Abstract Graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization. We examine processes of graphitic carbon emplacement and deformation in the active Alpine Fault Zone, New Zealand by analysing samples obtained from Deep Fault Drilling Project (DFDP) boreholes. Optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain. Raman spectroscopy allowed interpretation of the degree of graphite crystallinity, which reflects both thermal and mechanical processes. In the amphibolite-facies Alpine Schist, highly crystalline graphite, indicating peak metamorphic temperatures up to 640°C, occurs mainly on grain boundaries within quartzo-feldspathic domains. The subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions (500–600°C), resulting in clustered (in protomylonites) and foliation-aligned graphite (in mylonites). In cataclasites, derived from the mylonitized schists, graphite is most abundant (<50% as opposed to <10% elsewhere), and has two different habits: inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon. Tectonic–hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences. The increasing abundance of graphite towards the fault zone core may be a significant source of strain localization, allowing fault weakening. Supplementary material: Raman spectra of graphite from the Alpine Fault rocks is available at https://doi.org/10.6084/m9.figshare.c.3911797

Abstract There is a persistent body of literature that suggests low-angle normal faults (LANF) form and slip seismically; if true, the effective friction coefficient is much lower (<0.3) than that found in laboratory tests of rock friction ( c. 0.8) and in low-displacement faults that lack well-developed fault cores. This paper summarizes and discusses the mechanisms proposed to explain the low apparent friction of crustal-scale faults with low resolved shear stresses. Emphasis is placed on differentiating static weakening mechanisms, operating at strain rates c. 10 −12 s −1 –10 −15 s −1 , from dynamic weakening mechanisms, operating at strain rates >10 −1 s −1 . Previous published explanations for low fault friction do not appear to meet the key requirements of (i) reducing both static and dynamic frictional strength of LANF and (ii) operating only along crustal-scale faults. Fault rock assemblages in quartzo-feldspathic continental crust reveal that grain size reduction, or comminution, plays a fundamental role in fault zone development. As a fault accrues displacement, a fault core forms that contains granular material. We postulate that dynamic rock fragmentation occurs during the shearing of confined granular material; dynamic fragmentation is a volume-dependent mechanism responsible for reducing the static and dynamic frictional strengths of faults.