Syn-deformation temperature and fossil fluid pathways along an exhumed detachment zone, khao khwang fold-thrust belt, Thailand Rowan L. Hansberry a, ⁎, Alan S. Collins a , Rosalind C. King a , Christopher K. Morley b , Andy P. Giże c , John Warren d , Stefan C. Löhr e , P.A. Hall a a Centre for Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, the University of Adelaide, SA 5005, Australia b Petroleum Geophysics M.Sc. Program, Department of Geological Science, Chiang Mai University, Chiang Mai 50200, Thailand c Dept. of Geology, Faculty of Science, Chulalongkorn University, Bangkok, Thailand d Lucid Microscopy, Frodsham, United Kingdom e Department of Earth & Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia abstract article info Article history: Received 17 December 2014 Received in revised form 18 May 2015 Accepted 21 May 2015 Available online 12 June 2015 Keywords: Detachment Deformational temperature Fluid-flow Stable isotopes Shale detachment zones, their influence on deformational style, and their internal mechanisms of deformation are an understudied aspect of fold-thrust belts. Properties such as deformational temperature, lithology, and mineralogy are often recognized as having a key influence on the rheology and deformational style of detach- ment zones and overlying fold-thrust belts. However, little work has been conducted on rock properties of known detachment zones. A recently described upper-level detachment zone in the exhumed Khao Khwang Fold-Thrust Belt of central Thailand provides an ideal natural laboratory for investigation of the deformation conditions of the detachment zone, and association with its complex deformational style. The low-grade meta- morphic indicator illite crystallinty is used to broadly constrain deformational temperatures, while oxygen and carbon stable isotope analysis provides insight into fluid flow history and fluid-rock interaction. Illite crystallinity data indicate deep diagenetic, to low anchizonal conditions, and temperatures of ~160–210 °C in the shale detachment, interpreted as reflecting peak metamorphic conditions during the Triassic Indosinian Orogeny. No trend between the intensity (spacing, complexity) of structures and illite crystallinty is observed. However, shale shear zones of continuous-style deformation and inferred higher finite strain display uniformly higher illite crystallinty than surrounding packages of discontinuously faulted shales. We also note a positive association between total organic carbon content in the shales and the spacing and complexity of deformational structures. Data from limestones and syn-tectonic vein fills detail the history of fluid-rock interaction during early mesogenesis, through to orogenesis. The early covariant trend of increasingly negative δ 13 C and δ 18 O values is attributed to increasing burial, while a divergent orogenic trend of increasingly negative δ 18 O values is interpreted as the result of a loss of matrix permeability and interruption of fluid-rock re-equilibration. These hottest fluids were concentrated along large thrusts which facilitated fluid movement during orogenesis. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The structural behaviour of detachment zones and their influence on the structural style of fold-thrust belts has been widely documented (Corredor et al., 2005; Dahlen, 1990; Davis et al., 1983; Morley et al., 2011a; Rowan et al., 2004). Detachment zones can vary significantly in thickness and spatial distribution, and occur at multiple levels in a fold-thrust belt. Previous studies have focused primarily on interpreta- tion of seismic reflection data and well data from recent and active deepwater fold-thrust belts (DWFTBs) (e.g. Niger Delta – Bilotti and Shaw, 2005; Briggs et al., 2006) or are reviews of structural style and de- tachment layer influence of active fold-thrust belts across a variety of settings (e.g. Morley et al., 2011a; Rowan et al., 2004). The thickness, spatial distribution and rheology of detachment zones have been shown to exert significant control on fold-thrust belt geometries (e.g. Briggs et al., 2006; Simpson, 2009; Stewart, 1996). The vast majority of studies on, or incorporating, detachment zone behaviour are also focused at a foldbelt, or at least 10s of kms scale (e.g. Maloney et al., 2010; Rowan et al., 2004; Sherkati et al., 2005). A considerable range of factors contribute to shale detachment weakness, some are interrelated. These factors include tectonic setting, burial history, temperature of deformation, mineralogy at both the time of deposition and the time of deformation (diagenesis/metamorphism), stratigraphic thickness, mechanical stratigraphic relationships with adjacent layers, strain rate and fluid-distribution/overpressure history (e.g. (Davis et al., 1983; Stewart, 1996; Simpson, 2009; Ding et al., 2012; Hansberry et al., 2014). Consequently there are considerable Tectonophysics 655 (2015) 73–87 ⁎ Corresponding author. Tel.: +61 883131717. E-mail address: rowan.hansberry@adelaide.edu.au (R.L. Hansberry). http://dx.doi.org/10.1016/j.tecto.2015.05.012 0040-1951/© 2015 Elsevier B.V. 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