Crustal-scale fluid flow during the tectonic evolution of the Bighorn Basin (Wyoming, USA) Nicolas Beaudoin, *, † Nicolas Bellahsen, *, † Olivier Lacombe, *, † Laurent Emmanuel *, † and Jacques Pironon‡ *UPMC Univ Paris 06, UMR 7193, ISTEP, F-75005, Paris, France †CNRS, UMR 7193, ISTEP, F-75005, Paris, France ‡Universit  e, de Loraine, CNRS, CREGU, UMR 7359 Geo Resources, BP 70239, F-54506 Vandoeuvre-l  es, Nancy Cedex, France ABSTRACT Stable isotope measurements (O, C, Sr), microthermometry and salinity measurements of fluid inclusions from different fracture populations in several anticlines of the Sevier-Laramide Bighorn basin (Wyoming, USA) were used to unravel the palaeohydrological evolution. New data on the microstructural setting were used to complement previous studies and refine the fracture sequence at basin scale. The latter provides the framework and timing of fluid migration events across the basin during the Sevier and Laramide orogenic phases. Since the Sevier tectonic loading of the fore- land basin until its later involvement into the Laramide thick-skinned orogeny, three main fracture sets (out of seven) were found to have efficiently enhanced the hydraulic permeability of the sedi- mentary cover rocks. These pulses of fluid are attested by calcite crystals precipitated in veins from hydrothermal (T > 120 °C) radiogenic fluids derived from Cretaceous meteoric fluids that inter- acted with the Precambrian basement rocks. Between these events, vein calcite precipitated from for- mational fluids at chemical and thermal equilibrium with surrounding environment. At basin scale, the earliest hydrothermal pulse is documented in the western part of the basin during forebulge flex- uring and the second one is documented in basement-cored folds during folding. In addition to this East/West diachronic opening of the cover rocks to hydrothermal pulses probably controlled by the tectonic style, a decrease in 87/86 Sr values from West to East suggests a crustal-scale partially squee- gee-type eastward fluid migration in both basement and cover rocks since the early phase of the Se- vier contraction. The interpretation of palaeofluid system at basin scale also implies that joints developed under an extensional stress regime are better vertical drains than joints developed under strike-slip regime and enabled migration of basement-derived hydrothermal fluids. INTRODUCTION Foreland basins are often the location of multiple fluid flow events that have significant impacts on the chemical evolution of rocks, on the development of secondary porosity during diagenesis in carbonate rocks (e.g. Qing & Mountjoy, 1992; Bjørlykke, 1993, 1994; Roure et al., 2005; Katz et al., 2006; Vandeginste et al., 2012) and on fracture development (Hubbert & Willis, 1957; Rubey & Hubbert, 1959; Templeton et al., 1995; Billi, 2005). Major issues have been the understanding of the origin, pathways and interactions with rocks of fluids migrating in basins (Engelder, 1984; Reynolds & Lister, 1987; McCaig, 1988; Forster & Evans, 1991; Trav e et al., 2000, 2007; van Geet et al., 2002; Ferket et al., 2003; Roure et al., 2005, 2010; Vilasi et al., 2009; Bjørlykke, 2010; Evans, 2010; Li et al., 2011), large-scale faults being effi- cient drains or barriers (Sibson, 1981). More recently, the distribution and development of fracture populations at fold scale appeared to influence local-scale hydrological systems, upon conditions of good connectivity and/or notable vertical persistence (e.g. Laubach et al., 2009; Barbier et al., 2012a). Rubey & Hubbert (1959) showed that the presence of fluids could facilitate faulting, and some authors invoked fluids to explain anomalous fault kinematics (e.g. Templeton et al., 1995). Evans & Fischer (2012) highlighted the dynamic evolution of fluid system affecting strata during folding, and recent studies also suggest that structural style of deformation could influ- ence palaeohydrology in fold-related fractures: in thin-skinned tectonics, fluids generally migrate in the d ecollement levels (when activated) and remain mainly stratified above the thrust tip until development of syn- folding fracture sets (e.g. Trav e et al., 2007; Fischer et al., 2009; Dewever et al., 2011; Evans et al., 2012). In thick-skinned tectonics, the palaeohydrological systems Correspondence: N. Beaudoin, UPMC Univ Paris 06, UMR 7193, ISTEP, F-75005, Paris, France. E-mail: nicolas. beaudoin@upmc.fr © 2013 The Authors Basin Research © 2013 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 403 Basin Research (2014) 26, 403–435, doi: 10.1111/bre.12032 EAGE