Description:

_Journal of Structural Geology 33 (2011) 5e19_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Temporal constraints on fracturing associated with fault-related folding at Sant Corneli anticline, Spanish Pyrenees_ _J. Ryan Shackleton a,*, Michele L. Cooke a, Jaume Vergés b, Toni Simó c_ _a University of Massachusetts, Amherst, 611 North Pleasant Street, Amherst, MA 01003-9297, United States_ _b Institute of Earth Sciences “Jaume Almera”, CSIC, Lluís Solé i Sabarès street, 08028 Barcelona, Spain_ _c ExxonMobil, URC, 3120 Buffalo Speedway, Houston, TX 77098, United States _article info_ _Article history: Received 12 February 2009_ _Received in revised form 27 October 2010_ _Accepted 2 November 2010_ _Available online 13 November 2010_ _Keywords: Sant Corneli Fractures Joints Folding Growth strata Syn-tectonic_ _abstract_ _We mapped fractures at Sant Corneli anticline, an E-W trending non-cylindrical, fault-related anticline in the Spanish Pyrenees. Fracture mapping in syn-tectonic strata directly links fold-related fracturing to fold evolution because the relationship of fractures to syn-tectonic strata can constrain structural timing. Rather than using absolute fracture orientation as a primary means of grouping fracture sets, we used relative fracture timing, mineral fill, fracture size (length and height), and orientation with respect to bed strike to delineate five bed-orthogonal fracture sets that we interpret to be associated with folding and faulting events. We observe several early sets of joints with calcite fill (J1, J2, J3) many of which are interpreted to be related to fold axis perpendicular normal faulting of the anticline. Two late-stage joint sets with associated iron oxide mineralization in the surrounding wall rock (J4, J5) are oblique to bedding, maintain orientation with respect to bed strike, and are interpreted to result from flexure of the anticline. We infer the timing of J4-J5 jointing relative to syn-tectonic events. This research demonstrates how fracture studies that integrate syn-tectonic strata and distinguish fracture sets on the basis of multiple characteristics can better constrain the timing of fracturing relative to regional deformation events._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _The ability to quantitatively predict the orientation and density of natural fractures is advantageous in many industries because subsurface fracture networks form fluid flow pathways and reservoirs in groundwater aquifers, hydrocarbon reservoirs, and hydrothermally active basins. Fracture prediction in layered sedimentary rocks relies heavily on layer shape (fold shape) and fault shape. Fold shape affects fracturing because bending stresses control the density and orientation of fractures in three dimensions (Szilard, 1974; Fischer and Wilkerson, 2000). Faults exert a significant control on both the overall fold shape and the stress field near fault tips (e.g. Pavlis and Bruhn, 1988; McGrath and Davison, 1995; Rudnicki and Wu, 1995; Savage and Cooke, 2004; Bellahsen et al., 2006). Field studies of fracturing in fault-related folds serve two roles in fracture prediction. First, natural structures can be used to test theoretical predictions for fracturing based on plate bending, faulting, or other models for fracture formation (e.g. Hennings et al., 2000; Bergbauer and Pollard, 2004; Bellahsen et al., 2006). Second, field studies can inform subsurface predictions of fracture networks, which are commonly below the resolution of many geophysical techniques._ _Infering the fracture history and geometry in a fault-related fold requires an understanding of the evolution of the fold shape through time. Geologists have devised a number of approaches to understanding such folds, including forward geometric and geomechanical fold modeling (e.g. Suppe, 1983; Suppe and Medwedeff, 1990; Erslev, 1991; Poblet and McClay, 1996; Johnson and Johnson, 2002), structural restoration algorithms (Rouby et al., 2000; Griffiths et al., 2002; Thibert et al., 2005; Maerten and Maerten, 2006), and studies of natural folds and numerically modeled folds with syn-tectonic strata (Vergés et al., 1996; Ford et al., 1997; Poblet et al., 1998; Novoa et al., 2000; Salvini and Storti, 2002a). Natural folds with syn-tectonic strata provide direct evidence of changes in fold shape because each syn-tectonic horizon can be individually restored to document fold shape at a particular time (e.g. Vergés et al., 1996; Ford et al., 1997; Poblet et al., 1998; Novoa et al., 2000; Bernal and Hardy, 2002; Salvini and Storti, 2002b). While syn-tectonic strata have been used extensively to understand fold evolution, relatively few studies have utilized syn-tectonic strata to understand the evolution of fracturing and small scale structures associated with folding (e.g. Nigro and Renda, 2004), which is the purpose of this study._ _J.R. Shackleton et al. Journal of Structural Geology 33 (2011) 5e19_ _Fig. 1. Generalized map of the Spanish Pyrenees showing the major thrusts. The Southern Central Unit, a thrust-bounded block containing the Boixols thrust and Sant Corneli Anticline, is highlighted in the center of the map. (modified from Vergés, 1993)_ _Fig. 2. Geologic map of the Sant Corneli-Boixols-Nargo anticline. Thrusts shown at the surface south of the Sant Corneli and Boixols anticlines are relatively small displacement thrusts that splay off of the Boixols thrust. The dotted box indicates the boundaries of the study area and the location of Fig. 5. (modified from Bond and McClay, 1995)._ _J.R. Shackleton et al. Journal of Structural Geology 33 (2011) 5e19_ _Fig. 3. Cross sections through Sant Corneli anticline along or near the ECORS transect (Fig. 2). A) Dér Hammond et al. (1993), B), Vergés (1993), and C) Garcíada-Senz (2002)._ Ключевые слова: e, r, o