Evaluating the central Hikurangi Subduction Margin stress state from
geophysical logging
Abstract
Quantifying the orientation and magnitude of tectonic stresses is
essential to better understand active crustal deformation and faulting
in the Hikurangi Subduction Margin (HSM), North Island, New Zealand. In
this study, We estimate the horizontal stress magnitudes (Shmin and
SHmax ) utilizing leak-off test (LOTs) data, borehole breakout widths
measured from borehole image logs, and rock unconfined compressive
strengths (UCS) derived from empirical relationships using P-wave
velocity wireline logs. Stress field results are used to infer the
tectonic regime experienced in the region where three boreholes,
Makareao-1, Kauhauroa-5, and Tuhara-1A, are drilled. Relative stress
magnitudes in Makareao-1 at 260-900 m TVDss (True vertical depth from
sea level) suggest thrust or strike-slip tectonics (SHmax≥ Shmin= Sv).
Moving east to Kauhauroa-5, the stress results report a gradual
transition from shallow normal/strike-slip tectonics (Sv >
Shmin) to thrust or strike-slip tectonics (SHmax> Sv≥
Shmin) at depth. Further east again, at borehole Tuhara-1A, stress
results suggest normal/strike-slip tectonics (Sv≥SHmax>
Shmin) from 555-2264 m TVDss. The tectonic regimes in individual
boreholes are consistent with fault interpretations of seismic
reflection profiles from this region. These three boreholes are located
within the hangingwall of active, NE-SW striking thrust faults and from
borehole breakout azimuths we find a mean SHmax orientation of 065° ±
17° (NE-SW) for the deeper parts of these boreholes. The SHmax
orientation is broadly compatible with maximum contraction directions
determined from campaign GPS and sub-parallel to far-field relative
Pacific-Australian plate motion. This, combined with our stress
magnitude observations in Makareao- 1 and Kauhauroa-5 suggests these
NE-SW striking faults predominantly experience strike and/or oblique
slip despite appearing in seismic profiles as thrust faults. We suggest
that these faults originated as thrust faults during older stages of
subduction along this margin, which over time have become reactivated in
a more strike-slip manner.