca 50 km, NNE-SSW orientated stretch of the structure in southern Utah. To the NW and the SE, however, the fold trends WNW-ESE and in northern Arizona bifurcates into the Upper and Lower East Kaibab Monocline. Whilst field exposure in the latter area is poor, the structural geometry is exceptionally well defined by the modern day topography. Using a reconstructed digital elevation model (DEM) as a template surface, an initial static 3D model has been created in Midland Valley's Move” software. This model allows the geometry of the monocline to be analysed and provides a basis to test formation scenarios generated with kinematic algorithms; chiefly 3D trishear forward models. Furthermore, by considering slip tendency it can be demonstrated that under an ENE-WSW orientated maximum horizontal stress (σ1) the monocline in northern Arizona is likely to have a much greater dip-slip component than in southern Utah.