Abstract

Fission-track dating of zircons from the Alpine fault zone in New Zealand and associated granites and gneisses reveals at least three stages in the development of the present fault zone. Mylonites northwest of the Fraser fault give ages of ∼80 Ma. Zircons from the Fraser Formation, bounded by the Fraser and Alpine faults, give ages of ∼9 Ma. Samples east of the Alpine fault give ages below 5 Ma. These youngest ages result from the currently active uplift of the Alpine schists against the Alpine fault. The ∼9 Ma ages record a rapid uplift of the Fraser Formation owing to the onset of compression across the plate boundary. Mylonites to the west of the mapped Fraser fault that give zircon fission-track ages of ∼80 Ma indicate a third, much earlier phase of fault movement. Structural and other geologic evidence supports these conclusions.

The observation that the present plate boundary was located along a preexisting zone of weakness suggests that mylonite zones may merit greater attention as possible sites for initiation of rifting and/or continental transform faults.

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