Figure 1. (a) Tectonic
setting of East Coast of North Island, New Zealand. Fault traces from
Barnes et al., (2010), Langridge et al, (2016), Mountjoy and Barnes
(2011), and Pedley et al., (2010). Black arrows indicate the long-term
motion of the Pacific relative to Australian Plate from DeMets et al.,
(1994) and grey arrows show shortening rates at the Hikurangi Trough
from Wallace et al. (2012). (b) Interseismic coupling based on campaign
GPS velocities (1995–2008), shown in terms of coupling coefficient
(Wallace et al. 2012a). Green and pink shaded regions represent the
cumulative slow slips in 2002 and 2012 (Wallace et al., 2012a) and SSEs
beneath the Kaimanawa ranges in 2006 and 2008 (Wallace &
Eberhart‐Phillips, 2013; Wallace, 2020). Area of possible locked patch
between the East Coast and Kaimanaw SSEs is shaded in red. (c) Map
showing SHmax orientations from focal mechanisms
(Townend et al., 2012) and shear wave splitting fast orientations
(Illsley-Kemp et al., 2019). Boreholes are numbered 1: Makareo-1, 2:
Kauhauroa-2, 3: Waitahora-1, 4: Kauhauroa-5, 5: Tuhara-1A, 6: Kereru -1,
7: Whakatu-1, 8: Ngapaeruru-1, 9: Te Mai-2, 10: Rauni-2, 11: Orui-1A,
12: Titihaoa-1, 13: Tawatawa-1, 14: U1519A, and 15: U1518B.
Abbreviations: NIDFB, North Island Dextral Fault Belt; TVZ, Taupo
Volcanic Zone.
To date, several studies have been carried out to determine the
contemporary in-situ stress patterns along the East Coast of the HSM.
Analysis of earthquake focal mechanism solutions reveal that
SHmax orientations in the crust (≤60 km depth) change
from NE-SW (roughly margin-parallel; Figure 1c) in the northern and
central Hikurangi margin to a more margin-oblique, ENE-WSW orientation,
sub-parallel to Pacific/Australia relative plate motion, further south
(Figure 1c). These measurements are largely from earthquakes within the
subducting slab most located at depths >25 km depth
(Townend et al., 2012). In contrast, the seismic anisotropy fast
orientations determined from shear wave splitting methods that sample
the upper ~40 km (Figure 1c) suggest a dominant
SHmax orientation of NE-SW for most of the HSM forearc
(generally margin-parallel), while the northern HSM forearc displays
variable fast orientations, with a more dominant ENE-WSW inferred
SHmax orientation (Illsley‐Kemp et al., 2019). Shallow
(<3km) SHmax orientations have been determined
from limited analysis of borehole image logs from boreholes drilled
onshore and offshore HSM (Heidbach et al., 2018; Lawrence, 2018;
Griffin, 2019; Griffin et al., 2021; McNamara et al., 2021). Analysis of
borehole image data from four onshore boreholes show NE-SW to ENE-WSW
SHmax orientations in the central HSM (Heidbach et al.,
2018; Lawrence, 2018;), and an E-W to NW-SE SHmax orientation is
determined from two borehole image logs in the southern HSM (Heidbach et
al., 2018; Griffin, 2019; Griffin et. al, 2021). Boreholes offshore the
northern HSM drilled as part of the International Ocean Discovery
Program (IODP) Expeditions 372 and 375 show an E-W SHmaxorientation close to the Hikurangi trench, and a NW-SE
SHmax orientation in the offshore forearc (McNamara et
al., 2021), indicating strong variations in stress orientations across
the forearc.
In this study, we provide a detailed analysis of SHmaxorientations from stress related drilling-induced borehole failures, and
assess their variability within the upper plate of the HSM. We analyze
six borehole image and oriented four-arm caliper logs (not previously
used for stress orientation studies), and provide a reanalysis of the
seven borehole image logs investigated in Heidbach et al. (2018),
Lawrence (2018), Griffin (2019), and Griffin et al. (2021) with a focus
on acquiring higher resolution measurements (length, width, orientation)
of induced borehole features. We then discuss spatial variations in
contemporary SHmax orientations and their relationship
to far-field stresses and long-term patterns of tectonic deformation,
and their potential links to along-strike variations in subduction
megathrust slip behavior.
2 Geological Setting
The Hikurangi Subduction Margin (HSM) lies along the Pacific-Australian
plate boundary at the southern end of the Tonga-Kermadec Trench, off the
east coast of the North Island, New Zealand (Figure 1a). The Hikurangi
Subduction Margin accommodates westward subduction of the Hikurangi
Plateau (a Cretaceous large igneous province) beneath the continental
crust of North Island at the Hikurangi Trough (Davy, 1992). The
Hikurangi Plateau is ~10-15 km thick and transitions to
a more typical 5-7 km thick oceanic plate further north at the Kermadec
Trench (Davy et al., 2008; Davy, 1992; Ghisetti et al., 2016; Mochizuki
et al., 2019). The southern termination of the HSM is located somewhere
beneath New Zealand’s northeastern South Island, where oblique
convergence is transferred to the Marlborough Fault System and Alpine
Fault in the South Island (Barnes et al., 1998; Little & Roberts,
1997).
Three major tectonic phases during the Neogene to present-day have been
identified: 1) an early to middle Miocene compressional stage that
resulted in extensive reverse faulting, folding, and tectonic uplift
along the margin (Chanier et al., 1999; Barnes et al., 2002; Bailleul et
al., 2013); 2) a mixed compressional and extensional stage from the
mid-Miocene to early Pliocene resulting in widespread compressional
tectonics and localized subsidence and normal faulting in the inner
portion of the subduction wedge (Hawke’s Bay region) (Barnes et al.,
2002); 3) a compressional stage associated with and structural inversion
of listric thrust faults and folds during the Quaternary and rapid, late
Quaternary uplift of the Coastal Ranges, Axial Ranges, and Raukumara
Peninsula (Beanland & Haines, 1998; Nicol et al., 2002, 2007; Nicol &
Beavan, 2003; Bailleul et al. 2013). Late Quaternary extensional
faulting in the Raukumara Peninsula is likely the result of
gravitational collapse due to rapid uplift (Berryman et al., 2009;
Pettinga, 2004; Walcott, 1987).
Neogene to present tectonic deformation across the HSM is complex and
includes contributions from shortening associated with subduction at the
Hikurangi Trough, clockwise rotation of the East Coast forearc,
strike-slip faulting along the North Island Dextral Fault Belt (NIDFB),
and back-arc extension in the Taupo Volcanic Zone (TVZ) (Beanland &
Haines, 1998; Wallace et al., 2004; Figure 1a). The East Coast forearc
has rotated for at least the last few Myr at rate of 3°–4°/Myr relative
to the Australian plate (Nicol et al. 2007). This rotation results in
back-arc rifting in the central North Island’s Taupo Volcanic Zone
(TVZ), transpression in the southern North Island, and creates a large
along-strike change in convergence rate at the Hikurangi Trough, from
∼20 mm/year in the south to ∼60 mm/year at the northern Hikurangi Trough
(Wallace et al. 2004; Figure 1a). Wallace et al. (2004) suggest that an
along-strike change from subduction of the large igneous province
(Hikurangi Plateau) at the Hikurangi Trough, to normal oceanic crust
along the Kermadec Trench exerts a torque on the forearc, producing
clockwise rotation of the eastern North Island. Overall, relative motion
between the Pacific and Australian plates occurs through this region at
~40 mm/yr, and is oblique to the orientation of the
plate boundary. The oblique relative motion is partitioned into a
margin-perpendicular component and a margin-parallel component. The
margin-perpendicular component occurs along the Hikurangi subduction
interface and provides a NW-SE shortening which is accommodated via the
subduction interface and active thrust faults within the accretionary
wedge and overriding plate (Barnes et al., 1998; Nicol and Beavan,
2003). The margin-parallel component is largely accommodated by a
combination of right-lateral strike-slip on the North Island Dextral
Fault Belt (NIDFB) and vertical-axis clockwise rotation of the North
Island forearc (Beanland & Haines, 1998; Nicol et al., 2007; Wallace et
al., 2004).
3 Data and Methodology
We analyze borehole image logs acquired from eleven boreholes using a
range of tools including; the Schlumberger Fullbore Formation
Microimager (FMI™; Figure 2a) and Oil Based Mud Imaging tool (OBMI™),
Baker Atlas Simultaneous Acoustic and Resistivity Imager (STAR™), Tiger
Energy Services Acoustic Formation Imaging Technology (AFIT; Figure 2b),
and two orientated four-arm caliper logs (Figure 2c). The tool types and
their borehole wall coverage for each borehole are summarized in Table
1.