Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.

新西兰南韦斯特兰怀塔罗阿河阿尔卑斯断层深层断层钻探项目（Deep Fault Drilling Project，DFDP）第二阶段中，DFDP-2B钻孔在238.5~893.2米测量深度（Measured Depth，MD）处钻遇基岩。通过对全岩岩屑开展连续采样及中-微观尺度表征，研究明确该钻孔依次采集到角闪岩相、源自托勒斯复合地体的片岩、初糜棱岩与糜棱岩，最终在最大倾角为62°的阿尔卑斯断层主滑动带（Principal Slip Zone，PSZ）上方200~400米处完成取样。随着与PSZ距离逐渐缩短，最具判别意义的构造特征为剪切带的出现以及石英晶粒平均尺寸的降低。在约700米测量深度以下，云母与石英+长石的占比显著升高，这一成分变化被推测由不均一采样或与蚀变相关的岩性变化所导致。主量氧化物的变化特征表明，本研究未采集到此前在DFDP-1岩心中定义的、靠近断层的阿尔卑斯断层蚀变带。