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.
We thank the Friend family for land access and the Westland community for support; Schlumberger for assistance with optical fibre technology; the CNRS University of Montpellier wireline logging group of Pezard, Henry, Nitsch and Paris; Arnold Contracting; Eco Drilling; and Webster Drilling.
This manuscript was greatly improved in response to suggestions by Andy Nicol and another anonymous reviewer, as well as substantial comments from the Editor, Nick Mortimer. Helpful suggestions were also made by Diane Moore and Brooks Proctor. We are very grateful for these contributions.
- Alpine fault
- New Zealand
- scientific drilling