Laser-ablation 40Ar/39Ar step-heating analyses of 20 pseudotachylyte veins from a single location along the exhumed central portion of the active Alpine fault of New Zealand yield total gas age values between 1 and 19 Ma. Evidence shows that they are genetically related and were formed during coeval episodes of seismogenic melting at shallow crustal depth, contrasting with a spread in formation ages. The total gas ages show an exponential decrease with increasing proportion of melt matrix and K content, reflecting incomplete degassing and mixtures of radiogenic Ar sources. Calculation of intercepts for all–melted matrix and all–clast end-member components indicate ca. 570 ka (Quaternary) friction-melting ages of ca. 332 Ma (Lower Carboniferous) source rock. Assuming an average exhumation rate of 6–9 mm/yr for uplift and erosion, these results imply that friction melts were generated during major slip episodes at ∼3.5–5 km crustal depth. We conclude that reliable dating of young pseudotachylyte can be accomplished by combining chronologic study with clast-matrix quantification of genetically related vein assemblages.

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