Deformation of the Pacific Plate above the Alpine fault ramp and its relationship to expulsion of metamorphic fluids: An array of backshears
Deformation of the Pacific Plate above the Alpine fault ramp and its relationship to expulsion of metamorphic fluids: An array of backshears
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阿尔卑斯断层坡道上方太平洋板块的变形及其与变质流体排出的关系：一系列后切变
DOI：
10.1029/175gm10
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
T. Little
中科院分区：
文献类型：
--
作者：
R. Wightman;T. Little
新西兰南阿尔卑斯山中部约 2 公里宽的近垂直后切变阵列被解释为以类似自动扶梯的方式滑动，使太平洋板块上升到阿尔卑斯断层斜坡上，并在引导变质流体向上穿过这个活跃的造山带方面发挥了重要作用。下地壳中形成的斜滑后剪切力均匀分布（∼30 cm），平均偏移量为14 cm，呈脆性至延性，垂直长度延伸超过500 m。累积垂直位移表明，高山断层深处的致因斜坡阶梯的角度为 22 ± 8°。后剪切力之间的微尺度剪切可能导致地壳额外倾斜至~45°。我们推断这种剪切集中在基础斜坡台阶之上，是瞬态的，并且是抗震的。南阿尔卑斯山地震的震源机制表明，类似的反向剪切今天可能正在深处积累，与上地壳断层上的地震滑动有关。流体对于沿着后剪切力的剪切的形成和积累是不可或缺的。接近岩石静压的流体压力引发深层脆性剪切破坏（>20 公里）。陡峭的膨胀后剪切力使这些流体能够通过低渗透性（1 × 10 -18 m 2 ）片岩向上逸出。因此，流体排出可能完成了沿高山糜棱岩源区深度的脱挥发分和流变强化，同时也导致太平洋板块中流体侵入的岩石（特别是石英脉）的水解弱化。这些耦合的强度变化可能增强了高山断层上盘陡峭平面上变形的局部划分。
A ∼2 km-wide array of near-vertical backshears in the central Southern Alps, New Zealand, is interpreted to have slipped in an escalator-like way to up-ramp the Pacific Plate onto the Alpine Fault ramp, and to play an important role in channelling metamorphic fluids upward through this active orogen. The oblique-slip backshears formed in the lower crust, are evenly spaced (∼30 cm), and have an average offset of 14 cm that is brittle to ductile and extend over 500 m in vertical length. Cumulative vertical displacements suggest that the causative ramp-step in the Alpine Fault at depth had an angle of 22 ± 8°. Microscale shearing between the backshears probably accomplished additional crustal tilting to ∼45°. We infer this shearing was focused above the basal ramp-step, was transient, and aseismic. Focal mechanisms of earthquakes in the Southern Alps suggest that similar backshearing may be accumulating at depth today, where it is linked to seismic-slip on upper crustal faults. Fluid was integral to the formation and accumulation of shear along the backshears. Near-lithostatic fluid pressures triggered deep, brittle shear failure (>20 km). The steep, dilative backshears allowed these fluids to escape upwards through low permeability (1 × 10 -18 m 2 ) schist. Fluid expulsion may thus have accomplished a devolatilisation and rheological strengthening along the Alpine mylonite source region at depth, while also causing a hydrolytic weakening of the fluid-invaded rocks (especially quartz veins) in the Pacific Plate. These coupled strength changes may have enhanced the local partitioning of deformation onto steep planes in the Alpine Fault hangingwall.