Peridotite xenoliths entrained in magmas near the Alpine fault (New Zealand) provide the first direct evidence of deformation associated with the propagation of the Australian-Pacific plate boundary through the region at ca. 25–20 Ma. Two of 11 sampled xenolith localities contain fine-grained (40–150 μm) rocks, indicating that deformation in the upper mantle was focused in highly sheared zones. To constrain the nature and conditions of deformation, we combine a flow law with a model linking recrystallized fraction to strain. Temperatures calculated from this new approach (625–970 °C) indicate that the observed deformation occurred at depths of 25–50 km. Calculated shear strains were between 1 and 100, which, given known plate offset rates (10–20 mm/yr) and an estimated interval during which deformation likely occurred (<1.8 m.y.), translate to a total shear zone width in the range 0.2–32 km. This narrow width and the position of mylonite-bearing localities amid mylonite-free sites suggest that early plate boundary deformation was distributed across at least ∼60 km but localized in multiple fault strands. Such upper mantle deformation is best described by relatively rigid, plate-like domains separated by rapidly formed, narrow mylonite zones.

中文翻译：

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新西兰高山断层附近板块边界萌生过程中高度局部化的上地幔变形

夹带在高山断层（新西兰）附近的岩浆中的橄榄岩捕虏体提供了与澳大利亚-太平洋板块边界在约 25-20 毫安。11 个采集的捕虏体中的两个包含细粒 (40-150 μm) 的岩石，表明上地幔的变形集中在高剪切带。为了限制变形的性质和条件，我们将流动定律与将再结晶分数与应变联系起来的模型相结合。根据这种新方法计算出的温度 (625-970 °C) 表明观察到的变形发生在 25-50 公里的深度。计算出的剪切应变在 1 到 100 之间，给定已知的板块偏移率（10-20 毫米/年）和可能发生变形的估计间隔（<1.8 米），转换为 0.2-32 公里范围内的总剪切带宽度。这种狭窄的宽度和不含糜棱岩的地点中含糜棱岩的位置表明，早期板块边界变形分布在至少约 60 公里的范围内，但集中在多个断层链中。这种上地幔变形最好用相对刚性的板状区域来描述，这些区域被快速形成的狭窄糜棱岩带分隔开。