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Faulting and folding beneath the Canterbury Plains identified prior to the 2010 emergence of the Greendale Fault

R Jongens , DJA Barrell , JK Campbell & JR Pettinga

To cite this article: R Jongens , DJA Barrell , JK Campbell & JR Pettinga (2012) Faulting and folding beneath the Canterbury Plains identified prior to the 2010 emergence of the Greendale Fault, New Zealand Journal of Geology and Geophysics, 55:3, 169-176, DOI: 10.1080/00288306.2012.674050

To link to this article: http://dx.doi.org/10.1080/00288306.2012.674050

Published online: 21 Aug 2012.

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Download by: [103.242.70.121] Date: 23 November 2016, At: 19:00 New Zealand Journal of Geology and Geophysics Vol. 55, No. 3, September 2012, 169Á176

Faulting and folding beneath the Canterbury Plains identified prior to the 2010 emergence of the Greendale Fault R Jongensa*, DJA Barrella, JK Campbellb and JR Pettingab aGNS Science, Dunedin, New Zealand; bDepartment of Geological Sciences, University of Canterbury, , New Zealand (Received 1 December 2011; final version received 15 February 2012)

Prior to the 2010Á2011 earthquake sequence, several fault and fold structures were mapped beneath the Canterbury Plains using seismic reflection surveys and surface observations and depicted on the Christchurch and Aoraki 1:250,000 scale geological maps. Localised grabens associated with east-southeast-striking normal faults formed largely during the Late Cretaceous. South of River, some graben-bounding faults show minor normal offset extending into the late Cenozoic. Near Ashley River, proximity to a Late CretaceousÁPaleogene graben suggests that the active, predominantly contractional, east-striking Ashley Fault is at least in part a rejuvenated pre-existing normal fault. The easterly strike of the previously unknown Greendale Fault implies that it too may be a reactivated Late Cretaceous fault. Northeast-striking, southeast-facing reverse faults and fault- propagation folds beneath the western and northern parts of the plains are primarily late Cenozoic features. Variation in the distributions of Miocene sedimentary strata strongly suggests that contractional faulting was initiated as early as the Miocene. The overall late Cenozoic tectonic pattern is extension beneath the southern Canterbury Plains and contraction farther north. Keywords: Ashley Fault; Canterbury; Greendale Fault; Fault; late Cenozoic; Late Cretaceous; Racecourse Hill Anticline; seismic reflection; Springbank Fault; tectonics

Introduction across the Canterbury Plains. The Natural Hazards The 2010 Darfield Earthquake triggered surface rupture of Research Centre, University of Canterbury, was commis- the previously unrecognised east-striking Greendale Fault. sioned to interpret these surveys and integrate the inter- Subsequently, blind faults such as the Port Hills fault have pretations with surface geological observations, reprocessed been revealed during the aftershock sequence. All these newly 1963 BP Shell Todd (BP) seismic reflection lines and four discovered faults raise questions as to how many other blind existing exploration wells (Jongens et al. 1999). Follow-up faults exist beneath the Canterbury Plains. It is instructive to seismic surveys in 1999 and 2000 led to the drilling of two consider what was known prior to these earthquakes. further exploration wells: Arcadia-1 and Ealing-1. Subse- A network of oil exploration seismic reflection lines quently, the quarter-million-scale national geological map- across much of the Canterbury Plains, tied to exploration ping programme (QMAP) conveyed this information, along wells, gives insights into subsurface geology. This informa- with recently identified surface tectonic features, onto the tion, along with surface observations, provided the basis for a Christchurch (Forsyth et al. 2008) and Aoraki (Cox & regional geological model of faulting beneath the plains Barrell 2007) maps. (Jongens et al. 1999; Cox & Barrell 2007; Forsyth et al. 2008). The IP seismic reflection surveys (Schlumberger Geco- This paper documents the general geometries and movement Prakla 1998, 1999, 2000) are of good quality, commonly histories of faults and folds beneath the Canterbury Plains, as with well-defined reflectors down to the top of basement far as was known prior to the Darfield Earthquake, and (c. 1.2Á1.3 s two-way travel time), below which the quality provides regional tectonic context for newly discovered decreases markedly. In contrast, the late 1950sÁearly 1960s structures such as the Greendale and Port Hills faults. BP surveys (Kirkaldy et al. 1963) are of poor quality with The regional geologic and tectonic setting of the even the shallowest reflectors (base of Quaternary gravels) ill Canterbury Plains is described by Browne et al. (2012) and defined and difficult to interpret. The BP surveys none- Campbell et al. (2012). theless complement the IP surveys and help show the broad geological structure. Survey lines for both IP and BP cross much of the Canterbury Plains, with IP undertaking more Oil exploration data and stratigraphy closely spaced lines at Ashley River and between the Rakaia In 1998 an oil exploration company, Indo-Pacific Energy and Rangitata rivers (Fig. 1). Two BP lines cross the eastern (IP), undertook reconnaissance seismic reflection surveys end of the Greendale Fault but a re-examination of these

*Corresponding author. Email: [email protected]

ISSN 0028-8306 print/ISSN 1175-8791 online # 2012 The Royal Society of New Zealand http://dx.doi.org/10.1080/00288306.2012.674050 http://www.tandfonline.com 170 R Jongens et al.

Figure 1 Map of the Canterbury Plains showing location of Indo-Pacific Energy seismic lines and main faults and folds described in text. BP line 2, referred to in text, is also shown. Locations for all seismic lines, including all BP lines, can be obtained from http://www.gns.cri.nz/ Home/Products/Databases/PDQMap-Petroleum-Data-Query-Map. For concealed faults shown on the Canterbury Plains (dashed lines), ticks are shown to indicate downthrown side. Location of Figs 2, 3 and 4 seismic profiles are shown by a thickened grey colour on the respective line. AF Ashley Fault, CA Cust Anticline, CFF Cordys Flat Fault, EF Ealing Fault, GF Greendale Fault, GFz  Fault zone, HF Hororata Fault, Ha Hororata anticline, HPF High Peak Fault, Lf  fault, PPAFZ Porters Porters Pass-Amberley Fault Zone, RHA Racecourse Hill Anticline and SF Springbank Fault (and fold). IP1, 2, 3 and 4 refer to IP98-001, 002, 003 and 004 respectively, and IP101, 105, 106 and 109 refer to IP99-101, 105, 106 and 109 respectively. Map modified from Cox & Barrell (2007) and Forsyth et al. (2008). lines, including reprocessed versions, failed to show clear are interpreted as mid-Cretaceous Mount Somers Volcanics evidence of faulting because of their poor quality. Group (Fig. 2). Strong reflectors at or near the top of the Stratigraphy from exploration wells JD George-1 (Wood Late CretaceousÁPaleogene package are interpreted to 1969a), Chertsey-1 (summarised in Wood 1969a), and represent Oligocene limestone and/or Miocene volcanics -1 (Wood 1969b) was tied with 1998 IP seismic line (Figs 3, 4B). JD George-1, Kowai-1, Ealing-1, Arcadia-1 reflectors by Jongens et al. (1999). Subsequently, Ealing-1 and Kowai-1 (Hoolihan 1978) well records provide ‘time and Arcadia-1 (Indo-Pacific Energy 2000a, 2000b) and versus depth’ conversions that assisted in the compilation of outcrop geology were tied with 1998, 1999 and 2000 IP cross-sections on QMAPs Christchurch and Aoraki. seismic line reflectors for QMAP interpretation. Quaternary gravel, Pliocene Kowai Formation, Miocene and Late CretaceousÁPaleogene seismic reflection stratigraphic units Southern Canterbury Plains are readily identifiable beneath the Canterbury Plains Although Quaternary gravel accumulations mask the under- (Fig. 2; see Forsyth et al. 2008). Localised hummocky lying geology beneath the Canterbury Plains south of the reflectors below the Late CretaceousÁPaleogene package , seismic surveys IP98-001, 003 and 004 Faulting and folding beneath the Canterbury Plains 171

Figure 2 Selected Indo-Pacific Energy seismic reflection profiles over the southern Canterbury Plains; locations are shown in Fig. 1. A, Part of line IP98-001 where it intersects the JD George-1 well. Seismic packages (e.g. Kowai Formation) are bounded by prominent seismic reflectors which, for three of the four reflectors, correlate neatly with well-log stratigraphic boundaries using the ‘vertical reflection time vs depth’ chart of Wood 1969a. The MioceneÁPaleogene reflector does not correlate with Wood’s (1969a) well-log stratigraphy, but does tie in neatly with that of Ealing-1 and Leeston-1. The inconsistency has resulted in a reinterpretation of JD George-1’s well log (Jongens 2008). Boundaries are given in feet below ground level to directly relate to the well log of Wood (1969a). B, Southern part of line IP98-004 where it intersects the Rakaia Graben. C, Southern two-thirds of line IP98-003 where it intersects the Hinds Graben. For all figures, dashed blue lines represent major reflectors between seismic packages and thick black lines represent faults (arrows indicate sense of movement); thin blue vertical lines represent intersecting seismic lines and black horizontal lines represent two-way travel time (the top one 0 second and the next one below it 1 second). reveal two fault-bounded grabens at the base of the Late Hinds troughs from originally broader features identified by CretaceousÁPaleogene package (Figs 2B, 2C). These struc- Brown (1975). tures were depicted on QMAP Christchurch and Aoraki Most of the displacement on the Rakaia and Hinds cross-sections, and we formally define them here as the graben boundary faults accumulated during formation of ‘Rakaia Graben’ and ‘Hinds Graben’ (Fig. 1). Their the Late Cretaceous seismic package. Additional minor bounding normal faults can be interpolated between several normal displacement accrued on some of these faults during IP seismic reflection lines, indicating an east-southeast strike the late Cenozoic (Figs 2B, 2C). Late Cenozoic activity is (Fig. 1). Hummocky reflectors interpreted as Mount Somers well illustrated by rapid increase in thickness of the Miocene Volcanics are present on at least one horst margin of each seismic package south of the Rakaia Graben’s north- graben (Figs 2B, 2C). The general eastÁwest arrangement of bounding fault (Fig. 2B), and also highlighted by a change these grabens coincides with gravity residual anomaly lows in MioceneÁPliocene thickness at the Hinds Graben’s identified by Hicks (1989) who named them the Rakaia and south-bounding fault (Ealing Fault; Fig. 2C). Normal 172 R Jongens et al.

Figure 3 Two Indo-Pacific Energy seismic reflection profiles that cross the Ashley Fault Zone; locations are shown in Fig. 1. A, Full length of line IP99-106 crossing the Ashley Fault zone. Overall movement in the late Cenozoic is north-side down. The Ashley Fault (labelled) which has surface expression lies at the northern edge of the zone. The Late CretaceousÁPaleogene seismic package thickens towards the fault zone from the south. B, Northern two-thirds of line IP99-105 crossing the southern margin of the Ashley Fault zone west of Fig. 3A where there is no topographic expression of the fault. Line IP99-105 shows clear evidence of a Late CretaceousÁPaleogene half-graben on the south side of the fault zone. Refer to Fig. 2 for explanation of drawn lines. lst limestone. displacement on the Ealing Fault extends into the Quatern- (Forsyth et al. 2008). The surface position of this fold ary seismic package (Fig. 2C). Towards the range front (Fig. 2B) is close to the Rakaia Graben’s north-bounding however, where the two bounding faults of the fault, suggesting both structures may be related. Line Hinds Graben converge on to range-bounding reverse faults IP98-001, east of IP98-004 in Fig. 2B (see Fig. 1), shows (Fig. 1), the seismic imaging of the Ealing Fault is the same graben-bounding fault displacing the boundary ambiguous and could be interpreted as either steeply reverse between Quaternary and Pliocene units, indicating it was or normal (IP99-101). One possibility is that there has been active in the Quaternary and thereby supporting the idea reversal of movement on the Ealing Fault close to the range that the surface fold is an expression of the graben-bounding front. fault. West of Rakaia township, a subtle east-trending south- The seismic reflection surveys show minor faults with facing monoclinal fold deforms a late Last Glacial surface apparent normal displacement within the Hinds and Rakaia Faulting and folding beneath the Canterbury Plains 173 grabens. Farther west and northwest near the range front, westward to connect with the Glentui Fault zone (Fig. 1). surface mapping indicates northeast-striking contractional An alternative interpretation in this area is offered by faulting and folding (Cox & Barrell 2007). Overall, the data Ghisetti & Sibson (2012). Large-scale reactivation and suggest that late Cenozoic deformation beneath the southern intense deformation seen on IP99-106 near the AshleyÁ Canterbury Plains comprises minor extensional reactivation Okuku river confluence is not apparent farther west on of older graben-bounding normal faults that strike east- IP99-105. However, while IP99-105 may not have crossed southeast. Nevertheless, the orientation of the contemporary the entire Ashley Fault zone and may therefore not have stress field suggests that east-striking faults are currently imaged all the relative offset, it nevertheless shows optimally oriented for strike-slip movement (Sibson et al. Quaternary reflectors onlapping to tilted Pliocene reflectors 2011; Ghisetti & Sibson 2012) and a component of late (indicating significant movement pre-dating Quaternary Cenozoic strike-slip movement on these fault planes is deposition), as well as late Cenozoic reverse faults lying possible but difficult to demonstrate on 2D seismic reflec- just north of the Late CretaceousÁPaleogene graben-bound- tion lines. ing fault (Fig. 3B). These observations suggest that the bulk of late Cen- ozoic deformation on the Ashley Fault has transferred North-western Canterbury Plains south-westwards onto the Cust Anticline (see also Campbell Several obvious to subtle topographic irregularities that et al. 2012), although we infer that some late Cenozoic trend eastÁwest or northeastÁsouthwest on the north- deformation on the Ashley Fault zone continues westward western Canterbury Plains are attributable to active faulting onto the Glentui Fault zone (Fig. 1). and folding. In general, the surface expression of these faults Examples of northeast-oriented folds and faults are and folds is more subtle with increasing distance east of the preserved near Racecourse Hill west of Darfield and at range front. Springbank west of . At Racecourse Hill, and The east-striking Ashley Fault lies north of Ashley River continuing north-eastwards across the Waimakiriri River, and forms part of the Porters PassÁAmberley Fault Zone a ]1 km wide gentle anticline that warps the late Last (Cowan et al. 1996; Sisson et al. 2001). The Ashley Fault and Glacial surface (Fig. 1; Forsyth et al. 2008) is formally nearby Loburn fault have clear landscape expression, displa- named here as ‘Racecourse Hill Anticline’. Nearby ground- cing late Quaternary river terraces and channels (Sisson et al. water borehole logs (L35/0323, 324, 325) document 2001). East of Okuku River, apparent surface offset on the greensands (presumably pre-Quaternary in age) at only 11 m Ashley Fault is south-side down. In contrast, apparent surface depth on the northwest side of Racecourse Hill but thick offset west of Okuku River, and on the related Loburn fault, Quaternary gravels (]255 m) on the southeast side, is north-side down (Forsyth et al. 2008). Despite no clear suggesting a significant structural discontinuity. Seismic evidence for lateral movement (such as displaced terrace riser reflection line IP98-004, located 10 km to the southwest offsets), the overall fault trace morphology of the Ashley and along trend to the Racecourse Hill Anticline, reveals a Fault, including small fault-bounded depressions and wedges, northwest-dipping reverse fault with at least 200 m throw of suggests a strike-slip component (Sisson et al. 2001). Quaternary sediments and c. 700 m throw on underlying Seismic reflection lines IP99-106 and IP99-109 (Fig. 1) Kowai Formation Pliocene sediments (Fig. 4A). A similar, show complicated stratigraphic and structural relationships but less clearly resolved, fault was imaged northeast of across the Ashley Fault (Fig. 3A), whereas seismic line IP99- Racecourse Hill by Dorn et al. (2010). Collectively, faulting 105 farther west shows less intense late Cenozoic deformation along this northeast trend is referred to as the Hororata as well as clear evidence for a Late Cretaceous half-graben Fault (Fig. 1; Jongens et al. 1999; Dorn et al. 2010), and the and a thickened Paleogene sequence (Fig. 3B). The Arcadia-1 Racecourse Hill Anticline is inferred to be a fault-propaga- well, located south of the Ashley Fault between IP99-105 and tion fold overlying the Hororata Fault. The northwest- IP99-106, intersected a Late CretaceousÁPaleogene sequence dipping Hororata Fault is considered to be a major late that is unusually thick (c. 1200 m) for Canterbury. A Late Cenozoic reverse fault, with northeast-striking southeast- CretaceousÁPaleogene extensional structure, of similar char- dipping reverse faults to the northwest (e.g. the Cordys Flat acter to a Late Cretaceous half-graben described at the Birch and High Peak faults in the Malvern Hills shown on Fig. 1) Fault near the Waipara River c. 25 km to the north (Nicol inferred to be backthrusts to the Hororata Fault (Jongens et 1993), is therefore identified in the vicinity of Ashley River. al. 1999; Dorn et al. 2010). Based on this knowledge the Late CretaceousÁPaleogene half- Line IP98-004 also reveals a very broad anticlinal warp graben structure can be resolved in IP99-106 (Fig. 3A), with in the Quaternary seismic package east of the Hororata the additional observation that overall late Cenozoic move- Fault (Fig. 4A), and has been informally referred to as the ment is north-side down (Fig. 3A) in contrast to the late Hororata anticline (Jongens et al. 1999). Onlap of the base Quaternary sense of offset east of Okuku River. of the Pliocene seismic package onto the Paleogene south- We interpret the Ashley Fault to have partly reactivated east of the Hororata anticline (Fig. 4A) suggests growth of a Late CretaceousÁPaleogene normal fault that extends the fold began as early as Miocene. Growth of the Hororata 174 R Jongens et al.

Figure 4 Two Indo-Pacific Energy seismic reflection profiles that demonstrate northeast-trending contractional structures; locations are shown in Fig. 1. A, Northern part of line IP98-004. The Hororata Fault is shown at the northern end. Further south are possible footwall splays, with associated gentle folding at the surface (Hororata anticline). B, Eastern part of line IP98-002 crossing the Springbank fault- propagation fold structure, with an associated back thrust. Refer to Fig. 2 for explanation of drawn lines. lst limestone. anticline appears to reflect movement on northwest-dipping The Springbank Fault is a northeast-striking northwest- thrusts which are imaged at depth (Fig. 4A) that may dipping blind reverse fault imaged on two parallel seismic represent footwall splays of the Hororata Fault (Jongens lines IP98-002 and BP2 (Fig. 1). Clear fault dislocation et al. 1999). The anticline and its underlying thrusts coincide at depth passes upwards to monoclinally folded reflectors with an area of post-Darfield Earthquake uplift detected by (Fig. 4B). A gentle southeast-facing surface monocline forms differential interferometric synthetic aperture radar at the the southeast margin of an elevated middle Quaternary western end of the Greendale Fault (Beavan et al. 2010) as gravel terrace near Springbank. Line IP98-002 shows well as a northeast-oriented cluster of reverse-mechanism shallow reflectors approximately parallel to the topographic aftershocks (fig. 6 of Gledhill et al. 2011). These observa- relief of the surface monocline. The interpreted Miocene tions highlight blind thrust-fault deformation and growth of seismic package rapidly thickens east of the Springbank the Hororata anticline that occurred during the Darfield Fault, indicating initiation of the fault in the Miocene. The Earthquake. structure shows the characteristics of a fault-propagation Faulting and folding beneath the Canterbury Plains 175 fold. A similarly trending monoclinal warp deforming a late on these observations alone, the newly expressed east- Last Glacial surface is identified at Rangiora, 8 km east of striking right-lateral Greendale Fault most likely relates to the Springbank Fault (Forsyth et al. 2008), with a blind fault reactivation of a Late Cretaceous normal fault. Further inferred at depth. evidence supporting a reactivated origin for the Greendale Unlike seismic reflection lines that cross east-striking Fault is provided by seismic reflection lines immediately faults, lines that cross the northeast-striking Springbank and offshore, between the Ashley River and Christchurch, which Hororata faults show no indication of reactivation of an indicate PlioceneÁQuaternary contractional reactivation of older fault. That said, the northeast-striking Cordys Flat east-striking Late CretaceousÁPaleogene normal faults Fault in the Malvern Hills does show evidence of late (Barnes et al. 2011). Cenozoic reactivation with an inferred Late Cretaceous The Ashley Fault may be a more emergent but similar graben-fill conglomerate deposited on what is now the structure to that of the Greendale Fault (see Campbell et al. upthrown side of the fault (Browne et al. 2012). 2012). Both faults are optimally oriented for strike-slip Other seismic reflection lines along the north-western movement under the contemporary stress field (Ghisetti & Canterbury Plains display faults and folds similar to those Sibson 2012). Multiple fault strands of the Ashley Fault described above, and relate to either east- or northeast- zone resembling flower structure above the reactivated trending emergent structures, or a combination of both (e.g. graben-bounding fault in Fig. 3A, together with overall Cust Anticline). In contrast to southern Canterbury, late fault trace morphology, indicates the Ashley Fault has a Cenozoic structures below the north-western Canterbury strike-slip component. Bearing this in mind, the strike-slip Plains are contractional, although a component of strike-slip Greendale Fault may at depth consist of multiple strands, movement is suspected, particularly on those faults with rather than being a simple reactivated plane. easterly strike (Campbell et al. 2012).

Conclusions The missing Miocene Seismic reflection surveys reveal eastÁwest-aligned Late Apart from strong reflectors inferred to be localised volcanic Cretaceous grabens beneath the southern Canterbury Plains. rocks, the Miocene seismic package in all IP seismic lines is Some of the graben-bounding normal faults were reactivated very thin or absent in an area north of the Rakaia Graben, as late Cenozoic normal faults with minor offset. south of the Ashley Fault and west of the Springbank Fault. Late Cenozoic reverse faults and folds, including fault- Non-volcanic Miocene sediments are absent in the Arcadia- propagation folds, are revealed underneath the north- 1 and Leeston-1 wells, and in geological exposures in the western Canterbury Plains. In combination with emergent foothills within the area defined above (Andrews et al. topographic expression, these structures are oriented east or 1987). Reappearance or rapid thickening of the Miocene northeast. The best example of an east-striking structure is seismic package adjacent to faults such as may be seen in the Ashley Fault which displays late Cenozoic reactivation Figs 2B, 3A and 4B suggest that late Cenozoic faulting of a Late CretaceousÁPaleogene normal fault. The Hororata associated with the current plate boundary regime began as and Springbank faults are examples of northeast-striking early as the Miocene in this part of the Canterbury Plains faults at depth expressed at the surface as active fault- area, possibly as early as Early Miocene because the oldest propagation folds. regressive Miocene siltstones are of this age (Andrews The east-striking right-lateral Greendale Fault is most et al. 1987). likely associated with a reactivated Late Cretaceous normal fault. Implications for the Greendale Fault Seismic reflection lines demonstrate that eastÁsoutheast- Acknowledgements striking faults identified beneath the Canterbury Plains We thank Dave Bennett and Scott Langdale, formally of accrued normal displacement during Late CretaceousÁ Indo-Pacific Energy and Austral Pacific Energy, who provided Paleogene extension. In the south, graben-bounding faults the seismic reflection data for incorporation into QMAP. GNS show further minor normal offset in the late Cenozoic; in the Science provided funding to prepare this manuscript. We thank the north, the Ashley Fault has reactivated in the late Cenozoic external reviewers, Andy Nicol and Tim Little, for helpful as an apparent reverse fault. Residual Bouguer gravity comments that improved the clarity of the manuscript. anomalies (Bennett et al. 2000; Austral Pacific Energy 2004) highlight east west-oriented gravity lows correspond- Á References ing to thick sediments in the Rakaia and Hinds grabens. Of Andrews PB, Field BD, Browne GH, McLennan JM 1987. particular note is a less well-developed eastÁwest-aligned Lithostratigraphic nomenclature for the Upper Cretaceous gravity low immediately south of the Greendale Fault, and Tertiary sequence of Central Canterbury, New Zealand. suggestive of a graben or half-graben structure here. Based New Zealand Geological Survey Record 24. 40 p. 176 R Jongens et al.

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