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

摘要

Duringthesecondphaseof theAlpineFault, DeepFault Drilling Project (DFDP) in theWhataroa 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 degrees. 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.