Fault displacement-distance AUTHORS Amanda N. Hughes Department of Earth relationships as indicators of and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts contractional fault-related 02138; [email protected] Amanda N. Hughes is a structural geologist for folding style the Chevron Energy Technology Company in Houston, Texas. She received a Ph.D. in earth Amanda N. Hughes and John H. Shaw and planetary sciences from Harvard University (2012) and a B.S. degree in geology from Washington and Lee University (2006). Her research combines geological observations, ABSTRACT seismic reflection interpretation, and kinematic and mechanical modeling approaches in un- We present a method of using fault displacement-distance derstanding structural growth in the context of profiles to distinguish fault-bend, shear fault-bend, and fault- petroleum systems and seismic hazards studies. propagation folds, and use these insights to guide balanced John H. Shaw Department of Earth and and retrodeformable interpretations of these structures. We Planetary Sciences, Harvard University, 20 Ox- first describe the displacement profiles associated with different ford Street, Cambridge, Massachusetts 02138; end-member fault-related folding models, then provide exam- [email protected] ples of structures that are consistent with these model-based John H. Shaw is the Harry C. Dudley Professor of predictions. Natural examples are imaged in high-resolution Structural and Economic Geology and chair of two- and three dimensional seismic reflection data sets from the the Department of Earth and Planetary Sciences Niger Delta, Sichuan Basin, Sierras Pampeanas, and Cascadia at Harvard University. Shaw received his Ph.D. to record variations in displacement with distance updip along from Princeton University (advisor: John Suppe), and subsequently worked for Texaco’s Explora- faults (termed displacement-distance profiles). Fault-bend folds tion and Production Technology Department in exhibitconstantdisplacementalongfaultsegmentsandchanges Houston, Texas. Shaw’s research focuses on the in displacement associated with bends in faults, shear fault- nature of faulting and fault-related folding in the bend folds demonstrate an increase in displacement through crust, with applications to petroleum trap and the shearing interval, and fault-propagation folds exhibit de- reservoir characterization and regional earth- creasing displacement toward the fault tip. More complex quake hazards assessment. Shaw leads an AAPG field course in the Canadian Rockies, and offers structures are then investigated using this method, demonstrat- a short course on seismic interpretation ing that displacement-distance profiles can be used to provide methods based on an AAPG Seismic Atlas insight into structures that involve multiple fault-related fold- (AAPG Studies in Geology 53, coedited by ing processes or have changed kinematic behavior over time. C. Connors and J. Suppe). These interpretations are supported by comparison with the kinematics inferred from the geometry of growth strata overly- ACKNOWLEDGEMENTS ing these structures. Collectively, these analyses illustrate that the displacement-distance approach can provide valuable in- We thank CGGVeritas for their provision of data and support of the project, which was instru- sights into the styles of fault-related folding. mental. We also thank ExxonMobil and Chev- ron, which supported this research. We are grateful for the insightful comments and sugges- tions of our reviewers, M. Scott Wilkerson and Brent A. Couzens-Schultz, and editors Stephen E. Laubach and Rick Groshong. We are also in- Copyright ©2014. The American Association of Petroleum Geologists. All rights reserved. debted to Landmark Graphics Corporation, which Manuscript received January 9, 2012; provisional acceptance March 13, 2012; revised manuscript received provided software that was critical to this re- June 15, 2012; final acceptance May 31, 2013. search through their Strategic University Alliance DOI:10.1306/05311312006 AAPG Bulletin, v. 98, no. 2 (February 2014), pp. 227–251 227 Grant Program (Agreement 2007-CONT-005191). INTRODUCTION The AAPG Editor thanks Senior Associate Editor Richard H. Groshong and the following reviewers The ability to classify the structural style of a fault-related fold for their work on this paper: Brent A. Couzens- Schultz and M. Scott Wilkerson. is essential to many different applications. Understanding the kinematic history of fault-related folds can provide important constraints on the geometry and evolution of traps in petro- DATASHARE 51 leum geology. Similarly, various fault-related fold models make Uninterpreted seismic data are available in an different predictions about rock strains that may affect reser- electronic version on the AAPG Web site (www voir properties. Moreover, properly characterizing fault-related .aapg.org/datashare) as Datashare 51. folds can also be an important aspect of seismic hazard assess- ment. Various fault-related folding models predict character- istic relationships between uplift and displacement on the underlying fault, so the ability to identify the structural style of active fault-related folds is essential to properly defining the slip on an active fault based on an observed pattern of uplift. This is especially important in cases where faults do not reach the surface and only fold patterns are observable at the surface. Over the past two decades, kinematic models for several different types of contractional fault-related folds have been developed and successfully applied to describe a variety of natural structures (e.g., Suppe, 1983; Suppe and Medwedeff, 1990; Erslev, 1991; Allmendinger, 1998; Suppe, et al., 2004; Shaw et al., 2005). With this proliferation of fault-related fold models, it is sometimes challenging to properly identify which class of models most appropriately applies to a given structure. Additionally, many natural structures are sufficiently complex, or have exhibited different fault-related folding mechanisms over their history, that they are not well explained by a single model. Thus, it is desirable to have an independent means of discerning the style of fault-related folding present in the structure. One way of achieving this is by observing displace- ment as a function of distance along the fault (termed the displacement-distance profile) because each of the major types of contractional fault-related fold models—fault bend, shear fault bend, and fault propagation—has a distinctive pattern in its displacement-distance relationship (Figure 1). This study will first illustrate the displacement-distance profiles expected for end-member fault-related folding mod- els. These predictions will then be compared with a series of structures imaged with seismic reflection data to illustrate that the predicted displacement profile for each of these models is consistent with patterns of displacement observed in nat- ural structures. By establishing that the displacement profiles characteristic of each folding model are unique and appli- cable to natural examples, we are then able to interpret the displacement-distance profile for a complex, multistage natural structure. We restore parts of its deformation history that are 228 Displacement Variations and Folding Styles Figure 1. Illustration of how to construct a displacement-distance profile. (A) Example structure. Distance is measured from the inter- section of the lowest offset rock layer in the footwall and the fault (P) along the fault toward the tip (d), and displacement is measured (a, b, and c) for each layer. (B) The resulting displacement-distance plot, where distance along the fault from P to d is plotted along the x-axis, and the displacement of each layer that intersects the fault at that distance determines the y value of each point. consistent with the different fault-related-folding bends. Other researchers have sought to charac- mechanisms indicated by the observed displace- terize displacement profiles for natural structures. ment profile. Analysis of the geometry of overlying McConnell et al. (1997) made observations of growth strata further supports these interpreta- displacement-distance profiles in field outcrops tions, suggesting that analyzing displacement pro- in the Appalachians, whereas Briggs et al. (2006) files may be used as an important independent measured variations in displacement on structures means of identifying which fault-related-folding in the Niger Delta. Additionally, a significant body mechanisms may have been active throughout the of previous work on along-strike variations in dis- deformation history of a given structure. placement on thrust faults (for example, Wilkerson, 1992; Wilkerson et al., 2002; Bergen and Shaw, 2010) has provided insight into the lateral propa- PREVIOUS WORK ON gation, linkage, and termination of thrust faults. DISPLACEMENT-DISTANCE PROFILES The goal of our study is to develop a com- prehensive understanding of the patterns of dis- A displacement-distance profile is generated by placement along faults in the various classes of measuring the offset between the footwall and fault-relatedfolds,inamannerthatcaninformin- hanging-wall cutoffs in a cross section for several terpretations of natural structures. We will extend rock units in a fault-related fold (the displacement), the analysis of displacements of end-member fault- and plotting it as a function of distance along the propagation folds from previous studies (Hedlund,