Faulting and Folding Beneath the Canterbury Plains Identified Prior to the 2010 Emergence of the Greendale Fault
Total Page：16
File Type：pdf, Size：1020Kb
New Zealand Journal of Geology and Geophysics ISSN: 0028-8306 (Print) 1175-8791 (Online) Journal homepage: http://www.tandfonline.com/loi/tnzg20 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. Submit your article to this journal Article views: 527 View related articles Citing articles: 18 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tnzg20 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, Christchurch, 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 Rakaia 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; Hororata 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 Glentui Fault zone, HF Hororata Fault, Ha Hororata anticline, HPF High Peak Fault, Lf Loburn 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 Leeston-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 Rakaia River, 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