Normal-Fault-Propagation Folding in Mechanically Layered Carbonates and Shale: The Case Against “Fault Drag”
“Fault drag” is often interpreted as the product of frictional sliding along a fault and progressive tilting of beds with increased amount of sliding along the fault. We analyze a set of normal faults with displacement (throw) ranging from 0.5 m to 5 m, and the associated fault-related folds in mechanically layered upper Cretaceous carbonate and shale in central Texas. We interpret the set of fault-related folds exposed in outcrop to represent different stages in the deformation sequence. Fold wavelength is established early and at small displacement (<0.5 m throw). With increasing displacement, up to 1 m throw, the monoclinal fold limb steepens, and a half-meter thick competent bed within the fold accommodates bed-parallel extension by extension-fracture dilation and slip along bed-bound extensional faults within the fold limb. Further fault displacement, up to 5 m throw, causes the fold limb to continue stretching and produces boudinage in the most competent bed. Although outer arc extension fracturing is observed in fold hinges within the competent bed, layer extension is present throughout the fold limb and is concentrated in areas of steep dip rather than being restricted to high-curvature regions in fold hinges. Clay smear is well developed and a half-meter thick clay-rich shale is locally thinned to 0.1 m associated with fault throws of 0.5 to 5 m. We conclude that bed tilting and the development of apparent drag is not the product of frictional sliding but instead folding at the tip of an upwardly propagating normal fault. Field evidence shows that fault tip folding in mechanically layered rocks is accommodated by synthetic dip associated with normal faults and is initiated early in the fault development history. Synthetic dip associated with normal faults should not be assumed to be the product of frictional drag, but must be considered in the context of the mechanical stratigraphy.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California