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Evaluating the central Hikurangi Subduction Margin stress state from geophysical logging
  • Effat Behboudi,
  • David McNamara,
  • Ivan Lokmer
Effat Behboudi
University College Dublin

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David McNamara
University of Liverpool
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Ivan Lokmer
University College Dublin
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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.