Interplay between fluid flow and fault–fracture mesh generation within underthrust sediments: Geochemical evidence from the Chrystalls Beach Complex, New Zealand Å. Fagereng a,b, ⁎, C. Harris a a Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa b Department of Geology, University of Otago, PO Box 56, Dunedin 9054, New Zealand abstract article info Article history: Received 24 September 2013 Received in revised form 30 November 2013 Accepted 3 December 2013 Available online 11 December 2013 Keywords: Tectonic veins Fluid–rock interaction Subduction zones Episodic tremor and slow slip Fault–fracture mesh The Chrystalls Beach Complex, in the Otago Schist on the South Island of New Zealand, is a mélange comprising sheared trench-fill sediments and fragments of oceanic crust. It represents an exhumed analogue for underthrust sediments actively deforming along modern subduction thrust interfaces. The mélange is cross-cut by a fault– fracture mesh, comprising subvertical extension veins and subhorizontal slickenfibre-coated shear surfaces. Both shear and extension veins have a ‘crack–seal’ microstructure indicating episodic growth. Shear veins are associated with pressure solution selvages along the shear surface, whereas wall rock alteration is not observed adjacent to extension veins. Electron microprobe analyses of selvage seams indicate dissolution of silica from the immediate surroundings of slickenfibre shear veins, and therefore these slickenfibres probably grew by local dissolution–precipitation of silica. On the contrary, no depletion or addition of silica is detected around extension veins, indicating these veins grew by precipitation from advecting fluids. Oxygen isotope ratios measured in vein quartz show that shear and extension veins both precipitated from an aqueous fluid with 7 % ° b δ 18 O b 10 % ° , consistent with a fluid derived from low-grade metamorphic dehydration reactions. Fluid pressure therefore probably increased as fluids were introduced to a relatively impermeable mélange with increasing metamorphic grade and decreasing porosity. Fault–fracture mesh generation therefore involved localized shear assisted by dissolution–precipitation creep and concomitant extension fracturing. This led in turn to transient permeability associated with a fluid pressure drop, allowing episodic vein growth. This process may be analogous to geophysically observed episodic tremor and slow slip, which also involves a mixture of deformation styles that, put together, achieve shear slip along the subduction thrust interface. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Geophysical studies of active subduction zones commonly report a low velocity zone, hundreds of metres thick, directly beneath the inferred subduction thrust interface (e.g. Abers, 2005; Audet et al., 2009; Matsubara et al., 2009; Reyners and Eberhart-Phillips, 2009; Tobin and Saffer, 2009). This low velocity zone is interpreted as a fluid-saturated package of highly sheared, underthrust trench-fill sedi- ments, intermingled with fragments of oceanic crust and eroded hanging wall rock (e.g. Abers, 2005; von Huene and Scholl, 1991). Within these underthrust sediments, fast plate boundary slip rates (1–10 cm/year) are accommodated in a range of seismic styles including standard earth- quake rupture (M w ≤ 9.5), slow slip events, non-volcanic tremor, and continuous and episodic aseismic creep (e.g. Schwartz and Rokosky, 2007; Peng and Gomberg, 2010). Although the physical controls on the partitioning of slip between different seismic styles are poorly understood, it is commonly assumed that the presence of fluids has a significant effect on slip style (e.g. Audet et al., 2009; Katsumata and Kamaya, 2003; Liu and Rice, 2007; Peng and Gomberg, 2010; Saffer and Tobin, 2011; Shelly et al., 2006). Whereas active subduction thrust interfaces are only observable with remote techniques, exhumed subduction-related fault rocks should provide a directly observable, time-integrated record of the variety of slip modes occurring within active megathrust shear zones. The Chrystalls Beach Complex accretionary mélange, in the Otago Schist on the South Island of New Zealand, is an example of a subduction thrust analogue (e.g. Fagereng and Sibson, 2010; Fagereng, 2011a). Deformation in the Chrystalls Beach mélange was accommodated in a continuous–discontinuous style (Nelson, 1982), where dominant defor- mation mechanisms and bulk rheology depended critically on the rela- tive proportions of incompetent and competent materials (Fagereng, 2011a,b; Fagereng and Sibson, 2010). Discontinuous deformation is recorded in an anastomosing network of quartz–calcite shear and extension veins (Fagereng et al., 2010). Such veins are interpreted to represent sites of local extension fracture, followed by fluid infiltration and mineral precipitation (e.g. Barker et al., 2006; Cox and Etheridge, Tectonophysics 612–613 (2014) 147–157 ⁎ Corresponding author at: Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa. E-mail address: ake.fagereng@uct.ac.za (Å. Fagereng). 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.12.002 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto