129-1 Trishear Kinematic Modeling of the San Rafael Monocline, Utah: Reconnaissance Investigation of Contractional Fault-propagation Fold Evolution

Abstract:

Predicting the evolution of contractional fault-propagation folds has implications for the movement and storage of fluids in the subsurface, including hydrocarbon resources, ore mineralization, and for implementation of carbon capture and storage. Past studies have successfully used trishear kinematic numerical modeling to replicate the generation of fold geometries established in the field, demonstrating how underlying fault networks control the deformation of layers in front of the fault system.

We investigated two localities within the steepest east-dipping section of the Laramide-age San Rafael monocline in central Utah. There, canyons have dissected the fold, providing excellent exposure across the entire system, permitting us to document axis-perpendicular changes in dip at high-resolution. Previous researchers hypothesized that the fold formed in front of a propagating, steeply west dipping basement fault system, but those studies did not include detailed field-based structural data to constrain finite fold geometries. In this study, we used imagery from an unmanned aerial vehicle (UAV) and field-based structural measurements to document changes in fold geometry and dip through the two canyons. The UAV imagery was used to construct 3D virtual outcrop models of rock exposures based on structure-from-motion algorithms. We then used these models and field data to constrain cross-section construction.

Although cross sections for both canyons display similarly steep eastward dips in the forelimb of the fold, their cross sections display notable geometric differences, especially in the transition from the forelimb of the monocline to sub-horizontal bedding to the east. We used trishear forward modeling (FoldFault7) to replicate the real-world fold system and explain geometric differences. To do so, we tested variations in fault geometry, fault slip, propagation-to-slip ratio, number of faults, and trishear angle. Our initial results suggest that complexity in the underlying fault system has generated different curvatures in the forelimb transition, with a secondary fault splay a possible driving mechanism for geometric differences.

These kinematic forward models can be used to explain fold-axis-parallel changes in monocline dip to the north and south, where dip decreases to low (<20 degree) angles, likely related to lower displacement(s) on the underlying fault system. Our future research will focus on developing testable, high-resolution, field-based cross-sections along the fold, providing us a richer data set for constraining fold evolution.

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