The Alpine Fault Hanging Wall Viewed from Within: Structural and Lithological Analysis of Acoustic Televiewer Logs in the DFDP-2B Borehole, New Zealand

Abstract:

The Alpine Fault is the major dextral transpressive fault forming the Australia–Pacific plate boundary in the South Island of New Zealand. The DFDP-2B borehole, drilled in late 2014, reached a depth of 893 m (measured depth) and provides an opportunity to describe the protolith and structure of ductilely deformed schists in the hangingwall of this active fault. A total of 19 km of wireline logging data were acquired in the borehole, including 4.8 km of borehole televiewer (BHTV) acoustic image logs. The high resolution of the BHTV logs and the borehole’s inclination, which reached a maximum of 46° from vertical near the base of the hole, provide good conditions in which to intersect the predominant foliation, which dips SE at a high angle to the hole as well as other feature sets. Here, we present a detailed structural analysis and lithological interpretation of the BHTV logs from the 264–886 m interval. The borehole’s inclination and the attenuation caused by the drilling mud reduces the image quality in places. However, some zones of poor image quality also concur with indications of permeable fractures and are related to fractured zones with localized borehole wall damage.

Foliation can be regularly identified to a depth of 808 m (n=406). The mean orientation of foliation planes (dip of 57° towards 145) remains quite consistent throughout the drilled interval and is similar to that inferred for the structurally underlying Alpine Fault. Larger-scale changes in image amplitude suggest lithological or petrophysical variations not yet identified on cuttings. The majority of the 1650 planar features identified as fractures are subparallel to the foliation and may represent deformed veins or other mineral segregations that contribute to the foliation's internal structure. Other planar features have dominantly NW–SE strikes, approximately orthogonal to the foliation and are inferred to be fractures. Truncations or offsets are observed on ~20 features suggesting faulting or relative chronology between features.