Insights into fold growth using fold-related joint patterns and mechanical stratigraphy Heather M. Savage a, * , J. Ryan Shackleton b , Michele L. Cooke b , Jeffrey J. Riedel c a Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA b University of Massachusetts, Amherst, 611 North Pleasant St., Amherst, MA 01003 9297, USA c 2907 Secor Avenue, Bozeman, MT 59715, USA article info Article history: Received 5 April 2010 Received in revised form 30 August 2010 Accepted 10 September 2010 Available online 17 September 2010 Keywords: Sheep Mountain Anticline Joint pattern Mechanical stratigraphy Plate bending abstract Despite how common folds are as structural features, along-strike fold propagation has proven elusive to document. However, if a fold grows laterally along its axis, early-formed fold-related joints may differ significantly in orientation from joints that form later. In this paper, we integrate mechanical stratigraphy with joint pattern analysis to determine relative timing of jointing. Additionally, we demonstrate that joint patterns can be related to stresses on both the top and bottom of the bed during flexure. We present joint data from eight sedimentary beds on the fold terminus at Sheep Mountain Anticline, Wyoming, USA. The joint patterns around the terminus show two distinct patterns: joints in six of the beds show a radial pattern around the terminus whereas joint patterns in the two remaining beds differ from proximal units, despite being in the same structural position. Fracture resistance calculations confirm that the beds with mis-oriented fractures are less resistant to fracturing than other units in the study, and therefore would have fractured earlier in fold growth history. We present a plate bending model that illustrates potential joint patterns around a plunging fold nose from stresses along both the top and bottom of the bed. The joint strike predictions for the area in front of the inflection line on the fold nose match the orientations in our less resistant beds, which are now positioned behind the inflection line, suggesting that the fold grew laterally. The combined analysis of fracture pattern and mechanical stratigraphy provides a new way to investigate fold evolution. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Folds are ubiquitous deformational structures, found in every tectonic setting, with economic impacts in both oil exploration and mining. Understanding the formation and growth of these struc- tures will illuminate how progressive deformation is achieved, however direct observations of active folding are elusive. Active folding is assumed to be largely aseismic (Scholz, 2002), with some notable exceptions like the 1983 Coalinga earthquake (Stein and King, 1984), and GPS velocities attribute 100% of the observed deformation to fault slip. Nevertheless the omnipresence of folding within all deformational settings speaks to the role that this inelastic process must play in accommodating deformation during or between earthquakes. Past growth of folds along their strike has been observed in the field through quartz deformation lamellae (e.g. Pavlis and Bruhn, 1988) as well as through geomorphic indicators (e.g. Jackson et al., 1996; Keller et al., 1999). Fischer and Wilkerson (2000) used kinematic models to demonstrate how fold-related jointing may record fold growth. The key region for unraveling the evolu- tion of folding is at fold terminations where material is incorpo- rated into the fold structure. These inherently three-dimensional regions must be carefully investigated with attention to temporal and spatial variations in fold shape as well as variations in stra- tigraphy. Here we further explore the relationship between fold growth and joint orientation, by considering the mechanical properties of different layers to provide a relative timescale of fracturing at Sheep Mountain Anticline, WY, USA. Furthermore, we consider stresses on the bottom as well as the top of a folded layer and compare these patterns to observed joint patterns. Natural fractures are extremely common in folded sedimentary strata (e.g. Nelson, 1985). When bending stresses dominate, fold- related joints initiate along the outer arc of the fold where tangential longitudinal stresses are effectively tensile (e.g. Price and Cosgrove, 1990; Fig. 1A). These joints develop perpendicular to the direction of maximum curvature, which produces fractures that form a radial fanning pattern in map view around fold terminations * Corresponding author. E-mail address: hsavage@ldeo.columbia.edu (H.M. Savage). Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg 0191-8141/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.09.004 Journal of Structural Geology 32 (2010) 1466e1475