The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand

Abstract

Ultrasonic image logs acquired in the DFDP‐2B borehole yield the first continuous, subsurface description of the transition from schist to mylonite in the hangingwall of the Alpine Fault, New Zealand, to a depth of 818 m below surface. Three feature sets are delineated. One set, comprising foliation and foliation‐parallel veins and fractures, has a constant orientation. The average dip direction of 145° is subparallel to the dip direction of the Alpine Fault, and the average dip magnitude of 60° is similar to nearby outcrop observations of foliation in the Alpine mylonites that occur immediately above the Alpine Fault. We suggest that this foliation orientation is similar to the Alpine Fault plane at ∼1 km depth in the Whataroa valley. The other two auxiliary feature sets are interpreted as joints based on their morphology and orientation. Subvertical joints with NW‐SE (137°) strike occurring dominantly above ∼500 m are interpreted as being formed during the exhumation and unloading of the Alpine Fault's hangingwall. Gently dipping joints, predominantly observed below ∼500 m, are interpreted as inherited hydrofractures exhumed from their depth of formation. These three fracture sets, combined with subsidiary brecciated fault zones, define the fluid pathways and anisotropic permeability directions. In addition, high topographic relief, which perturbs the stress tensor, likely enhances the slip potential and thus permeability of subvertical fractures below the ridges, and of gently dipping fractures below the valleys. Thus, DFDP‐2B borehole observations support the inference of a large zone of enhanced permeability in the hangingwall of the Alpine Fault.

Seismic P Wave Velocity Model From 3‐D Surface and Borehole Seismic Data at the Alpine Fault DFDP‐2 Drill Site (Whataroa, New Zealand)

“…In a similar experiment in Sweden, Simon et al () were able to determine Thomsen parameters and thus an anisotropic velocity model (vertical transverse isotropy, VTI) making use of the different dominant ray paths in surface and borehole analysis. However, the Alpine Fault in the Whataroa Valley dips at ∼50° (Lay et al, ; Toy et al, ) and the fabric dip is between 45° and 55° (Massiot et al, ; Townend et al, ) so that the simplest anisotropy approach would be a tilted transverse isotropy case. This tilt makes the derivation of anisotropy parameters more challenging and it is not possible to only use the described differences in traveltimes observed by surface and borehole receivers.…”

“…The second phase of drilling, DFDP‐2, was intended to intersect the Alpine Fault at a depth of ∼1 km to determine temperature, stress, and fluid pressure regimes (Sutherland et al, ; Townend et al, ; Toy et al, ). The borehole is nearly vertical from the surface to about 400 m depth but is deviated by as much as 44° to the northwest at greater depths (Massiot et al, ; Sutherland et al, ). Due to this deviation, the true vertical depth of the 893 m long borehole is ∼820 m and the bottom of the borehole lies 720 m below sea level.…”

Fluid Flux in Fractured Rock of the Alpine Fault Hanging‐Wall Determined from Temperature Logs in the DFDP‐2B Borehole, New Zealand

“…(d)Lithological profile of DFDP‐2B. Figures 1c and 1d modified after Massiot (). MD refers to measured drilled depth, TVD refers to true vertical depth of the borehole.…”

Fault zone architecture of a large plate-bounding strike-slip fault: a case study from the Alpine Fault, New Zealand

“…2c). This inner damage zone is surrounded by an up to a few kilometers wide 'outer zone', which might be a near-surface, topographically-controlled feature only (Townend et al, 2017;Massiot et al, 2018;Williams et al, 2018).…”