Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract 16Recent friction experiments carried out under upper crustal P-T conditions have shown that 17 microstructures typical of high temperature creep develop in the slip zone of experimental 18 faults. These mechanisms are more commonly thought to control aseismic viscous flow and 19 shear zone strength in the lower crust/upper mantle. In this study, displacement-controlled 20 experiments have been performed on carbonate gouges at seismic slip rates (1 ms -1 ), to 21 investigate whether they may also control the frictional strength of seismic faults at the higher 22 strain rates attained in the brittle crust. At relatively low displacements (< 1cm) and 23 temperatures (≤ 100 °C), brittle fracturing and cataclasis produce shear localisation and grain 24 size reduction in a thin slip zone (150 µm). With increasing displacement (up to 15 cm) and 25 2 temperatures (T up to 600 °C), due to frictional heating, intracrystalline plasticity mechanisms 26 start to accommodate intragranular strain in the slip zone, and play a key role in producing 27 nanoscale subgrains (≤ 100 nm). With further displacement and temperature rise, the onset of 28 weakening coincides with the formation in the slip zone of equiaxial, nanograin aggregates 29 exhibiting polygonal grain boundaries, no shape or crystal preferred orientation and low 30 dislocation densities, possibly due to high temperature (> 900 °C) grain boundary sliding 31 (GBS) deformation mechanisms. The observed micro-textures are strikingly similar to those 32 predicted by theoretical studies, and those observed during experiments on metals and fine-33 grained carbonates, where superplastic behaviour has been inferred. To a first approximation, 34 the measured drop in strength is in agreement with our flow stress calculations, suggesting 35 that strain could be accommodated more efficiently by these mechanisms within the weaker 36 bulk slip zone, rather than by frictional sliding along the main slip surfaces in the slip zone. 37Frictionally induced, grainsize-sensitive GBS deformation mechanisms can thus account for 38 the self-lubrication and dynamic weakening of carbonate faults during earthquake propagation 39 in nature. 40

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Carbonate faults commonly contain small amounts of phyllosilicate in their slip zones, due to pressure solution and/or clay smear. To assess the effect of phyllosilicate content on earthquake propagation in carbonate faults, friction experiments were performed at 1.3 m/s on end‐members and mixtures of calcite, illite‐smectite, and smectite gouge. Experiments were performed at 9 MPa normal load, under room humidity and water‐saturated conditions. All dry gouges show initial friction values (μi) of 0.51–0.58, followed by slip hardening to peak values of 0.61–0.76. Slip weakening then ensues, with friction decreasing to steady state values (μss) of 0.19–0.33 within 0.17–0.58 m of slip. Contrastingly, wet gouges containing 10–50 wt % phyllosilicate exhibit μi values between 0.07 and 0.52 followed by negligible or no slip hardening; rather, steady state sliding (μss ≪ 0.2) is attained almost immediately. Microstructurally, dry gouges show intense cataclasis and wear within localized principal slip zones, plus evidence for thermal decomposition of calcite. Wet gouges exhibit distributed deformation, less intense cataclasis, and no evidence of thermal decomposition. It is proposed that in wet gouges, slip is distributed across a network of weak phyllosilicate formed during axial loading compaction prior to shear. This explains the (1) subdued cataclasis and associated lack of slip hardening, (2) distributed nature of deformation, and (3) lack of evidence for thermal decomposition, due to low friction and lack of slip localization. These findings imply that just 10% phyllosilicate in the slip zone of fluid‐saturated carbonate faults can (1) dramatically change their frictional behavior, facilitating rupture propagation to the surface, and (2) significantly lower frictional heating, preventing development of microscale seismic markers.

In ancient basement regions such as the Lewisian Complex, NW Scotland, the ages of brittle deformation events are commonly poorly constrained due to a lack of datable fills. An array of NW-SE sinistral and antithetic E-W dextral faults related to a regionally recognized episode of brittle shearing cut Neoarchaean gneisses and c. 2.25 Ga quartz-pyrite veins close to the trace of the unexposed, regional-scale NW-SE fault.Copper-iron mineralisation occurs at an intersection between an antithetic dextral fault and an older c.2.25 Ga quartz vein. Optical microscopy, SEM and XRD analyses reveal an array of intergrown, co-genetic copper-iron sulphides, hematite and barite. Complex mm-thick zoned alteration rims rich in epidote occur at contacts between the sulphides and gneisses. Rhenium-Osmium copper-iron sulphide geochronology yields an age of c. 1.55 Ga for the hydrothermal mineralization event associated with faulting. Fault movements demonstrably overlap with mineralisation based on the asymmetric fibrous growth forms of these minerals within local dextral shears which acted as local channelways for mineralizing fluids during and after faulting. We tentatively propose that this regionally recognised strike slip faulting, previously termed the 'Late Laxfordian', should be referred to as the 'Assyntian' in order to distinguish it from kinematically distinct Laxfordian events. [end] text file