Jamming of the Hikurangi Plateau (large igneous province) subduction, within the Chatham Rise convergent margin of Gondwana at circa 105 Ma, led to offset and rotation of the convergent margin before subduction ceased in the New Zealand region at circa 100 Ma. The southern limit of the plateau, following leading slab break off, is highlighted by a lineament of prominent horst blocks in the southern Bounty Trough. Subduction jamming of the Gondwana margin, and accompanying compression of the onshore margin and/or extension of the offshore margin, has led to two 60 km left-lateral SSE offsets of the Chatham Rise convergent margin at the coast and in inland Canterbury. Recognition of the onshore Chatham Rise using the gravity data also highlights the correlation of the inland Chatham Rise and central South Island seismicity. In a similar manner to the rotation of Cretaceous spreading-ridge and transform-fault fabric adjacent to the Osbourn Trough spreading ridge, the convergence direction at the Gondwana margin was rotated anticlockwise to N-S between 105 and 100 Ma. Most of this rotation has been accommodated by offshore extension and margin offset. The divergence between the anticlockwise rotation of offshore crustal structure and the jammed onshore margin led to the development of the Great South Basin at 105–100 Ma. Further offshore in the Bounty Trough, extensional zones, formed between crustal blocks rotated to adjust to a changed Cretaceous direction of subduction, are evident in gravity and seismic profiles.
A variety of orientations for Cretaceous subduction at the New Zealand segment of Gondwana have been proposed [Luyendyk, 1995; Bradshaw, 1989]. The near-linear, greater than 1000 km long, east-west oriented, bathymetric margin of the northern Chatham Rise (Figure 1) and seismic sections showing a subducting basement at the northern slope base led Davy and Wood  to interpret north-south subduction of the Hikurangi Plateau beneath the northern Chatham Rise. The Cretaceous Gondwana margin is generally assumed to have been at the northern limit of, and parallel to, the bathymetric Chatham Rise.
Few, if any authors, consider the onshore extension of the Chatham Rise accretionary prism structure. The Esk Head Melange Belt (EHM), a 10–20 km zone of highly dissected accretionary fan deposits, limestone, and chert [Silberling et al., 1988], which trends ESE to where it intersects the Canterbury Plains (Figure 1) approximately 50 km north of Banks Peninsula, has been the most discrete belt linked to accretionary structure. It marks an accretionary boundary between Jurassic-Cretaceous Pahau Torlesse Terrane to the north, and Triassic-Jurassic Rakia Torlesse Terrane to the south (Figure 1 inset).
Hikurangi Plateau guyots (Polar Bear and Shipley Seamounts—Figure 1) which formed at 90–99 Ma [Davy et al., 2008] indicate the Hikurangi Plateau was as much as 2 km shallower when subducted at the Chatham Rise Gondwana margin.
Gravity data analysis presented in this paper suggests that the Triassic-Jurassic Chatham Rise accretionary prism structure may have once been the location of a Jurassic-Cretaceous volcanic arc although extreme uplift following Hikurangi plateau subduction will have eroded much of the erupted volcanism. The analysis also indicates that jamming of the subducting buoyant 15 Myr old Hikurangi Plateau at circa 105 Ma has offset and rotated the once continuous accretionary margin and possible volcanic arc and set in train deformation eventually leading to Gondwana supercontinent breakup. This paper uses Sun-illuminated free-air and Bouguer gravity anomaly plots (a water density correction of 1.67 × 103 kg/m3 is applied) to highlight Gondwana margin structure and derive an explanation for the onshore-offshore structure and fabric differences and to trace the gravity signature of the Chatham Rise within the South Island. Extensive use is also made of the horizontal gravity slope to highlight subsurface faulting in the manner of Davy et al. . The horizontal gravity slope is a measure of the maximum unsigned slope of the gravity anomaly (in any direction) at a point. This value is calculated from the known gravity anomaly field using the “GRDGRADIENT” tool from the Generic Mapping Tools [Wessel and Smith, 1995]. Within the gravity slope image isoclines are readily recognizable—hence the use of the term “gravity gradient” when referring to the figures in the text.
(Note: the terms offshore and onshore margins used throughout the text are based on the modern margin configuration and do not imply anything about sea level or crustal thickness during Mesozoic times.)
Adams et al.  identified the Esk Head Melange Belt (EHM) in the northeastern South Island (Figure 1) as the equivalent of the central North Island Kaweka Terrane, a Jurassic accretionary terrane. Adams et al.  mapped the onshore geographic extent (Figure 1 inset) and zircon ages of the Pahau Terrane (principal occurrences being 130–112 Ma), an accretionary basement with a youngest age of 104 Ma, which occurs north of the EHM and lies beneath Champagne Formation sandstones of the inland Kaikoura region. The Triassic-Jurassic segment of the Permo-Triassic Rakaia Terrane [Adams and Maas, 2004] is mapped (Figure 1 inset) from inland central Canterbury to immediately north of Banks Peninsula region, and Jurassic schist is mapped on the Chatham Islands [Adams et al., 2008].
Although Triassic-Cretaceous accretionary margin basement extends from inland central Canterbury to the NE limit of the South Island [Adams et al., 2012; Rattenbury et al., 2006; Forsyth et al., 2008], Cretaceous felsic volcanism is mapped on the Chatham Islands, near the eastern end of the Chatham Rise, and in the Banks Peninsula/Canterbury foothills region [Forsyth et al., 2008; Tulloch et al., 2009]. In both areas this volcanism lies within Triassic accretionary margin deposits landward of the Jurassic-Cretaceous margin. Accreted Early to middle Cretaceous sediment extends for 200 km north of the Esk Head Melange Belt onshore to the NE tip of the South Island [Adams et al., 2012] and may have extended further north prior to Hikurangi Plateau entry into the margin and consequent margin erosion. Gondwana Jurassic-Cretaceous sediment may also have once overlain the Chatham Rise crest but following Chatham Rise uplift and crest erosion accompanying subduction of the buoyant Hikurangi Plateau much of it is likely to have been removed by erosion. Mid-Cretaceous (120–100 Ma) Tupuangi Sandstone, exposed on the Chatham Islands, has 45% of the zircon age pattern [Adams et al., 2008] likely derived from local Cretaceous volcanic sources.
2.2 Tectonic Setting and Convergent Margin Rotation
An updated tectonic model of that presented by Davy et al.  is illustrated in Figure 3 with many features highlighted also apparent in the gravity image of Figure 2. The updated model uses the greater southern extent of the Hikurangi Plateau interpreted by Reyners et al. . This interpretation of the subducted Hikurangi Plateau extending as far south as Fiordland, at the southern end of the South Island (Figures 1 and 2), implies a revised timing of subduction events (Table 1) from that of Davy et al. .
Table 1. Timing of Subduction Margin Events and Deformation
Ontong Java/Manihiki/Hikurangi Plateau breakup
Hikurangi Plateau first enters the Gondwana margin
Subduction jamming onshore western South Island
Subduction direction rotation from SSW to S mainly offshore
Onshore margin compression and Chatham Rise offset
Davy et al.  interpreted subduction of oceanic crust at the Chatham Rise section of the Gondwana margin, postbreakup of the Ontong Java/Manihiki/Hikurangi Plateau circa 120–115 Ma, entering the margin from the NNE (relative to the present-day eastern South Island framework used throughout this paper).
Identified geological basement terranes [Mortimer, 2004] occurring on the southeastern side of the South Island all trend WNW and lie parallel to the trends in gravity highlighted by white arrows in Figure 2, in contrast to E-W trending half-graben lineaments on the Chatham Rise. This onshore fabric is orthogonal to the NNE convergence direction at the margin from 120 to 105 Ma (Figure 3). At 110 Ma the Hikurangi Plateau is interpreted to have first entered the Gondwana margin (Figure 3b). This date, 5 Myr earlier than that estimated by Davy et al. , is a consequence of the Hikurangi Plateau subducting 400 km further south than estimated by Davy et al. and is consistent with the 110 Ma onset of extension in the Gondwana margin [Mortimer et al., 1999, 2002].
The fabric of the ocean crust north of New Zealand is comprised mostly of abyssal hill fabric which lies orthogonal to the direction of earlier seafloor spreading [Downey et al., 2007] and fracture zone faults which are parallel to the seafloor spreading direction. In convergent margins where the subducting crust is not as elevated and buoyant as the 110 Ma Cretaceous Hikurangi Plateau, strain partitioning results in convergent margin fabric which need not be orthogonal to the margin. However high interplate coupling resulting from collision/subduction of a buoyant subducting plateau [Willingshofer et al., 2013] leads to deformation structures parallel and orthogonal to the margin. Within the Chatham Rise and along the eastern South Island compressional deformation associated with the Hikurangi Plateau subduction has imprinted and overprinted basement fabric (e.g., Figure 2) generally orthogonal to the Cretaceous direction of plateau subduction.
Seafloor transform fabric from NNE of the Hikurangi Plateau up to ~400 km south of the Osbourn Trough paleospreading ridge is NNE oriented, orthogonal to the basement and terrain fabric in the eastern half of the South Island (Figure 2). North of the interpreted 105 Ma labeled line in Figure 2 the younger fracture zone fabric rotates to approximately N-S at the Osbourn Trough intersection. The Osbourn Trough is semiparallel to half-graben lineations visible in gravity from the Chatham Rise (Figure 2). Downey et al.  observed paleoseafloor spreading fabric within an area <400 km north of Osbourn Trough (Figure 2), where the fabric was rotated about 16° anticlockwise to east-west and the overall spreading-rate signature indicated a slowing to 30% of its previous value. Whereas Davy et al.  interpreted rotation of seafloor spreading starting at 110 Ma, 5 Myr prior to Hikurangi Plateau subduction beneath the Chatham Rise, the plateau is now interpreted to have entered the margin at 110 Ma with jamming and margin rotation interpreted to have started at approximately 105 Ma. This jamming is likely to have initiated near the southern limit of plateau subduction, possibly in the Fiordland region (Figure 3c).
Flowers et al.  document rapid crustal burial possibly associated with crustal thickening in the Fiordland region at 112–111 Ma. This date of compressional burial lies close to the date of inferred Hikurangi Plateau impingent on the Gondwana margin and may potentially be causally related. However the subducting Hikurangi Plateau is not expected to extend to Fiordland until circa 105 Ma when the buoyant leading edge may have jammed against such a zone of crustal thickening.
Prior to 105 Ma Gondwana margin subduction was oriented SSW (Figures 2 and 3). Basement fabric within the eastern South Island correlates with this subduction period and in particular the deformation associated with the long, approximately 400 km, reach of buoyant Hikurangi Plateau subduction between 110 and 105 Ma. At about 105 Ma subduction is interpreted to have slowed throughout the New Zealand region but more extremely in the western South Island. From 105 to 100 Ma the direction of plate convergence rotated to N-S. The Osbourn Trough and Chatham Rise both exhibit this final orientation (Figure 2). This fabric orientation is particularly apparent in half-graben basin lineations found throughout the length of the Chatham Rise (Figures 1 and 2). These sediment-filled half-graben are known from seismic data (e.g., Figure 4) to exhibit ≥3 km of basement relief.
By 100 Ma subduction ceased at the New Zealand segment of the Gondwana margin where the Hikurangi Plateau had subducted. Similarly, seafloor spreading at the Osbourn Trough ceased although reactivated volcanism in the Osbourn Trough region associated with later plate rearrangement stresses, e.g., the onset of New Zealand-Antarctic spreading, is possible and may explain younger rocks dredged from the region [Mortimer et al., 2012]. To the east, and presumably to the west in a zone since subducted, of where the Hikurangi Plateau subduction jammed, seafloor spreading ridges continued to migrate south (Figure 3d). Extension was fed into the New Zealand Gondwana interior via transform faults of the Wishbone Ridge Fault System immediately east of the jammed Hikurangi Plateau (Figure 2). This transtensional motion occurred along a series of faults such as the Campbell Fault System [Sutherland, 1999].
By circa 85 Ma, with the arrival of the south migrating spreading ridge in the Eastern Chatham Rise region, it became more dynamically favorable to initiate seafloor spreading between New Zealand and Antarctica (Figure 3e).
3 Data Interpretation
3.1 Chatham Rise Gondwana Margin Offset and Differential Rotation
The crest of the Chatham Rise is bracketed by a pair of gravity anomaly lows (Figure 4), which, although relatively continuous, show discrete breaks in continuity (discussed later), the most pronounced of which is mid-Chatham Rise. The paired gravity lineaments can be traced onshore into the South Island where they initially bracket Banks Peninsula. Two approximately 60 km SSW offsets of the gravity low lineaments associated with the half-graben, and by implication the Chatham Rise crest, are highlighted in Figures 4 and 5.
The horizontal gravity-gradient pattern, visually apparent in the horizontal gravity slope image of Figures 5-8, and based onshore on the gravity station density of Figure 6a, highlights major fault locations (or steep fold structures) many of which are buried and no longer active structures. Figure 6b highlights the generally high correlation between gravity-gradient lineations and mapped basement fabric [Mortimer, 2014]. The gravity-gradient plots also highlight the onshore continuation of the half-graben on either side of the Chatham Rise. Figure 5 matches offshore free-air gravity horizontal anomaly gradient to Bouguer gravity horizontal anomaly gradient onshore. In addition to the offset graben on the Chatham Rise crest, Figure 5 highlights that the 65 km long NNE offsetting fault extending from the northern rise crest toward the eastern Banks Peninsula underlies, and will likely have been responsible for the formation of, the Pegasus Canyon (Figure 1). On the southwestern Chatham Rise a 110 km long, similarly oriented fault is recognizable in the gravity gradient extending NNE from the northern limit of the shelf break to the southern rise slope. This latter offset is close to the sum of the two Chatham Rise/Gondwana Margin offsets.
The interpretation of fault-offset prolongation of the Triassic-Jurassic Chatham Rise convergent margin into the central South Island, west of the Canterbury Plains, is based upon the gravity reading data distribution shown in Figure 6a. For much of the central South Island region the interpreted half-graben in the Bouguer gravity anomaly match the valleys and riverbeds of the Rakaia and Rangitata Rivers (i.e., G1″ and G2″,7 respectively, in Figure 9) and indeed the river locations are likely a consequence of the underlying half-graben. A number of gravity readings are concentrated along these riverbeds. Cross sections of Bouguer gravity anomaly (not shown) across the interpreted central South Island convergent margin extension are similar to profile AA′ across Banks Peninsula extension—i.e., that of a back-tilted basement high complex with included half-graben gravity lows. The approximately 100 μN/kg magnitude of the “graben” Bouguer gravity lows in these cross sections is similar to that of the G1 and G1′ (Figures 8 and 9) gravity lows in North Canterbury and the northern Chatham Rise.
The onshore-offshore half-graben gravity fabric continuity is disrupted over an approximately 100 km wide zone in the Banks Peninsula region, and immediately eastward, by volcanism, a broad band of strike-slip offset faults (Figures 5, 8, and 9) and a region of crustal extension (Figure 8b). This margin disruption is due to rotation and offset of the Gondwana margin post 105 Ma Gondwana subduction jamming. The offshore-onshore prolongation of the Chatham Rise gravity fabric, characterized by the paired gravity half-graben is easily identifiable across the above zone of disruption and is highlighted in Figure 7. The Esk Head Melange is mapped west of the faulted offset of the Chatham Rise. Sixty-five kilometers of fault offset implies that the unmapped coastal expression of the EHM should be close to “C” in Figure 7. Also shown is the predicted plate motion path over the last 6 Ma of the interpreted western limit of the Chatham Rise relative to the western South Island. Deformation in the North Canterbury-Kaikoura region may reflect some of the deformation associated with the Chatham Rise passage.
NNE oriented horizontal gravity-gradient lineations (orange lines of Figures 8 and 9) are interpreted as Cretaceous strike-slip faults which disrupt the continuity of half-graben extending WNW from Banks peninsula [Davy et al., 2012].
The onshore half-graben are generally aligned WNW parallel to the interpreted convergent margin orientation prior to 105 Ma. Onshore analysis of sediment and basement velocity structure by Reyners et al.  strongly supports the correlation of these gravity low (G1 and G2) with underlying sedimentary basins > 3 km deep (Figure 8). From seismicity investigations in the Christchurch region [Reyners et al., 2013] gravity low G1 and the observed velocity low B1 colocate well and the 3 km depth is consistent with the vertical relief of graben on the Chatham Rise southern crest (Figure 4). The gravity low lineament G2′ (Figure 8) falls outside the area of Reyners et al.'s valid solutions, and hence, Reyners et al. do not map the underlying basin. From the same study, the green shaded zone FD, broadly bound by major NNE faults (orange lines), marks a highly fractured domain of Vp/Vs < 1.575 at 3 km depth occurring immediately after 4 September 2010 Mw7.1 earthquake which later returned to values over 1.6 [Reyners et al., 2013].
An exception to the WNW trend of onshore bounding half-graben is the southern side of the Chatham Rise basement high in southeastern Canterbury, mapped by the Bouguer gravity anomaly (Figure 9) where lineation G2′ trends more east-west. Gravity profiles (AA′ and BB′, Figures 8 and 9) across the Banks Peninsula extension suggest that on the western profile AA′ the Chatham Rise/Banks Peninsula basement high is more extended and thinned in the south. This more east-west fabric lies southwest of the fractured domain FD in Figure 8, the eastern margin of which was a zone of intense seismicity in the 2010–2011 Canterbury earthquake sequence [Davy et al., 2012]. The E-W orientation of G2′ may be due to Cretaceous anticlockwise rotation and thinning of the basement on the southern margin of the Banks Peninsula continuation of the Chatham Rise, coincident with Chatham Rise margin rotation but prior to transform offset between the onshore and offshore segments. The modern plate boundary faulting may also influence basement fabric in the region.
Locations for earthquakes taken from the New Zealand GeoNet catalogue (http://quakesearch.geonet.org.nz/) and occurring between 1990 and 2013 have been plotted over the gravity gradient in Figure 10. Seismicity is concentrated along, and close to, the onshore continuation of the Triassic-Jurassic Chatham Rise margin. The correlation between earthquake locations and the gravity-gradient fabric will be discussed later.
3.2 Bounty Trough Extension and Rotational Thinning Following Subduction Jamming
With the Hikurangi Plateau interpreted to have subducted as far south as Fiordland [Reyners et al., 2011], Figures 11 and 12 present gravity and seismic reflection data which highlight interpreted offshore Cretaceous extensional deformation consistent with the Reyners et al. interpretation.
There is interpreted to be oceanic crust underlying basement within the inner Bounty Trough [Van Avendonk et al., 2004] which probably formed as back-arc crust to the Triassic-Jurassic Chatham Rise margin to the north. Figures 13 and 14 use gravity-gradient and seismic reflection data to further highlight extensional zones within the crust resulting from rotation of the direction of offshore subduction between 105 and 100 Ma.
4.1 Chatham Rise Onshore Prolongation
The Chatham Rise is characterized by two half-graben, typically approximately 3 km in basement relief, which bracket the rise and can be traced throughout its length from east of the Chatham Islands, through Banks Peninsula, to the Alpine Fault. The origin of the half-graben is uncertain, but the northern basin may be a fore-arc basin terminated at its northern margin by fault uplift associated with subduction of the buoyant Hikurangi Plateau (e.g., profiles HKDC1 and HKDC3 [Davy et al., 2008]). Based on more extended gravity profiles (not shown) showing multiple half-graben blocks south of the rise, the southern basin is likely to be the shallowest of a series of half-graben basins associated with back-arc extension again uplifted and extended by Hikurangi Plateau subduction.
Cretaceous age Gebbies Pass Rhyolite and McQueens Andesite (QMAP—www.gns.cri.nz) exposed on Banks Peninsula (Figure 9) are interpreted to lie within a once continuous Triassic-Cretaceous Chatham Rise convergent margin and possible volcanic arc (Figure 15). A major Cretaceous strike-slip fault which left-laterally offset the Chatham Rise (imaged by its gravity lineaments) by approximately 60 km, in a similar manner to the offshore/onshore offset farther west (Figure 9), lies near the Malvern Hills compressional Miocene-Recent interface at the western margin of the Canterbury Plains. Mount Somers Group ignimbrite and rhyolite, exposed in the Malvern Hills, Mount Somers region (Figure 9), lie close to the above inferred Cretaceous offset fault. The Banks Peninsula and Mount Somers Group volcanism was emplaced between 97 Ma and 89 Ma, up to 10 Ma after Cretaceous subduction ceased. The occurrence of the felsic volcanism coincident with the Chatham Rise crest and offsets suggests that residual volcanic arc lava, resident within or beneath the crust, from an earlier Chatham Rise volcanic arc, may have been brought to the surface as part of volcanism widespread throughout the South Island, Chatham Rise, and Hikurangi Plateau between 99 and 87 Ma [Hoernle et al., 2010] and subsequent to Gondwana slab detachment. Earlier erupted volcanism from the Cretaceous-Triassic convergent margins is likely to have been eroded away due to major (>1 km) uplift associated with Hikurangi Plateau subduction.
4.2 Gondwana Margin Jamming and Rotation
The relative continuity of magnetic anomalies lineations [Sutherland, 1999] onshore-offshore south of the Dun Mountain Terrane (Figure 1) implies that the interpreted 120 km offset of the Gondwana margin between the Chatham Rise and central South Island is due to onshore crustal shortening, offshore crustal extension, or a combination of both mechanisms. It is possible that a collisional event in the southern South Island, probably in the Fiordland region [Klepeis and Clarke, 2004; Flowers et al., 2005], jammed the Hikurangi Plateau subduction there (Figure 15) with consequent compression and shortening of the onshore margin offsetting the Chatham Rise and Banks Peninsula segments. Such compressional shortening, at a time of Hikurangi Plateau subduction at least as far south as the Dun Mountain-Matai Terrain area, may have been associated with uplift and exhumation of the Otago Schist in the Late Cretaceous [Mortimer, 2000; Weinberger et al., 2010]. While compression and uplift of the Otago Schist belt onshore (QMAP—www.gns.cri.nz) could form part of onshore compressional margin collapse, it is not obvious how this could be accommodated with the thick (<35 km) [Reyners et al., 2011] underlying Hikurangi Plateau.
Alternatively, following subduction jamming onshore in the south, the offshore subduction margin rotated anticlockwise offsetting the margin near the coast and extending crust of the inner Bounty Trough (Figure 3c). The interpreted 110 km fault offset of the slope break SW of the Chatham Rise (Figure 5) favors this latter interpretation. The resulting change in plate motions at circa 105–100 Ma [Wessel and Kroenke, 2008] also led to anticlockwise rotation of the Chatham Rise segments of the still active convergent margin (Figure 16) to E-W. This rotation of subduction direction has caused extension within the Great South Basin starting at approximately 105 Ma [Cook et al., 1999] (Figure 14). NE-SW oriented graben fabric within the Great South Basin, orthogonal to onshore terrane grain (Figure 14), has been observed in basement and 104 Ma sedimentary grain [Constable and Crookbain, 2012].
4.3 Offshore Extensional Fabric
Spreading fabric immediately adjacent to the Cretaceous Osborn Trough spreading center slowed and rotated synchronously prior to seafloor spreading cessation (Figure 2) [Downey et al., 2007], and this synchronous motion may be partly mirrored in the Bounty Trough fabric. The extensional fabric of Figure 11, apparent only in the western inner Bounty Trough west of 176°E, may include underlying Triassic-Jurassic back-arc spreading fabric (similar to, but not the same as, that proposed by Davy ) south of the Chatham Rise, particularly in the symmetric fabric between the two orange dashed lines in Figure 12.
Faulting in the seismic sections of Figure 12 is extensional as would be expected given the symmetric basement fabric highlighted. The center of the eastern symmetric zone on line CD (Figure 12) has been planed off by sea level erosion when basement was about 2 km shallower accompanying underlying Hikurangi Plateau subduction. This basement has subsided to the present depth as the underlying large igneous province has cooled in a similar manner to plateau subsidence north of the Chatham Rise.
It is difficult to conceive how extension could occur in the Bounty Trough crust with strong compressional coupling likely between this crust and the underlying, buoyant, subducting Hikurangi Plateau. However, extensional basement faulting, rotated graben erosion, and E-W extension disruption observed in the seismic reflection data (Figures 11-14) imply that some of the extension occurred coincident with Hikurangi Plateau subduction. Some bending-related extension may have accompanied the 1–2 km uplift due to the underlying introduction of the subducted buoyant plateau. The observed extension is probably more likely to have occurred during northward migration of the Chatham Rise as the margin offsets and rotates relative to the onshore margin. Such northward migration of the subduction margin may have created a back-arc extensional regime in the inner Bounty Trough crust.
With segmentation and rotation of the offshore Gondwana margin to an E-W orientation, a set (>5) of approximately N-S oriented E-W extensional zones, linked to the segmentation breaks in the Gondwana margin, have developed in the Bounty Trough crust (Figures 13 and 14) overlying the subducting Hikurangi Plateau. One of the zones extends south to form the Great South Basin, and another extends north to Banks Peninsula. Reyners et al.  interprets a basalt zone shallowing to approximately 8 km depth beneath the latter volcanic peninsula. The intersection of a NNE margin offsetting fault with one of the above crustal extension zones will have provided a major conduit for the persistent volcanism observed at the peninsula.
The two easternmost zones of east-west extension in the Bounty Trough (Figure 13) are inferred from gravity gradient but do not have crustal-scale seismic available to confirm their nature. Gravity-gradient fabric on either side of the highlighted extension zones (Figures 11 and 14) appears relatively undeformed and offset E-W suggesting block rotation between the extension zones in response to a changing direction of the shallow-angle/flat Hikurangi Plateau subduction. The E-W extension has disrupted the pattern of N-S extension (Figures 11 and 12) visible in the symmetrical linear gravity-gradient fabric and interpreted seismic basement of the inner Bounty Trough (Figures 11 and 12).
The mapped extension zones of Figure 13 give an understanding of the deformation of crust in the western Bounty Trough accompanying rotation in the subduction direction of the underlying Hikurangi Plateau from south-southwest to south. Linked to breaks in the Chatham Rise convergent margin continuity, the crust in the Bounty Trough has deformed much like a set of tilted dominoes. Seismic reflection data in Figure 14 reveal there are zones of crustal thinning corresponding to the boundaries of rotated crustal blocks consistent with anticlockwise basement rotation away from the South Island east coast.
4.4 Extent of Hikurangi Plateau Subduction
The curvilinear set of prominent offshore horst structures (20–40 km wide × 2–3 km high) (Figures 11 and 12) intersects the South Island coast at the Dun Mountain-Matai Terrane gravity boundary (Figures 2, 11, 13, and 16 (QMAP—www.gns.cri.nz)). This boundary onshore is marked by a major gravity high lineament (Figures 2 and 16) with uplifted Permian ophiolite exposed at its core. This approximately 600 km long set of uplifted onshore and offshore basement high is interpreted to be crust that has been uplifted vertically following break off and upward rebound of the underlying leading subducted oceanic slab circa 100 Ma. The uplifted crust is likely to have been close to the southern limit of the subducted Hikurangi Plateau and represents the modern southern limit of the plateau. The 400–500 km distance between the northern Chatham Rise accretionary margin and the above southernmost interpreted Hikurangi Plateau extent lies close to the maximum width of “flat” subduction of oceanic plateaus and ridges reported elsewhere [Gutscher et al., 2000]. Farther east where the plateau subduction terminus is interpreted to lie beneath the Chatham Rise (Figures 13 and 16), uplift associated with leading edge rebound may have exposed shists observed only on the eastern rise [Wood and Herzer, 1993].
4.5 Post Chatham Rise Subduction
At approximately 100 Ma seafloor spreading at the Osbourn Trough and subduction beneath the Chatham Rise ceased [Davy et al., 2008]. While seafloor spreading near the Osbourn Trough slowed, rotated, and eventually ceased at the paleoridge, seafloor spreading continued from a southward migrating spreading ridge farther east (Figure 3d). Extension from the still active spreading ridge to the east will have been fed into the Gondwana interior via transform fault structures such as the Wishbone Ridge [Davy et al., 2008], which intersects the Chatham Rise Gondwana margin at the eastern edge of the jammed Hikurangi Plateau (Figure 1).
Following subduction cessation the oceanic slab leading Hikurangi Plateau subduction will have detached and subsequent widespread volcanism throughout the Hikurangi Plateau, Chatham Rise and onshore South Island in the interval 100–89 Ma followed mantle upwelling through the slab window [Davy et al., 2008; Hoernle et al., 2010].
4.6 Neogene North Canterbury-Kaikoura Deformation
The curved basement fabric of the eastern Kaikoura region (A in Figure 7) has been previously noted [Vickery and Lamb, 1995, Hall et al., 2004] as an up to 100° rotation. The gravity anomaly gradient near points “B and C” in Figure 7 suggests up to 180° rotation further south. The 100° rotation represents the angular difference between the Cretaceous WNW margin-parallel structure (e.g., Esk Head Melange—Figure 7) and curvilinear NNE faults [Van Dissen and Yeats, 1991; Bourne et al., 1998, Rattenbury et al., 2006], accommodating motion between the Australian Plate (west coast South Island) and Pacific Plate (east coast South Island) over the last 20 Myr [Walcott, 1998; Cande and Stock, 2004]. The up to 180° rotation farther south may include deformation from Cretaceous margin offset. The thick, dashed, white line in Figure 7 highlights the relative plate motion path over the last 6 Myr for a point on a nominally undeformed underthrust Pacific Plate near the western limit of the interpreted Chatham Rise beneath the Southern Alps. Deformation of the Chatham Rise complex by this motion is interpreted in the western Alpine area (orange dashed segment of the Chatham Rise—Figure 7). In reaching the present configuration transform displacement branching off, the Alpine Fault has distributed deformation through the Kaikoura Region [Van Dissen and Yeats, 1991] along active faults (Figures 7 and 10), displacing basement fabric north of the Banks Peninsula region. Accompanying Neogene relative strike-slip displacement between plates there has been approximately 80 km of convergence at the latitude of the Chatham Rise extension through the South Island. This will have been accommodated via a combination of underthrusting (e.g., beneath the Malvern Hills) and compression.
4.7 The South Island Crustal Root
Deep crustal seismic reflection lines SIGHT1 [Van Avendonk et al., 2004] across the central South Island will have clipped the southern margin of the Cretaceous accretionary prism (Figure 10). Much of the structure observed on this line may reflect the increased crustal thickness associated with the paleo-accretionary prism. The central South Island crustal root [Stern et al., 2000], highlighted by a low-velocity Vp (<7.25 km/s) region [Eberhart-Phillips et al., 2010] (Figure 17) starts immediately south of the Cretaceous paleo-accretionary prism. The regional residual gravity field, after crustal root correction, also exhibits an approximately 5 km westward step in values (at pink and white arrow in Figure 17) immediately south of the southern margin of the Cretaceous paleo-accretionary prism [Davy et al., 2013] reflecting both the discontinuity in crustal structure at this latitude and the older, thickened low-density crust to the south.
4.8 Seismicity Patterns
Seismicity in the Canterbury Plains-West Coast region between 1990 and 2013 (Figure 10) highlights the correlation between Chatham Rise structure and seismicity. The Greendale Fault, formed during the September 2010 Mw7.1 earthquake, lies within the southern segment of the Chatham Rise, beneath the Canterbury Plains, which has been partly rotated to an E-W orientation, subparallel to graben G2′ of Figure 8. The Chatham Rise accretionary complex basement, including intruded arc volcanic deposits, protrudes both above into the overlying sediment and below against the underlying Hikurangi Plateau crust. Such a large rise structure with embedded linearly extensive half-grabens and NE-NNE offsetting faults has a number of identifiable faults acting as a foci for seismicity associated with motion relative to the underthrusting Hikurangi Plateau. Davy et al.  document a number of the Cretaceous faults in the eastern Canterbury Plains along which much of the 2010–2011 seismicity occurred.
The seismicity distribution in the alpine zone of the Chatham Rise (ACR in Figure 10) [Boese et al., 2012] immediately east of the Alpine Fault, partially correlates with high gravity-gradient lineaments although sparse gravity station spacing in the region (up to 10 km apart, Figure 6a) limits gravity-gradient definition. There are several WNW trending seismicity bands in the ACR (highlighted by pink arrows in Figure 10c) which are potentially occurring on the equivalent of faulting which bounds half-graben within the coastal Chatham Rise (Figure 8d).
In the same near-ACR zone but immediately north of, and parallel to, the interpreted Chatham Rise, there is a band of seismicity, labeled C1 in Figure 10, associated with the 1994 Arthurs Pass earthquake [Abercrombie et al., 2000]. Described as “enigmatic” by Abercrobie et al., the mainshock and largest aftershock occurred on NE faults bounding the NW trending aftershock sequence. These bounding faults intersect the 1–10 km deep aftershock sequence, a possible strike-slip sequence, at a high angle giving a seismicity pattern somewhat reminiscent of the 2010–2011 Canterbury seismicity pattern [Davy et al., 2012].
Crustal Vp velocities greater than 8.5 km/s at 38 km depth [Eberhart-Phillips et al., 2010] approximately 60 km north of Banks Peninsula (Figure 17) conform to the identified onshore offset of the Chatham margin. Reyners et al.  interpreted lower crustal velocities > 8.5 km/s as diagnostic of the base of the Hikurangi Plateau. The shallowing of the Hikurangi Plateau in the north Canterbury region is consistent with a 2001–2010 seismicity profile by Reyners et al.  along the eastern South Island where seismicity shallows as the latitude of Christchurch is approached. Why this occurs is an interesting outstanding question. Possibly, the plateau shallows to fill a thinned crustal zone created by southward offset of the Chatham Rise at circa 105 Ma. Alternatively, the shallowing of the plateau may represent a fore-bulge north of compressional Hikurangi Plateau underthrusting, associated with modern plate motion, to the SSW beneath the buoyant Chatham Rise. Unfortunately, the Eberhart-Phillips solution does not have valid values eastward along the offshore northern Chatham Rise to compare with. Despite Cretaceous uplift and erosion of the Chatham Rise by the subducted Hikurangi Plateau, the 15–20 km thick basement rock of the rise offshore [Davy et al., 2008], remains a highly buoyant structure. There is significant seismicity at the northern Chatham Rise margin in the ACR region (Figure 10) but only minor equivalent seismicity between the ACR and the Canterbury Plains area over the last 24 years. It is also notable that most of the plate boundary displacement in the northern South Island occurs on the strike-slip Marlborough Fault System faults [Van Dissen and Yeats, 1991] which channels fault movement from the eastern South Island through to the Alpine Fault deviating around the onshore Chatham Rise convergent margin.
Basement fabric visible in shaded gravity, and horizontal gravity gradient, data have revealed about 60 km of left-lateral SSW offset between the offshore Chatham Rise Triassic-Jurassic Gondwana convergent margin and its onshore component beneath Banks Peninsula. A further 60 km of SSW offset occurs at the western margin of the Canterbury Plains beneath the Malvern Hills.
An offset and rotated buoyant Chatham Rise extending beneath the central South Island correlates with a transitional zone in modern seismicity and deformation patterns within the central South Island, and the southern margin limits the crustal root. The northern margin of the Chatham Rise in the South Island alpine region has acted as a structural barrier to transpressive stress associated with the modern plate motions through the central South Island. Like the Canterbury 2010–2011 earthquake sequence, much of which occurred along existing E-W oriented half-graben faults and offsetting NNE faults [Davy et al., 2012], seismicity in the central Alpine region is occurring on faults parallel to the paleomargin as well as high-angle offsetting faults.
North-south basement extension within the western Bounty Trough revealed by gravity and seismic reflection data is interpreted as having occurred during northward migration if the Chatham Rise convergent margin. The recognized gravity fabric within the Bounty Trough enables visualization of the >400 km extent of offshore Hikurangi Plateau subduction. A curvilinear set of prominent offshore flat-topped horst structures (20–40 km wide × 2–3 km high) clearly identifiable in gravity data, and trending SW across the southern inner Bounty Trough toward the Dun Mountain-Matai Terrane gravity boundary, are interpreted to mark the southern limit of the Hikurangi Plateau after leading slab detachment following subduction cessation.
Rotation of the offshore Chatham Rise margin to near E-W following interpreted jamming of the onshore Hikurangi Plateau subduction at approximately 105 Ma led to east-west extension between onshore and offshore east of the South Island and formation of the Great South Basin. Similar extensional zones in the western Bounty Trough, linked to segmentation breaks in the rotated Chatham Rise, highlight basement block rotation in overlying crust on and south of the Chatham Rise accretionary margin in response to the changed subduction direction post 105 Ma.
The rotation of crustal fabric in the Bounty Trough region provides unique insights into the deformational process following jamming of low-angle/flat large igneous province subduction and the early stages of supercontinent breakup.