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II. Rock Products of Thrusting |
Institute of Nuclear Sciences, DSIR, Lower Hutt, New Zealand
Rates of vertical and horizontal movement at the Alpine Fault Zone and within the adjacent Alpine Schists (Haast Schist Group) are deduced from K-Ar age data of metamorphic and mylonitic rocks. The relative contributions of Jurassic-Cretaceous (Rangitata) and Miocene-Recent (Kaikoura) orogenic phases are also estimated.
After Jurassic regional metamorphism, the Haast Schists were uplifted regionally 18–20 km at rates 0.3–0.4 mm/yr until mid-Cretaceous times. No differential vertical movements occurred on the Alpine Fault at this time. During the Miocene-Recent orogeny, a maximum of 10 km uplift, produced mylonitic rocks at the Alpine Fault and brought Alpine Schists (Haast Schist Group) to the surface. Ages of mylonite, blastomylonite and pseudotachylyte indicate intense fault movement was occurring 5–10 Ma ago. Ages as young as 0.7 Ma occur in the adjacent Alpine Schists. From these data, uplift rates at the Alpine Fault for the period late Miocene-Pliocene are in the range 2.5–5 mm/yr and late Pleistocene-Recent, 7–14 mm/yr.
The estimation of horizontal displacements at the Alpine Fault in the Jurassic-Cretaceous and Miocene-Recent orogenic phases depends critically on the reality of a Cretaceous (?) lamprophyre dyke swarm cut by the Alpine Fault. If this is accepted, then about 360 km horizontal movement is pre-Upper Cretaceous and 120 km is younger. If the dyke swarm evidence is dismissed, then a total 480 km displacement on the Alpine Fault is entirely post-Cretaceous and may have occurred during the Miocene-Recent orogenic phase. This is, then, broadly associated with large-scale motions of the Indian and Pacific plates in the late Cenozoic and Quaternary.
Analysis of the age patterns within the Alpine Fault mylonites and the adjacent Alpine Schists support the possibility that a 15 km wide thermal aureole, about the Alpine Fault or at zones of intense shearing within the Alpine Schists, has been created by frictional generation of heat during relatively rapid aseismic shear sustained over the last 5 Ma. This thermal effect has been sufficient to cause complete argon loss from rocks over a 10 km wide zone immediately E of the Alpine Fault.
