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.