Elsevier

Tectonophysics

Volumes 576–577, 5 November 2012, Pages 78-85
Tectonophysics

Extensional fault-propagation folding in mechanically layered rocks: The case against the frictional drag mechanism

https://doi.org/10.1016/j.tecto.2012.05.023Get rights and content

Abstract

“Fault drag” (deflection of beds or other markers into folds that are convex in the direction of relative slip) is often interpreted as the product of frictional sliding along a fault and progressive tilting of beds with increased amount of displacement along a fault. We analyze two sets of normal faults, with throws ranging from 0.5 m to 5 m, and associated fault-related folds in mechanically layered upper Cretaceous carbonate, anhydrite, and shale in central Texas. For each fault set, we interpret the fault displacement and fault-related folds exposed in outcrop to represent different stages in the developmental sequence. In both fault sets, faults in dolostone or limestone lose displacement and tip into less competent anhydrite or shale where deformation is accommodated by folding and smaller scale deformation. Fold wavelength is established early and at small displacement (< 1 m throw). With increasing displacement the monoclinal fold limb steepens and is extended parallel to bedding, locally producing boudinage in the most competent bed between incompetent beds. Clay smear is well developed where a 0.35 m thick clay shale is locally thinned to 0.1 m associated with fault throws of 0.5 to 5 m. Bed tilting and the development of apparent drag is not the product of frictional sliding but instead folding at the tip of an arrested, in this case upwardly, propagating normal fault. We conclude that synthetic dip associated with steep normal faults (i.e., fault drag) should not be assumed to be the product of frictional drag, but must be considered in the context of the mechanical stratigraphy. Instead, fault-tip folding in mechanically layered rocks produces synthetic dip (drag) early in the fault development history prior to propagation of the fault tip through the folded layer.

Highlights

► “Fault drag” is often interpreted as the product of frictional sliding. ► Apparent “drag” was investigated along normal faults in central Texas. ► Faults in competent beds lose displacement and tip into incompetent beds. ► Displacement gradient produces fault tip monocline and shale smear. ► Apparent “drag” structure not the product of frictional sliding.

Introduction

“Fault drag”—defined as deflection of beds or other markers adjacent to a fault into folds that are convex in the direction of relative slip—is often interpreted as the product of frictional sliding along a fault and progressive tilting of beds with increased amount of sliding along a fault (e.g., Billings, 1972) (Fig. 1A). Davis (1984) described that “Strata close to the fault surface are deformed by frictional drag into folds that are convex in the direction of relative slip. The truncated ends of dragged layers point away from the sense of actual relative movement.” In the case of steep normal faults cutting horizontal or gently dipping strata, this deformation is generally represented by synthetic dip of layers (i.e., layers dipping in the same direction as the related fault). Although the concept of “fault drag” is well established in geology, a growing body of evidence suggests that frictional drag may not be the cause. Numerical modeling studies have concluded that fault friction is an untenable explanation for most “fault drag” structures (Grasemann et al., 2005, Reches and Eidelman, 1995) and that the drag most likely represents pre-faulting deformation (Fig. 1B). Deformation at and beyond the fault tip during fault propagation—extensional fault propagation folding—has been demonstrated as the source of normal-fault related monoclines and breached monoclines in clay models (Jin and Groshong, 2006, Withjack et al., 1990) and in experimentally generated faults in rock (Patton et al., 1998). Field investigations have shown that normal-fault related monoclines and breached monoclines are often produced at fault tips (e.g., Ferrill and Morris, 2008, Ferrill et al., 2005, Ferrill et al., 2007, Ferrill et al., 2011, Gawthorpe et al., 1997, Grant and Kattenhorn, 2004, Hardy and McClay, 1999, Janecke et al., 1998, Schlische, 1995, White and Crider, 2006), and numerical modeling has replicated this behavior (e.g., Patton and Fletcher, 1995, Smart et al., 2009, Smart et al., 2010).

In this paper, we investigate two sets of small-displacement faults in mechanically layered rocks to understand the relationship between normal faulting and related folding. We find that monocline formation is directly related to faulting in a mechanically layered rock sequence, influenced by mechanical layer thickness and mechanical contrasts between layers. The inability of a fault to propagate across a mechanically weak layer during continued fault slip leads to a large displacement gradient at the fault tip. Folding and related localized deformation occurs beyond the fault tip to accommodate the displacement gradient. Ductile deformation in the shale results in local thickening and thinning to fill in around competent beds experiencing brittle extensional deformation and in some cases boudinage. Collectively this deformation would give the appearance of “drag” along a fault that broke through the monocline, although the origin is not the product of friction on the fault.

Section snippets

Geological background

The Balcones fault system is an extensional fault system along the Balcones escarpment that defines the margin between the Edwards Plateau and the Gulf Coastal Plains province, and marks the surface expression of the limit of normal faulting along the northwestern margin of the Gulf of Mexico Basin (Ferrill and Morris, 2008) (Fig. 2). The Balcones fault system extends from Del Rio, Texas in the west, to near Dallas, Texas in the north, changing trend by approximately 80° through the central

Faults in Beckmann Quarry

Beckmann Quarry is an active quarry in northwest San Antonio where limestone is quarried and crushed for use as aggregate. The quarry is in the Kainer Formation (part of Edwards Group) within the recharge zone for the Edwards Aquifer, which provides the main water supply for San Antonio and the surrounding region. The Kainer Formation consists of generally clay-poor limestone, dolomitic limestone, and dolostone, with rare argillaceous and evaporite beds deposited in shallow-marine subtidal to

Faults in Green Mountain Road exposure

In northeast San Antonio, a roadcut exposure along Green Mountain Road (Fig. 6) reveals small-displacement normal faults cutting the contact between the Austin Chalk and the Pecan Gap Formation (part of the lower Taylor Group) (Ewing, 2010). The Austin Chalk at the exposure is represented by massive micritic limestone, and the Pecan Gap Formation is represented by clay-rich shale, and both clay-poor and clay-rich limestone (marl) (Ewing, 2010).

Fault propagation folding process

In the outcrop-scale normal fault examples analyzed in this paper, it is clear that mechanical stratigraphy strongly influences the fault propagation process. Fault propagation is facilitated by competent (high rebound, high Young's modulus) beds, and inhibited by incompetent clay-rich or anhydrite-rich beds. This is consistent with experimental tests that have demonstrated that shale and anhydrite can accommodate significantly greater prefailure strain than limestone and dolomite (see

Conclusions

The relative rate of fault propagation tends to be high in competent beds (e.g., limestone, dolomitic limestone) that do not accommodate significant prefailure strain, and low in incompetent beds (e.g., clay-rich shale or marl, anhydrite) where significant prefailure strain is accommodated. Consequently incompetent beds inhibit fault propagation. If displacement continues on a fault after its tip is arrested in an incompetent bed, then fault tip folding may occur, especially if the rocks beyond

Acknowledgments

We thank Michelle Lee and Martin Marietta Materials for allowing us access to Beckmann Quarry in November and December 2001. We thank Danielle Wyrick and Gary Walter for their reviews of the manuscript. We thank Tectonophysics reviewers Bernhard Grasemann and Carlos Liesa, Guest Editor Olivier Lacombe, and Editor Fabrizio Storti for their reviews and recommendations that led to further improvements to the manuscript.

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