Subducted sediments, upper-plate deformation and dewatering at New Zealand's southern Hikurangi subduction margin

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Subducted sediments, upper-plate deformation and dewatering at New Zealand’s southern Hikurangi subduction margin G.J. Crutchley a,b,∗ , D. Klaeschen b , S.A. Henrys a , I.A. Pecher c , J.J. Mountjoy d , S. Woelz d a

GNS Science, 1 Fairway Drive, Lower Hutt 5011, New Zealand GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, Kiel 24148, Germany c University of Auckland, 23 Symonds Street, Auckland 1142, New Zealand d National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, 6021 Wellington, New Zealand b

a r t i c l e

i n f o

Article history: Received 10 May 2019 Received in revised form 30 October 2019 Accepted 3 November 2019 Available online xxxx Editor: M. Ishii Keywords: Hikurangi subduction margin subduction to strike slip transition subducted sediments dewatering pre-stack depth migration reflection tomography

a b s t r a c t The southern end of New Zealand’s Hikurangi subduction margin accommodates highly oblique convergence between the Pacific and Australian plates. We carry out two-dimensional (2D) seismic reflection tomography and pre-stack depth migrations on two seismic lines to gain insight into the nature of subducted sediments and upper plate faulting and dewatering at the toe of the wedge. We also investigate the NE to SW evolution of emergent upper plate thrust faulting using 47 seismic lines spanning an along-strike distance of ∼270 km. The upper sequence of sediments that ultimately gets subducted (the MES sequence) has an anomalously-low seismic velocity character. At the southwestern ¯ end of the margin, ∼150 km east of Kaikoura, the MES sequence has experienced greater compaction (for an equivalent effective vertical stress) than it has some 200 km further to the northeast. This difference is likely attributable to greater horizontal compression in the southwest caused by impingement of the Chatham Rise on the deformation front. Relationships between velocity and effective vertical stress suggest that the MES sequence is well-drained in the vicinity of frontal thrusts, corroborated by evidence for upper plate dewatering along those thrusts. Effective drainage of the MES sequence likely promotes interplate coupling on the southern Hikurangi margin. The décollement is generally hosted near a seismic ¯ reflector known as “Reflector 7”. East of Kaikoura, however, Reflector 7 becomes accreted, indicating that subduction slip at the southwestern end of the margin is no longer hosted at (or above) this reflector. Instead, the décollement steps down to a deeper stratigraphic level further inboard. Further to the SW, ¯ approximately in line with the lower Kaikoura Canyon, the offshore manifestation of subduction-driven compression ceases. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Subduction zones are responsible for Earth’s largest earthquakes and tsunamis. The initiation and propagation of tectonic deformation that underlies these hazards is inextricably linked to the distribution and movement of fluids (Saffer and Tobin, 2011). Controlled-source seismic methods can shed light on both tectonic deformation and fluid flow, helping to unravel the processes occurring from the subduction interface to the seafloor (Bangs et al., 2015). The subduction to strike slip transition zone of New Zealand’s Hikurangi margin has experienced a range of earthquake sequences in recent years, including the 2013 Cook Strait earth-

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Corresponding author at: GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, Kiel 24148, Germany. E-mail address: [email protected] (G.J. Crutchley). https://doi.org/10.1016/j.epsl.2019.115945 0012-821X/© 2019 Elsevier B.V. All rights reserved.

¯ quakes (Holden et al., 2013) and the 2016 Mw 7.8 Kaikoura earthquake and aftershocks (Hamling et al., 2017) (Fig. 1a). Plate motion through this region is accommodated by a variety of crustal structures from the subduction interface to upper plate dextral transform faults (Barnes et al., 1998; Wallace et al., 2012a). Intriguing relationships between upper plate faulting and subduction interface afterslip and slow slip events (SSEs) highlight the complex tectonic processes occurring in this region (Mouslopoulou et al., 2019; Wallace et al., 2018). In this study, we re-process seismic data acquired for petroleum exploration and present new academic seismic data to shed new light on the nature of tectonic deformation in this area. We use these data to address the following aims: 1. Characterise the nature of the incoming sequence and décollement, through two-dimensional reflection tomography and pre-stack depth migrations

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Fig. 1. a) Large-scale tectonic setting. Blue box is the area shown in (b). Framework depth migrated seismic sections are shown by coloured lines (05CM-38 in Plaza-Faverola et al., 2016; PEG09-19 in Plaza-Faverola et al., 2012; PEG09-05 and PEG09-23 in this study). Labelled vectors are relative plate velocities (mm yr−1 ) from the MORVEL plate motion project (DeMets et al., 2010). Broken red line is the approximate combined region affected by earthquakes >Mw 3 in the 2013 Cook Strait earthquake sequence ¯ and the 2016 Kaikoura earthquakes (Holden et al., 2013; Hamling et al., 2017). Yellow dot is IODP Expedition 375 Site U1520 (Wallace et al., 2019). Note: ODP Site 1124, also mentioned in the text, is ∼350 km east of Site U1520. b) Enlargement of the southern Hikurangi subduction margin (map projection is UTM Zone 60S, WGS84 datum). Land is grey. Offshore shading is an overlay of slope dip (greyscale) and water depth (colour shades). Inter-plate vector (white) after DeMets et al. (2010). Red line is the approximate location of the subduction thrust, after Litchfield et al. (2014). “CSC” = Cook Strait Canyon. Black lines are seismic profiles from the PEG09 and APB13 industry seismic surveys. Yellow lines are segments of Line PEG09-05 and PEG09-23 that we have re-processed. Blue lines are research seismic lines collected aboard RV Tangaroa in 2018. (For interpretation of the colours in the figure(s), the reader is referred to the web version of this article.)

2. Map out faults at the leading edge of subduction deformation and investigate changes from NE to SW, i.e. into the transition to strike-slip motion. 3. Investigate dewatering near the deformation front, where both the décollement and upper plate faults are well imaged with seismic data. 4. Improve the characterisation of the southern extent of subduction and compressional deformation off north-eastern South Island. By addressing these aims we gain improved understanding of compaction, dewatering and tectonic deformation in this unique plate boundary transition zone that has hosted complex upper plate faulting and subduction interface afterslip in recent times. 2. Tectonic setting The Pacific Plate has been subducting beneath the Australian Plate since ∼24-30 Ma (Ballance, 1976; Stern et al., 2006), manifesting itself in the Hikurangi subduction margin (Fig. 1). The margin displays much variability along strike, with convergence obliquity increasing significantly from north to south (Barker et al., 2009; DeMets et al., 2010; Nicol et al., 2007). At the southern end of the margin, the subduction zone transitions into dextral strike slip faults of the Marlborough Fault Zone that, further south, merge

into the Alpine Fault continental transform plate boundary (Fig. 1a, b). Transition from subduction to strike slip is due to a combination of an along-strike change in the plate motion vector and the plate boundary orientation with respect to the plate motion vector (Barnes et al., 1998; Wallace et al., 2012a). The Hikurangi subduction zone experiences widespread SSEs, including shallow (<15 km), short-lived (a few weeks) and frequent (1- to 2-yr recurrence) SSEs in the northern and central parts of the margin, as well as deep (25-40 km), long (∼1 yr) and infrequent (∼5 yr recurrence) SSEs on the southern part of the margin (Bartlow et al., 2014; Wallace and Beavan, 2010; Wallace et al., 2012b, 2018). The absence of shallow SSEs beneath southern North Island, and further eastward to the trench, marks a zone of relatively high interseismic coupling (locking) on the southern extent of the subduction interface (Wallace et al., 2012b). Further south, beneath Cook Strait and the north-eastern region of South Island, afterslip was detected following the 2016 Mw 7.8 ¯ Kaikoura earthquake (Wallace et al., 2018). We note that afterslip does not necessarily indicate lower coupling and can occur on locked patches of a plate interface (Rolandone et al., 2018). Input sediments to the Hikurangi subduction zone have been sampled at Ocean Drilling Program Leg 181, Site 1124, and used for friction experiments (Boulton et al., 2019; Rabinowitz et al., 2018). A key result of these experiments was that plate-rate velocities promote velocity weakening behaviour, suggesting that sub-

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Table 1 Processing sequence for seismic lines PEG09-05 and PEG09-23. Steps (4)-(7) and Steps (8)-(11) are iterative loops, where the output velocity models were used to run successive pre-stack migrations to improve the velocity models at each iteration. 1. 2. 3. 4. 5. 6. 7. 8. 9.

Crooked line common midpoint (CMP) binning Low cut frequency filtering (3 Hz ramped to 5 Hz), anomalous amplitude attenuation, deconvolution CMP sorting, hyperbolic move-out (using smooth stacking velocity field) Pre-stack Kirchhoff time migration Automatic reflection picking using non-linear and parabolic curve scanning 2D travel time inversion using picks from (5) Velocity model building (smoothing result from (6))

10. 11.

Pre-stack Kirchhoff depth migration Automatic reflection event picking using non-rigid matching on a regular grid Reflector dip estimation Grid based ray tracing and 2D tomography

12. 13.

Residual move-out correction Stacking

ducting sediment could be capable of hosting SSEs without fluid over-pressure (Rabinowitz et al., 2018). 3. Data and methods 3.1. Seismic data We incorporate seismic data from two industry seismic surveys (the 2009 PEG09 survey and the 2013 APB13 survey), as well as new research seismic data collected in 2018 (the TAN1808 survey). Acquisition and processing parameters of these datasets are given in Supplementary Material A. We re-processed two seismic lines from the PEG09 survey (Lines PEG09-05 and PEG09-23). Our processing included iterative 2D reflection tomography and pre-stack time migrations (PSTM) and pre-stack depth migrations (PSDM). We carried out the processing sequence (Table 1) using Schlumberger’s Omega2 seismic processing suite. Steps (4)-(7) were done iteratively, where the output velocity models from Step (7) were used to run successive pre-stack time migrations. Imaging improved with successive velocity model refinement until a final velocity model (in time) was attained that produced the best flattening of common image point gathers. We then used this final PSTM velocity model to conduct the first PSDM (Step 8), which we again improved iteratively by repeating Steps (8) to (11) until we converged on a satisfactory final PSDM and velocity model. We carried out a total of eight iterations through Steps (8) to (11) before applying a residual moveout correction to flatten minor remaining moveout and then stacking to produce the depth sections. The final iteration was chosen when there was no visible improvement to the resolution of the velocity model. We estimate a conservative velocity error of 5% for the final velocity models. An important aspect of the reflection tomography and velocity model building in the depth domain is that it was grid based with non-hyperbolic reflection event picking on migrated common image point gathers, without being constrained by interpreted horizons. This has the advantage that we can confidently compare coherent velocity layering with seismic stratigraphic sequences in the knowledge that the velocity structure was not influenced by prior geological interpretation. Unlike full waveform inversion, our method cannot achieve vertical velocity resolution at the wavelet scale, meaning the upper and lower limits of velocity anomalies will be somewhat smeared at reflection boundaries. However, our final velocity models provide very good resolution of sedimentary sequences that exist over a series of reflections.

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3.2. V p -derived porosity predictions To aid the interpretation of our seismic velocity models, we have considered Erickson and Jarrard’s (1998) empirical relationships between compressional wave velocity (V p ) and porosity in siliciclastic sediments to derive estimates of porosity from velocity models (e.g. Calahorrano et al., 2008; Ellis et al., 2015; Tobin and Saffer, 2009). Erickson and Jarrard (1998) describe a range of V p -porosity relationships that depend on the shale volume fraction and consolidation history. For predictions of porosity from velocity further north on the Hikurangi margin, Ellis et al. (2015) assumed a shale volume fraction (V sh ) of 0.3 and considered both “normal” and “high” consolidation histories, as defined by Erickson and Jarrard (1998). Core samples from IODP Expedition 372/375 Site U1520, located on the incoming plate of the northern Hikurangi margin, indicate that sediments typically contain total clay minerals of ∼40% of the bulk sample (Wallace et al., 2019). A study from Tudge and Tobin (2013) from the Shikoku Basin, where sediment inputs to the Nankai Trough have been sampled, revealed much higher V sh values (∼75%) in hemipelagic mudstones. As such, it is appropriate to consider a wide range of V sh to examine how it will affect the range of possible porosities from a given velocity model. In this study, we explore the effect of varying V sh between 0.3 and 1.0. Likewise, we compare predicted porosities using both the “normal” and “high” consolidation relationships of Erickson and Jarrard (1998). We note that for non-siliciclastic sediments, Erickson and Jarrard’s (1998) method may not reliably estimate porosity. For comparison, we also considered Gardner et al.’s (1974) relationship between velocity and density to derive porosity indirectly from density assuming a sediment matrix density (ρm ) and a water density (ρ w ) of 2650 kg m−3 and 1035 kg m−3 , respectively. Details of the V p -derived porosity approaches are included in Supplementary Material B. Our aim with the porosity predictions on Lines PEG0923 and PEG09-05 is to be able to compare the two profiles, with less emphasis on the absolute porosity values since we acknowledge the associated uncertainties. From the porosity models, we also estimate effective vertical stress (σ v ) as the overburden pressure subtracted by hydrostatic fluid pressure:

σ v = ρb × G × z − ρ w × G × z,

(1)

where z is depth, G is gravitational acceleration (9.81 m s−1 ), ρb is bulk density = ρm (1 − ϕ )+ ρ w × ϕ , ρm is sediment matrix density, ρ w is water density and ϕ is porosity. As above, we assumed ρm and ρ w to equal 2650 kg m−3 and 1035 kg m−3 , respectively. We calculate σ v using the wide range of V p -derived porosity models noted above, thereby propagating the uncertainty into the σ v estimations. Constraining σ v allows us to compare velocities within a given stratigraphic unit at different sub-seafloor depths across different seismic profiles. 4. Results 4.1. Seismic depth imaging 4.1.1. PEG09-23 (off the Wairarapa coast) For Line PEG09-23 we focused our re-processing on the outer part of the profile, extending from the undeformed sedimentary sequences (to the SE) into the frontal thrust that represents the onset of deformation (to the NW) (Fig. 2a). (The un-interpreted PSDM image is in Supplementary Material C.) The thrust fault deforms the sedimentary sequence above a prominent reflector, referred to in previous studies as “Reflector 7”, which has been interpreted as the boundary between Miocene

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Fig. 2. Final pre-stack depth migration of (a) Line PEG09-23 and (b) Line PEG09-05. Upper plate faults are annotated as solid yellow lines. Broken lines are selected seismic reflections (e.g. Reflections 7 and 8; after Plaza-Faverola et al., 2012). Broken purple line is the interpreted décollement. In Line PEG09-05, we interpret a step down in the décollement around 10 km (along profile) to explain coupled deformation above and below Reflector 7. MES = Cretaceous sedimentary rocks termed MES by Davy et al. (2008). MS4 is the “Marlborough Slope 4” fault after Litchfield et al. (2014). Green lines are the bottom simulating reflection (BSR). Inset-a1) Broken purple line is the level where the décollement forms, above Reflector 7. Inset-b1) High gain shows Reflector 7 continuing to the NW of the profile. Inset b-2) Folding and offset reflections indicate deformation across Reflector 7. Inset-b3) Broken purple line is the level where the décollement forms, above Reflector 7. Inset-b4) Subtle but distinct uplift of the BSR (green line) at the main thrust (yellow arrows) beneath the Hikurangi Channel.

sediments (above) and a condensed section of Late CretaceousEarly Oligocene (70-32 Ma) sediments (below) (Plaza-Faverola et al., 2016, 2012). Davy et al. (2008) refer to this Late CretaceousEarly Oligocene condensed sequence as “Sequence Y”. As such, Reflector 7 correlates to Sequence Y. Following the nomenclature of Plaza-Faverola et al. (2012, 2016), we also identify the weaklyreflective sequence beneath Reflector 7 that has been inferred to comprise Cretaceous sedimentary rocks, termed “MES” in Davy et al. (2008) (Fig. 2a). Beneath the MES sequence is “Reflector 8”, marking the top of the Early Cretaceous Hikurangi Plateau oceanic volcanic sequence, termed “HKB” by Davy et al. (2008). Plaza-Faverola et al. (2012, 2016) noted that the décollement often sits some distance above Reflector 7. On Line PEG09-23 it is approximately 200 m (or more) shallower than Reflector 7, as revealed by the shallowest reflection that has not been disrupted by the base of the splay fault (Fig. 2a; inset-a1). We define the start of the décollement as the point on a 2D section where the splay fault projects down onto the deepest undisturbed reflector. The onset of deformation above the décollement in Line PEG09-23 is characterised by a complex, but very well imaged, fault-propagation fold (Fig. 2a). In the lower half of the sedimentary section, the thrust fault system is split into two prominent seaward-verging thrust faults, dipping at ∼30-35◦ . The upper part of the sequence is characterised by numerous subtle but distinct seaward-verging and landward-verging proto-thrusts (Fig. 2a), features that have been investigated in detail further north on the margin by Barnes

et al. (2018). The proto-thrusts extend ∼5 km landward into the hanging wall of the main thrust, and ∼2 km seaward into the footwall. ¯ coast) 4.1.2. PEG09-05 (off the Kaikoura For re-processing Line PEG09-05 we selected a section of the line extending across the major thrust “MS4”, through the Hikurangi Channel, and up onto the Chatham Rise (Fig. 2b). The name MS4 is after Litchfield et al. (2014), standing for “Marlborough Slope 4”. As with Line PEG09-23, Reflector 7, Reflector 8 and the MES sequence are all identifiable on this profile (Fig. 2b). (The uninterpreted PSDM image is in Supplementary Material C.) The fault MS4 is manifested by the nature of fault propagation folding in the hanging wall. The base of the fault is not imaged, but Reflector 7 can still be traced to the northwest (Fig. 2b; inset-b1), and it is likely that the fault soles out in the vicinity of Reflector 7. Further seaward of fault MS4 there is still distinct folding, for example in the region ∼10 km along the profile at ∼3 km depth (Fig. 2b). Interestingly, Reflector 7 is also folded in this region, ∼11 km along profile and ∼4 km depth, and offset by a local thrust fault (Fig. 2b; inset-b2). Further seaward, between ∼15 and ∼19 km along profile, is a region of dense seaward- and landwardverging thrust faults beneath the Hikurangi Channel (Fig. 2b). We hereafter refer to these, informally, as the “Hikurangi Channel Thrusts”. The main thrust within this network dips at ∼40◦ . The lower extent of the faults is well imaged; they sole onto a dé-

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collement at least 100 m above Reflector 7 (Fig. 2b; inset-b3). As such, the stratigraphic level of the décollement is similar to that on Line PEG09-23, albeit at a much shallower sub-seafloor depth. At the point where the fault crosses a bottom simulating reflection (BSR)—the seismic indicator of the base of gas hydrate stability (BGHS)—we observe a subtle but distinct uplift of the BSR (Fig. 2b; inset-b4). The deformation of Reflector 7 at ∼11 km along profile suggests that the décollement has stepped down deeper into the sedimentary succession (Fig. 2b).

Hikurangi Channel Thrust where compressional folding and thrusting initiate. Velocities within LVZ1 (Fig. 3e) remain relatively stable (∼2000 m s−1 –2500 m s−1 ) along the southeastern half of the profile, between traces 4100 and 6500, i.e. SE of the Hikurangi Channel Thrusts. On the northwestern side of the profile, between traces 2300 and 4100, velocities within LVZ1 increase steadily as they become more deeply-buried and experience higher vertical effective stress (Fig. 3e, 3f).

4.2. Seismic velocity models

4.2.3. V p and V p -derived porosity in LVZ1, comparing Lines PEG09-05 and PEG09-23 To detect differences in the MES sequence between Line PEG0923 and Line PEG09-05, we plot V p along the MES sequence as a function of effective vertical stress σ v on the MES sequence (Fig. 4a). The different coloured lines in Fig. 4a correspond to a range of V p -porosity model scenarios, as outlined in the Methods (Section 3.2). The MES sequence in Lines PEG09-23 and PEG0905 exists under notably different ranges of σ v ; in general, the MES sequence in Line PEG09-23 is significantly deeper (higher σ v ) than it is in Line PEG09-05. There is, however, a range of σ v on the order of 25-35 MPa where the MES sequence exists on both lines, which reveals that velocities within the sequence on Line PEG09-05 are distinctly higher than they are in Line PEG09-23 at a given σ v (Fig. 4a). These higher velocities at a given σ v within Line PEG09-05 point to higher compaction and lower porosities (Fig. 4b, c, d), with uncertainty in actual porosity values because it is unknown which V p -porosity relationship best suits the MES sediments.

4.2.1. PEG09-23 velocities (off the Wairarapa coast) The sediments above Reflector 7 are, in general, characterised by steadily increasing seismic velocity with depth (Fig. 3a). Exceptions to this are at the BGHS, where free gas beneath gas hydrate-bearing sediments leads to a pronounced velocity inversion and often quite thick low velocity zones beneath the BGHS. LVZ3 and LVZ4 (Fig. 3a) are caused by free gas beneath the BGHS. Much deeper in the section are two other pronounced low velocity zones, herein referred to as LVZ1 and LVZ2. LVZ1 is a widespread low velocity zone that occurs within the MES sequence beneath Reflector 7. With increasing depth of burial towards the NW, LVZ1 gradually diminishes and ultimately disappears, i.e. the velocity inversion at Reflector 7 ceases to exist (Fig. 3a). This disappearance of LVZ1 occurs seaward of the onset of thrust faulting and before sediments within the MES sequence are subducted. LVZ2 is a region of anomalously low velocities around the base of the splay fault, in a region where there is pronounced folding and faulting. Despite the deformation, there are clear seismic reflections that have formed the basis of detecting this low velocity anomaly; it is unlikely that the low velocity zone is an artefact of seismic processing. The evolution of velocities within LVZ1 from seaward of the deformation front (SE) to landward of the deformation front (NW) is shown in Fig. 3b (note: estimated effective vertical stress along this horizon is given in Fig. 3c). This plot of velocities (Fig. 3b) within a 200 m window of the upper part of MES shows how velocities increase from SE to NW along the line. In the region beneath the splay fault, velocities are no longer lower than velocities of the sediments above Reflector 7. Between traces 7500 and 9000, over a distance of ∼10 km, velocities within LVZ1 remain relatively stable at just above 3000 m s−1 (Fig. 3b). To the northwest of trace 9000 velocities steadily increase from just above 3000 m s−1 to approximately 5000 m s−1 at the landward end of the profile. ¯ coast) 4.2.2. PEG09-05 velocities (off the Kaikoura Reflector 7 occurs at a significantly shallower sub-seafloor depth on Line PEG09-05 than on Line PEG09-23. As on Line PEG0923, Reflector 7 marks the boundary between LVZ1 within the MES sequence and the higher velocity sediments above (Fig. 3d). From SE to NW, LVZ1 extends from the Chatham Rise, through the Hikurangi Channel and into the deforming wedge. It is unclear how far beneath the wedge (to the NW) LVZ1 continues, as we cannot resolve it beyond approximately Trace 2500 (Fig. 3d). The continuation of LVZ1 beneath the deforming wedge contrasts with LVZ1 on Line PEG09-23, where, on that line, LVZ1 already ceases to exist SE of the wedge. Other anomalous low velocity zones on this line (LVZ5, LVZ6 and LVZ7; Fig. 3d) are due to free gas beneath the BGHS. The velocity resolution is not sufficient to inspect velocities around the base of the MS4 thrust fault. Velocities around the Hikurangi Channel Thrusts, on the other hand, are well resolved because of good seismic imaging in this region. In clear contrast to the splay fault beneath Aorangi Ridge (Line PEG09-23; Fig. 3a), there is no significant low velocity zone around the root of the

4.3. NE–SW evolution of frontal deformation We have used PSTM seismic sections from the PEG09 and APB13 surveys to map out the distribution of thrusts near the deformation front. To extend the mapping further south, we incorporate high-resolution data from the TAN1808 voyage. The results are shown in Fig. 5, in a map that also shows the dominant structures published by Litchfield et al. (2014). Many of the faults we have mapped provide new insight into upper plate deformation. For example, seaward of Opouawe Bank, and further southwest to Kekerengu Bank, there is a distinct increase in the role of backthrusts (landward-verging thrusts) in accommodating upper plate deformation (Fig. 5) (Barnes et al., 2010). Another observation further to the southwest is the widespread occurrence of thrust faulting within the western branch of the Hikurangi Channel. Examples of these faults are shown in Fig. 5b-e. Most of the Hikurangi Channel thrusts do not extend down to near Reflector 7 (Fig. 5b-d). However, in the southwestern extent of the channel, where it moves progressively closer to the deformation front of the wedge, the thrusts begin to extend deeper down in the stratigraphic section (Fig. 5e). By Line PEG09-05, the main Hikurangi Channel thrust extends down to the same stratigraphic depth (i.e. close to Reflector 7) as where the décollement has formed further inboard (Fig. 2b). To display relationships between upper plate faults shown in Fig. 5 and the distribution of Reflector 7, we show selected sections from NE to SW in Figs. 6 and 7. The most significant changes in the nature of deformation occur toward the southern end of the margin, in the region where the subduction zone eventually terminates. Across most of the southern Hikurangi margin, from Line PEG09-23 in the North to Line PEG09-05 in the South, the décollement develops just above Reflector 7 (Fig. 6). Changes in the expression of Reflector 7 and the folding of overlying strata occur southwest of Line PEG09-05. Here we explain these observations moving from Northeast (Fig. 7a) to Southwest (Fig. 7d). On Line TAN1808-141 (Fig. 7a), we observe similar

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Fig. 3. Overlay of pre-stack depth migrated seismic reflection data and interval velocity models for seismic lines PEG09-23 (a) and PEG09-05 (d). Note the different velocity range and plotting scales for these two lines. Both are vertically exaggerated by a factor of 2. Fine black lines are faults, broad broken white line is the base of gas hydrate stability (BGHS), dotted white lines are Horizons 7 and 8. The solid white line is a horizon picked through the MES sequence (i.e. where LVZ1 occurs). The blue line in (b) and (e) represents interval velocities extracted along the MES sequence on Lines PEG09-23 and PEG0905, respectively. The velocities in (b) and (e) are averaged within a 200 m vertical window around the MES horizons (i.e. from 100 m above to 100 m below the solid white lines in (a) and (d)). The pale blue regions around the blue lines represent a conservative estimate of the error (5%) in the velocities. Black circles in (b, near Trace 9000) and (e, near Trace 4100) are locations discussed in the text where there is a pronounced change in the V p trend along the profile. c and f) Effective vertical stress σ v along the MES sequence (at the same depth as the V p extractions in (b) and (e)) for Lines PEG09-23 and PEG0905, respectively. The range (grey band) comes from a range of V p to porosity estimations as outlined in the Methods and in Supplementary Material B.

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Fig. 4. a) V p plotted against effective vertical stress σ v , from the “white MES horizon” in Figs. 3a and 3d. Different coloured lines represent different V p -porosity relationships (see legend). The lines plotting in the upper part of the plot (i.e. at lower σ v ) are from Line PEG09-05, where the MES sequence is, in general, shallower beneath the seafloor. The lines in the lower part of the plot are from Line PEG09-23. The MES sequence exists in both lines over a common σ v (around 30 MPa). b-d) Predicted porosity versus σ v along the MES sequence (white MES horizon from Figs. 3a and 3d) based on different V p -porosity models (see legend), as described in the Methods and in Supplementary Material B. Legend: “Erickson and Jarrard” refers to Erickson and Jarrard (1998); “Vsh” is shale volume content (proportion); “Gardner” refers to Gardner et al. (1974). Horizontal grey bars in all plots represent the approximate σ v range where LVZ1 disappears on Lines PEG09-05 and PEG09-23 – i.e. on each line, LVZ1 exists at σ v values < the grey bars, but does not persist to σ v values > the grey bars.

structures to Line PEG09-05; thrust faulting beneath the Hikurangi Channel and the existence of the MS4 splay fault beneath the slope. Further to the southwest (Line TAN1808-138; Fig. 7b) the data display pronounced folding on the north-western side of the profile. The north-westward continuation of Reflector 7 on this line shows that, rather than continuing deeper beneath the seafloor, it is folded up into the first anticline of the slope sequence. Thrust faulting beneath the channel persists on this line and terminates well above Reflector 7. Further to the southwest (Line TAN1808-139; Fig. 7c), there is pronounced folding of Reflector 7 beneath the Hikurangi Channel. A normal fault from the North Mernoo Fault Zone (Barnes, 1994a, 1994b) is also imaged beneath the channel. On the north-western end of the profile there is some ambiguity about where Reflector 7 continues, but it remains apparent that it is folded up into the slope stratigraphy. The slope stratigraphy on this line is still characterised by pronounced folding, but these data do not penetrate deep enough to image underlying thrusts associated with this folding. Although constant velocity (1500 m/s) processing of these data leave some uncertainty in reflector geometry, the nature of the pronounced folding we observe (e.g. Fig. 7c) will have been only effected to a minor degree by the seismic processing velocity. Our final seismic line to the southwest (Line TAN1808-136; Fig. 7d) shows that the shallow slope stratigraphy on the north-western half of the profile are not folded. This represents a stark contrast to the same slope stratigraphy on the line immediately to the northeast (cf. Lines TAN1808139 and TAN1808-136, Figs. 7c and 7d, respectively). Some subtle folding on Reflector 7 persists (Fig. 7d), but to a much lesser degree than in the lines to the northeast. On the south-eastern end of the profile, Reflector 7 is offset by a previously identified deep thrust (Barnes, 1994a) that has presumably formed as a result of impingement of the buoyant Chatham Rise on the southern end of the subduction margin.

5. Discussion 5.1. Nature of the subducted (MES) sedimentary sequence 5.1.1. Existing understanding of the MES sequence The weakly-reflective MES sequence beneath Reflector 7, inferred to comprise Cretaceous sedimentary rocks (Davy et al., 2008; Plaza-Faverola et al., 2012), is easily distinguishable on a regional scale on the southern Hikurangi subduction margin. Anomalously low velocities in this sequence have been identified previously over broad regions, extending at least as far as from 41◦ South to 42◦ South (Plaza-Faverola et al., 2012, 2016). Our results show that the low velocity zone (referred to as LVZ1 in this study, and in Plaza-Faverola et al., 2012) extends at least another 100 km along strike to the southwest. LVZ1 is therefore a widespread feature that is inherently characteristic of the MES sequence (Fig. 8). Plaza-Faverola et al. (2012) suggested small amounts of gas, possibly thermogenic, within the MES sequence (at ∼42◦ S, on Line PEG09-19) could be at least partly responsible for LVZ1. In assessing the equivalent low velocity zone further north on the margin (i.e. at ∼41◦ S, on Line 05CM-38), Plaza-Faverola et al. (2016) interpreted it as the manifestation of fluid-rich over-pressured sediments, capped by a low-permeability condensed layer of chalk and interbedded mudstones (i.e. Sequence Y). The degree to which Sequence Y behaves as a regional seal remains unknown. 5.1.2. Compaction in the MES sequence: comparing Lines PEG09-23 and PEG09-05 The occurrence of LVZ1 within the MES sequence on both Lines PEG09-23 and PEG09-05 (Fig. 3) provides an opportunity to investigate the evolution of this sequence from northeast to southwest as it moves into the transition from subduction to strike slip. Since the MES sequence is so widespread on the margin and of such consistent seismic character, we deem it unlikely that lithology within the sequence varies significantly between Line PEG09-23 and PEG09-05. A key difference between the two lines is the

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Fig. 5. a) Map of thrust faults at the deformation front of the wedge (black toothed lines, where the teeth indicate fault dip direction). Red lines are the active faults from ¯ Litchfield et al. (2014), for reference. The yellow contours are centimetres of afterslip in the year after the 2016 Kaikoura earthquake, reported by Wallace et al. (2018). The reader is referred to Wallace et al. (2018) for more details about this event. Grey lines are seismic profiles; labelled orange and blue segments are shown in Figs. 6 and 7, respectively. b-e) Segments of seismic profiles (see purple lines in Fig. 5a for locations) at the Hikurangi Channel. Thrust faults (black lines) and Reflector 7 (broken yellow line) are annotated.

stark difference in sediment thickness above the décollement in the vicinity of the deformation front (∼1.2 km sediment thickness on Line PEG09-05 versus ∼4 km on Line PEG09-23; Fig. 2). In Line PEG09-23, where LVZ1 disappears before the MES sequence is subducted (Fig. 3a), significant compaction has occurred from σ v . Indeed, the same was observed in Line PEG09-19, where LVZ1 disappears prior to MES being subducted (Plaza-Faverola et al., 2012). Since LVZ1 disappears close to the frontal thrusts on both lines, it may be possible that horizontal stress associated with the thrusting starts having an effect on compaction immediately ahead of the thrusts. In any case, the disappearance of LVZ1 requires effective drainage of fluid out of the MES sequence during compaction. On Line 05CM-38, LVZ1 within the MES sequence persists further down dip and ultimately occurs beneath the décollement (Plaza-Faverola et al., 2016), an observation that has been made at other subduction margins (e.g. the Ecuadorian margin, Nankai and Alaska; Calahorrano et al., 2008; Li et al., 2018; Tobin and Saffer, 2009; Tsuji et al., 2008). On Line PEG09-05, LVZ1 also exists down dip of the thrust faults beneath the Hikurangi Channel (e.g. Fig. 3d).

In the plot of velocity against

σ v , it is clear that the MES

sequence in Line PEG09-05 hosts significantly higher seismic velocities for a given sub-seafloor burial depth than it does in Line PEG09-23 (Fig. 4a), some 200 km to the northeast (Fig. 1). Despite the uncertainties in porosity predictions from V p , it is likely that this correlates to a history of greater compaction of the MES sequence in Line PEG09-05 than in Line PEG09-23 (Fig. 4b, c, d). The greater degree of compaction in the MES sequence in Line PEG0905 is expected given that we observe horizontal compression in the form of both folding and faulting of this sequence (e.g. Fig. 2, inset-b2; Fig. 7) in the more southern reaches of the subduction zone. We do not observe such compressional structures in the MES sequence beneath the undisturbed basin fill in Line PEG09-23. We interpret the horizontal compression within the MES sequence further south (i.e. in the vicinity of Line PEG09-05) to be the result of the impingement of the bathymetrically-elevated Chatham Rise (e.g. Figs. 2b, 7a) into the deformation front of the subduction margin.

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Fig. 6. Segments of seismic profiles across the deformation front, from NE to SW (a-d) – see Fig. 5a for locations. Broken yellow line is Reflector 7; black lines are thrust faults. In (a)-(c) it is clear that the décollement (broken purple line) approximately coincides with Reflector 7, NW of the frontal thrusts. In (d), coupled deformation above and below Reflector 7 (in the region circled red; see also Fig. 2b) suggests that the décollement steps down beneath Reflector 7 in the region of the red circle. This is represented schematically in Fig. 9b.

5.1.3. Lateral variations in drainage of the MES sequence It is important to consider the inference from Plaza-Faverola et al. (2016) that LVZ1 represents significantly over-pressured fluids within the MES sequence, as well as other global examples of over-pressured subducted sediments. The underthrust section of the Nankai Trough subduction zone provides a useful comparison; it is characterised by relatively low and stable velocities (between 2000 m s−1 and 2500 m s−1 ) over a lateral distance of at least 8 km from the trench, despite a steady increase in overburden stress (Tobin and Saffer, 2009). This lack of a correlation between overburden stress and seismic velocity within the underthrust sec-

tion (in particular beneath the outer part of the wedge) can be explained by invoking higher overpressure within the underthrust section. There is evidence for these types of patterns in the MES sequence on both Lines PEG09-23 and PEG09-05. The most pronounced region of relatively stable velocities with increasing σ v on Line PEG09-23 occurs over a distance of ∼10 km east of Trace 9000, where the velocities are ∼2900-3400 m s−1 (Fig. 3b, c). On Line PEG09-05, such a region of relatively stable velocities (∼2000-2500 m s−1 ; Fig. 3e) occurs for at least 10 km from Trace 4100 to the southeast. We interpret these regions of stable velocities within the MES sequence on both lines as evidence for

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¯ Fig. 7. Segments of high-resolution seismic profiles across the slope from the TAN1808 survey east of Kaikoura. From NE to SW (a-d) (see Fig. 5a for locations). Broken yellow line is Reflector 7, black lines are thrust faults.

over-pressured fluids due to relatively poor drainage, both upward across Reflector 7 and laterally upward along the MES sequence to the southeast. Steadier increases in velocity with σ v on the northwestern halves of both profiles (Fig. 3) is likely due to better drainage of the MES sequence. Drainage in both cases, Lines PEG09-23 and PEG09-05, is likely facilitated by fluids migrating upward through Reflector 7, possibly through small scale faults and fractures, and into the upper plate fault networks beneath Aorangi Ridge (Fig. 3a) and the Hikurangi Channel thrusts (Fig. 3d), respectively. We depict these fluid flow scenarios for both lines conceptually in Fig. 8, using the geometry from the pre-stack depth migrations. For comparison, in Fig. 8 we also draw the relationships revealed by pre-stack depth migrations from Plaza-Faverola et al. (2012, 2016) for Lines PEG09-19 and 05CM-38. Plaza-Faverola et al. (2016) observed LVZ1 within the MES sequence persisting over

a distance of up to 40 km from the deformation front, as the overlying wedge thickens. They inferred that relationships between a sealing layer above (Sequence Y) and an oceanic seamount (Fig. 8a) might provide conditions that enable fluid compartmentalisation (i.e. poor drainage) and the development of high fluid pressures within the MES sequence. This is distinctly different from what we observe on Line PEG09-23 and what Plaza-Faverola et al. (2012) observed on Line PEG09-19, where LVZ1 has disappeared before it even enters the subduction zone (Fig. 8b, c). It is also clear that sediment thickness above the incoming sequence is highly variable along strike, a factor that controls the degree of vertical stress-derived compression of the sediments before they become subducted. Such differences along strike on the margin could have fundamental importance for plate interface coupling and seismic style.

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Fig. 8. Line drawings of parts of depth migrated seismic lines (a) 05CM-38 (after Plaza-Faverola et al., 2016), (b) PEG09-23 (this study), (c) PEG09-19 (after PlazaFaverola et al., 2012) and (d) PEG09-05 (this study). Brown line = seafloor; thick grey line = décollement; cyan region = LVZ1; orange arrows are interpreted fluid drainage pathways. Long arrows represent effective drainage; short arrows represent poorer drainage. Note: the locations of the seismic lines, upon which these line drawings are based, are shown in Fig. 1a.

5.2. Upper plate fluid flow Numerous studies on the Hikurangi margin have highlighted examples of large-scale, focused, upper plate fluid flow, which is often manifested in changes to gas hydrate stability (Crutchley et al., 2011; Pecher et al., 2010, 2017; Plaza-Faverola et al., 2016) and methane seepage out of the seafloor (Barnes et al., 2010). The pressure gradients that drive fluid flow along faults can come from long-term vertical and horizontal compaction (Townend, 1997). It is also possible that short-term strain associated with SSEs or earthquakes can cause localised fluid pulses (e.g. Brown et al., 2005; LaBonte et al., 2009; Pecher et al., 2017; Plaza-Faverola et al., 2016). Gas accumulations within anticlinal closures (e.g. beneath Aorangi Ridge; Fig. 3a) are often ambiguous in terms of interpreting fault-controlled fluid flow because it is also possible that fluid accumulation within such closures is a function of layer-parallel fluid flow focusing, independent of faulting. However, the existence of a low velocity zone and free gas at the intersection of protothrusts seaward of Aorangi Ridge (Fig. 3a), where the sediments are not folded, indicates that these faults are indeed preferred dewatering pathways. Without these proto-thrusts, there would be no mechanism to accumulate thick gas pockets in this part of the

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sub-surface because there are no folded strata to focus fluid flow. Line PEG09-05 also shows evidence for dewatering along the primary fault beneath the Hikurangi Channel, in the form of a subtle but distinct upward bend in the BSR coincident with the point where it crosses the fault (Fig. 2, inset-b4). This upward bend in the BSR is a typical manifestation of advective heat flow along a fault, perturbing the shallow thermal field and thereby the BGHS (e.g. Zwart et al., 1996). Thus, our imaging of the splay faults, free gas accumulations and BSR perturbations at the leading edge of deformation on Lines PEG09-23 and PEG09-05 corroborates that focused upper plate dewatering is a widespread feature of the Hikurangi subduction margin. Further, we show that it occurs right at the deformation front and is not restricted to splay faults further inboard (cf. Pecher et al., 2010; Plaza-Faverola et al., 2016). This is important because this onset of upper plate fluid flow can provide an explanation for the apparent well-drained nature of the MES sequence in the northwestern extents of the two profiles (Lines PEG09-23 and PEG09-05), manifested as a steady increase in V p with increasing σ v (Fig. 3b and 3e) and depicted conceptually in Fig. 8. LVZ2 (Fig. 3a) is situated over the base of the thrust fault, in a region where there is some topography and discontinuity on Reflector 7. The existence of LVZ2 only within the region of compressional deformation around the base of the thrust fault, and not within the same strata at greater depth toward the NW, suggests that LVZ2 is a manifestation of high fluid pressure in this region (e.g. Chopra and Huffman, 2006; Dutta, 2002). Our interpretation is that upward drainage from the MES sequence, as well as compressional deformation around the base of the fault, has led to excess fluid pressure in these sediments, which in turn is driving focused fluid flow upward along the thrust fault network. We do not observe any localised amplitude anomalies at depth along the fault networks, suggesting that free gas is not sourced from the base of the thrust fault. The free gas that accumulates at the BGHS is therefore likely to have come out of solution at relatively shallow depths after being transported in solution from greater depth or being generated relatively close to the BGHS (Kroeger et al., 2015). 5.3. Insights into the southern end of the subduction margin The southern end of the Hikurangi margin is a complex subduction termination and transition into strike slip motion. The region has been studied using a range of data types and analyses, including fault slip rates, controlled-source seismic imaging, aftershock distributions, GPS velocities and strain and kinematic modelling (e.g. Barnes et al., 1998; Holt and Haines, 1995; Mouslopoulou et al., 2019; Wallace et al., 2012a, 2018). The distribution of thrust faults within the Hikurangi Channel off the East Coast of northern South Island represents a major seaward-directed step in the expression of compressional deformation at the plate boundary. This step occurs just to the west of the confluence between Cook Strait Canyon and the Hikurangi Channel. We interpret that the step seaward has developed because of the impingement of the bathymetrically-elevated Chatham Rise into the southern end of the active subduction margin (Fig. 9). Because of the highly-oblique convergence in this region, it is likely that the Hikurangi Channel thrusts also accommodate dextral strike slip motion (Fig. 9b). This may be manifested by the steeper fault dip at the Hikurangi Channel Thrust (Fig. 2b, inset-b4) compared to the splay fault on Line PEG09-23 (Fig. 2a). Since some of the Hikurangi Channel thrusts extend down to the same stratigraphic level as the décollement observed further inboard (most clearly observed on Line PEG09-05; Fig. 2b), we consider it likely that active subduction extends east as part of this seaward step in compressional deformation. In this case, we expect the subduction interface to step down below Reflector 7 in the region of Fig. 2b (inset-b2). We

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Fig. 9. 3D perspective conceptual sketch of the southern end of the Hikurangi subduction zone. a) Topography and bathymetry draped, showing locations of three cross sections (x-x ; y-y ; z-z ) which are approximately projected along the locations of Line PEG09-15 (Fig. 5a; Fig. 6c), Line PEG09-05 (Fig. 5a; Fig. 6d) and Line TAN1808-139 (Fig. 5a; Fig. 7c), respectively. Solid white line = coastline; broken black lines = surface projections of faults (this study and Litchfield et al. (2014); NI = North Island; SI ¯ = South Island; OB = Opouawe Bank; KB = Kekerengu Bank; HC = Hikurangi Channel; KP = Kaikoura Peninsula; OSTF = Offshore Splay Thrust Fault (after Mouslopoulou et al., 2019); dotted red line = approximate northern extent of Chatham Rise; broken purple line = décollement; blue line = Reflector 7. Coordinates are metres in UTM Zone 60S. White interplate vector is after DeMets et al. (2010). Note: the Awatere, Clarence and Hope Faults are dominantly dextral strike slip faults, with a secondary component of reverse motion (Litchfield et al., 2014). b) Same view as (a) but with topography and bathymetry stripped back to cross section x-x . Peach coloured surface is the décollement. Between cross section y-y and z-z the décollement steps down deeper than Reflector 7. Label (1): Hikurangi Channel Thrust at the impingement of the Chatham Rise with the deformation front. Thrusts here likely accommodate some dextral strike slip motion. Labels (2) and (3): Folding of Reflector 7 suggests the décollement has stepped down deeper than Reflector 7.

depict this scenario conceptually in Section y-y of Fig. 9b. With the thrusts soling out at various sub-seafloor depths (Fig. 5b-e, Fig. 7a-b), subduction propagation out to the Hikurangi Channel is complex and may be limited to the region directly southeast of the MS4 Fault (Fig. 5a); a region where the northern margin of the Chatham Rise encroaches close to the deformation front (Fig. 9b, “Label 1”). ¯ Afterslip following the 2016 Kaikoura earthquake gave new insight into the southward extent of active subduction on the Hikurangi margin (Wallace et al., 2018; Fig. 5a). Our observations from ¯ seismic lines east of Kaikoura provide further constraints on the southern extent of subduction, since they reveal key relationships between Reflector 7 (i.e. Sequence Y, the approximate stratigraphic marker for subduction) and deformation in the overlying strata. The seismic lines crossing the Kowhai Sea Valleys area (Fig. 5a) display clear folding in the slope stratigraphy (Fig. 7a-c), highlighting the pronounced compressional deformation still taking place in this highly transpressional zone of the plate boundary. The distinct change in the behaviour of Reflector 7 in this region is important (Fig. 7b, c; Fig. 9b, “Labels 2 and 3”) because it shows that Reflector 7 (or any horizon above it) no longer hosts the subduction décollement. This marks a profound change in the subduction system (depicted in Fig. 9b). Since there is evidence that active subduction still occurs this far south (Wallace et al., 2018; Fig. 5), the décollement must step down deeper in the sedimentary succession than Reflector 7 (Fig. 9b). Our data do not penetrate sufficiently deep nor extend far enough to the west to interrogate the character of a deeper subduction interface. The step down of the décollement beneath Reflector 7 is likely to be related to the continual reduction in sediment thickness above Reflector 7 toward the southwest, as well as effective drainage of the MES sequence around the Hikurangi Channel. Mouslopoulou et al. (2019) propose that a major splay fault (that they term “OSTF”) to the plate interface was a key compo¯ nent of the Kaikoura Earthquake, and that upper-plate fault rupture with a minor component of plate interface slip dominates the subduction zone termination. In Fig. 9 we show the approx-

imate location of the OSTF (after Mouslopoulou et al., 2019), as well as other major inboard strike slip faults in the context of our interpretation of where subduction associated with Reflector 7 is abandoned in the offshore region between the OSTF and the Hikurangi Channel. Mouslopoulou et al.’s (2019) model is compatible with our observations in the sense that the OSTF lies directly inboard of the region where Reflector 7 is no longer exploited as a subduction décollement. 6. Conclusions We have used controlled-source seismic data from three seismic surveys to give new insight into key components of the subduction to strike slip transition zone on the Hikurangi margin. Pre-stack depth migrations with iterative reflection tomography for velocity updating (on Lines PEG09-23 and PEG09-05) confirm the existence of a widespread low velocity zone in sediments that ultimately become subducted (the MES sequence). The thickness of sediments above the MES sequence is variable along strike, with sediment thickness decreasing markedly to the southwest. Velocities and velocity-derived porosity estimates within the MES sequence in Line PEG09-23, approximately 200 km to the northeast of Line PEG09-05, indicate that it is less compacted for a given effective vertical stress than it is in Line PEG09-05. Greater compaction of the sequence in Line PEG09-05 is likely due to greater horizontal compression in the area where the Chatham Rise impinges toward the deformation front. The MES sequence in both lines appears to be relatively poorly drained in regions well seaward of the deformation front, where velocities remain low despite increasing effective vertical stress toward the deformation front. Closer to the deformation front, however, and further landward beneath the frontal thrusts, steady increases in velocity with increasing effective vertical stress suggest that the MES sequence is well drained and therefore unlikely to host excess fluid pressure. Effective drainage of the subducted sequence, upward through frontal thrusts, may be an important factor for high interplate coupling known on the southern Hikurangi margin.

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Upper plate dewatering at the deformation front is evidenced by free gas accumulations at the intersections of proto-thrusts and the BGHS, as well as from local uplift of the BGHS which is indicative of advective heat flow. We expect both vertical compaction and horizontal compression to provide the driving force for the dewatering. Subduction throughout the southern Hikurangi margin exploits a décollement that occurs approximately at a horizon referred to as Reflector 7, interpreted in previous studies as a condensed layer of chalk and interbedded mudstones (also referred to as Sequence Y). ¯ Slip on or above this horizon ceases off the coast from Kaikoura, where Reflector 7 itself becomes accreted into the slope stratigraphy. The décollement inboard of this region must step down deeper in the sedimentary succession, and slip may also be transferred into a large upper plate splay fault (the OSTF; Mouslopoulou et al., 2019). Subduction-related compression of slope stratigraphy ¯ persists almost as far south as the Kaikoura Canyon, where folding and faulting of offshore slope sediments appears to cease. Our insight into the nature of subducted sediments, upper plate dewatering and the southern extent of subduction, are important for improving the understanding of subduction-related seismic hazard in central New Zealand. Future research is required to understand how fluids within the subducted sequence might contribute to mechanical strength of the décollement. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Seismic data presented in this paper are available from New Zealand Petroleum and Minerals, a division of the Ministry for Business, Innovation and Employment. The data in Fig. 6 were processed by CGG Services (Singapore) Ltd. under contract to Anadarko New Zealand Company in 2014. We thank the captain and crew of R V Tangaroa and extend our particular gratitude to Will Quinn (NIWA) and the TAN1808 science team for their excellent technical support to acquire the data presented in Fig. 7. We are grateful to JY Collot and two anonymous reviewers for their thorough and insightful reviews that significantly improved this paper. We also thank Laura Wallace (GNS Science) for valuable discussions and for providing slip magnitude data to create the contours in Fig. 5. This research was supported by NZ Government funding to GNS Science (MBIE Strategic Science Investment Fund and MBIE Endeavour Fund “Hikurangi Subduction Earthquakes and Slip Behaviour”, Contract no. C05X1605). Appendix A. Supplementary material Supplementary material related to this article can be found online at https://doi.org/10.1016/j.epsl.2019.115945. References Ballance, P.F., 1976. Evolution of the Upper Cenozoic Magmatic Arc and plate boundary in northern New Zealand. Earth Planet. Sci. Lett. 28 (3), 356–370. Bangs, N.L., McIntosh, K.D., Silver, E.A., Kluesner, J.W., Ranero, C.R., 2015. Fluid accumulation along the Costa Rica subduction thrust and development of the seismogenic zone. J. Geophys. Res., Solid Earth 120 (1), 67–86. Barker, D.H.N., Sutherland, R., Henrys, S., Bannister, S., 2009. Geometry of the Hikurangi subduction thrust and upper plate, North Island, New Zealand. Geochem. Geophys. Geosyst. 10 (2). Barnes, P.M., 1994a. Continental extension of the Pacific Plate at the southern termination of the Hikurangi subduction zone: the North Mernoo Fault Zone, offshore New Zealand. Tectonics 13 (4), 735–754.

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Barnes, P.M., 1994b. Inherited structural control from repeated cretaceous to recent extension in the North Mernoo Fault Zone, western Chatham Rise, New Zealand. Tectonophysics 237 (1–2), 27–46. Barnes, P.M., de Lépinay, B.M., Collot, J.-Y., Delteil, J., Audru, J.-C., 1998. Strain partitioning in the transition area between oblique subduction and continental collision, Hikurangi margin, New Zealand. Tectonics 17 (4), 534–557. Barnes, P.M., Ghisetti, F.C., Ellis, S., Morgan, J.K., 2018. The role of protothrusts in frontal accretion and accommodation of plate convergence, Hikurangi subduction margin, New Zealand. Geosphere 14 (2), 440–468. Barnes, P.M., Lamarche, G., Bialas, J., Henrys, S., Pecher, I., Netzeband, G.L., Greinert, J., Mountjoy, J.J., Pedley, K., Crutchley, G., 2010. Tectonic and geological framework for gas hydrates and cold seeps on the Hikurangi subduction margin, New Zealand. Mar. Geol. 272 (1–4), 26–48. 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