--> The Influence of Mechanical Stratigraphy on Thrust-Ramp Nucleation and Thrust Fault Propogation: Outcrop Data, Cross Section Reconstructions, and Finite Element Models of Thrust Structures in Utah

AAPG ACE 2018

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The Influence of Mechanical Stratigraphy on Thrust-Ramp Nucleation and Thrust Fault Propogation: Outcrop Data, Cross Section Reconstructions, and Finite Element Models of Thrust Structures in Utah

Abstract

We examine an alternate model of thrust fault nucleation that takes into account vertical variation in the mechanical strength of sedimentary rocks. This model maintains that thrust ramps may nucleate in structurally strong units then propagate upward and downward into weaker units, often creating hanging wall anticlines and footwall synclines. Evidence for the ramp-first style of faulting is demonstrated in small-scale outcrop data, but there is a crucial need to study large-scale structures and the basic mechanics of the problem to determine if the faulting style scales up. We hypothesize that the mechanical stratigraphy of faulted rocks may create a significant stress heterogeneity within the system and exert a first-order control on these factors.

We present two examples of this faulting style: the Ketobe Knob Thrust and the Providence Canyon Thrust, which are both thrust faults with associated hanging wall anticlines and footwall synclines. The Ketobe Knob thrust, Central Utah, cuts the earthy facies of the Entrada Sandstone, which is a mechanically layered sedimentary package of fine grained sandstone and siltstone. The Providence Canyon Thrust, in northern Utah, offsets the Monroe Canyon Limestone. We use these structures to obtain a suite of outcrop data including bedding and fault orientation data, stratigraphic descriptions, and mechanical strength of the faulted units and input this field data into 2D and 3D reconstructions of the thrust in Midland Valley’s Move and models in finite element modeling program ABAQUS in which we alter rheology, geometry, and spacing of thrust ramps.

Seismic interpretation strategies are strongly influenced by kinematic fault models. Our ability to accurately project structures at depth, predict the location of smaller faults, and determine fault timing depends on how well we understand the behavior of faults. Quantifying the effects of mechanical stratigraphic changes on the location of the failure, fault geometry, stress heterogeneity, and fault propagation direction could prompt the reconsideration of interpreted structures in cross sections and seismic data.