Geophysical exploration and dynamics of the Alpine Fault Zone
Geophysical exploration and dynamics of the Alpine Fault Zone
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高山断裂带地球物理勘探与动力学
DOI：
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发表时间：
2013
期刊：
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通讯作者：
F. Davey
中科院分区：
文献类型：
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作者：
T. Stern;D. Okaya;Stefan Kleffmann;M. Scherwath;S. Henrys;F. Davey
新西兰南岛中部的阿尔卑斯断层，可以用地震反射方法追踪到深度为1.35 km的具有强反射率的利斯特里克形表面。地表最大倾角在15公里深度处为0.60度，然后倾角随深度减小，直到反射率在0.35公里处接近水平。广角地震方法表明，在断层面以上的区域，岩石的P波速度比正常值低10%。在横截面上，这个低速阿尔卑斯断层带是细长的，位于断层面之上，尺寸约为45 × 20 km。大地电磁研究表明，低电阻率异常与低地震速度区大致一致。一个简单的解释是，电气和地震异常是由相互关联的流体在岩石静压力。下地壳中流体的推断得到了特定炮集地震反射属性分析的支持，其中阿尔卑斯断层反射可以明确识别。我们参考的幅度和相位的断层带反射的独特的侧扫反射在遥远的普卡基湖海岸。阿尔卑斯断层反射的高反射系数估计为0.25，这可能需要各向异性和流体来解释。我们解释水源是侵入造山带的片岩-杂砂岩的变质脱水。在一条80公里长的未偏移地震反射剖面上，一个沿着逆冲的滑脱面被认为是一个强反射带。这一明显与阿尔卑斯断层反射相融合的强反射带与莫霍面的深度无关，而是与片岩-杂砂岩基底（Vp = 6-6.2 km/s）和下地壳（Vp = 7-7.2 km/s）之间的边界有关。我们解释这个边界上的强反射率是由于剪切组构。地质学和地球物理学的观测结果都表明，地壳下部和中地壳以及地幔的变形似乎是由韧性和脆性行为的结合造成的，没有岩石圈弯曲的证据。我们解释阿尔卑斯山断裂带作为一个深刻的热，湿，和大陆地壳薄弱的地区。
The Alpine Fault of central South Island New Zealand, can be tracked with seismic reflection methods to depths of ∼35 km as a listric-shaped surface with strong reflectivity. Maximum dips of the surface are ∼60 degrees at 15 km depth and the dip then lessens with depth until the reflectivity is sub-horizontal at ∼35 km. Wide-angle seismic methods are used to show that the P-wave velocities of the rocks are up to 10% less than normal in the zone above the fault surface. In cross-section this low-velocity Alpine Fault Zone is elongate, sits above the fault surface, and has dimensions roughly 45 by 20 km. A magnetotelluric study shows a low-resistivity anomaly that is roughly coincident with the zone of low seismic velocity. A straightforward interpretation is that both the electrical and seismic anomalies are caused by interconnected fluids at lithostatic pressure. The inference of fluids in the lower crust is supported by an attribute analysis of seismic reflections on specific shot gathers where the Alpine Fault reflections can unequivocally be identified. We reference both the amplitude and phase of the fault-zone reflections to the distinctive side-swipe reflections generated at the far shore of Lake Pukaki. High reflection coefficients of ∼0.25 are estimated for the Alpine Fault reflections, which may require both anisotropy and fluid to explain. We interpret the source of water to be metamorphic dewatering of the schist-greywacke rocks that thicken into the orogen. A detachment surface along which the greywacke-schist rocks are obducted is recognised as a zone of strong reflectivity on an 80-km-long, unmigrated seismic reflection section. This zone of strong reflectivity, which apparently merges into the Alpine Fault reflections, does not correlate with depth to the Moho but rather with the boundary between the base of the schist-greywacke rocks (Vp ∼6-6.2 krn/s) and the lower crust (Vp ∼7-7.2 km/s). We interpret the strong reflectivity on this boundary as being due to a shear fabric. Both geological and geophysical observations imply deformation in the lower and mid-crust and mantle that appears to be caused by a combination of ductile and brittle behaviour, with no evidence of lithospheric flexure. We interpret the Alpine Fault Zone as a profoundly hot, wet, and weak region of continental crust.