Figure 6. (a) Map of
maximum contraction directions from GPS data (Haines & Wallace, 2020)
and borehole-derived SHmax orientations (orange arrows)
and IODP borehole image analysis from McNamara et al. (2021) (blue
arrows). (b) Map of SHmax orientations and inferred
faulting regime from focal plane mechanisms (0-60 km; square:
extensional, circle: strike-slip, and triangle: compressional regime;
Townend et al., 2012), seismic anisotropy measurements (0-40 km; black
lines). Dashed blue lines show depth of subduction interface from sea
level (Williams et al., 2013). (c) The map showing depth to basement map
of North Island adapted from FROGTECH (2014). The clockwise rotation of
SHmax orientation along the HSM strike follows the
basement topography. 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). Red dots show the borehole locations.
Although borehole-derived SHmax orientations at the HSM
are broadly consistent with maximum contraction directions,
SHmax orientations are not uniform within the forearc
and in places deviate from the observed maximum contraction directions
(Figure 6a). Some of these variabilities may be due to longer-term
partitioning of oblique relative motion and strain into margin-parallel,
margin-perpendicular, and strike-slip components. Similar deviations
from maximum contraction directions are observed within the Nankai
subduction zone (Chang et al., 2010) and Costa Rica margin (Malinverno
et. al, 2016). For example, borehole-derived SHmaxorientations in the accretionary prism across the Nankai subduction zone
are subparallel to the plate convergence vector between the Philippine
Sea plate and Japan, perpendicular to the plate boundary, and for one
borehole in the shelf edge is parallel to the plate boundary (Lin et
al., 2010; Chang et al. 2010; Wu et al. 2012). The variations of stress
among these sites and the deviation of borehole-derived
SHmax orientations from the plate motion vectors derived
from GPS data are suggested to be attributed to the strain partitioning
into trench‐parallel, right‐lateral slip and thrust tectonics due to
oblique plate convergence (Tobin et al. 2009; Lin et al. 2010).
Alternatively, such stress variations have also been attributed to other
factors such as thrusting and bending within individual geologic domains
and local extension deformations derived by gravitation collapse of the
prism in the forearc (Chang et al., 2010; Lin et al., 2016).
Townend et al. (2012) derived SHmax orientations from
earthquake focal mechanisms using a Bayesian approach for earthquake
data between 2004-2011 with ML> 3. Their
observations reveal that SHmax orientation changes from
NE-SW in the central HSM to a more oblique SHmaxorientation, sub-parallel to Pacific-Australia relative plate motion, in
the southern HSM (Figure 6b). Only two focal mechanism solutions are
located within the upper plate of the HSM or near the subduction
interface; one strike-slip event with a NE-SW SHmaxorientation (latitude 38°S; North of central HSM) at 7 km depth, and one
compressional event with a WNW-ESE SHmax orientation
(latitude 40.5°S; southern HSM) at 25 km depth (at the top of subducting
plate near the subduction interface; Figure 6b). In the central HSM
(where the plate boundary is largely creeping) borehole-derived NE-SW
SHmax orientations and maximum contraction directions
agree with the NE-SW SHmax orientations derived from
focal mechanism inversions within the upper plate and subducting plate,
implying that far-field plate boundary forces exerted at the HSM
primarily control the present-day crustal stresses in the upper plate in
this area. In contrast, the southern HSM shows stress field rotation
with depth from borehole-derived WNW-ESE/NW-SE SHmaxorientations (roughly margin-perpendicular) in the shallow crust of
upper plate to NE-SW/ ENE-WSW focal mechanism-derived
SHmax orientations within the subducting plate (Figure
6b). The NE-SW/ ENE-WSW SHmax orientations derived from
focal mechanism inversions indicate a normal or strike-slip tectonic
regime within the subducting plate of the southern HSM. There is one
compressional event at ~25 km depth (near the subduction
interface) where the borehole-derived SHmax orientation
agrees with a focal mechanism-derived WNW-ESE SHmaxorientation (latitude 40.5°S; Figure 6b). This implies that the
overriding plate broadly is in a condition of horizontal compression
parallel to convergence direction, potentially as deep as the plate
interface (±10 km) (barring localized variations) and the tectonic
regime becomes more strike-slip and normal within the subducting plate.
The change in stress orientation with depth in the southern HSM is more
compatible with a theoretically mechanically weak plate interface with
low resolved shear stress and normal effective stress on the plate
interface. The fact that the upper plate is in the state of horizontal
compression parallel to plate convergence, on the other hand, implies
that contemporary SHmax orientation is being driven by
subduction of Hikurangi Plateau beneath North Island. Taking these
observations into consideration, we propose that plate interface remains
mechanically weak along the whole HSM and that a small increase of
elastic compression strain and stress caused by the interseismically
coupled southern HSM interface is sufficient to rotate
SHmax orientation in the upper plate toward the
subduction direction.
More recently, Illsley-Kemp et al. (2019) measured seismic anisotropy
using shear wave splitting fast orientations from earthquake data in
2005–July 2018 with ML>0. Some studies
consider fast orientations as a proxy for SHmaxorientation if other significant crustal anisotropies are not present
(e.g. fracturing, faulting, grain and crustal preferred orientations).
The seismic anisotropy was derived from events ≤40 km deep and fast
orientations were dominantly NE-SW (margin-parallel) for most of the HSM
forearc, and more ENE-WSW in the northern and central HSM (Figure 6b).
With the exception of some portions of the central HSM, the fast
directions from shear-wave splitting studies do not agree well with the
SHmax orientations we measure in the shallow boreholes.
This could mean either (a) that the shear wave splitting fast directions
are not controlled by contemporary SHmax orientation, or
(b) that the fast directions are mainly controlled by structural
features such as faults and the stress state at greater depths, which
may be different than the near-surface which our study samples. In fact,
propagating waves sample the volume of crust above the hypocenter
without taking the variation of anisotropic with depth into account.
Long-term tectonic deformation resulting from rapid clockwise rotation
of the Hikurangi forearc (Wallace et al. 2004) may also possibly explain
the along-strike variation in SHmax orientation. The
clockwise rotation of forearc, which accommodates the margin-parallel
component of oblique Pacific-Australian plate motion, results in
significant tectonic transitions along the strike (Nicol et al., 2007;
Wallace et al., 2004). Tectonic regime varies from back-arc rifting
within the TVZ in the central part of North Island, strike-slip and
normal faulting onshore of the northern and central HSM to transpression
and reverse faulting at the southern HSM (Figure 5). The rotation of
SHmax orientations from margin-parallel in the onshore
central HSM to margin-perpendicular in the southern HSM likely reflects
this transition in the tectonic regime, Such that the NE-SW
SHmax orientation in the central HSM reflects
margin-normal extension due to the transmission of slab rollback forces
across the forearc and into the adjacent extensional back-arc rift
(Wallace et al., 2012b). The clockwise rotation of the Hikurangi forearc
may also explain why our observed contemporary SHmaxorientation of NE-SW is not consistent with active large-scale NE-SW
striking compressional faults in the central HSM. The imaged intervals
of the central HSM boreholes are located within the hanging walls of
active NE-SW striking compressional faults and within NE-SW oriented
fault-bend fold axes identified from seismic reflection profiles
(Western Energy New Zealand, 2001; Barnes et al., 2002). It is possible
that the deformation and growing rate of these reverse faults within the
central HSM have changed through time, allowing forces exerted by
forearc rotation surpass the NW-SE compression stress on these active
faults. In this case, the rotation of the forearc, which accommodates
the margin-parallel component, induces a NE slip motion on these
pre-existing faults. This also suggests that these reverse faults in the
context of current stress field could be currently active as
strike-/oblique-slip faults. Another possibility is that due to frequent
earthquakes in this region, the SHmax orientation within
the central HSM has changed from a potential NW-SE orientation
perpendicular to the margin, compatible with the orientation of observed
reverse faults and fold axes, to the contemporary NE-SW orientation
subparallel to the margin. During the interseimic period, strain and
stress accumulation along the reverse faults and subduction interface
drives compression in the upper plate parallel to convergence direction.
Following great earthquakes or frequent earthquakes, the
margin-perpendicular component is reduced due to post-seismic stress
release, allowing a strike-slip motion in direction of long-term oblique
relative plate motion on these faults. This implies that the stress
state associated with seismicity perturbations is quite long lasting
(these boreholes are drilled over the course of 4 years) such that
stress state require a long time to recover or the perturbations were
significant enough to permanently reorient the stress. In this scenario,
the intermediate principal stress (Shmin) and minimum
principal stress (Sv) would switch and principal
stresses re-orient in response to forces exerted by long-term relative
plate motion and fluctuations in the magnitude of stress parallel to
plate convergence, modulated by the seismic cycles. Sacks et al. (2013)
and Moore et al. (2013) used a similar concept to explain the contrast
between SHmax orientations and normal fault strikes
within Kumano Basin of the Nankai subduction zone.
Lateral variations in surface and basement topography may also play a
role in the observed along-strike variation in upper plate
borehole-derived SHmax orientations (Figure 6c).
Basement structures such as faults, folds, and seamounts that are not
covered by thick sediments can cause significant stress rotation at both
regional and large scales by introducing geomechanical inhomogeneities
and lateral discontinuities (Gale et al., 1984; Enever et al. 1999;
Rajabi et al., 2016a). In fact, the presence of seamounts on the
incoming plate have been suggested as a possible localized effect on
stress field orientations and shear-wave splitting fast directions in
the offshore northern HSM (McNamara et al., 2021; Zal et al., 2020). The
SEEBASE™ model of New Zealand, a depth to basement map based on the
integrated interpretation of gravity, magnetic, borehole, and seismic
data (Figure 6c) clearly shows a significant change in sediment
thickness overlying the basement rocks, and basement topography along
the HSM margin (FROGTECH , 2014). The onshore northern and central HSM
basement is located at depths ~9-15 km and covered by
thick Neogene sediments, whereas at the onshore and offshore southern
HSM the Mesozoic greywacke basement is uplifted and located at
relatively shallow depths (<5 km), and in some locations above
sea level. Thin sediment packages directly overlying mechanically strong
basement rocks, may be more reflective of the tectonic stresses
experienced by the basement. Hence, it is possible that the basement
compressional structures striking NNE/NE-SSW/SW control the observed
margin-perpendicular, NW-SE SHmax orientation within the
shallow crust of the southern HSM.
In addition to large-scale stress variations, localized variations have
been observed between and within individual boreholes which could be
attributed to local active geological structures such as faults, surface
topography, and variable mechanical properties. For instance a localized
NW-SE SHmax orientation (144° ± 11°) is noted from
four-arm caliper data in borehole Whakatu-1 (south of Napier; Figure 5),
perpendicular to the dominant NE-SW SHmax orientation of
the central HSM. The NW-SE SHmax orientation in
Whakatu-1 is margin-perpendicular implying a localized compressional
tectonic regime at this locale, which agrees well with observed onshore
and offshore active, NNE-SSW striking, compressional faults in this
region (Figure 5; Litchfield et al. (2014); Langridge et al, (2016)),
such as the reverse fault responsible for growth of the Cape Kidnapper’s
anticline (Hull, 1987). Furthermore, Dimitrova et al. (2016) reported a
possible locked patch in southern central HSM between shallow SSEs on
the East Coast and deep Kaimanawa SSEs west of the East Coast (Figure
1b). Downdip changes in interseismic coupling behavior of the subduction
interface and the accumulated strain associated with the locked patch
could also be influencing the compressional tectonic regime in this
region and borehole Whakatu-1.
6 Conclusions
This paper presents the first comprehensive analysis of contemporary
SHmax orientations along the HSM, and discusses stress
field orientation variability within the context of variable tectonics
and slip behavior of this subduction margin. SHmaxorientations in the central HSM are predominately NE-SW (sub-parallel to
plate boundary), which rotates to a dominantly WNW-ESE
SHmax orientation in the southern HSM (approximately
perpendicular to the plate boundary). Our borehole-derived
SHmax orientations agree with the maximum contraction
strain directions from GPS measurements along the HSM suggesting that
the observed stress orientations along the HSM are influenced by elastic
strain accumulation due to interseismic coupling on the Hikurangi
subduction interface. The long-term tectonic deformation arising from
rapid rotation of the Hikurangi forearc, causing reverse faulting and
strike-slip in the southern part of the margin and a combination of
extension and strike-slip in the northern and central margin and the
basement topography may also be at play in influencing the along-strike
variations in observed stress orientations. In the southern HSM,
borehole-derived SHmax orientations are inconsistent
with SHmax orientations derived from focal mechanism
solutions in the subducting plate, implying that the southern HSM
interface is mechanically weak. Further interpretation of HSM stress
state could be achieved by constraining stress magnitudes, which will be
the focus of future studies.